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==INDEX==
A TEXT-BOOK
 
 
 
OF
 
 
 
EMBRYOLOGY
 
 
 
FOR STUDENTS OF MEDICINE
 
 
 
BY
 
John Clement Heisler, M.D.
 
 
Professor of Anatomy in tiif Mkdico-Chiriikgical Collecb,
 
Philauelfhia
 
 
 
http://archive.org/details/atextbookembryo01heisgoog
 
 
WITH 2J2 ILLUSTRATIONS, 32 OF THEM IN COLORS
 
 
 
XCbir5 E5ition, xrboroiujbls lRcrisc&
 
 
 
l>HILAI)i:i,I'inA AND LONDON
 
W . B . S A U X \) E R S C O M P A N Y
 
1907
 
 
 
Set up. elcctrotyped. printed, and copyriKhted October, 1899. Revised, reprinted.
 
and recopyrightcd September, i^:>i. Reprinted September, 1902. and
 
September, 1905. Revised, reprinted, and recopyrighted March, 1907
 
COPYKIGHT, 1907,
 
By W. B. SAUXDERS COMPANY.
 
 
 
 
 
 
• • •
 
• • •
 
 
 
kLECTROTVPCD BY
WISTCOTT a THOMaON. PHILAOA.
 
 
 
PRESS OF
W. B. BAUNDCBS COMPANY
 
 
 
 
 
 
PREFACE TO THIRD EDITION
 
 
 
The great activity along the lines of embrvologieal
research during the past half-dozen years has brought forth
much literature and some new facts. In some cases existing views have thereby been modified or set aside; in
others, they have been more firmly established. In presenting the third edition of this book the effort has been
made so to revise the text as to harmonize it with the
results of recent researches. To this end, certain of the
sections have been practically rewritten, while others have
been slightly altered, and still others have been merely
somewhat amplified. Where changes have been made the
authorities therefor have usually been cited. The revised
portions of the book include the sections dealing with the
ovum, the spermatozoon, the blastodermic vesicle, the amnion, the vascular system, the pancreas, the spleen, the
larynx, the thymus, the thyroid, the parathyroid, the
adrenal, the kidney, the spinal cord, the vitreous, the
musculature, and the vertebral column.
 
J. C. H.
 
3829 Walnut St., Philadelphia,
March, 1907.
 
 
 
PREFACE
 
 
 
The facts of embryology having acquired in recent years
such great interest in connection with the teaching and with
the proper comprehension of human anatomy, it is of first
importance to the student of medicine that a concise and yet
sufficiently full text-book upon the subject be available. It
was with the aim of presenting such a book that this volume
was written, the author, in his experience as a teacher of
anatomy, having been impressed with the fact that students
were seriously handicapj>ed in their study of the subject of
embryology by the lack of a text-book full enough to be
intelligible, and yet without that minuteness of detail which
characterizes the larger treatises, and which so often serves
only to confuse and discourage the beginner.
 
In the arrangement of the subject-matter of the book, it
has been the aim not only to present a connected story of
human development, but also to make each chapter as nearly
as possible complete in itself, for the sake of convenience of
reference. It is for this reason that some repetitions occur
in the text. The frequent allusions to certain facts of comparative embryology are rendered necessary by the very
nature of the subject, but it has been the writer's aim to make
these allusions as simple and as easily intelligible as possible.
 
In the selection of the illustrations, great care has been
exercised to employ those of the greatest teaching value, and
to arrange them, with reference to any one chapter, as nearly
 
 
 
 
 
 
 
10 PREFACE,
 
as possible in proper chronological sequence. Due acknowledgement is made in each case for every illustration borrowed
from other works.
 
With few exceptions, no attempt has been made to cite
authorities in the text, and the author would here express
his obligations to the writings of His, O. Hertwig, Kolliker,
Schultze, Bonnet, Balfour, Marshall, Piersol, Minot, Tourneux, and many others.
 
J. C. xl.
 
3829 Walnut St.,
 
PllILADliI.PUIA.
 
 
 
CONTENTS.
 
 
 
CHAPTER I.
 
The Male and Female Sexual Elements ; Maturation ; Ovulation; Menstruation; Fertilization 17
 
The Spermatozoon 20
 
The Ovum 24
 
The Hen's egg 27
 
Oogenesis 29
 
Maturation of the Ovum 32
 
Ovulation 80
 
Menstruation 38
 
The Relation of Menstruation to Ovulation and Conception . . 40
 
Fertilization 41
 
Artificial Fertilization 44
 
 
CHAPTER II.
 
The Segmentation of the Ovum and Formation of the Blastodermic Vesicle 45
 
Segmentation 45
 
The Stage of the Blastula 49
 
 
CHAPTER III.
 
The Germ-layers and the Primitive Streak 52
 
The Two-layered Stage of the Blastodermic Vesicle . . 52
 
The Embryonal Area 58
 
The Primitive Streak 59
 
The Development of the Mesoderm 62
 
The Derivatives of the Germ-lavers 67
 
 
CHAPTER IV.
 
The Beginning Differentiation of the Embryo; the Neural Canal ; The Chorda Dorsalis ; the Mesoblastic Somites m
 
The Neural or Medullary Canal 7()
 
The Notoohord or Chorda Dorsalis 7.'>
 
The Neurenteric Canal 74
 
The Somites or Primitive Segments 75
 
CHAPTER V.
 
The Formation of the Body-wall, of the Intestinal Canal, and of the Fetal Membranes 79
 
The Formation op the Body-wall and of the Intestinal Canal op the Embryo 79
 
The Amnion 82
 
The Yolk-sac 87
 
The Allantois 89
 
The Chorion 92
 
 
CHAPTER VI.
The Decidual and the Embedding of the Ovum; the
 
Placenta; the Umbilical Cord 95
 
The Decidua and the Embedding of the Ovum 95
 
The Placenta 98
 
The Umbilical Cord 102
 
Relations of the Fetal Membranes at Birth 104
 
 
CHAPTER VII.
 
The Further Development of the External Form of the Body 105
 
The Stage of the Ovum 106
 
The Stage of the Embryo 107
 
The Visceral Arches and Clefts Ill
 
The Stage of the Fetus 118
 
 
CHAPTER VIII.
 
The Development of the Connective Tissues of the Body, and of the Lymphatic System 124
 
The Connective Tissues 124
 
The Development of the Lymphatic System 127
 
CHAPTER IX.
 
The Development of the Face and the Mouth Cavity .... 130
 
The Evolution of the Face 130
 
The Mouth 134
 
The Teeth 137
 
The Salivary Glands 143
 
The Tongue 143
 
The Nose 145
 
 
CHAPTER X.
 
The Development of the Vascular System 147
 
TnK Vitklline Circulation and the Origin of the Blood 147
 
TlIK I)KVELOrMKNT OF THE HeAUT 152
 
The Metamorphosis of the Single into the Double Heart .... 156
 
The Valves of the Heart 161
 
The Allantok- and the Placental Circulation 163
 
 
The Fetal Arterial System 165
 
The Fetal Venous System 169
 
The Formation of the Pericardium, the Pleur^ and the
 
Diaphragm 174
 
The Portal Circulation 177
 
The Final Stage of the Fetal Vascular System . . . 181
 
 
CHAPTER XI.
 
The Development of the Digestive System 185
 
The Mouth 192
 
The Pharynx 193
 
The Tongue 194
 
The Tonsil 194
 
The Anus • 195
 
The Differentiation of the Alimentary Canal into Separate Regions 197
 
Increase in Length and Further .Subdivision 201
 
Alteration in the Relative Position of Parts, and Further Development 202
 
Histological Alterations 205
 
Meckel's Diverticulum 207
 
The Development of the Liver 207
 
The Gall-bladder 209
 
The Ligaments of the Liver 209
 
The Development of the Pancreas 211
 
The Development of the Spleen . . 212
 
The Evoution of the Peritoneum 214
 
CHAPTER XXL
 
The Development of the Respiratory System 222
 
The Thyroid, the Parathyroid, and the Thymus Bodies . . 226
 
CHAPTER XIIL
 
The Development of the Qenito-urlnary System 232
 
The Development of the Kidney and the L'reter . . . 232
 
The Mesonepliros or Wolffian Body 234
 
The Metanephros or Permanent Kidney 237
 
The Suprarenal Bodies 241
 
The Development of the Internal Generative Organs . 243
 
The Indifferent Type 243
 
The Male Type 24')
 
The Female Type 249
 
The Bladder and the Prostate Gland 25')
 
The External Organs of (t feneration 2'>s
 
The Female External Genitals 25l>
 
The Male External Genitals . 2t)l
 
Summary 2<>4
 
 
 
14 CONTENTS,
 
CHAPTER XIV. PACK
 
The Development of the Skin and its Appendages 268
 
The Skin 2G8
 
The Appendages of the Skin 270
 
The Nails 270
 
The Hair 271
 
The Sebaceous and Sweat Glands 273
 
The Mammary Gland 274
 
 
 
CHAPTER XV.
 
The Development of the Nervous System 278
 
The Development of the Spinal Cord 281
 
The Development of the Brain 286
 
The Fifth Brain-vesicle 289
 
The Hind-brain Vesicle 292
 
The Mid-brain Vesicle 294
 
The Inter-brain Vesicle 296
 
The Fore-brain Vesicle 302
 
The Development of the Peripheral Nervous System . 316
 
The Development of the Sympathetic System 324
 
The Carotid Body, the Coccygeal Body, and the Organs of Zuckei^
 
kandl * 325
 
 
 
CHAPTER XVI.
 
The Development of the Sense Organs 32(}
 
The Deveix)pment op the Eye 326
 
The Retina and Optic Nerve 328
 
The Crystalline I-«ns 336
 
The Vitreous Body 338
 
The Middle and Outer Tunics of the Eye 339
 
The Eyelids and the Lacrimal Apparatus 343
 
The Development of the Organ of Hearing 345
 
The Internal Ear 346
 
The Middle and External Ear 355
 
The Development of the Nope 358
 
 
 
 
 
CHAPTER XVII.
 
The Development of the Muscular System 363
 
The Striated or Voluntary Muscle 363
 
The Muscles of the Trunk Proper 363
 
The Metamorphosis of the Muscle-plate 366
 
The Branchial Muscles 369
 
The Muscles of the Extremities 370
 
The Involuntary or Unstriated Muscular Tissue .... 371
 
The Cardiac Muscle 371
 
The Development of the Skeleton and of the Limbs .... 372
 
The Axial Skeleton 373
 
The Development of the Trunk 373
 
The Stage of the Chorda 373
 
The Membranous Stage 374
 
The Cartilaginous Stage 377
 
The Osseous Stage 379
 
The Development of the Ribs and Steraum 382
 
The Development of the Head Skeleton 384
 
The Membranous Cranium liSo
 
The Cartilaginous Cranium 38()
 
The Osseoufi Stage 389
 
The Appendicular Skeleton 402
 
The Pectoral and Pelvic (iirdles 403
 
The Bones of the Extremities 404
 
The Development of the Limbs 40t>
 
The Position of the Limbs . 407
 
Tabulated Chronology of Development 409
 
Index 417
 
 
 
CHAPTER I.
 
THE MALE AND FEMALE SEXUAL ELEMENTS;
MATURATION ; OVULATION ; MENSTRUATION ;
FERTILIZATION.
 
Embryology is that department of biology which treats
of the generation and development of organisms. It may
refer to the development of the race or stock — ^Phylogeny — or
to that of the individual — Ontogeny ; again, it may treat of
animal or of vegetable development.
 
Since no observations have been made upon embryos
of an age less than four or five days, and but few, indeed,
upon those younger than sixteen or eighteen days, we
cannot be said to possess definite knowledge of the very
earliest processes of development in man. There is, however, sufficient analogy between the known facts of human
development and those of corresponding stages in allied
groups of animals, as well as between the various groups of
animals themselves, to establish certain broad general principles of agreement in essential features. In tracing the history of human development, therefore, frequent recourse
must be had to the development of animals, since in this
way only is it possible at present to fill up the gaps in our
knowledge of human embryology.
 
That a new individual may be called into existence, the
union of the male element, or spermatozoon, with the female
element, or ovum, is necessary. Such union is variously
called fertilization, fecundation, and impregnation.
 
Prior to the beginning of the present century, little or
nothing was definitely known concerning reproduction and
development. The opinions of the biologists of early times
found expression in a theory which was then called the theory
of imfoMing or of evolution, but which more recently has
 
2 17
 
 
 
18 TKXT'IiOOK OF KMIiRYOLOGY.
 
\wf*u d(?Hi^natf'<l flic; prefnrmntion theory, Acconling to this
dortririe, th^* egg or germ contained all the parts of the adult
org^misrn in an exeeniingly minute condition, and <lcvelopment eonsisted in the simple growth or nnfolding of already
formed |Mirts. v\h the throri/ of unfoltlinf/ impliiH^I the pn:formation not only of the immediate hnt of all subsc»quent
offspring, its votaries were ahle to eompnt(; that the ovary
of VjVi* e4>nt4iinHl 2()0,(KK) millions of hnman germs.
 
With th<' (liw^overy of the s|M*rmatozoon in 1H77 by Hamm,
a pnpil of Ii<»nw<'nh(M'ek, a ermtroversy arose as to whether
it was th<' spermatic^ fdament or the ovum that contained the
germ. Those who maintained the former view were known
as animahiiliHtH ; thow who h<!ld the latter, as or/Vx. According to the opinions of th<* anirnalcniists, tin* spermatozoon
was the complete organism in miniature, and it re(|uired for
its growth the soil or environment which the ovum alone
could furnish.
 
The enunciation l)y Wolff, in I TolJ, of his (Utcfrhic ofepif/vneSM completely overturned the preformation theory. Wolif
maintained that the g<'rm was ininrf/ftnizrd mntfcr^ and that the
union of male and female mat<Tial was <'ss(>ntial to repnnhiction. While Wolff's thcorv was in the main corn'ct, it remained for lat<»r in vest igji tors to show that the ovum did not
consist of unorgjini/ed mntt^M*, as he thought, i>ut that it |missesswl definite^ structund characteristics. Thus, tin* g(»rminal
vesicle of the hen *s egg was discovered in 1825 |)y Purkinje,
and the germinal spot in lS2tl i)v Wagner. SM)n after the
enunciation of the celI-<loctrine i>v S'hicidcn and S-hwann,
it was seen that the ovum was in reality a typical cell, possessing all the jMirts of such a structure
 
It was not, however, until about th<* year IS 10 that it was
shown, by KoUiker, Reiehert, and others, that the spermatozoa
are the active agents in fecundation. I*n»vious|y it had Inrn
held, since the refutation of the pivformation theory, that the
seminal fluid pcrfomic<l this function, and that tli<' spermatozoa were parasitic organisms.
 
Thclengtli of time necessary for the devt»lopmcnt of the
new individual varies according to the spcnics ; in man it
 
 
 
MALE AND FKMALE SEXUAL ELEMEHTS. 19
 
occupies nine calendar months or about ten lunar moDths —
that is, from 273 to 280 days. The period of human gestation is arbitrarily divide(] hy His into three stages : (I) The
 
 
 
ArtirnmaUn part qf paranudeia.
 
 
 
Nucienloi in dtvision.
 
 
 
 
■lo. 1.— I to B, Various slaxeB of
 
ioa<,fthediig;10,M
Boaaetl.
 
 
 
stage of the ovum, comprising the first two weeks of development ; (2) The stage of tlie embryo, extending from the en<l
of the second woek to the fifth week, during which time the
germ begins to assume definite form ; and (3) The stage of
 
 
 
20
 
 
 
TEXT'IiOOK OF EMIiHYOLOGY,
 
 
 
the fetus, whicli incliides the rc;niain<ler rif tlie term of intrauterine existence.
 
It n)ay he pointe<l out that the term omm^ as employe<l in
cmhryoh)^y, has three (liflerent significations : it designates
the female sexual cell prior to its impregnation ; it is used in
the sense note<l alxivt? to doignate the fertilized egg; and it
is somewhat Ioos<'ly appliitl to the pnxluct of conception
during various stages of d(fV(!lopment.
 
THE SPERMATOZOON.
 
It is noteworthy that l)oth sp<!rmatozna an<l ova — that is,
lM>th s(»xual <*ells — are priMJucts of metamorphoses taking
place in epithelial structures, the former heing derived from
the spermat<»g<Miic cells found in the sc»miniferous tubules of
 
the testicle, while the latt<T come from
tlie germinal epithelium of the ovary.
Till* form of the seminal fdameut varies greatly in dillrrciit spc'<*i<»s i Fig. 1),
hcin;j: usually an el«»uirat<'d Hairellate
cell. I'hc human spermatozoon (Fig. 2)
is ahout O.tJo mm. ( . ,\„ inch) in length,
consisting of a head, a middle piece, a
tail or flagellum, and an end-piece.
 
The head i> murh thii^kcucd as eom|mriMl with llu' otlu'r M'giut'uts, appearing egg >ha|H'd as m-cu upon its hroader
surtacc, the >malh'r (»xi remit v l>riii«r
iviuiccIihI with the middle pircr ; srcn
in pri>Sle, it is rt»nvr\ on one >i»li' ami
concave on the other. Tho middle
piece is somewhat longer and much
thinner than the lu'ad. whilo the tail
is a slender filament ^lii^htlv mnn* than
four-fifths of the entiri' Irnirih i»l' tin5^vnuato/.<H»n. I-ying in ilir i-iiitrr
of the sjwrmato/iHin, and «\t«iidiii:^
thrnu$:\iou' h- eiiim- ImriK i? the slender axial fiber, wliiili
i^ \»nj\uii|:»f<.l tii;riiiiy tt^-yimd ihe wil as the end-piece «m- ter
 
 
Fib n.— Human ♦'per
iniuumi laftuT l:fiz.u> :
'1. ipeniuunxiiim »it'n <%
)wt . h. bwd . a., middle
iJt ^'i MRSU irOlL. tiK' iidb.
 
 
 
THE SPERMATOZOON.
 
 
 
21
 
 
 
minal flUment. At the anterior end of the axial fiber is a
small body, the end-knob (not shown in tlie figure).
 
The power of locomotion which the spermatozoon exhibits
b conferred by the vibratile movement of its tail, aec«impanied
by a rotation about its long axis through an arc of 90 d^rees.
The rate of progression is about 0.05 or 0.06 mm., or its own
length, per second.
 
Spermatozoa possess remarkable vitality, remaining active
in the male genital tract for several days after death. In the
genital passages of the female, they may retain their activity
for several weeks, and when mounted and protected from
evaporation they have been known to show vibratile motion
after the lapse of nine days (Piersol). Weak alkaline solutions render them more active, while acids, even quite dilute,
destroy them. The spermatozoa of the bat, being deposited
in the female genital passages in the autumn, retain their
power of fecundating ova until the following spring.
 
Spermatogenesis. — The details of spermatozoon-formation,
or spermatogenesis, vary in different animals. A cross section of a seminiferous tubnle of a mammal (Fig. 3) shows a
layer of cuboidal cells called parietal cells, lying in contact
 
 
 
 
'•cella; it, Bjicmiatoblaels.
 
 
 
with the ba.semenf membrane of the tubnle wall. This layer
consists of tlio so-called Bertolli'a colnmns, or snstentacnlar
cells, and of the spermatogenic cells or spermatogonia. The
su^tentaculnr cells are merely supporting ; the spermatogenic
cells give rise to the siiermatozoa.
 
 
 
22 TEXT-BOOK OF EMBRYOLOGY.
 
Tlie spermatogonia undergo re{)catcHl mitotic division with
a concomitant decrease in size. The last generation of the
spermatogonia, after an intervening jxiriod of growth, give
rise, also hy mitotic division, to the mother-cells or primary
spermatocytes, which lie nearer the lumen of the tubule than
do their predecessors. The primary s|K»rmatocytes now divide to form the daughter-cells or secondary spermatocjrtes,
and these in turn undergo division to form the spermatoblasts
or spermatids. From the s}>ermatids, by rearrangement of
their constituent elements and certain special modifications
in form, are produced the spermatozoa.
 
Xot all the details of the differentiation of the sjxjrmatozoon from the spermatid are as yet clear; moreover, these
details vary somewhat in different spi*cies. As observed in
mammals, the nucleus of the s|K»rmati(l becomes somewhat
flattened and elongated to become finally the head (nucleus)
of the spermatozoon. The centrosonie migrates to that side
of the nucleus which is toward the lumen of the tubule, becoming attache*! here to the nuclear membrane, while the
attraction sphere (arehoplasm) goes to the opposite side. The
attraction sphere produces the head-cap and lance which are
present in the spermatozoa of some mammals. From the
centrosome a delicate filament grows through the cytoplasm,
toward the lumen of the tubule, the centrosome itself — or
centrosomes, as there may be two — giving rise to at least the
neck of th(» middle piece, that is, the part adjoining the head
(Meves and Iji'uhoss^k) or, acconling to others, persisting
as th(^ end-knob. Although the axial filament seems to grow
forth from the centrosome it i?s believed l)V Meves that it is differentiated from the cytoplasm of the spermatid, which latter
also gives rise to the remaining part of th(» middle piece an<l its
sheath as well as to the tail and its sheath. Meanwhile the
cytoplasm in relation with the nucleus is reduced to an exceedingly thin layer, a portion of it being cut off* in some eases.
 
During th(» metamorphosis of the spermatids the Sertoli!
cells incri^ase in size, elongjuing toward the lunien of the
tubule. To each such S<TtoIli column a number of s]>ermatids become attached, the Sertolli cell iH'ing apjKirently used
up in yielding nourishment to the deveh)ping s|>erm-cells.
 
 
 
THE SPERMATOZOON. 23
 
The descent of the spermatozoa from the si)ermatogonia is
accompanied by a peculiar modification of ordinary mitosis
known as the reduction of the chromosomes, or reduction-division. The spireme, or chromatin thread, of the ordinary
cells of the body, known as body-cells or somatic cells in
contradistinction to the reproductive or germ-cells, breaks up
at the beginning of mitosis into a definite number of segments or chromosomes, which number is constant and characteristic for the species. Thus in man, the guinea-pig, and
the ox, there are sixteen chromosomes ; in the mouse, the
salamander, and the trout, twenty-four; in some sharks,
thirty-six ; in the grasshopper, twelve. After the division
of the chromatin into the characteristic number of chromosomes, in the case of a somatic cell, each chromosome splits
longitudinally, so that each daughter-nucleus receives as
many chromosomes as there were in the parent cell. Thus
the number of chromosomes remains constant notwithstanding re])eated cell-divisons. In the division of the germ-cells,
however, an important mo<lification of this process has been
observed, resulting in the reduction of the number of chromosomes. This reduction-division occurs both in the development of the spermatozoon and in the " maturation '' of
the ovum. The essence of reduction-division is that in the
germ-cell the chromatin divides into half as many chromosomes as in the case of the somatic cell, but these chromosomes are tetrad and hence are equivalent to double the
number of the somatic chromosomes ; and that two subsequent cell-divisions occur without intervening reconstruction
of the nucleus, so that one element of each tetrad passes to
each one of the four descendent cells. Thus each of the four
cells descended from any one germ-cell containshalf as many
chromosomes as a somatic cell, and when, during fertilizaticm,
such a (male) germ-cell unites with a similar (female) germcell the normal numl)er of chromosomes is restored.
 
During spermatogenesis the multiplication of the spermatogonia is effected by the usual method of mitosis, as is also
tlie formation of the primary spermatocytes from the last
generation of the spermatogonia, but ^vhen the primary
spermatocyte enters upon the process of mitosis its chromatin
 
 
 
24
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
divides (in the case of the Ascaris megalocephala whose
somatic chromosomes number four) into two chromosomes,
each chromosome being tetrad. Without reconstruction of
the nucleus the primary spermatocyte divides into two secondary spermatocytes (Fig. 4), each tetrad chromosome
dividing into two dyads, one for each new nucleus. Again,
without reconstruction of the nucleus, each secondary spermatiKjyte divides into two spermatids, each dyad breaking
 
 
 
Pritnordtal sexual cell.
 
o
 
 
 
Zone of proliferation. {TAe j^ene rations are tnuch
larger.)
 
 
 
TUmt of growth.
 
 
 
y,perMittiHyte I ntdrr.
 
'.petmatiK yte, II onler.
!,f'frmiitul»
 
 
 
Zone of maturation.
 
 
 
Vui. i. - Hi:h<!ffiiit Irr <lin((ruin of HfHfrmatogenesis as It occurs In ascaris (after
Ii<»virl;. (• KrK'«:tm. <1. Aimt. u. Entw.," Bd. L)
 
u|) into two rtiii^i<? (rhromosomes, one for each spermatid.
Thiirt ciu'h ^|NTlllati(l (contains half as many chromosomes as
the Hituiiiiir iiuiiilxT (•liara(!teristic for the sj>ecies, and each
primary HporrnatiM-yU? is the parent of four spermatids, which
riubn<M|U(;iitly become fum^tional spermatozoa.
 
THE OVUM.
 
The female sexual cell or ovum is remarkable among animal
cells for its size, it beinj^ a rule, to which there are no known
exceptions, that it is much larger than any other cell in the
b<Mly of the parent. The human ovum measures, in the mature
stjite, 0.2 nun. in diameter.
 
In structure, the ovum presents the parts of a typical cell ;
 
 
 
THE OVUM.
 
 
 
25
 
 
 
namely, a cell-wall, here called the vitelliae membrane, the
cell-contents, or vitellna ur yolk, a nucleus or germinal vesicle, and a nucleoliis or germinal spot.
 
Surrounding tbe ovum is a st)niewliat loosely -fitting transl)arent, elastic envelope, the zona pellucida, and outside of
this is the corona radiata. These two layers are often re
 
 
 
Ftn. y—Eft!! Ctom a rnbbll's rolUclc nhicli was n.:> Dim, (th loflhl In diameter
(•(ler W«lderer). It is mrroundtd by Ihu- Ii>no pelliirldu (t p.). ou which IhiTe
rvsl It one pince rolliealur L-ells I/, i.). Ttie yolk conUiint deutoplasmjc gruiulci
('tl. In the grtinlUBllve vwklo il. b.\ the niirlMr uHtwork (l, n.) Ib eapwlallr
 
ferrcd to as the egg-eoTelopes ; but since they are contributed
by the discus proligerus of the Graafian follicle, it must be
remembered tliat they arc iif>t. properly speaking, a part of
the ovum . Between the zona jielliicida and the ovum is the
small perivitelline space. The radial striation of the zona is
generally regarded as due to the presence of minute lauals
opening into this space. The canals are thought by some to
facilitate the ingres."? of spermatozoa, thus corresponding in
function to the micropyle, a small aperture found in the leas
easily penetrable egg-envelopes of many invertebrates and
of some lis lies.
 
 
 
26 TEXT-HOOK OF EMBRYOLOGY.
 
The TiteUine membrane does not call for extended description. It tu:iy be regarded aw a slightly speciullzed condeDsation of the i>eriplit.Tal jwrt of the eel 1-cont eats.
 
The viteUiu, or cell-contents, here, as in other cells, ia
essentially protoplaam or cytoplasm, to which is added material called deutoplaam, designed for the nutrition of the ovtim
at the beginning of development. The pmtoplasm is also
called the fonnative yolk ami the egg-plasm, while thedeutoplasm is known as the nutritiTe ya\^. In the hiimuD ovum
these elements are more or less uniformly distribulLtl j there
is, however, a difioreutiation into an inner, slightly less clear
region, containing more yoik-granules (deiitoplaam) and a
peripheral, clearer zone. The chahictoristic transparency of
the human e^-eell is due to tlie fact that the deutoplasmic
particles found in it arc not clondy as in the ova of other
mammals. The following classification of ova by Balfour is
based upon the arrangement of these constituents :
 
1. Aleclthal ova are those in which the pnitoplasm and
 
 
 
Ftn. a.— Dl«Bfiiin n' "n egg wmi thp V\., -.■nllhllic
 
niilriHve yolk in ■ pol»r iimitioQ. The nulriiu. ■: ■ riii.r. TIib
 
fljim>tlTe;iiUconHl<[uU!siittbcaiiltda1 gcnniiiiiiiv. vc^kic i>. '> i <><'<:u|ik-B llie
 
poleU.P.|.«germ-aitkiA-."-A,linwhi(Th inidill.^ .,f Oit^ miirltlve yolk (n. d,),
 
the germlnatlvi! Teslcle It.b.l In en- which In viivclopeil la a mannc iif
 
ulMied. The nutritive yolk m.rf,! nils furniiilivB yolk (d-d.) (Hertwlg),
the rMl of Ihe ciw up lo Ihe vpgetutlvo
polu (r.P.) (Htrtwig),
 
deiitoplasm are uniformly distributed, as in the ova of Mammalia, hiclilding man), and of aniphioxus (Fig. o).
 
2, Telolecithal ova are those in which the reialivelv abuii
 
 
THE OVUM. 27
 
dant dcutoplasm is accumulated at one side of the ovum,
called the vegetative pole, while the protoplasm appears as a
flat germ-disk at the animal pole on the op]K)site side. Here
belong the eggs of birds, reptiles, and bony fishes (see Fig. 6).
 
3. Centrolecithal ova are those in which the deutoplasm is
centroly the protoplasm completely surrounding it, as in the
eggs of arthropods (Fig. 7).
 
Ova are classified also according to their method of segmentation. This will be described later.
 
The germinal vesicle or nucleus is the most imj)oi'tant part
of the cell, since, as will be seen hereafter, it is essentially
by the conjugation, or more accurately by the fusion, of the
nuclei of the male and female parent-cells that generation is
effected. As a rule, there is but one nucleus, though there
may be two. Its position is usually — if not universally —
eccentric, this being more marked where there is a distinct
differentiation into animal and vegetative poles, in which case
it is found always near the animal pole. It is nearly spherical in shape, and like the nucleus of any other typical cell,
it is composed of the nuclear network consisting of linin and
cliromatin, and nuclear juice or achromatin, the former containing the latter within its meshes. Surrounding the nucleus
is the well-marked nuclear membrane, while within it is the
nucleolus or germinal spot. The latter may be single or
multiple, according to the species, though the number is fairly
constant for each species. Nagel ascril)es ameboid movement to the germinal spot.
 
Polarity. — The polarity of the egg has been incidentally
referred to. Apparently it owes its existence to the eccentric
position of the nucleus, the animal pole being that point on
the surface to which the nucleus is nearest. Polarity bears a
significant relation to the specific gravity of the ovum, since
the nucleus reaches the surface of the latter at the animal
pole and there extrudes the polar globules ; and it is also
related to the segmentation of the fertilized e^g.
 
The Hen's Egg. — As the hen's Q^g is so largely utilized
for the study of development, it will be profitable to consider
briefly its structure. The ovum or egg-cell is represented by
 
 
 
28 TEXTBOOK OF EMBRYOLOGY.
 
tlie yolk or yellow of the egg, the alhuinen or white, as well
as the shell ami filiell-meiiibraiie, being egg-envelopes contributed by the oviduct. As in other ova, the egg proper is
a Bingle cell, having a vitelline membrane and a germinal
vesicle. The enormoUH size of the cell is due to the large
quantity of nutritive material or dcutoplasm present, this
contriiiuting by far the greater part of the bulk, while the
nmch Bmaller formative yolk or protoplasm, containing the
germinal vesicle, is so eccentrically placet! that it seems to
float upon tliu surface of the deutoplosra. The little whitish
flpot on the surface of the yolk, known as the cicatricnla or
garmlnative disk, consists of the germinal vesicle with the
Niirrouniling formative yolk. It is in the germinative disk
niorii- ihiit .-^fomentation takes place, and it is for this reason
that i-gtfK iif iliis class are designated meroblaatlc, or partiallydMdiag «BKB'
 
The dnitopluMm is made up of white ami of yellow yolk
 
 
 
 
 
 
Ill iinlnru
 
 
Mted hi-n's egt! (ntti-.T
 
 
 
 
 
 
while y..lfc.
 
 
*hl<'h
 
 
 
 
umlwr »t <■
 
 
mc«.,tric Uy
 
 
 
 
■ tlrhln*
 
 
 
 
z. & (omgwli
 
 
Ifluiil
 
 
i.ivloi*.
 
 
the yolk: u
 
 
albumpn, eo
 
 
i.p<»cd
 
 
.. «ul<l portion.; fh.
 
 
. chain™ : a.
 
 
(*, air
 
 
 
tl|l%. *k 'VW VWMMfr 4Vtellkl« uf » thin tnyer spread over
^«UfcidbM ^'i ^ ^^ *^ * "^^^^ ^**^ known a.s Pander's
 
 
 
i
 
 
 
THE OVUM. 29
 
nnclens, situated under the germinative disk; of a larger
mass, the latebra, more deeply placed ; and of several concentric layers separated from each other by the yellow yolk.
 
Such is the egg as it leaves the hen's ovary. In the beginning of the oviduct it is fertilized by the spermatozoa already
there. After fertilization it passes into the longitudinally
furrowed second part of the tube, where it receives a copious
coating of albuminous material, the white of the %gg\ thence
it goes into the villous third part of the oviduct, where it
acquires a calcareous coating, the shell; finally, passing
through the fourth part of the canal, it is "laid."
 
The layer of albumen immediately surrounding the yolk
is relatively dense ; it is prolonged to either extremity of the
egg, somewhat spirally twisted, as the chalazse. Enclosing
the albumen is the thin tough shell-membrane. This consists of two layers, which se]>arate at the blunt pole of the
egg soon after it is laid, giving rise to the air-chamber. The
shell, composed largely of lime salts, is very porous and thus
readily permits of the necessary gas-interchange between the
contents of the egg and the external air during incubation.
 
Ova do not possess the remarkable vitality which is characteristic of spermatozoa. An unimpregnated ovum perishes in from seven to nine davs.
 
Oogenesis. — The formation of ova takes place throughout
the greater part of fetal life and continues for a short time
(two years, according to Waldeyer, Bischoff, and others)
after birth. Their number is estimated to be about seventy
thousand.
 
The ovum, the direct derivative of the germinal epithelimn covering the free surface of the ovary, is situated in the
cortical part of the latter organ, being enclosed in the
Graafian follicle. As a rule, each Graafian follicle or ovisac contains but one ovum, though sometimes two, and more
rarely three are present.
 
The Graafian follicle, in its mature condition, is a vesicle
from 4 to 8 mm. in diameter, which is surrounded by a
sheath, the theca foUiculi or tunica vascolosa, consisting of a
condensation of the ovarian stroma. The outer, more fibrous
 
 
 
TEST-BOOK OF EMBRYOLOGY.
 
 
 
 
ru.g.-SoCIIonnr hUlPBO uvary. Includlne ^rlex : a.^ennintl epithelium of
tn* (urlkrii; b, lunlu •FliiiKlnca ; r. pcTlr>heral Mrama cunMinlQg immatura
Cliullali tolllcJncDi r, wcllMilvanrcd fulllclH rrum ulKwe wall latinbriuiB )-raiiuluaa liaa partially *cpaHl«l ;/, cavity of liquor folKculi: ;;. ovum sutroUDdcd by
Mll-tuaa HiiiitltulliiK dlwiu |imllt^nit it>lenii)l|.
 
zone of tho tliota, containing large blooil-vcsaels, is distinguifthcd as the tonlca fibrosa ; the inner more cellular layer,
rich in »imall vessels ami capillarien, as the tunica propria.
 
 
 
THE OVUM. 31
 
The fibrous wall of the follicle is lined by the membrana
granulosa, which consists of many layers of epithelial e^lls ;
these, at the point of contact with the ovum, project in such
a manner as to surround it completely, the cellular envelope
thus formed constituting the discus proligerus. The inner
cells of the discus are arranged in two layers, the individual
elements having their long axes radially directed. From the
appearance of radial striation, conferred partly by this circumstance, the inner zone has been called the zona radiata
or zona pellucida, and the outer the corona radiata. The
cavity of the Graafian follicle is filled with fluid, the liquor
folliculi.
 
The stigma, or hilum folliculi, a yellowish-white spot devoid
of blood-vessels on the free surface of the Graafian follicle,
indicates the point at which rupture will take place. After
this event, which occurs when the ovum is " ripe,'' the latter
passes into the Fallopian tube.
 
The ultimate origin of the egg^ is to be sought in that important group of cells on the surface of the ovary to which
Waldeyer gave the name germinal epithelium. This first
appears at about the fifth week of intra-uterine life, as a
localized thickening of the cells of the structure that subsequently becomes the peritoneum. The thickened areas comprise two longitudinal elevations on the dorsal side of the
future abdominal cavitv, one on each side of the median
plane of the body ; these are the genital ridges. Owing to
the development of connective tissue beneath the epithelium,
the ridges increase in thickness, and, with the progress of
other changes, finally become, in the female, the ovaries.
At about the sixth or seventh week — the germinal epithelium
now consisting of several layers of cells instead of being a
single stratum thick, as at first — cord-like processes, the
sexual cords, or primary egg-tubes, or egg-columns, grow from
the surface into the underlying connective tissue, carrying with
them certain of the surface-cells (see Fig. 128). Conspicuous
among these are the large sexual cells, or primitive ova ; while
smaller cells, likewise from the germinal epithelium, are
 
» See p. 2.=)0.
 
 
 
32 TEXT-BOOK OF EMBRYOLOGY.
 
also present. The sexual cords become divided into groups
of cells, each group containing one or more primitive ova
and many of the smaller cells. Gradually, the small cells
of the group surround the primitive ovum, at first as a single
layer of flattened cells, which are succeeded by several layers
of j>olygonal cells. From these enveloping cells come the
membrana granulosa and the theca of the Graafian follicle.
 
The primitive ova or oogonia — analogous to the spermatogonia — having undergone repeated mitotic division, cease to
divide at a certain period of their history and enter upon a
period of rest and growth. They thus increase in size and
become fully formed ovarian egi^ or oocytes, the nucleus enlarging and the cytoplasm becoming more or less laden with
deutoplasmic material or food-stuffs.
 
The youngest ova are found nearest the surface of the
ovary, the eggs as they develop advancing toward, but never
entering, the medulla of the organ. Finally, in the fullydeveloped condition of the ovum and the follicle, the size of
the latter is such that its diameter equals or exceeds the
thickness of the ovarian cortex, its position being usually
indicated by a small prominence on the surface of the ovary.
 
MATURATION OF THE OVUM.
 
By maturation or ripening is meant that series of changes
by which the ovum is prepared for fertilization and without
which the latter process is impossible. In nearly all mammals, including man, it occurs while the ovum is still in the
Graafian follicle ; in some other groups it takes place after
the egg has reached the oviduct.
 
Briefly, maturation may be said to consist in the extrusion
from the cell of a j)art of its nucleus and of a small pan of
its cytoplasm. The nucleus undergoes changes ])ractically
identical with those of ordinan^ cell-division. First, the
nuclear membrane disappears, the nucleolus disintegrates,
the nuclear juice becomes mingled with the surrounding protoplasm, and the nucleus moves toward the periphery of the
egg (Fig. 11). There is now formed a nuclear spindle from
the achromatin substance of the nucleus. The long axis of
 
 
 
 
raUiig til? Hpsmeiitalliin d
ingrnm JlliiilrulUli; lliu rdillUiu i
vlly \it ibU BUBe uaneapoudiiiK
 
 
 
fnm (AUt'Q ThuDiBon,
1 |irluiHry layerj ot the blastothe lUGheiileroQ
 
 
 
 
MATURATIOX OF THE oru^f.
 
 
 
33
 
 
 
tlie spindle lies parallel with one of tlie radii, and its direction is ilctermineil hy the position of the pole-corpuscles.
Each pole-corpiisole is surrounded by a radiatiou, the attraction-sphere or polar striation. These bodies exercise a contmlling intlnence u|>on t)ie nuclear spindle, so that it assumes
 
 
 
 
Fro. U.— Pnrtliini of the ova or Atleriai glariatft.nho^iaK rliiiiigi't Hni'i.-l Inutile
 
t, germtnal Bpot, cnrnpoei-i] nI aiiclvln niid lArauuclEtn (r; : <t, niirtear uplnille In
praceii of RirmMlon.
 
such a position that each of its apices points towanl a polecorpu.scle.
 
The outer extremity of the nuclear spindle, being made to
protrude by the continued onward movement of the nneleiis,
becomes detached (Fi^. 12); this separated piece, with the
 
 
 
 
Fro. 12.— Fonnallcn of the poUr bodlea In the dy« of Aileriai gUicinUi iHeitwtg): pt, polar spindle : pb', flnt polar body : pb", ■econil polar hod;r. "i nucleiu
returning Id condition of reat.
 
small 8urrt>unding constric ted-off toasa of protoplasm, constitutes the first polar body. Krom the remnant of the first
 
 
 
34
 
 
 
TKXT-HOOK OF KMBRYOLOOY.
 
 
 
niic\vav !^piiit]li>, a sccun<] ono is funiiGd, \vhicli in the same
nianuer extrudes the second polar body. M'liat reiniiuis of
the nucleus dow moves toward the center of the cell and
U kii'>wn as the female pronncleos. The [u^ition of the
female proniicleiis is nearlv or absolutely central. The egg
U now ready for fertilization.
 
 
 
 
For some timo after their extrusion, and pending their
final disaiipeurance and disint(>gration, the jxilar l>odies are
to be seen lying in the perivitelline syaix. The formation
of polar globulcM is prolrably almost universal throughout
the animal world. It is of interest to note tliat in some partbenogenetic eggs — that is, e^s capable of developing into a
new individual without contact with the male element, as, for
example, the summer e^;s of plant lice and of some other
arthropiHls— only one polar globule is said to be formed, and
it has recently l>een shown (Sobotta) that in the maturation
of the ovnm of the mouse only one polar body was tormed
in the miijoritv of cases.
 
Tiic maturation of tiie ovum is essentially ii rediielion of
the chromosomes precisely analogous to the re<^l net ion-di vision
seen in the descent of the s[>ermatozoon from tiie primary
spermatocyte. The List generation of oogoniu having increase<l in size after iheir stage of rest, uud liavinj; thus
become the ovarian eggs or primary oocytes, now niiderj^
mitosis, but in a uianner differint; from that of their pndecessors. The chromatin lhrea<l of the nucleus, instead of
 
 
 
MATURATION OF THE OVUM.
 
 
 
35
 
 
 
dividing into the number of chromosomes characteristic for
the somatic cells, divides into half that number. These
chromosomes are tetrads, that is, each one consists of four
more or less loosely associated elements conceived to result
from a primary longitudinal splitting of the chromosome,
followed possibly by a transverse division of the two halves.
In the division of the primary oocyte (Fig. 14) to form two
secondary oocytes (one of which is the first polar body) each
tetrad is halved so that the same number of dyads goes to
 
Primordiai egg-cell.
 
 
 
Odgonia
 
 
 
 
Germinal zone.
 
Zone of mitotic division.
( The nutnber of generations is much larger
than here represented.)
 
 
 
' Zone of growth.
 
 
 
Oocyte I. order
 
Odcyte II. order
Matured cvntn.
 
 
 
 
o I. It-fiL I Zone of maturation.
 
 
 
11. P. B.
 
Fig. 14— Scheme of the development and maturation of an ascaris ovum (after
Boveri) : P. /?., Polar bodies. (From '* Ergebn. d. Anat. u. Entw.," Bd. I.)
 
each new nucleus. The secondary oocytes now undergo
mitosis, but without reconstruction of the nucleus, each dyad
chromosome giving one of its elements to each of the new cells,
that is, to the now mature ovum and the second |K)lar body.
It will be apparent, therefore, that the casting off of the
polar bcxlies is a cell-division, but one which results in the
production of cells of very unequal size. It is noteworthy,
as pointed out by E. B. Wilson, that the chromatin of the
nucleus is exactly halved at each division, notwithstanding
the disproportion in the division of the cytoplasm.
 
 
 
36 TEXT-BOOK OF EMBRYOLOOY.
 
In comiiaring the phenomena of maturation with those of
s[>ern]ut<^encisis it i^ to he nuted tliat iu tlie latter case all
four pr<^etiy of the primary spermatocyte become funotiooal
si>erniatozou, while in the furiiier case three of the progeny
cunie to naught, only one of the number, the mature ovum,
being functioually imjKjrtant. E. B. Wilson points out
that the nfluction of tiie chromosomes in the germ-cells is
for the piir[M»sc of maintaining the constancy of the number
of chnimoisomes which is peculiar to the specieij, since, if
reduction did not occur, the number would be doubled at
each generation ; lie further points out, however, that " the
real jirobleni is why the number of chromosomes should be
held constant."
 
OVULATION.
 
Extrusion of the ovum from the Graafian foUicle, or ovt!lation, occurs upon the completi<«n of the process of maturation. As the time for this event approachetr, the wall of the
follicle at the site of the stigma iKxximes much thinned and
finally ruptures, ati<l the ovum passes into the Fallopian
tube (Fig. l^). If, instead of ])assing into the tube, the
 
 
 
 
ovum maintains its connection with the ovary and is fertilized there, it may undergo partial development in xitu; such
a condition constitutes one variety of eztra-nteiine pregnancy
or ectopic Kestation.'
 
Ova are extruded from the ovary, one or niorc at a time,
' Other varictiw of orlc.pic j^lntion are ahdominal «,nA tubal, ibe naniea
of whicb are miffii-iontly iluHTiptiro.
 
 
 
OVULATIOy,
 
 
 
■ali-, from imhorty to the
 
 
 
at regular, generally nmnthly,
climacteric.
 
After tbe eseajie nf the ovum, hemorrliage into tlit- empty
follicle occurs, the resulting clot being the corpus hemoirhagicnm. According to Leopold, if niptiir« occurs during the
iDtermenstrual period instead of at the time of menstruation,
hemorrhage will be amall or entirely wanting, the resulting
corpus lut«um being called then atyplcid, to distinguirih it
from the ii/plca/ body formed in tbe ordinary manner.
 
Tbe hl(md-c!ot is soon permeated by cells originating in
tbe Willi of the follicle, some of wiiicb are fusiform connective-tissue cells, while others are large cells containing
the yellow pigment, lutein. Meanwhile, the follicular wall
thickens and becomes plicated. I^ater, upon tbe replacement
of the mass of clot and cells by fibrous tiwsne and the development of capillaries within it, the body assumes a yellowish
cicatricial appearance and is known as the corpna Intenin.
(Fig. 16). The color of the corpus varies considerably in
 
 
 
 
- Flo. IS,— Oviifli
 
 
 
different species of animals, the yellow color being characteristic for the human subject.
 
If the ovum is not fertilized, the («rpU8 luteum attains its
maximum development in less than a week and begins to
shrink at about the twelfth day, becoming completely ab
 
 
38 TEXT-BOOK OF EMBRYOLOGY.
 
sorbet! in a few weeks. If fertilization occurs, it continues
to grow for two or three months and accjuires a size onefourth or one-third that of the entire ovary ; persisting till
toward the end of gestation, it finally shrinks to a small
white scar, which may not totally disappear until a month or
more after labor.
 
It has been customary to designate the larger, better developed yellow body, the true corpus lateuxn, or the corpus
Inteuxn of pregnancy, in contradistinction to the so-called
fedse corpus Intemn of menstruation, and to regard the presence of the former as absolute proof of previous impregnation. This view is no longer tenable, since bodies identical
in appearance with true corpora lutea have been found in
virgin ovaries (Hirst).
 
The relation of ovuhition to the menstrual function has
been much discussed. While the two processes usually occur
at the same time, they are not to be reganled as dependent
one upon the other. It has been shown by Coste, whose
observations have been confirmed by Leoi)old, that as a rule
Graafian follicles burst during menstruation, tliough they may
rupture before or after this event. It has also been shown
that in the rabbit sexual intercourse hastens the rupture of
the follicle.
 
MENSTRUATION.
 
Menstruation, or the eatamenial flow, is considered here
because of its natural association with the function of ovulation.
 
Menstruation may be defined as a periodical discharge of
bl(HHl and disintegrated epithelium and other structural elements of the mucous membrane of the bodv of the uterus,
mixwl with mucus from the uterine glands and the vagina,
occurrinii: normallv about everv twentv-eii^ht davs, an<l
ass(MMat(Hl with more or less disturbance of the entire sexual
system. The inauguration of the function marks the age of
puberty, the beginning of the sexual life of woman ; its cessation, known as the climacteric, or menopause, indicates the
termination of the child-1 Hearing j)eriml.
 
In temi)erate climates, the menses are established between
 
 
 
MENSTRUATION, 39
 
the thirteenth and seventeenth years and cease between the
ages of forty and fifty. In the tropics, they appear somewhat earlier; in cold climates, somewhat later. The function
is suspended during pregnancy and, usually, during lactation.
 
The quantity of the discharge, though subject to considerable variation, is usually from 4 to 6 fluidounces. The blood
is venous in character, and, owing to admixture of alkaline mucus, does not coagulate unless present in excessive
amount.
 
The menstrual cycle of twenty-eight days may be divided into four periods : the constructive stage, comprising
from five to seven days ; the destructive stage, lasting about
five days ; the stage of repair, covering a period of three or
four days ; and the stage of quiescence, including the remaining twelve to fourteen days.
 
In the constructive stage, which occupies the six to seven
days preceding the discharge, the mucous membrane of the
uterus becomes markedly swollen, the normal thickness of
from 1 to 2 millimeters being more than doubled. The uterine glands become wider and longer and also more branched.
The blood-vessels, especially the capillaries and veins, undergo great increase in size, and the connective-tissue cells
are increased in number. The thickened mucous membrane
resulting from these alterations is the decidua menstrualis.
The term "constructive" is applied to this series of changes
for the reason that their apparent purpose is the preparation
of the womb for the reception of a fertilized ovum.
 
The destructive stage, corresponding to menstruation
proper, lasts from three to five days. It consists essentially
in the partial destruction of the hypertrophied mucous membrane, the menstrual decidua, accompanied by hemorrhage.
The initial step is the infiltration of blood into the subepithelial tissue ; according to Overlach, this takes place, not by
rupture of capillaries, but by diapedesis. In a day or two
the superficial layers of the mucous membrane disintegrate
and are cast off, those portions of the enlarged uterine glands
included within this stratum sharing the same fate. By the
loss of the epithelium and the subjacent strata, the blood
 
 
40 TEXT-BOOK OF EMBRYOLOGY.
 
vessels are exposed. Subsequently these rupture, giving rise
to the characteristic hemorrhage. Fatty degeneration accompanies the death of the cast-off tissue, and was thought by
Kundrat and Engelman to be the direct cause of the hemorrhage ; it is probable, however, that fatty degeneration is not
present until after the flow of blood has begun.*
 
The stage of repair, comprising the three or four days following the period of the discharge, witnesses the return of
the uterine mucosa to its usual condition. With the gradual
subsidence of the swelling, the superficial layers, which were
lost, are replaced by the growth of new tissue from the
deeper layers, which persisted. The formation of the new
epithelium begins at the mouths of the uterine glands.
 
The stage of quiescence extends from the close of the preceding stage to the end of the cycle, or, in other words, to
the beginning of the next constructive stage.
 
Other parts of the sexual apparatus, including the ovaries,
the Fallopian tubes, and the mammary glands, show more or
less sympathy with the uterus during menstruation, the
changes in them consisting chiefly in swelling, hyperemia,
and tenderness.
 
The Relation of Menstruation to Ovulation and
 
Conception. — The function of menstruation and the extrusion of ova from the Graafian follicles, though closely
associated, are not depeiKl(?nt upon each other. Ovulation
occurs perhaps most commonly during the time of the menstrual discharge, but it may take phic(» before or after this
event. While it is now general ly accepted that the two
functions are not mutually interdependent in the sense that
one is a necessary part of the other, yet, since the turgescence
incident to sexual intercourse has been shown to hastim the
rupture of the follicles, it seems reasonable to sup(K)se that
the ovarian hyperemia attendant upon the menstrual epoch
woyild exert a like influence.
 
Since the function of menstruation is normally susj)ended
during pregnancy, the relation Ixitween menstruation and
 
 
 
' MarHhair8 "Vertebrate Embryology;" Miiiot's '^ Human Embryol
 
 
ogy
 
 
 
»»
 
 
 
FERTILIZA TION. 41
 
ovulation, and of these to conception, are of practical interest in determining the date of labor. The duration of pregnancy is from 270 to 280 days, nine calendar, or ten lunar,
months, and it dates from the moment of conception. But
since the ovum retains its vitality for about a week after its
extrusion from the Graafian follicle, and since the activity
of the spermatozoa may continue for several weeks after their
entrance into the female genital tract, it is impossible to fix
accurately the date of conception even in those cases in which
there has been hut one coitus. It is now believeil by most
embryologists that the ovum is fertilizable only while it is
in the Fallopian tube, a period probably of about seven days ;
if this be true, it follows that conception must occur within
a week after ovulation, although it may be effected as late as
two weeks after coitus. Since the ovum is usually discharged
from the ovary during the menstrual j)eriod, it is evident that
the time most favorable for conception is the week following
menstruation ; and inasmuch as the latter function is suspended during pregnancy, it is obvious that the most reliable
basis for calculating the probable date of conception is the
last menstruation. The method usually employed is to count
nine months and seven days from the first day of the last
menstruation. After what has been said it is perhaps needless to remind the reader that this can furnish only approximately the date of labor. In a case where conception occurred a few days prior to the first omitted period, there
would be a discrepancy of several weeks between the actual^
and the calculated, termination of pregnancy.
 
FERTILIZATION.
 
Fertilization is that peculiar union of spermatozoon and
^g-cell which initiates the phenomena resulting in the formation of a new individual. As implied in a preceding section,
impregnation is possible in the higher organisms only after
the completion of maturation, while in others, as for example
the maw-worm of the horse, spermatozoa enter the ovum
before the extrusion of the polar bodies, and thus one process
overlaps the other.
 
 
 
42 TKXT'HOOK OF EMBRYOLOGY.
 
 
 
'XW* iiMifit primitive methrxl of fertilization is that effected
witli/>ut ^5ri|>iilation of the parent organismi^y or extemil fertUizattoi; thin (K^rturH in 088eous fishes, in some amphibiarm, and in many invertebrates. In these groups, both
ova and Mcmen are discharged into the water and there
\%%iH*i, In frogK, however, there is a quasi-copulation, the
nml<? <?mbnuiing the female during the breeding season and
i\i*\Hm\!\\\^ mwM'W u|K)n the eggs as they are evacuated. In
all liiirh'T animals, internal fertilization occurs, this being
i*tViutU*il by Mf'Xiial congress.
 
In man, f(f*rtili/Jition normally occurs in the outer third of
iUit Fallopian tnlH». The semen having been deposited in
iUtt vagina, or the utcnis, or even upon the vulva, the speruuiiny4rti umU(* tlir»ir wav into the oviduct bv the vibratile
motion of tl^'ir tails. Meeting the ovum, they swarm around
it, and mtttio, of i\u'U\ pass through the zona pellucida into
tli« |Mfrivit<*lline HjiarM*. It is believed by many investigators
that lUii ranaln of i\w. zona (ronHtitiite the avenues of entrance
for iUt* n\u*nuaUy/Ani, In the rather firm egg-en veIoj>es of
Umu*iii and ^ime finheH, there is a small aperture, the microVfU, through wliirli the spermatozoa gain entrance.
 
Whihi many Hpcrmatozoa may pass through the zona, only
mi' — thai onn wliow? liciMJ first impinges against the vitelline
m^mlimnn nnf<*rH \\m\ ovinn. Why others do not or cannot
MUlnl' l» unknown; ponhiMy ht^i^ause the egg's power of attraction U Mnnilll^d (Minolj. Polyspermia, or the i)enetration of
mivim'mI ttpMrniMlo/oii, may o<*(Mir, however, if the ovum is
linlutMllliV I Mud In Home Iowit types it is sjiid to be normal.
 
A(i (liM lapMriMMloKoon Ih ahout (o strike the vitelline merabnmni lIlM pnilnplilnm MWelU up at tlH> iM)int of contact into
(hit li>oiiMMVi> MnMMlHotioi* ( l**i^;. 17). Through this the spermHh»#»«»n ImiH'o IIo nvmv, !{« tail heing absorbed by the cyto\\\\\^\\\ \^^ Mu^ MVnm or tiring \v\\ outside in the ease of the
h^M^ Uh^hhr V\\\' middli' pife*', t»r at least a j>art of it,
h\\^b^vl^»y4i \W v\\^\ lxh»*h» \\\\\v\\ luHrr rcpix'smts the eentro*>vw>^ \^\ v\\N <^vUHHU^Ii \^\\W\^ iIm' oNUUi with the head (uu\^W^^ \^* ^hv i^s VUS4^\^'»\^N^l\ V\\\n uueieus or head now en^MV>^ ^S\v^*^^'^dL ^^^^y^ ^^^v^ "*^*'^ ^*<MuM«eA'M/*. The male and
 
 
 
FERTILIZATION.
 
 
 
43
 
 
 
female pronuclei approach eat'h other and finally meet in the
center of the ovum, the two bodies apparently fnsing to form
the single segriientotioit-Hiivleas or vlmvage-mideus. It must
not be mulerstood, however, that an actual single membranatc nucleus is formc<I, As the proniiclot approach each
other the centrosome of the spermatozoon lies between them
surrounded by its attraction-sphere, and gives rise to a unclear
spindle after the manner of onlinary mitosis, the chromatin
threads of the two pronuclei lying in relation with its equator,
but on opposite sides from each other. In other words, the
chromatin contributed respectively by the two pronuclei
 
 
 
 
of AtterUtt glacial!*, ahowlng the approach >
le ovum (Hertwlg) : a. fcrtillstnK n.ftle eleme
>/, b", aUgts u( fUalon or the head of Ihe si
 
 
 
retains in each case its identity, the " segmentation-nucleus "
entering iii^n the processes of mitotic division without
previous intermingling of the chromatin of the ovum with
that of the spermatozoon. As will be shown in Chapter II.,
one-half of each chrontatiii thread goes to one pole of the
spindle, while the other hidf of each goes to the ()p{>osite
pole, to give rise to the two daughter-nuclei resulting from
this first segmentation of the segmentation-nucleus. It will
be evident that the segmentation -mid ens consists of chromatin
substance derived from each {Mirent. As this fact has been
thought to explain, anatomically, the otTspring's inheritance
of both paternal and maternal characteristics, it has l>een
 
 
 
TEXT-BOOK OF EMBRYOLOQT.
 
 
 
made tho basis of a theory of heredity formulated l>y Hertwig and independently advanced by Strasburger.
 
 
 
 
L, fcrtlKntovumnrerlilDus iHertwlg): I
) nre apprOBcblnR: In B Iher hnre almait
ion of fenlltiallon (Hertvrlg) : 1.71., irgmei
 
 
 
e ratio {a) aod the female
 
 
 
Artiflclal fertiUzatioti, or the bringing about of the development of the ovum by urtifioial (chemieal) means, without the
partioijMtion of the male element, has l>cen rcpently experimentally effected with the e^s of the sea-nrchin by Ijoeb,
of Chicago, These e^s, when first immersed for about two
hours in a mixture of sea-water and a weak Boliition of
magnesium chlorid, and then transferred to normal seawater, were found to undent complete and normal development, producing perfect larvse. This artificially induced
development diflere<] from that of the ordinary method only
in Ijeing slower.
 
 
 
CHAPTER II.
 
THE SEGMENTATION OF THE OVUM AND FORMATION OF THE BLASTODERMIC VESICLE.
 
While the fertilized ovum is passing along the Fallopian
tube to the uterus — a journey believed to require seven or
eight days in man * — it undergoes repeated segmentation, or
cleavage, becoming a more or less globular mass of cells or
blastomeres. This mass is the mulberry-mass or morula.
 
The details of the process of division correspond closely
to those of ordinary indirect cell-division, or karvokinesis.
The first indication of approaching cleavage is seen in the
segmentation -nucleus, just as, in other cells, the sequence of
changes leading to cell-division is inaugurated in the nucleus.
 
The achroraatin-substance of the segmentation-nucleus
forms a nuclear spindle in the ordinary manner, with a centrosome or pole-corpuscle at each apex. The centrosome
is surrounded by the polar striation or attraction-sphere.
After the usual preliminary changes, the chromatin-substance
assumes the form of V-shaped loops arranged around the
equator of the spindle in such a manner as to produce the
wreath or aster. Each chromatin loop splits longitudinally,
and the resulting halves^of each move to opposite poles of
the spindle, where they become grouped about the pole-corpuscle to constitute the daughter-wreaths of the new nuclei.
The vitellus now begins to divide, the first step being the
formation of an encircling groove on its surface ; this groove
deepens more and more until finally division of the cell is
complete. In like manner, each daughter-cell divides into
two, and each of these two into other two, the cell-division
continuing until there results the mass of cells, or morula,
already mentioned (Plate I., Fig. 1). The two cells or
blastomeres resulting from the division of the segmentationnucleus do not always divide simultaneously ; if the division of
 
* Ret'ont investigations by Peters, of Vienna, upon an ovum of three or
four (lays, already embe<l(ie<l in the uterine nnicosa, woukl indicate that less
time than this is occupied in tr.ivei'sing the oviduct.
 
45
 
 
 
46 TEXT-BOOK OF EMBRYOLOGY,
 
one cell precedes that of the other there will be a stage when
three bla«tomeres are present. Further i rregularity in division
nr«ijlt?i in the production of five-celled and six-celled stages.
 
Thf'jw? processes have been followed the most accurately in
the frj^g of the sea-urchin ; in reptilian eggs, as well as in
i\um'. of the rabbit and other mammals, they have been
ni%u\m\ ai?^) and have been found to agree with the former
\u all eHH^;ntial respects. Certiiin modifications dependent
nyntu the relations and proportions of formative-yolk and
(tttA-yiAk will Ik* pointed out hereafter.
 
Whihr no one has seen the segmentation of the human
itynm, th^fre in no reason to suppose that it differs materially
from that of other mammals.
 
\u ifiterenting and probably significant modification of the
Utt*^\iiA of eh-jivage as just described has been observed by
Var» \U*uMi'U in the ova of the maw-worm of the horse. In
tl|ij« #'a«M' male and female pronuclei do not fuse but merely
\U* t'\tt*4* t^rj^ethrr. At the beginning of segmentation, the
rUri$um\\\\ of #»jjeh pronucleus assumes the form of a ex)nvolul^'d lliri;aii, which divides transversely into two sisteriUrt'iuU, In thin manner are produced four loops of chromtiUUf whi^'h \u'(U}U\o ^roup<»d around the equator of the
UWi'U'iw t-pindh* juht formed, and each one of which then
tephlu loh^ifudififilly inio two threads. In the migration of
thn tM'{iuu*u\n tliiit now ensues, each pair of sister- threads
fee|mnih'h, on*^ flin'nd ^oin^ to one pole of the spindle, one
lo \\n' oiIm-I', lliMMM', at each pole, and taking i>art, therefore, in iIm' (oniiiifion of each new nucleus, are two male
and fvvo (f'hiiilc liircndH of chromatin. Thus the male and
fcni/ilc pronuclei contribute e(jual shares of chromatin to
each dan^litcr-niiclcuH.
 
Hincc tlic H-j^MicMtiition-ini<^lciis of the ovum gives rise to
all the ccllh of the ImmIv, every cell of the adult orgjinism
niMrtt conhi^l of cijual amounts of material from each parent.
 
Cleavage-planes.— The direction of the planes of cleavi\\l^i^ is deterniine<l by certain laws. The ilirection of the
plains of the first cleavage bears a definite relation to the
long axis of the nuclear spindle, whose j>osition, in turn, depends upon the manner of distribution of the egg's proto
 
 
THE SKGMJCyTATIOX OF Till': OVUM. 47
 
plasm, lis (lirpction <;»iiicii1in}r with tlic limgest diamoter of
an oval e^, but lying in any diameter of a spherical one.
Tbe first cleavage-plane always cuts the axis of the nuclear
spindle perpendicularly at its center; thp second bisects the
tiret, also perpendicularly ; and the tliird is perjxtudicular to
the two others and passes through tiie middle of their uxifi
of intersection.
 
Kinds of Cleavage. — Tlie mode of cleavage of the
ovum is infinenced by the relation of the protoplasm and
the deutoplasm to each other, and by their relative proportions. The classification of ova according to their method
of cleavage is as follows ;
 
1. Holoblastic ova are those in which segmentation is total
— that is, the entire ovum undergoes division. If the resulting cells are of equal size, there is said to be
 
(a) Total equal cleavage ; to this class belong the alccithal
ova of uiammals (Fig. .5) and of amphioxns, to the segmentatiou of which the above descriptitm may be said to apply.
Strictly speaking, the cells are not of exactly equal site,
tiiose in the region of the vegetative (H)le of the egg being
slightly larger than those at the animal pole. Contrasted
with this is
 
(b) The total unequal cleavage of amphibian ova, whose
figments are of unequal size (Fig. 19), These eggs being
 
 
 
 
B beginning tn be dtvlcJeil
Mted sDrftce of the cfni a
eher In pmtopl««n; rf, thi
 
Indie (HcnwiB).
 
 
 
gg: j<, stage of (tie firitdlvfginn,
. The four segnipnls ofthesepontl Bt»gc of division
ijr nn equatorial farrow into elaht Btginents ; p, pigthe nnlmal pole: pr, [he pari iif the e^ wblch ii
part which Is richer In deiitoplaam ; ip. nudeac
 
 
 
elolecithal, the ligmer protoplasmic animal \xAe is directed
Ipwanl, while the deutoplasmic vegetative pole is under<^
 
 
 
J
 
 
 
' »»■
 
 
 
48
 
 
 
TEXT-BOOK OF EMBRYOLOGY,
 
 
 
neath. The inequality of the resultinjij segments, as well as
the <lirection of the cleavaj^e planes, may be appreciated by
reference to Fig. 1 9, whicli reprevseuts a frog's ovum.
 
2. Meroblastic ova are those in which the segmentation is
partial, division being limited to the formative yolk, or
pn)toplasm.
 
(a) Partial discoidal cleavage is the variety of meroblastic
cleavage that occurs in those tololecithal ova having a germdisk (Fig. 8), to which latter the segmentation is limited.
This method of s(»ginentation is seen in the eggs of birds,
re[)tih;s, and fishes. In the L'^^y; of the bird, which may be
tiiken as a typical example, the germ-disk, in whatever position the i'\r^ may be placed, floats on the top of the yolk.
The beginning of the first segmentation is indicate*! by a
furrow in the center of the surface of the germ-disk (Fig.
20). This furrow deepens, cutting vertically from the
 
 
 
 
 
 
ABC
 
Kl«. 21).— Surfiioo view i»f the first stntrcs <«f rli-avnir*' in tin; hen'B epij (after
Coiito) : o. IwnKT of tlui jftTm-disk ; />, vertiful furrow ; r, sinuU central segiiieut ; d,
Innr^' lK»riplu»ml neinnent.
 
upjM^r to the h)wer surface (»f the germ-disk, dividing it into
 
two iHpial l>Jirts. Another groove, crossing th(» first at a
 
right angle, bisiHi^t^ each of the two segments, and each of
 
thi^» is in turn bisected by a radial furrow, so that the
 
p^niwlisk now consists of eight sector-shap<'d cells. (Voss
 
fumnvs, api>oaring near the center of the disk, vwi off the
 
apiixv^of the sectors, adding small central .s<'gments. CVll
d\vi*\on cimtinues until the germ-disk consists of a flatt(Mied
 
ntt.^ of Ml*, ^veral strata thick, lying on the surfiice of the
 
volk,
 
TV wcond methoil of meroblastic segmentation is
^>^ ¥«i|lMnl ctomttt, which occurs in the eentn^leeithal
 
 
 
THE STAGE OF THE BLASTODERMIC VESICLE. 49
 
bered, tbe nutritive-yolk isccDtrally placed and is surrounded
by the fonnative-yolk. The segmentation-nucleus lies in the
center of the nutritivc-yolk, and in this position undei^oes
division and subdivision. The new nuclei now migrate into
the peripherally placed formative-yolk, when the latter divides into as many jiarta as there are nuclei, and thus the
central unsegmented nutritive-yolk becomes enclosed in a
sac composed of small cells.
 
 
 
THE STAGE OF THE BLASTODERMIC VESICLE.
 
The blaatomeres of the morula soon show a diiTereutiation
into two groups of cells, a peripheral or outer and a central
or inner group (Plate I., Fig. 2, left figure). This differentiation is indeed foreshadowed by the fact that of the first
 
 
 
 
Fig. 21,— Orom of the bsl, ihiiw- Fiq. ■."■i-Oi-ura of (he bat, nhowl
 
IntC vacuulallon of tlii; in.f!mpntod olallon iif Uii; sugintiiiwl I'Kg lo form the
 
eggtfifomi theblaslodenuiccavily. blaatndennJu oavltf . y IJUO(Van BeDedea].
X 500 (Van Btneden).
 
two blastomercs, one is slightly larger than the other.
Vacuoles now appear in some of the central cells, and these,
becoming larger, finally coalesce to form a fissure-like space,
the cleavage-cavity or segmentation-cavity, the lecithocele
of Van Beneden (Figs. 21 and 22), The ovum is now in the
stage of the blastodermic vesicle.
 
It will be profitable to compare this stage of the mammalian ovum with the corresponding blastnla stage of the
 
 
 
5^; TEXT-HOOK OF EMBRYOLOGY.
 
\aw:i-\':\, tit 3iii|ilit<>\ii:s iaiict'olatus, out: of the lowestvcrte\iTaH:^, a fi.-li-likf unimal several iiiuhos in length inliabiting
tli<- M<i]iu:rniii<-aii S«.-a. The blastnla in this ca^ id a simple
N«; <-"rii]»fi-<-il of cells which siirniiiii(] the cleavage -cavity as
fi .-itifrU; layer (Fig. 2(i, A), The celU in the region of the
vi-({**tativc |>ol<; are larger and niore tiirltid, liecatise more
<leiit(i[ila^niii', (h:in those at the animal pile, as shown in the
«.m.-lipirf.
 
The mammaHftn blaatwdennic veBlcle, varying in shape in
'ItfTert'iit .-[H-cie:*, i-un^i^ts of a layer of i^omowhat flattcDcd
(ri'IU, the enveloping or subzonal layer (the "outer cell-mass"),
MirroinnHng a sp:ic<-, tlie cleavage -cavity or aegmentation'
cavity, and of an irn'guhir mass of more spherical cells, the
 
 
 
 
inner ceU-nasB, whi<-Ii hitter is attached at one jioint to the
enveloping hiyer and encroaehes upni the >|Kice {Fig. 23
iind Plate I., Fig- 1). The pleavage-cavity contains an
ailMiniiii'iiiH fluid. It is during this stai.f that the germ, in
the eiiH! of mammals, n'aches tin- ntenis. As a pecnliarity
of mammalian development the hlast<Hleritii<- vesicle now
rapidly ineniaws in size, the cleavage-civity l.e.-oming relatively much larger. The zi>nii pcUiioiila, \vhi< li siill snrroimds
iIk! ovnm, in hy this time i|uite attcnnated unci is called
iIk. ptodiorion.' The cells of the cnv<loping hiycr tliin
•Ml I't constitute what is known as Eauber's layer.= In
II,*' rahhil *mil.r>'o, Hauler's layer, heing fniicih.oless, .lisnpjo-ar- «t alfiiit the seventh day ; in most mamnials, how»-..^, it i^rrrisls to play a part in fnrtiicr development
7- vr* /..•rffcwwn U bIho av.pliea tn * c.ntir« i.f »lhijtiiiiinii« niiiKr'9. •f*'i, -iit •nam mtivB* ■« >l pawe* »'""K ''"^ nviilmt.
 
— .,. (wVirm 4^«Mr Itaubci'* lny" ■■ "«" I""" "' ''"^^ . nvl■l..|.inB
*'•• rii-1. -Am tkr cmbrraoic ihieM
 
 
THE STAG£ OF THE BLASTODERMIC VESICLE. 51
 
(Van Bcnedeu). Tlie form of the blastnia of amphibians
and of the Snuropsida (birds and reptiles) is greatly nio<li(ied
by the relatively abundant niitritivc-yolk with which their
ovR are endowed. An
amphibian ovum in the
blastula glage is shown
in Fig. 24. It will be
iieen that its walls consist of several layers of
cells, and the cleavagecavity is encroached
npon to a considerable
extent by the large and
abundant cells of the
vegetative pole, which
lire esnecially rich in t^c ^t.—BiaituU of trftun Mnlatiu:/'!. leg
~\ ^ nicnUtlon-MTlty : n. mnrKln^ mae; di. MIi
 
deutoplasm. In the with abundBnl yoU (Herlwlg).
 
eggs of birds and reptiles — that is, in the telolecithal eggs that undergo partial
discoidal segmentation — the blastula fiirm is so markedly
modified as to be scarcely recognizalile. In this case, as
shown in Fig. 25, the cleavage-cavity is a narrow fissure
 
 
 
 
 
riQ. 25— Mi!i]iBnstM.-IJnn through Bgerm-iUtik of prlmluriu la the blutnlaiUga
(kIIi-t JtUekert): B,cavily of the blululH: b. ■egmentvilgcnn: <U, Buely gninnlar
yoU with yolk-nuclel.
 
whose niof is the perm-disk, and whose floor is the unsegmented nutritive-yolk, which latter corresponds therefore to
the large vegetative cells forming the floor of the amphibian
egg shown in V\^, 24.
 
 
 
CIIAPTEK 111.
 
THI£ GERM-LAYERS AND THE PRIMITIVE STREAK.
 
TMi: TWO-LA YI:RED STAGE OF THE BLASTODERMIC
 
VESICLE.
 
In .•lii<lyiiij»; tint complicated miuI obscure iilienomciui of
I lie ioriii.'itioii oi* the p*rni-layei*s in inamnials, eoin]Kiri.son
willi what iK'riiis in c(?rtain lower forms is liel})fnl. In
lln' rax' of tin* ampiiioxus^ the one-layered stajrc, the blastula in .-iir<'eed<-d liy the sac-like douhle-layered gastmla
stage. TIh' ^a-lriila, in its typical form, consists of t\vt> layers
of ctlls f-nrroijiidin^a rmtral cavity, \vhi<*h latter comniunicales w iih th<* exterior Iiy means of a small ajuTtuiv, the blastopore. 'VUr ravily is tiie archenteron or cu'lenteron or intestiiio-liodv cavitv. 'rh<* outer laver of <'ells is the ectoderm
or epihla.-^l ; the inner layer is the entoderm or hy|K)l)liLst.
riii-« t'onu of the pM'ni is hmmi in holohlastic invertehnite, as
v^ell ar^ in Mtnie vertchrate ova, and is typically exemplified
:;j :he developmenf of the amjihioxus. The hiastiila of this
liiiijjai i«* a ^inlpl^• >a<', iIm' wall of which is a single layer t)f
ipir K-Iial eelU ^nrnMlnding the <"h'avajre-cavity ( Fi<::. li<J, -l).
!Jv 1 :j'i;.>riiu^-in i*!' the vi'^<*lativr rells, the cleava«;:e-cavity
s -.urn/aji.-iuxl ii|Hiu and fntaiiy i:^ cnmpletely nl)literate<l, l>eing
i'iiiuv/-.'i vy the an*lientei'on iVi*^. -»». O. From this it is
•» . .'-Hr JUi' r^^irulation occurs here hy a simple pnnM'ss of
^••:«^! Lu.il I III, lu ova with a larjrc amonnt of f^Mwl-yolk, as
. .4i.f- II P/^N, UixU, ami finhes. the process is m<Mlifi(»d
uL , .1 .u«'i ^ « tbti^ivudltion. Still iurther nnHlifications
I- i-.v- .P-. -*;».•!! ill ihe development of mammal-, >ince
^.:.. .h.xv J. !>AluetiiM» in thr amount t)ftoo«l-yolk.
I ir. -»»-..-.! ",aI gaMni la lln-ory n(* llarrkrl, all
• • - .i- : ...»!;».*:». ibraulnials a^ di-tinjrni-hnl fmni
*.i- '-.•..'..»• liTCiiUiMns — pa>^ ihrnujih a typiral
■ ^ . ii> .-.^iv*: .r! \iivir ilevelopmrni.
• i'»4 •; .. ihfii:{7«l \ffilloipl«' that thr hiirher
 
 
 
TWO-LAYERED STAGE OF BLASTODERMIC VESICLE. 53
 
animals during their development repeat, to a greater or less
extent, the embryonic or the larval forms of the lower members of the group to which they belong. Huxley has pointed
out the morphological identity of the adult form of the coelenterata with the two-layered gastrula.
 
 
 
 
Fig. 26.— Gastrulution of amphioxus (modifled from Hatschek). .1. Blastula:
az, animal cells; vz, vegetative cells; fh, cleavage-cavity. J!. Beginning invagination of vegetative pole. C. Gastrula stage, the invagination of the vegetative cells
being complete: ok, outer germ-layer; ik, inner germ-layer; ud, archenteron; a,
bla8toiX)re.
 
It must not be understood, however, that we find in mammals a gastrula stage such as that of amphioxus ; we do find,
indeed, that the single-layered blastodermic vesicle as described
above acquires two and, still later, three cellular layers, but
to what extent the lavers of the diderniic blastodermic vesicle
correspond to the ectoderm and entoderm of the gastrula of
lower types is not quite clear ; nor are all the details of
the growth of these layers as yet clear. The phenomena
have been studied in the mole, the rabbit, the bat, the sheej),
and the dog, as well as in some other mammals. Acconling
to the investigiitions of Van Beneden upon the development
of the rabbit and the bat, the inner cell-mass spreads out
upon the inner surface of the enveloping layer and shows a
differentiation into two groups of cells. One group, occupy
 
 
54 TEXT-BOOK OF EMBRYOLOGY.
 
ing the center of the mass, Van Beneden's embrj'onic
bud (Fig. 27) consit^ts of cells that are at first globular,
but later cuboidal ; the second group, composed of flatter
and darker cells, covers continuously that surface of the
mass which looks toward the blasto<lerniic cavity and soon
extends beyond the limits uf the muss to line tlic inner
surface of the enveloping layer, thus constituting the entoderm (Fig. 27). Some of tiie cells of the lighter group now
vacuolate, the several vacuoles later becoming confluent to
form one cavity, which i.s the future amniotic cavity (Fig. 28).
In the rabbit this vacuolation docs not occur, at least not at
this stage. The more or less globular mass of cells remaining after the vacuolation is known as the embiyonic bnd or
 
 
 
 
Fto. Ti.—n\am of bat : dIlTcrcnl1at1i>n nf cmbryntii
 
 
 
embryonic button or embryonic di^k (Fig. 20). It will be
obsorvtHl that tiie amniolii: cavity lies between the embryonic
bud and the enveloping layer. It is important to note thai the
embryonic buil is the anla^c of the bixly of the embryo, that
it is from this group of cells alone that the embryonic body
 
The enveloping layer, which disappears in the rabbit at
alwnt the seventh d.iy, brit which |M'rsists in <)tiier mammals
so far as known, resolves ilsclf into two laminie in the region
overlying the embryonic bnd and the amniotic cavity, an
inner luycr, the cystobUst, an<l an outer, the plasmodoblast
or placentobUst, eomiK>sed of flattened cells. The cystoblast
now constitutes the inimediatc roof or vault of the nnmiotic
cavity, while a layer of cells ditfcrentiated from the amniotic
surface of the embryonic bu<l forms the floor of the cavity.
This latter lamina becomes the outer laver of tlic dtdermic
 
 
 
TWO-LA YERED STAGE OF BLASTODEBMIC VESICLE. 55
 
 
 
embryo, that is, it represents the cmbrvonic ectoderm, while
the enveloping layer would correspond to the extra-embry
 
 
 
onic ectoderm. Somowliiit later the eystoblast in the vault of
the amniotic cavity disappears and the himina of cells referred to above && forming the floor of the amniotic cavity
 
 
 
TEXT-HOOK OF EMBRYOLOGY,
 
rv of the cmljrv'i
 
 
 
u btld
 
 
 
Ijccomes contiiuioiis at the \
with the enveloping layer.
 
ConijMirison of but and mole embryo,-; with the oviira of
Peters (Fig. .10) shows that essentially the same jiheDomeDa
tKCMT in the development of the human ovum. The Dviim of
Peters was estimated by him to be Iwtween three and fonr
davH ohl and wiis the youngest hnmun ovura as yet studied.
One muv see here the embrvonie bud or disk E. Sch., the
 
 
 
ODk meaoblMi : •
 
 
 
 
. tgiiMicy
Embryonic L'^illiliut; £ltil.. vmbryuiilv liyimUiut: iti'ti,,Kiatity..uuMUvAl v<aMe; X<H..cliurli>n[c tpthlut; St.. foltl in cxoi|[cciiv(i)r liiH'il bf»»liiBl« lijcror flailMicl veil*, whlrh are
klngpunlrutwlib Ihi-liyi-ruf cylludrtcr editor the einbryunlc<-|ilb1iiBl.
 
L'mbrj-onic eetodenu formitodcrm Enl,, and likewise
» somewhat more ad vanced
■ under cousiderutioii.
 
may see that tlie singlemnnils becornos eonvertcd
 
 
 
ainniotio cavity A. H., the exti
ing the vault of this cavity, the
tlie yolk-aac 1). S., sinee this ovni
iu development tliun the stage ii
From the foi-egoing aeeount i
Inyered blastodermic vesicle of
into tlie two-layered or diploblastic vcsiele, consistiDg of the
entoderm and the ectoderm, not, as in the amphioxns, by a
simple process of invagination, but largely by a rearrange
 
 
TWO-LArSRED STAOE OF BLASTODERMIC VESICLE. 57
 
meat of tlie cells of the inner cell-mass ; and that, partly as
a consequence of ibis rearrangement of cells, the amniotic
cavity is proilnced and a different in tiun l)econies manifest
between a group of cells, the embryonic hiid, which is to
serve for the development of the embryonic body, and other
cells which arc destinwl to produce the accessory organs or
envelopes of the developing embryo.
 
As ])reviotisly stated, the process of gastrulation and the
form of the pastnila are modified in the t".ise of ova possessing a large proportion of deutoplasm. In the case of the frog,
for example, as well as in other amphibians, the blastula has
the form shown in Fig. 24. By an invagination of the bla»tula-wall at the place of transition from the animal cells to
the vegetative cells, all of the latter and a part of the former
arc carried into the interior of the blastula to form the lining
of the archenteron (Fig. 31). Compare this with the ampht
 
 
 
Fte. 3L— Bi^tUI ■ecllon Ilinmgli uii I'ltenfirlliiii (nncrthcendorfcutrulntlon):
o*. onltr BOnn-laycr; ijt, ii
ventral lipa of the ccdenteron;
genu-layer (Hertwlg).
 
 
 
oxus gastruln as shown in Fig. 26. In the bird's e^, the
form of wiiose gastnila is shown in Fig. 32, an infolding
or invagination occurs, as in the frog's egg, at the place of
transition from the animal cells to the vegetative cells, or, in
other words, at the margin of the germ-«lisk. The gastrnla
thus formed is represented in Fig. 32. Its archenteron is a
 
 
 
58
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
narrow fissure, and its blasto]>orc, situated at the posterior
margin of the gerni-ilisk, is exceedingly small.
 
The Embryonal Area. — tTpon the surface of the germ
at the lieginning of gastrulation — that is, at about the fifth
day of devclopmeut in the ease of the rabbit's germ — there is
 
 
 
 
a roimd whitish spot, the embryonal area. Its position eorre6jK)uds to that formerly held by the inner cell-mass of the
bla^to<lormic vesifle, as shown in Plate I., Fig. 2. It is only
in this region that the wall of the vesicle is, at this particular
etag*", composed of morn than a single layer of colls, the octotlvrm and the cnloilcrm not I'Xti.-ndinp; much, if at all, beyond
its periphery.
 
Tin? embryonal area, soon becoming oval (Fig. 'i'i) and,
later, pear-shaped, exhibits, at it^ posterior margin, a trans
 
 
 
nsed with
 
 
 
TWO-LAYERED STAOE OF BLASTODERMIC VESICLE 59
 
reference to the future body, the narrow end of the area embryonalis corresponding to the posterior pole or caudal extremity of the fetus.
 
In the chick's e^, the embryonic area (Fig. 34), or embryonic shield, appears while the egg is yet in the oviduct.
 
 
 
bryonk shield : pr, primlllru gruuvu.
 
Its embryonic crescent correH|>on»ls to the manialian terminal
ridge. Segmentation being limited tu the gcnii-<li»k in the
chick's egg, the resulting blastoderm, which is not a vesicle,
but a flattened mass (Fig. 32) composed of several layers of
cells, rest.-? by its margin upon the jKirtially liquefiwl yolk.
Tlie central region iif the blastoderm, which overlies tlie
liquefied portion of the yolk, from its tnuisluccnee ia known
as the area pellncida (Fig. 34), while the dark opafjue rim,
resting iipun the yolk is the area opaca. The inner rim of
the area opacii is tiie area vasculosa. These regions are
observed also in the mammaliiin e{^.
 
It in In Ihe eiiihri/o)iri! firea tilotie that the bofly of the
embryo is developed ; the other parts of tlie germ produce
extra -embryonic .structures, such a?* the amnion, the yolksac, etc.
 
Partial longitndinal division of the embryonic area during development result.s in the pi-o<hiction of some form of
doable monster ; its complete cleavage gives rise to homologous
or homogeneons twins, wiiicli arc twins of the same sex and
of almost absolutely identical structure. Oniiuary twins are
developed frtim separate ova, which may or may not have
come from the same ovary.
 
The Primitive Streak. — The primitive streak is a linear
 
 
 
00 TEXT-BOOK OF EMBRYOLOGY.
 
median marking lying ju the long axis of tlic ombiyonal
area and containing a median furrow, the primitive groove
(Fig. 35j. A traoMverse section tiirough the primitive streak
 
 
 
 
fFigs. 36 and 37) shows tliat Una surface-marking is produced by a thickening of the ectoderm along the median
liae, owing to a proliferation of wlls from its under aide.
TV lenfth of the streak is alwut two-thirds of that of the
■oafafTOBal area. In the rabbit's ovum it is seen at about
t vtrealh day ; in the hmiiun germ the time of its appear
 
 
 
h ItHi » pfobably about ihe third or fourth
 
» .^ tmtt a ^strula as that of the amphi
»iit»«fl dte blastopore appniadi cuch
 
 
 
TWO-LAYERED STAGE OF BLASTODERMIC VESICLE. 61
 
other and fuse in a line corresponding to the median longitudinal axis of the future embryonic area, the fusion or eoncrescence b^inning at the anterior extremity of this line
and proceeding toward its caudal end. The surface-marking
produced by the apposition and partial union of the blastoporic lips was called the primitive streak, and its median
furrow was known as the primitive groove, long before their
true significant^e was appreciated. Since the edge of the
blastopore marks the place of transition from the entoderm
to the ectoderm (Fig. 26), the two germ-layers afkr the
 
 
 
.SSJSi
 
 
 
 
union of the edges of this opening are in intimate association
under the primitive streak, as shown in Fig. 37.
 
Morpholi^ically the primitive streak of the higher vertebrates is regarded as the fnsod and extended hlastoiwre of
lower types. The terminal ridge of the mammalian embryonic area, as well as the crescent of the ombrvonic shield of
avian and reptilian e^js, represents, as 6tate<] above, the
anterior lip of the blastopore. Since the embn.-onal area is
increasing in circnmferpnee while the li^js of the blast()p<)re
are nndei^iing union or concrescence, the transversely directed terminal ridge, wliich lies at the ]>osterior edge of the
embryonal area, and which remains a fixed point, becomes a
 
 
 
G2
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
longitudinal marking, and tiiis marking or primitive streak
<*oinc.s to lie, therefore, behind the site of the . blastopore.
Hc'ference to DuvaFs diagram (Fig. 38) will make this clear.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Kill. "•H.— I)lHKriiiii fliicidiitiiiK the formation of the primitive groove (aAer
lUi\tit . Thi' liicrt'ii.tiiiK Hi/j' of the f;erm-disk in the course of the <Ieveh)pment is
iiiiht'HU'tl l»y iloltr«l rirtMihir lines. The heavy lines n-present the ereseentic
<niii\i- mill the primitive groove which arises from it by the fusion of the edges
 
A tier ihe devc'lopnient of tlie primitive .streak, there is
'*4Tii, ill I Ik* nirdian line of the enibrvonal area, anterior to
\\w MifaU, another marking, the head-process of the ])rimitive
^irrak. This is ahnost i(h'ntieal with the primitive axis of
\liiioi, whieh tlial investigat(>r (les<*ril)es as a median band of
«i'il.^ «*«iiiiiivtol with tlie ent<Mlerm and extending forward
'niiii ih«' Ma.slo|M»re.
 
HouMu's Utfdi^ In an neennnilation of cells on the nnder
 
-iiii'h-f .'t" I ill* t'ohHh'rni at the anterior end of the ])rimitive
 
-.ir«iv. M »^ iiinH»ruint lieean.s<» of its relation to the nenren
r-* . mal. wiiiili will be ilt'seribcd later.
 
. iMOttmi iu' primiiive streak and l)last«>pore j)l:iy no part
 
•I -KM ^u»c«'^ "' deNrlopnuMit, it is worthy (jf note that
 
„, 1.^ ti :iu' line of the longitudinal axis of the
 
— .^.K. imI iuii ilie |Hi?»ilion of th(? bht-^topore niarks
-,.:■.., . .iiiuai end of the embrvo.
l>evctO|Mtt«iiC of the Mesoderm. — Hie mesoderm
- ,; -.iMiiitiv vi'miH^stHJ of sevenil layers of eells
r . fKurui :uul the ento<K'rm. It is earliest
 
,N i' 'lie fn>nt end of the primitive
 
--..V- i'Mt4i\ iu'ld by the i)lastoj)ore. From
 
■ ■♦- .i t.:si.> ukI Tvr^eriorlv and, later, ant<'ri
^. '•^^%v\er, until (»ther imj)ortant
 
v.*^ rvt «•. 1 -Atciuls eompletely around
 
 
 
1 •♦-1 1
 
 
 
TWO-LAYERED STAGE OF BLASTODERMIC VESICLE 63
 
The terms gastral memderm and peristomal mesoderm
are used to designate respectively that portion developing
from the region of the head-process of the primitive streak
and that portion growing from the region of the blastojwre.
 
Concerning the origin of the mesoderm much difiPerence of
opinion prevails. The simpler and more primitive method
is seen in the amphioxus, in which it develojis as two evaginations from the dorsal wall of the arclienteron, one on each
side of the mid-line. These entodermic folds, containing
each a. cavity, the enterocoel, grow out laterally between the
inner and the outer germ-layers. By transverse constriction,
each fold divides into a series of segments, the somites, which
lie on either side of the median line from the head-end to the
tail-end of the embryo. Each somite divides into a dorsal
part, the " protovertebra," and a ventral jmrt, the lateral
plate. By the fusion of the lateral plates of each side their
several cavities become one, the body-cavity or coelom.
 
The origin of the middle germ-layer in higher vertebrates
is far less clearly made out. Some investigators hold that it
arises in essentially the same manner as does that of amphioxus — that is, by evagination or outfolding of the entoderm
bounding the coelenteron ; the investigations, however, of
Bonnet and of Duval respectively upon sheep and chick
embryos, point to a different conclusion. Bonnet's observations show that the mesodermic tissue, starting from Hensen's
node, grows out laterally between the ectoderm and the entoderm, and that at some distance from the median line of the
embryonic area there is a delamination or splitting-off of
cells from the entoderm ; and, further, that these two primitive areas grow toward each other and unite to form one
continuous sheet of mesoderm. It may be said, therefore,
that the mesoderm originates from a double source, chiefly
from the entoderm, hut also from the ectoderm, since the cells
giving rise to the part that grows from the region of Hensen's node are ectoderm ic. A section of tlie germ transverse
to the long axis of the embryonic area (Figs. 36 and 37)
shows the mesoderm to be a distinct and independent layer,
sharply defined from the other germ-layers everywhere except
 
 
 
64
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
\a the region of the mid-line, in which position the three
layers are so closely related as to constitute one striu-tur&
The mesoderm does not extend <wniplet«ly around the germ
at this stage, being deficient on the side opposite the embryonic area.
 
The mesoderm, after its formation, grows by tlie proliferation of its own cells, independently of the eotiKlerm and the
entoderm.
 
If the ex])ansion of the mestxlerra, as indicated by the
surface appearance of the germ (Fig. 391, be noted, it will
 
 
 
 
Fio, »9.— DlagmniniMlc »urfiies vli-w i.r r«bl)i('» uvum of SOS hours (alter Tnnrneuil. The dtrkly ahadHl area indicate! Ihe pilGtit of Ihe meandcmi. n. Pvriphenl llrnll nf area n|>ar> ; A. of area pellucldai e, nf parietal tune: d, cif itvin-ioDe ;
/. Henien'a nodi: ; g. proamnion.
 
be seen that at first it is present throughout a pear-shaped
area whose narrow end is directwl forward. Somewhat later,
two wing-like expansions grow forward fivim the front end
of this area (Fig. 40); these winga, meeting at their tips,
enclose a sjKice, the proamnion, which is devoid of mesoderm.
Referring again to the transverse section (Fig, 37), it is
evident that the middle germ-layer in the vicinity of the
median line is composed of a somewhat irregular mass of
cells, while farther away it constitutes u lamina on eiich side.
 
 
 
TWO^LAYESKn STAGE OF BLASTODERMIC VESICLE. 65
 
As development advaix^x, these two portions become more
differentiated from eacli oilier, althungli thej- are not entirely
separated niilM mmli lutt-r. The thick ma^^ adjiiwnt to tlie
median line is tin- Tertebral plate, ()r primitive Hegment plate,
or paraxiaJ mesoderm; the niorf Haitcnod hitor.il [wrlion is
the lateral plate. The mesodtjrm at this stage, therefore,
consists of four fwrts — the two pamxial musses, lying one on
each side of the median line, and extending from the head
 
 
 
Fra. «--DU((r«mm«tlp surluui: iku- uf n
nenxl. The darklf sliuded nrua IndlcBlei tb
enl limll vf ■roopoiMi; 2. of area pellnc[da: S. of pariiiti
6, Hemen's node; 7, proamnion.
 
end to the tail-end of the cmbrj-onal area, and the two lateral
plates, situated upon the outer sides of the paraxial colnmns.
 
Each primitive segment plate nndor^ocs transverse division
into a inimhrr of irrofiiihirjy rnlncal masses, thu mesoblastic
Bomites, or primitive segments, often improperly called the
protovertehrffi. The presence and position of the primitive
s^ments are indicated by transverse jiarallel lines on the
surface of the germ, which constitute a series on either side
of the primitive streak and its head-process (Fig.*. 40 and
46). The formation of the somites begins at the cephalic end
of the emliryo and progresses tailward.
 
The lateral plate of the mesoderm splits into two lamellaj,
 
 
 
1]
 
 
 
66 TEXT-BOOK OF EMBRYOLOGY.
 
of which the outer or parietal layer ig tlie Bomatic mesodenn.
and the inner or visceral layer is ihe splanchnic mesoderm.
The somatic mesoderm unites with the cctiHlerin, forming the
Bomatoplenre ; the splanchnic mesoderm unites with the entoderm, forming the BpIasctmopleuTe. The tissiire-like cavity
between the soniatoplciire and the splanchnopleure is the
cffilom, or body-cavit:r, or p leu rojieritj meal cavity (Fig, 45).
The great serous i-avities of the adult body — pleural, pericardial, and peritoneal — are later subdivisions of the ooe
The mesodermio cells bounding the body-cavity become
flattened and endothelioid lu character, and constitute the
mfluthelinm ; from them are descended the various endotlielial cells lining tlie serous cavities of the mature organism.
According to some authorities, among whom Hertwig may
be especially mentioned, there develop from the mesothelium
at an early stage certain (x.d]s whose jtarticular function is
the formation of the differt-nt kinds of connective tissue,
8uch as bone, i-arlilitge, fibrous ti.ssue, etc ; these elements
are often distinguished as mesencliyinal cella, or collectively,
as mesencliyine. According to this classification, the importance of wliich is insisted ujwn by Minot, the mesenchyme
includes all the mesodennic tissue except the flattened cells,
tlie meBothelium, lining the body-cavity.' "Mesenchyma
consists of widely separated cells which form a continuous
network of protoplasm, the meshes of which arc originally
filled by a honn^neous intercellular substance or matrix."
—Minot.
 
His claims a double origin for the mesoderm. He maintains that the mesothelium and the smooth musculature of
the h«iy are of intra-enibryimic origin, and these structures
he terms the archibUst; while all other parts of the mesoderm, which he designates the parablast, have, in his opinion,
an extra-embrvonic source, being derived [»ossibly from the
granulosa cells of the ovary. These views are not shared,
however, by the majority of cmbryologists.
 
> Minot holds Willi Cio«tt« Hint the uiesondiyin:!! fcll^ are tlio product
or till! mfwillii'liiim. IIprtwiK mninlninH (lint the mesenchyma artsei from
all the other germU.Teni by the emigration of i«ol*tcd cellA.
 
 
 
TWO-LAYERED STAGE OF BLASTODERMIC VESICLE. 67
 
The Derivatives of the Germ-layer8. — From the
 
three primary germ-layers are developed the various tissues
and organs of the body by metamorphoses which may be
referred to the two fundamental processes of spedalizcUiony
or the adaptation of structure to fimction, and of uneqvud
growthy which latter results in the formation of folds, ridges,
and constrictions.
 
From the ectodemi are produced : —
 
The epidermis and Us appendages^ including the nails, the
epithelium of the sebaceous and sweat-glands and their involuntary muscles, the hair, and the epithelium of the mammary
glands.
 
The infoldings of the epidermis^ including the epithelium
of the mouth, with the enamel of the teeth, the epithelium
of the salivary glands, and the anterior lobe of the pituitary
body :
 
The epithelium of the nasal tract with its glands and communicating cavities :
 
The epithelial lining of the external auditory canal, including the outer stratum of the membrana tympani :
 
The lining of the anus and of the anterior part of the
urethra :
 
The epithelium of the conjunctiva and of the anterior part
of the cornea, the crystalline lens.
 
The spinal cord, the b7*ain with its outgrowths, including
the optic nerve, the retina, and the posterior lobe of the
pituitary body.
 
The epithelium of the internal ear.
 
From the entoderm are produced : —
 
The epithelium of the respiratory tract.
 
The epithelium of the digestive trad, from the back part
of the pharynx to the anus, including its associated glands,
the liver, and the pancreas.
 
The epithelial parts of the middle ear and of the Eustachian
tvhe.
 
 
 
68 TEXT-BOOK OF EMBRYOLOGY.
 
The epithelium of the thymus and thyroid bodies.
 
The cpitheliuni of the bladder, and of the first pari of the
mide urethra J and of the entire female urethra.
 
From the mesoderm are developed : —
 
Connective tissue in all its modified, formsy such as bone,
dentine, cartilage, lymph, blood, fibrous and areolar tissue.
 
Muscular tissue.
 
All endothelial cells, as of joint-cavities, bursal sacs,
lymph-sacs, blood-vessels, pericardium and endocardium,
pleura, and peritoneum.
 
The spleen.
 
The kidney and the ureter.
 
The testicle and its system of excrdoi*y ducts.
 
The ovary, the Fallopian tube, the utei*us, and the vagina.
 
From the foregoing tabulation it may be seen that, generally speaking, all epithelial structures originate from either
the ectoderm or the entoderm, the notable exception to this
rule being that the epithelium of the sexual glands and their
ducts, and also that of the kidney and of the ureter, proceed
from the mesoderm.
 
 
 
CHAPTER IV.
 
THE BEGINNING DIFFERENTIATION OF THE EMBRYO; THE NEURAL CANAL; THE CHORDA
DORSALIS; THE MESOBLASTIC SOMITES.
 
The germ, in the stages thus far considered, has the form
of a hollow vesicle more or less irn^giilarly spherical. It
will he seen, in following the further history of development,
that the layers of cells constituting the walls of the vesicle
give rise to the alterations of external form and to the rudiments of the various organs of later stages hy processes which,
though seemingly com])lex, are referable to certain simple
fundamental principles. It is, namely, in the unequal growth
of different parts of the gt^rm, in outfoldings and infoldings,
and in the furrowing and constricting-off of parts, as well as
in the adaptation of structure to function, that we find an
explanation of the various develo})mental processes.
 
The first indication of the formation of the embr\'o and of
its differentiation from the parts of the germ that are destined
to produce, wholly or in part, the several extra-embryonic
structures, is the marking out of the embryonic area by the
thickening of the cells of the vesicle-wall in a definitely circumscribed region. The structures designated as extra-embryonic are the umbilical vesicle, the amnion, the allantois, and
the fetal part of the placenta. The development of these and
the production of the external form of the body of the embryo will be considered in the next chapter.
 
The primitive streak and its head-process have been already
described. After their appearance the further evolution of
the embryonic bodv is closelv associated with three fundamentally important processes — namely, the formation of the
neural canal, of the chorda dorsalis, and of the mesoblastic
somites.
 
69
 
 
 
70
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
The Neural or Medullary Canal. — The neural canal
 
is an elongatwl tulic Ij iiig Iji'iK'atli the ectoderm in the median longitudinal axis ol' tlie eniltryonio body, its [wsition
ct)rre)ipi)ndinf; tu that of the future e>pinat canal. Its walls
are cump4i!^ed of cylindrical epithelial cellw.
 
To follow the development of the medullary canal, it is
necessary to study the surface np]»earanee of the ovimi at. the
stage when the mesoderm is beginning to gniw out from the
r^ion of the head-process of the primitive streak. Upon
the surface of such a germ (Fig. 35), one may see the primitive iiitreak and, in front of it, also in the median line of the
embryonic area, the head-procesB of the primitive streak. The
ectotlermtc cells overlying the head-process thicken so as to
 
 
 
 
become columnar, while tliose on each side of it Ix'come flattened. This differcntiiition results in the prtMluctitm uf a
relatively thick axial plat« of ectoderm, the medullary plate,
which is present at the hcRinning of the ei^rhth day in the
rabbit's germ, and in the human germ at about the fimi'teeiith
 
 
 
THE SEURAL OR MKTiVLLARY CANAL. 71
 
 
 
 
day. Almost as soon as the plate is formed, its liitm-al and
anterior edges b^n tu curl up, producing tlie mednlUiy for*
 
 
 
 
72
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
thinner ectoderm; these projections constitute the mednllarr
fold!. A Hitrf'aee view shows the medullary folds to he
(MintitiiioUM witli euch other ia front, while their posterior
MwU iiri! w|mrutwl and embrace l>etween them the front end
iif ihi! priniilivti Htrcuk (Fig. 41). Since the formation of
\\ii\m: ntrii'^liiri-N '\r, always mure advanced in the ant«rior part
(»(' th" cfiiliryoiiii! iin'u, their posterior extremities are not
Mhiirply 'li'tln(«i hut fiwle away (Fig. 41). The edges of the
tnithiihiry plnte <>»iitinnc to curl until they meet, when they
llliil4-, forriiiliK tJie mftdullary or neural canal (Figs. 44 and
 
 
 
 
I'll 'V\w mm I II I III I'v tiililM mill plate (continuing to advance
hivvmtl llu> lull imkI nl' \\w i<iiilirvi)nii> area, and the closure
(>l llii> dihi' UilvliiH |iliii<4' iVmi lii'fon' Imokward, the entire
tulii(lllvi>«li«4tk la iiimli' ltMllpiiip|H<nrliy lieing included within
\\w tivtiml \\\W
 
\\w u\v*\\\\\-\\\ y\\\\U liiivliiu: grown tiiwnnl each other a
Aw^xi um^> t'lltav \\w uiil»ii of th<> ciigcH of the me<Inllary
yi»\W "I'w iiihum'wi' tl«> |wrrliilly riirnii'd neural tube. By
vhv yiKWlh \iC \,\\v w.\\\\\\\\w\ liijilrt itiid their subsequent
v^\tK. 1 1 iii^', lhi> t4>iii)'U'Usl ut'tinil 1tilH> <>oni(<M to lie under the
mUm\' viUhKhh. iu ^xuktHVthm wllh wliioli is afterward lost
 
 
 
THE SOTOCHORD OR CHORDA DORSALIS. 73
 
It is apparent, therefore, that the neural tube is a structure
whose walls are cumposed of ectodermic cells, and that it has
originated from the ectoderm by what may be called a process
of infolding.
 
The medallanr canal is the fundament of the entire adult
nervous system. The first step in the conversion of a structure so simple into one so complex consists in the rlilatation of
the cephalic end of the ucural tube and the subsequent division
of this dilated extremity into three imperfectly separated compartments, named respectively the fbre-brain, the mid-brain,
and the Iiind-brain Tesiclea. It is by the multiplicatiou and
 
 
 
Paritlai UHuJirm,
 
 
 
 
Tia. IS.— TiSDSvene lection or a gcTenteen-and-A'balf-daT Bheep-embrro (Bonnel).
 
 
 
specialization of the cells composing the walls of Hie medullary tube that the cerebrospinal axis is produced, the brainvesicles giving rise to the brain-mass, while the remainder of
the tube produces the spinal cord. Approximately one-half
of the length of the (u!>e is devoted to the formation of the
brain, the other half forming the spinal cord.
 
The nenral tube closes first in the future cervical region,
the cephalic part of the canal remaining o]>en for a time.
FVom the neck region the closure of the tube progresses
toward either end of the embryo.
 
The Notochord or Chorda Dorsalis. — The notochon!
is a solid cylindrical column of cells lying parallel with the
medullary tube, on the dorsal side of the archenteric cavity.
 
 
 
74
 
 
 
TKXT-BOOK OF EMURYOLOOY.
 
 
 
IIh position is that of a line passing througli tlio centers of
the bodies of the future vertebrie, Tlie development of tlie
chorda occurs at the .same time aa that of the neural tube,
and ill a verj- similar manner. A thickening of ihe cells of
the entoderm in a longitudinal line extending along the
dorsal aspect of the weleateron produces the c}iordal plate.
Along cither edge of the chordal plate a small fold of entoderm projects veutralward. By the curling around of the
edgcii of the chordal plate, the latter becomes a solid cylinder
of cells, which is isepunited from the entoderm proper by the
union of the chordal folds, as shown in Figs. 44 and 4-5.
 
The appearance of the notoehord is the first indication of
the axis of the embryo, since around it the permanent spinal
column is built up. The relative size of the chorda is lees
in the higher vertebrates than in the lower members of this
group. It is (me of the distinctive features of a vertebrated
animal.
 
The chorda is essentially an ombrjonie structure, since it
gives rise to no adult organ. Its only representative in
postnatal life is the pulpy substance in the centers of the
intervertebral disks. It is a [KTroanent structure in one
vertebrate only, the amphioxus. In this animal it is the
representative of the spinal (*lumn of higher vertebrates.
The notoehord affords another illustration of the principle
that higher organisms rp|)eat, in their development, the
structure of the lower members of the group to which they
k-lon^r.
 
The Neurenteiic Canal. — The nenrenteric canal is
closely associattnl with the ilevelopmcnt of the medullary
canal and with the disappearam-e of the primitive groove.
We have learned tliat the blastopore is the orifice through
which the ccelenteroii ojiens to the exterior, and also that in
birds and mammals the position of the blastopore, as indicated by the presence of the terminal ridge, corresponds to
the anterior end of the primitive streak, and therefore of the
primitive groove. Reference to Fig. 41 will show that the
medullnry folds have extended so far posteriorly that they
embrace between them the primitive groove; therelore when
 
 
 
THE SOMITES OR PRIMITIVE SEGMENTS 75
 
they unite to form the neural canal, the primitive streak falls
within its limits.
 
In a gastrula with an ojKin blastopore, such as that of the
amphioxus and those of amphibians, the blastopore is included between the me<lullary folds, and, after the completion
of the neural canal, it constitutes an avenue of communication between the latter and the coelenterou or primitive enteric
cavitv; this communication is the nenrenteric canal. In
mammals, as also in birds, reptiles, and selachians, classes in
which the primitive streak is the representiitive of the closed
blastopore, a small canal is found at the anterior end of the
primitive groove, passing through Hensen's node, and opening into the coelenteron. With the covering in of the primitive groove by the medullary folds, this canal becomes the
neurenteric canal. According to Graf Spec, a ueurenteric
canal is found in the human embryo, as well as in the groups
above mentioned. The canal is a temporary structure and
gives rise to no organ of the adult.
 
The Somites or Primitive Segments. — The mesoblastic somites are cuboidal masses of cells, arranged in two
parallel rows, one on each side of the notochord, extending
the entire length of the l)ody of the embryo. They are
sometimes called protovetiebvcey but this term if use<l at all
should be restricted to a subdivision of them that appears
later.
 
The development of the somites was incidentally referred
to in the descripticm of the mesoderm. As mentioned in that
connection, the paraxial plates of mesoderm, lying as parallel
longitudinal columns, one on each side of the notochord,
break up, each one into its corresponding series of primitive
segments. The division throughout the entire length of the
boily takes place not simultaneously, but consecutively, beginning at the head-end.
 
The segmentation of the axial mesoderm is indicated by
certain surface markings. The surface of the embryonal
area, at the stage when the primitive streak and the medullary groove are present, shows a dark zone on either side of
the median line, the so-called stem-zone, which marks the
 
 
 
76 TEXT-BOOK OF EMBRYOLOGY.
 
litiiiti! of the iixial pliit« uf mesoderm (Fig. 46) ; the position
of the lateral platos in indicated by the peripheral lighter
puietal zone. The stem-zone soon exhibits, on eucli side of
the primitive streak and medullary groove, a series of parullel
transverse lines, produced by the transverse furrowing of the
axial plates, preparatory to their divisiou into the primitive
e^ments. The first pair of somites is formed in the future
cervicid region, before the medullary folds have united to
form the neural tube, and when tiie primitive streak is yet
preseut. After tlie appeuranee of the first ]>air, the forma
 
 
 
*4 ^ X
 
 
 
¥ 4*
 
Fra. «,— fi«bbll embryo of the ninth A»j, seen from Ihi- doraal side (aflet
KClItker). Mignlflert SI illiiraate™. The Blem-Bcinu («i:i und Uieiittrieml lone (pil
are lo be diHtlnKiilBhccI. In the fonner 8 pairs of prlmlllve scEtnentii have been
estobllBhed al the aide of the ehorda and nentral tube: op. area pellnclda: r/,
medullary trnrare: fA, fore-brain: nft, eye-vealele: nA, mM-braln: M>. hind-brain:
uH>, primitive eemaetitT id, stem-Hine : jv, partetsl mnei A. heart', »*, pericardial
part of the body-cavity: vd, marjcin of the entrance to the benil-inic tvorStre
Da/mpfotie'. acen through the overljrinit stmclnn's; nf. amniotic fold: ™. vena
omphalomeienterlea.
 
tion of other segments proeceds hradward and tailward. In
selachians the number of head-segments has been shown to
be nine; in higher vertebrates the number is possibly less.
The trunk-segments are added in regular order from the
neck-region to the tail-end of the embryo. In the hiimau
embryo there are thirty-eight (wirs of neck and trunk
somites and ix;rhaps four pair* in the i»coipital region of the
head.
 
The first somites appear on the eighth day in the nibbii,
and between the twentieth and twenly-semnd hours in tlie
chick. While they are forming the nciinil canal is closing,
 
 
 
THE SOMITES OR PRIMITIVE SEGMENTS, 77
 
the notochord is difFerentiating from the entoderm, and the
lateral plates of mesoderm are splitting to form the bodycavitv or ccelom.
 
In structure the primitive segments of lower vertebrates
consist of columnar cells arranged around a central cavity
(Figs. 43 and 45). The cavity, in the amphioxus, communicates for a time with the ooelenteron, since the segments
are in this case developed as entodermic evaginations ; in
selachians, the method of formation of whose primitive segments may be regarded as the primitive method for vertebrates, the cavity is for a time in communication with the
body-cavity, since the segments in these animals develop as
if by evagination from the dorsjil side of the mesoderm after
it has separated into its parietal and visceral layers and before
it has divided into the axial and lateral plates. The size of
the cavity is quite variable; in some cases, as in the Amniota,
it is almost if not entirelv obliterated bv the encroachment
of the cells of the walls of the somite.
 
Belonging to the somite, though not apparent on the surface, is a mass of cells which connects, for some time, the
somite proper with the lateral plate (Fig. 45). This is
known as the intermediate cell-mass or middle plate. Later,
the separation of these is effected, the mesial part of the
somite being the myotome, the intermediate cell-mass
becoming the nephrotome. Each one of these parts
contains a cavity, that of the myotome being called the
myocoBl. From the inner, mesial side of the myotome,
embryonic connective-tissue cells (mesenchyme) develop,
constituting the sclerotome, or skeletogenous tissue. The
sclerotomes, made up of loosely-arranged embryonal connective tissue, grow around the medullary canal and chorda
dorsalis, spreading out and fusing with each other. Subsequently this tissue produces the vertebral column and its
associated ligamentous and cartilaginous structures. The
outer part of the myotome, sometimes called the cutis plate,
gives rise to the corium of the skin of the trunk or perhaps to muscular tissue. The remaining part of the myotome, that situated dorsolate rally, constitutes the muscle*
 
 
 
78 TEXTBOOK OF EMBRYOLOGY.
 
plate or myotome proper ; it gives rise to the voluntary musculature of the trunk.
 
The segmentatioii of the body of the embryo is an embryological process of great significance.
 
The s^mented condition is common to the developmental
stage of all true vertebrates, and in some invertebrates it
persists throughout adult life. The development of the
axial skeleton and of the muscular system, it will be seen
later, bears an important relation to the process of segmentation, as does also the evolution of the genito-urinary
system.
 
Upon reflection, it will be seen that in the region of the
embryo corresponding to the future neck and trunk, the
segmentation affects only the dorsal part of the body, while
the ventral mesoderm, the so-called lateral plate, which contains the ccielom, remains unsegmented. On the other hand,
in the head-region, the segmentation is both dorsal and ventml, the fornx^r being in series with the trunk-segments,
while the latter, affecting the ventral mesoderm, and therefore also, in the corresponding region, the coelom, produces
j*ognu»nts known as branchiomeres, in connection with which
tho vistH^nil arches are developed (see Chapter VII.).
 
Thr ivhition of the primitive segments to the differentiation t»f the skeleton and of the musculature of the trunk, and
ttl«o of the visceral arches to the muscles of the jaws, will be
inm^idrrtnl in subsequent chapters.
 
 
 
CHAPTER V.
 
THE FORMATION OF THE BODY- WALL, OF THE
INTESTINAL CANAL, AND OF THE FETAL
MEMBRANES.
 
 
 
The formation of the fetal membranes occurs coincidcDtally
with the production of the external form of the body of the
embryo. These changes mark the division of the hollow
sphere or vesicle of which the germ consists up to this stage
into two essentially diflFerent parts — namely, the embryonic
body and the fetal appendages, the latter of which are destined
for the nutrition and protection of the growing embryo.
Although the several processes by which are produced the
diflFerent parts of the embryo and its various appendages go
on simultaneously, it is necessary, for the sake of clearness,
to consider successively the development of each structure
from its inception to its completion.
 
THE FORMATION OF THE BODY-WALL AND OF THE
INTESTINAL CANAL OF THE EMBRYO.
 
In the stages of development thus far considered, the part
of the ovuni that is to become the embryo — that is, the
embryonic area — is represented by a localized thickening of
the wall of the blastodermic vesicle, of the shape and relative
size shown in Fig. 33, which presents a surface view of the
germ. On each side of the embryonic axis, represented by
the notochord, is the paraxial mass of mesoderm, which has
undergone partial segmentation to form the somites ; on the
distal side of the paraxial column, the mesoderm has split
into the somatic or parietal, and the splanchnic or visceral
lamellae, between which is the body-cavity or coelom. The
cavity of the germ until the occurrence of the transformations about to be described is one undivided compartment
which is bounded by splanchnopleure ; and a conspicuous fea
79
 
 
 
80 TEXT-BOOK OF EMBRYOLOGY.
 
ture of the changes under consideration is the division of this
cavity into two by the folding-in of the splanchnopleure composing its walls. It will be well to consider first the formatior
of the body-wall and the accessory structures of the chick, and
then to take up the special mollifications that are presented
by mammals generally and by the human ovum in particular.
The first indication of the foldings that lead to the differentiation of the embryo from the fetal appendages is seen
upon the surface of the germ at a very early stage. A surface view of the germ — in the case of the chick on the first
day of incubation — shows, at what becomes the head-end of
the embryonic area, a transverse crescentic groove, with its
concavity looking backward (Fig. 41) ; a similar groove is
seen at the opposite extremity of the area, and also one at
each lateral margin. These marginal grooves are depressions in the somatopleure. The elevated outer edges of the
grooves form folds of somatopleure, designated respectively
the head-fold, the tail-fold, and the lateral folds of the amnion.
As these marginal grooves increase in length they meet each
other and now constitute one continuous furrow, which encircles the embryonic area ; its outer elevated edge is the aninionfsihL This furrow, which may be called an inverted fold
composed of splanchnopleure and somatopleure, progressively
deepens and at the same time its bottom is carried inward
toward a point vertically under the central region of the
cmbryimic area ; that is, a fold composed of somato})leure
•■4 gfJanchnopleure grows from all parts of the periphery
«f Ac embryonic area toward the point indicated above, a
fMik which corresponds to the site of the future umbilicus.
Bf At iDgiowth <rf the edges of the fold, the cavity of the
 
» moie and more constricted (Plate II., Figs.
 
Sy, vatil Cmdly, with the completion of the infolding,
 
^^^'•"•■"•Avifcd iaio two parts of unequal size ; the smaller
 
<# t)liii»-fi|Ma^iftdiagliiiMnci, or intestinal canal of the em
Vaffyil^ wWi» di^ luf j u m the yolk-sac or umbilical vesicle.
 
which the gut-tract commute vttelliiic duct (Plate II.,
 
kfcf of the ingrowing fold
 
 
 
 
 
niBgninu lUusLrntiiit: tha fbrmatlon of Itau fl-tal membniiiGs {miHlllIed frum Rnule).
"The ipapes miirt.il ■ lioclj-uaviiy ' In Flgun^s X and 4 nn; merely Itie etirs-embri'oiilo
portions of llie body-cavlly. For more recent mncepUona »a in the romiatiuii «t tbOM'
 
 
 
THE FORMATION OF THE BODY-WALL. 81
 
thos outlines and forms the walls of the intestinal canal, the
aomatopleuric layer, which acconiiianios it, constitutes the
lateral and ventral body-walls of the enibrvo. During the
progress of this infolding of the splanchnopleure and the
somatopleure, the part of the latter nicn)l)rane that forms
the outer wall of the groove becomes lifted up to constitute
the anmion-fold (Plate II., Fig. 3) ; by the continued upward
growth of this amnion-fold and the {simultaneous settling
down of the embryo upon tlio yolk-sac, the margins of the
fold come to lie above the cml)ryonic IkkIv, and, approacliing
each other, they fuse over its ba(^k, in this manner enclosing
it in a cavity. It is obvious that the fold just described is a
double layer of somatopleure. After the union of its edges,
the two layers become completely scjwinited, the inner one
constituting the amnion, while the outer layer is the false
amnion, or serosa, or chorion (Plate 11., Fi^s. 4-()).
 
Since the infolding of the sphmchnopleure begins at the
periphery of the much elongatcMl eml>ryonic area, the resulting gut-tract has the form of a straight tube extending from
the head-end to the tail-end of the embrvo (Plate IWX
When the caudal and the cephalic portions of the s|)]am'lm(H
pleuric fold have advanced but a comparatively short distance, in consequence of which the coinnuinicatinn between
the gut-tnict and the umbilical vesi<*le is still widely o]>on,
as shown in Plate II., Fig. 5, there is a eiil-de-^iie or ]>oeket
formed of splanchnopleure at the head-end (»!' tlut embryo
and a similar one at its tail-end ; these reee<ses are respectively the foregut and the hindgut, the orifiees of whieli are
designated the intestinal portals. At this particular sta^rts
therefore, the cavity of x\w crut-tnirt i< ineompletely closed
off from that of the uinbilieal vesicle.
 
It is evident that the gut-tract, beiuL^ a tiiluilar cavitv
enclose<I by splanehno]>leurc, is lined with entodernial cells;
this simple strai<j:ht tube develops subsequently into the adult
intestinal canal and its associated ^^andular apparatus.
 
It has alreadv been pointed out that the lavcT of somatopleure which is iohh'd under the embrvonie area in coni]>anv
with the splanchnopleure constitutes the lateral and the ven
 
 
82 TEXT-BOOK OF EMBRYOLOGY.
 
tml walU of the Ixxly of the embryo. The fold continues
to 'jji\\"dnoM from each side and from each end, and its
erJ^ri? t'jniiti together and fuse in the median line of the
v*?fjtral hurface of the bo<Iy.* At one place, however, fusion
of the mli^t^aof the fold dfies not occur; this r^ion correkf{x>tMlif U} the umbilicus and is often designated the dermal
nsreL Here the {jart of the somatopleure that form^ the
UmIv'^wiiII ih c;ontinuous with that part of this membrane
whi^'h ^^>n»^tituUfH the amnion (Plate II., Fig. 6). By the
iiifoldiiiir of the H^imat^>pleure the body-cavity or pleuro|X'rit/>fj<-sil K|KUM« \HH^mien divided into an intra-embryonic
and an ^'Xtni'^frnbryonic [lortion, the two communicating for a
liffie through the Hmail annular space that encircles the
proximal t'tnl of the vitelline duct; this is represented in the
a/'X'.'/miia living figures.
 
Uy iUii- Huiphf prrntesH of folding, associated with the
iin<''jiial growth of different parts, the leaf-like fundament
i'diui^ilutU^l bv the embrvoiiie arc«i is differentiated into the
\p4nly of the I'lnbryo; the ventral jK)rtion of. this body now
i'4nmti^tH of two tiibeh, one within the other, of which the
hrrmlh'r, lK>iiiidi'd by the Hphiiiehnopleure, is the intestinal
canal, and the Iarg<'r, I'lu-lowd by the somatopleure, is the
bodjr-caritjr, th** wallK of which are tin? walls of the body of the
embryo. In the doival region is a third tube, the medullary
canal; iM'twecn it and the dorsal wall of the intestine is the
notochord, on itwh side of wliieh an* the somites (Fig. 44).
The further evolution of this biKJv and the differentiation of
its various orgjuis and systems will be? described in subse^juent sections.
 
THE AMNION.
 
The amnion is a inembmnous fluid-filled sac, which surrounds the fetus of certain gmups of vertebrate animals
during a part of their perio<l of (l<>velopment. In mati, it is
 
' Failuro of uni<m of tlio wimntoplciiric fohlM in the median line of the
 
thorax jjrrKluww the deformity known uh cleft Htennim ; while lack of fusion
 
of the hiteral halven of the alxloiniiial wall n^Mnlts in an extra-alxlominal
 
* INmition of the inteHtincH, or, if in lesKer dej^ree, in exstrophy %£ the bladder.
 
 
 
THE AMNION. 83
 
found as early as the fourteenth day,^ before the medullary
groove has closed to form the neural canal ; it attains its
maximum size by the end of the sixth month and persists
until the end of gestation. It constitutes a loose envelope
for the fetus, being attached to the abdominal wall of the
latter at the margins of the umbilicus, and loosely enveloping
the umbilical cord (see Plate III., Fig. 2).
 
An amnion is found in birds, reptiles, and mammals, these
groups being classed together as Anmiota, while fishes and
amphibians, which are without an amnion, constitute the
class Anamnia.
 
The first indication of the growth of the amnion is apparent
at a comparatively early stage of development. A surfaceview of the blastodermic vesicle of the first- day of incubation in the case of the chick shows a curved line or marking
at the anterior edge of the embryonic area (Fig. 41) ; this is
the anterior marginal groove, in front of which is another
marking, the head-fold of the amnion. Very soon the lateral
and posterior marginal grooves appear at the sides and posterior edge respectively of the embryonic area ; the outer elevated edges of these marginal grooves constitute the lateral
folds and the tail-fold of the amnion. The grooves and folds
increase in length in each direction until they meet, when
they form one continuous furrow, which circumscribes the
embryonic area, and the outer elevated edge of which is the
amnion fold. The groove involves both the somatopleure
and the splanchnopleure, constituting the inverted fold of
these two structures that grows in to form the body- wall and
the wall of the gut-tract, while the amnion fold is composed
of somatopleure alone (Plate IT.). This separation of the
somatopleure and the splanchnopleure enlarges the extraembryonic portion of the bofly-cavity. The amnion fold
continues to grow upward, and finally its edges meet and fuse
over the back of the -embryo, the line of union being the amniotic suture ; the suture closes first at the head-end of the
embryo and last at the tail-end. After the union of the edges
 
' Ret*enlly it lias been found complete in an ovum estimated to be four
(lavs old.
 
 
 
84 TEXT-BOOK OF EMBRYOLOGY.
 
of the fold, its inner layer, consisting of ectoderm and parietal mesoderm, separates from the outer layer to constitute the
true amnion, whose enclosed s|>ace is the amniotic cavity ; the
outer layer, which is merely a part of the general somatopleure, is the false amnion or serosa. It is apparent from
this description that the amniotic cavity is lined with ectodermal epithelium and that its walls consist of somatopleure
— that is, of ectoderm and parietal mesoilerm.
 
While the amnion fold is growing upward, the embryonic
area — now undergoing ditferentiation into the embryonic
body — is sinking down ujK)n the yolk-sac. The amnion fold
does not grow uniformly in all parts of its periphery. The
head-fold is produced first and constitutes a cap or hood covering the head of the embryo, which is forming simultaneously
by the vcntnid growth of the somatopleure at the bottom of
the marginal groove. It is only after the development of the
head-fold is well a<lvanced that the lateral, and, later, the
caudal, portions of the amnion-fold grow up to meet it. The
head-fold is, for a time, destitute of mesodermic tissue, since
it corresponds to that region of the wall of the blastodermic
vesicle described on ])age 64 as the proamnion.
 
It htis lK»en shown (p. 54) that the amniotic cavity of
mammals is produced not by the upgrowth of folds of somatt>pleure, but by a vacuolation of a portion of the cells of the
inner ii»ll-mass (Fig. 28, p. 5")). Since the enveloping layer,
which forms the roof or vault of the amniotic cavitv, constitutes the extra-embrvonic cct<Mlerin (p. 05 1, this cavitv in
mammals as in binls is lin<»d with ccttMhTmal cells, the floor
t»t the cavitv being also ectodermal since it is i'ormed by the
<Mni^r^'«mic disk, the amniotic* surface of which constitutes the
•'mnrviioic eou>ilerm (Fig. 21>). Covering the ectodermal roof
> A .uv^-r '^t nu*SiHlenn continuous with tlu* mesiHJerni of the
Li'n- -iiic dLsk. The embryonic bud or <lisk, at first con"■' •«! rs> ^mwiitfii* <urf?HH\ the future dorsjil surface, becomes
*ia\vx^ Its eilgi*s curving towanl th<' o])jM»site
 
r -^HTTs. -=iriL'^- It should not be forgotten that the
-5P-tt-:=. — -^^^ "iis ventral surface and at the ]MTipherv
 
Jeti'p surface of the enveloping
 
 
 
THE AMNION. 85
 
layer, and also that the embryonic ectoderm is likewise
continuous at the periphery of the embryonic bud with tliat
part of the enveloping layer which forms the vault of the
amniotic cavity ; hence, after the ventral curvature of the
embryonic bud, the periphery of which carries with it toward
the ventral surface the amniotic ectoderm and mesoderm,
we have practically the same conditions as obtain in the
avian embryo as shown in Plate II., Figs. 4 and 5. While in
the latter case the amnion is produced by the formation of
folds, in the mammalian germ the same result is attained by
the vacuolation of the inner cell-mass.
 
As the curving ventrally of the embryonic bud continues,
the originally flat mass of cells composing it is converted
into an imperfect tube, the lateral and ventral surfaces of
which correspond with the former dorsal surface. This
ventral folding of the embryonic bud produces the body of
the embryo. As the folding includes the entodermal layer
on the ventral surface of the embryonic bud, the blastodermic
cavity is divided, as in the bird's germ, into the primitive
intestinal canal and the yolk-sac (Fig. 48).
 
The amnion of man presents an important variation
from that of all other Amniota, since the inner layer of the
amnion-fold does not entirely sever its connection with the
outer layer, but remains attached to it over the caudal pole
of the embryo. In consequence of this attachment the true
amnion is connected with the false amnion, and since the true
amnion is continuous with the body- wall of the embryo, the
caudal end of the embryonic body is attached to the false
amnion or chorion bv a mass of mesodermic tissue called the
allantoic stalk or belly-stalk, as seen in Fig. 47. The relation of the belly-stalk to the development of the allantois
will be pointed out hereafter.
 
The space within the amnion — the amniotic cavity — is filled
with the amniotic fluid or liquor amnii.
 
The amnion at first envelops only the sides and dorsum of
the embryonic body, occupying the upper part of the cavity
enclosed by the chorion, as shown in Plate II., Figs. 5 and
 
 
 
86 TEXT-BOOK OF EMBRYOLOGY.
 
6 ; the groove, or farrow, however, of which the amnion fold
is the peripheral or outer elevated edge, becomes deeper, and
the bottom of the groove is carried toward the middle of the
future ventral surface of the embryo, its ventrad growth continuing until it reaches the position of the future umbilicus
(Plate II. : Fig. 4, transverse section ; Figs. 5 and 6, longitudinal section). The layer of somatopleure constituting the
inner wall of the groove — that is, on the side toward the
embryonic area — becomes the lateral and ventral walls of
the body of the embrvo, as described above ; in this manner
is effected the transition from the flattened or layer-like embryonic area to the definite form of the embryonic body.
The ventral body-wall is continuous at the margins of the
umbilicus witli the amnion, since the somatopleure, forming
the outer boundary of the original groove, is a part of that
membrane. After its completion, therefore, the amnion envelops tlic body of the embryo on every side, lying closely
applied to it, since the amniotic cavity is at first very small.
With the progress of development and the increase of the
fluid the amnion requires more room, until, in the third
month, it fills out the entire space within the chorion, with
the inner surface of which it acquires a loose connection.
The umbilical vesicle and the alhintois have meanwhile
imdergone regression. The walls of the amniotic sjic contain contractile fibers; it is to these that the rhvthmical contractions observed in the amnion are due. Its lining is at
first a single layer of flattened epithelial cells; at the fourth
month the cells are cubical for the most part, but to some
extent columnar.
 
The liquor amnii is a watery fluid having a s]>ecific gravity
of 1.007, and containing about 1 per cent, of solids (albumin,
urea, and grape sugar). The origin of the fluid is believed
to be in the blood of the mother, the liquid portion of which
transudes into the amniotic cjivity. The amniotic fluid increases in quantity until the sixth month of pregnancy ; from
this time until the close of gestation it generally diminishes
about one half. A pathological excess of the fluid constitutes
the condition of hydramnios.
 
The Amction of the amniotic fluid is two-fold ; it serves as
 
 
 
THE YOLK-SAC. 87
 
a buffer for the fetus, protecting it from mechanical violence,
and it supplies the fetal tissues with water, since portions of
it are from time to time swallowed. Evidence that the fetus
swallows the fluid is aflTorded by direct observation of chicken
embryos, and by the presence of epidermal cells, hairs, and
fatty matter in the fetal alimentary canal. After the development of the bladder, the urine of the fetus is from time to
time evacuated into the amniotic cavity.
 
The epidermis of the child in utero is protected against
maceration in the amniotic fluid by the presence of a fatty
coating, the vemix . caseosa, which is a modified sebaceous
secretion.
 
At the end of pregnancy, the amnion is loosely united
with the chorion and the deciduae ; during birth it ruptures,
and its fluid escapes.
 
THE YOLK-SAC.
 
The yolk-sac, or umbilical vesicle, as seen in the higher
vertebrates, is a capacious sac attached by a narrow i>edicle,
the vitelline duct, to the ventral surface of the embryonic
intestinal canal, the duct passing through the umbilical aperture (Plate II., Fig. 6).
 
In order to appreciate more fully the function and the
morphological relations of this structure, it is necessar}' to
glance at the conditions that obtain in the several classes of
vertebrate animals. In ova that develop outside of the body
of the parent organism, a special dower of pabulum is provided for the nutrition of the embryo ; this dower is represented by tlie deutoplasm so abundant in telolecithal ova.
In the case of amphibians, whose cleavage, it will be
remembered, is holoblastic or total, the cells richest in deutoplasm are accumulated, after segmentation, in the floor of the
archenteron ; this accumulation produces on the future ventral
surface of the embryo a marked bulging, which constitutes
the amphibian yolk-sac. As the embryo grows, it draws
upon this store for its nutrition, in consequence of which the
sac gradually shrinks, its cells being, for the most part.
 
 
 
88 TEXT-BOOK OF EilBRYOLOGY.
 
liquefied nod absort)ed, while Bome of them cx>ntribute to the
lining of the iiiteittiiial canal.
 
Ill a UghaT type, as exemplifled in sharks and dog-fiahGS,
the yolk-sac is produced by a folding-in of the spIaDchnopleure and the soraatopleure, the walls of the sac being
therefore constituted by both of these layers ; this folding-ia
divides the arehenteroa into a smaller part, the intestinal
canal, lying within the body of the embryo, and a larger
cavity, the yolk-sac, situated outside of that body. The
eplanchnopleuric layer of the yolk-sac is continuous with the
wall of the intestuial canal, while its somatopleuric layer is
continuous with the body-wall. A system of blood-vessels
develops upon the yolk-sac, their function being to convey
the nutritive material into the Ixidy of the embryo. These
blood-vessels constitute the so-called Tascnlar area, which
appears, in surface views, as a zone encircling the embryonic
area, and, later, the embryo, since the latter reposes upon the
proportionately much larger yolk-sac. As the contents of
the sac becj)me absorbed, the latter shrinks, the splanchnopleurie layer slipping into the abdomen iif the embryo
through the umbilical oi>ening, the somatopleuric layer contracting to close that aperture.
 
In the Asmiota (p. 83) the development and structure of
the yolk-sac are modified by the presence of the amnion. In
these groups the yolk-sac and the gut result from the division
of the blastodermic cavity by the folding-in of the eplancli■oplcure alone, since the somatopleure grows away from the
Bplanchnopleure to form the amnion-fold, and thus only
partially invests the yolk-sac (Plate 11., Fig. 4),
 
Since the yolk-sac contains the store of food destined for
the nutrition of embryos that develop outside of the maternal
body, and since the mammalian embryo, which le.idsan intranterine existence, is endowed with a relatively small quantity
of such store, the yolk-sac of mammals would seem to indicate the dcwent f)f the latter from oviparous ancestors.
Further and strimger evidence of such descent is found
in the fact that the eggs of the lowest order of mammals,
the Monotrcniata, comprising the echidna and the ornithorhynchus, are " laid" and undergo triro-uterine development.
 
 
 
THE ALLANTOIS. 89
 
In the hninan embryo the umbilical vesicle is found partially constricted off from the intestinal canal by the end of
the second week ; by the end of the third week the separation of the two cavities has advanced to such an extent that
the vitelline duct is present, the sac attaining its maximum
size by about the fourth week.
 
The fanction of the umbilical vesicle, as above intimated, is
to serve as the organ of nutrition for the embryo during a
certain period. The manner in which its blood-vessels develop will be considered in Chapter X. Their growth precedes that of the intra-embryonic portions of the vascular
apparatus, the vascular area of the yolk-sac being the seat
of the earliest blood-vessel formation. The vessels find
their way into the body of the embryo along the vitelline
duct, and consist of two vitelline arteries and two vitelline
veins.
 
With the development of the allantois the yolk-sac retrogresses, the allantois succeeding it as the organ of nutrition
and respiration. By the end of the sixth week the sac has
shrunk to a narrow stalk, which is surrounded by the enlarged amnion, and which terminates in a knob ; at birth,
the knob lies near the placenta (Plate V., Fig. 2), and the
atrophic remnant of the stalk is one of the constituents of
the umbilical cord.
 
THE ALLANTOIS.
 
The allantois is an embryonic structure which is found in
those vertebrates possessing an amnion. Its growth is correlated with the retrogression of the umbilical vesicle, which
structure it supplants as the organ of nutrition and respiration for the embryo.
 
Appearing at first as a little evagination or out-pocketing
of the ventral wall of the gut-tract, the allantois finally becomes a pedunculated sac lying in the extra-embryonic part
of the coelom (Plates II. and III.)> its stalk leaving the
body-cavity proper through the umbilical opening. Being
an outgrowth from the intestinal canal, the walls of the
allantois are made up of splanchnopleure — that is, of ento
 
 
90
 
 
 
TEXT'BOOK OF EMBRYOLOGY,
 
 
 
derm and visceral mesoderm. Blood-vessels develop in the
mesodermic stratum^ the principal trunks^ the two allantoic arteries and veins, being connected at their proximal
ends with the primitive heart ; this system of vessels constitutes the allantoic circulation and is the avenue through
which the growing eml)rj'o is supplied with nutritive material and oxygen. As the fundus of the allantois increases
in size, it spreads itself out ujwn the inner surface of the
false amnion or chorion (Plate III., Fig. 1), into whose villi
its vascular tissue penetrates, and with which it becomes
intimately blended. The union of the allantois and the false
amnion produces the true chorion of some authors.
 
The human allantois presents a striking i)eculiarity as compared with that of birds and reptiles ; in man, the allantois
 
 
 
 
 
 
 
Fio. 47.— DIaprammatic sections roproRcntinp: growth nnd nrrnnpomcnt of the amnion in the earlioHt HtagcH of the human embryo (His).
 
develops not as a free sac projecting into the extra-embrj-onic
bo<ly-cjivity, but as a mass of splanchnopleuric tissue which
contains only a rudimentary cavity and which grows into
the alxlominal stalk (Fig. 47 and Fig. 5S, hfd\^ Iwing guided
bv that structure to the chorion. Moreover, while the
human allantois is in effect an evagination of the ventral
wall of the primitive gut-tract, its evagination begins before
the gut-tract is const ricted-otf from the yolk-sac (Fig. 48).
 
 
 
THE ALLANTOIS, 91
 
The ftmctioii of the allantois in ogg-laying animals, and
possibly in some others, is to serve as a nutritive and respiratory organ and as a receptacle for the fetal nrine : in
man its cavity is exceedingly minute, and its chief function
is to furnish a means of conveying blood-vessels from the
embryo to the chorion.
 
 
 
 
Fig. 48.— Mesial section through an early human ovum (Oraf Spec) : a, AMominal Btalk ; b, amnion : r, yolk-.sac : d. hypoblaut ; c, me»obla!»t; /, vessels on wall of
yolk-sac : g, primitive streak : h. allantois : i. medullary [»late ; j, early heart : k,
mesoblast of chorion ; /, early villi ; m, chorionic mesoblast extending outward
into villi.
 
The part of the allantois contained within the body of the
embryo produces three structures of the adult organism : 1,
the nrachus, an atrophic cord extending from the summit of
the bladder to the umbilicus;^ 2, the urinary bladder; and 3,
the first part of the urethra of the male, or the entire female
urethra. The extra-embryonic portion shrinks after the
appearance of the placenta and forms one of the constituents
of the umbilical cord, its blood-vessels becoming the umbilical arteries and veins.
 
^ If the urachas remains patulous instead of becoming impervious, urine
may escape at the umbilicus, and the condition is a variety o( urinary JUtukL
 
 
 
92 TEXT-BOOK OF EMBRYOLOGY.
 
THE CHORION.
 
At the time when the false amnion is forming, the attenuated zona pellucida still surrounds the embryonic vesicle
as the so-called prochorion, which unites with the false amnion, producing the primitive chorion. After the allantois
has grown forth from the gut-tract and has spread itself over
the inner surface of the primitive chorion, it becomes blended
with the latter to constitute the tme chorion of some authors.
The chorion, according to the above nomenclature, may be
defined as the membrane which encloses the germ at the
stage following the appearance of the amnion and the false
amnion, and which has resulted from the fusion of the allantt)i8 with the primitive chorion ; or, ignoring the zona pellucida, the chorion results from the fusion of the allantois and
the false amnion. Minot, however, defines the chorion as all
that part of the extra-embryonic somatopleure which is not
used in forming the true amnion, and hereafter in this work
the wonl will be used in this sense. This definition limits
the term to the outermost covering of the germ after the
formation of the amnion (Plate III., Fig. 1).
 
The chorion consists of an outer ectodermic layer and
 
an inner lamella of mesodermic tissue. The mesoblastic layer
 
it thini being composed of from two to four layers of round,
 
OVtl|Or fusiform cells, and is at first devoid of blood-vessels.
 
The latter, in the form of capillaries, make their appearance
 
il KNne time during the second week, probably as extensions
 
^ the bUKxl-vessels of the allantois.
 
*t\m outer ectodermic or epiblastic cells of the chorion at
ft ttty tftrly period, certainly as early as the third day,
pffoliferatioD to form a layer of tissue called the
wbioh IB from one to several layers of cells in
The tfophoblast layer is thickest at the place of
eC dM ovum to the uterine mucosa. The inner
ef il» iMfhebbH aie cttbical, and have large, finely
 
•vtl niicleL In the youngest human
 
of Peters, estimated to be three
 
was found to present many
 
qC etiiiids and buds, these
 
 
 
 
THE CIHiRfOX.
 
 
 
93
 
 
 
being tlie foundations^ of tlip future villi nf tlio cliorion. The
tropliohlaet layer was not solid, but was honeycombed with
little spacra or vacuoles filled with maternal blood, which
spaces were partly lined with a nnclealed protoplasm, the
early syncrtitun (Plate lA'., n). Even at this early stage,
therefore, when the trophoblast strands or early villi are as
yet devoid of a mesnblastic element, they are bathed with
the maternal blood. Very soon the niesoblastic tissue of
the chorion grows into the trophohlast strands, thus forming
the permanent villus stems ; and during the seeond week
capillaries extend into the stems, completing the foruiatioD
of the fully developed villi of the chorion.
 
The early develu|»mont of villi in characleristio of the
human chorion {Fig. 40 and Plate II, Fig. 6). At first the
 
 
 
 
 
TlQ
 
 
19.— H
 
 
man ovum o
 
 
mb'jutlU'L'lve
 
 
Fl'
 
 
('•
 
 
—Front view of
 
 
 
 
 
 
Keioherl), «
 
 
U vie*.
 
 
 
 
 
 
 
 
FIK.4B.
 
 
 
 
la tKitb ncurui the
 
 
illi are limlled 1
 
 
rtfH
 
 
rll
 
 
 
 
lea vine th
 
 
 
villi, either covering the entire surface of the chorion or
leaving the two opposite poles free, are of uniform size ; in
the latter half of the first month, however, there begins to
be a differentiation into a r^on containing smaller, and one
hiiving larger and more nnmerous, pmjections. The difference between the two area* becoming more marked, the
relatively smooth part of the membrane, possessed of rudimentary villi, is designated the chorion love, while the
region with well-develoiHsd villous projections is distinguished as the chorion frondoBom (Plate III., Figs. 1 and 2
 
 
 
the latter a
 
 
 
jqim-es
 
 
 
of the uterus and
 
 
 
close relation with the mucous membrane
i the fetal part of the placenta.
 
 
 
94 TEXT-BOOK OF EMBRYOLOGY.
 
The villi in their earlier condition are somewhat club-shaped
elevations, which later become branched to form secondary
villi. Each fully developed villus consists of a core of
mesoblast, covered with ectodermic epithelium and containing blood-vessels (Fig. 54 and Plate IV., a). Their microscopic appearance is so characteristic that they afford a
means of positively determining whether a mass discharged
from the uterus is or is not a product of conception. The
further alterations in the villi as well as in the trophoblast
in general, including the syncytium, will be considered in
Chapter VI.
 
A chorion is present, as a rule, in those animals whose
embryos develop witliin the uterus ; this would include the
entire class Mammalia, with the exception of the monotremes, wliose eggs undergo extra-uterine development, and
the marsupials, whose embryos, though nourislied in the
womb, never acquire villi on the serosa, nutriment being
absorbed by simple contact of the latter with the uterine
mucous membrane. Tlie Mammalia are therefore divided
into the Achoria, comprising the monotremes and marsupials,
and the Ohoriata, including all other mammals.
 
 
 
 
CHAPTER VI.
 
THE DECIDU>E AND THE EMBEDDING OF THE OVUM.
THE PLACENTA. THE UMBILICAL CORD.
 
THE DECIDU-C AND THE EMBEDDING OF THE OVUM.
 
The decidnn (deciduous ur cadncons membranes) are the
hy[)ertrophie(l mucosa i»f the uterus so dcvelojwd as to
form not only a lining for the uterine t-avity, liut also an
envelope enclosing the ovum, and a
specially all^reil part which serves
as a bond of connection between
the ovnin and the womb.
 
During tlic four or five days
preceding lucnslruation. the socalled constructive stage of the
mensiruiil eytle, the miienus membrane of the womb liecomes mnch
thickened and unusually vascular,
the purpose of these changes being
evidently the preparation of the
uterus for the reception of the
ovum in the event of impregnation. If impregnation has not
occurred, the thickcueil mucosa,
the decidua menstmalis, is in great
part easi off ns a purt i^f (he menstrual discharge; if, on the other
I, conception has taken place,
^e raucous membrane undergoes
I greater hyj^ertrophy. On scc, it is seen lo consist of a super1 compact stratum and a deeper b^'™"""*'
^nrr layer rcjKising directly upon the muscular wall of the
{.Uterus, In the compact layer are the necks of the much
 
 
 
 
jl.— Crnan Beellon through
iici.iia nicttibrane of Iho
Hi the begliiiilng nf preg^
(aner Kiindral knd Bn
 
 
 
96 TEXT-BOOK OF EMBRYOLOGY,
 
enlarged uterine glands, while in the spongy layer are their
greatly branched and often tortuous bodies (Fig. 51). The
tortuosity and division of the deeper extremities of the glands
produce the characteristic appearance of a section of the
spongy stratum.
 
The alterations necessary to convert the menstrual decidua
into the decidusB of pregnancy take place in part while the
ovum is still in the Fallopian tube ; when it reaches the
uterus it becomes attached to the mucous membrane of the
latter, usually along the upper part of the posterior wall. A
portion of the mucous membrane eventually comes to enclose
the ovum as in a distinct envelope (Plate V., Fig. 1). The
part of the uterine mucosa which thus surrounds the ovum
is the decidua reflexa; the part still lining the cavity of the
womb 13 the decidua vera ; the part that is in contact with
the chorion frondosum is the decidua serotina. The decidua
sor\>tina afterward becomes the maternal part of the placenta,
iMtimatoly uniting with the chorion frondosum.
 
r«til recontlv it was believed that the ovum became imnUnUnl ti/K)n the surface of the mucosa, and that the latter
irr\^\v up nr\>und and over it to form the decidua reflexa.
VUi^ tluH^ry has Ik^ou completely set aside by the recent
m\\v<turHtions of IVters of Vienna, whose results have been
v\^«Knm>i by Webster of Chicago. Peters' observations
\iv*v uu^\io u|H\n the gravid uterus of a suicide, the ovum
ts^a^ v^^nln^UUnl in a triangular i>rominence on the upi)er
»Hxx^<%»* ^\<:<\m of the i>osterior uterine wall. The ovum
»Mxs«.xut^>J »u tlmv diameters respectively 1.6, 0.8, and 0.9
»wu». »w vN^^mKAUnl HS^^ iHMUg three or four days.
 
tX^ ■im* tt i t lft'if ^Iht OTum (Plate IV.), or its sinking into
 
Ik u,4x>s.^*, X v^uK^kly mHH>mplished by the c-osion of the
 
. , s ;4w ;;<: 'u>> t^ xv|^ iho hitter, presumably by the phagocytic
^. u .. i »iK M^»^4^UxU*t. Actual erosion is evident from
 
.», . <..;.>> ^4 »^>^ xnH^ivv epithelium at this place. The
.»..^ KUH*^^at vM^x ivlation with the deeper layers of
 
\ , X ,v ♦ *K^ vN^K^^ v^' tW excavation are undermined,
 
.1, v.. .a '^ jmhK ww^hI by the mucosa, the area
 
i. . v.;vvl \iu^ v\vu|^\s) by an organized blood clot.
 
 
 
THE DECIDUJE. 97
 
the tisane ftmgns (Plate IV.). The overhanging edges of the
excavation constitute the beginning of the reflexa, which is
obviously, therefore, not produced by the upgrowth of a
circular fold of mucosa. The trophoblast strands or early
villi extend toward and into the serotina, to which some of
them become attached. It is thought by Webster that they
may absorb fluid and imtriment, and that by phagocytic action
they open up the blood-spaces of the serotina, thus bringing
them into communication with the lacuna? of the trophoblast.
 
The blood-lacunse of the trophoblast form a system of intercommunicating spaces, the beginnings of the later intervillous spaces of the placenta ; they are filled with maternal
blood from the serotina, and are lined with syncytium (Fig.
63), the latter being thin in places and resembling an endothelium. There is no extension, however, of the endothelium
of the serotinal vessels, either upon the villi or into the
spaces (Peters and Webster).
 
The ssmcytiiun is the more or less irregular layer of nucleated protoplasm which appears upon the surface of the
ovum toward the end of the first week, lining the trophoblast lacunae, and later |>enetrating as irregular masses into
the serotina, where it is found until the end of pregnancy.
No traces of it are found on the veni after the sixth week.
The origin of the syncytium has long been in dispute. Peters
has shown that it results from the tninsformation of the
superficial part of the trophoblast, probably from contact of
the latter \\\i\\ maternal blood, which, he thinks, exercises a
blending influence upon the trophoblast cells, so that as individual cells they disappear, the result being a non-cellular
but nucleated protoplasm. Peters also believes that corpuscles of the maternal blood are appro])riated by the
syncytium, and that the latter, covering villi and chorion as
it does, has something to do with the interchange of nutriment and waste products between the maternal blood and
the ovum. What remains of the early trophoblast after the
formation of the svncvtium is the layer of Langhans.
 
The ciliated epithelium of the uterine mucous membrane
disappears by the end of the first month of pregnancy (Minot) ;
 
 
 
98 TEXT-BOOK OF EMBRYOLOGY,
 
somewhat later, that of the uterine glands is also lost. By
the end of the fifth month the fetus and its appendages have
increased in size to such an extent that they completely fill
the cavity of the womb, and the space between the vera and
the reflexa is obliterated. After the second month the vera
becomes progressively thinner and the reflexa undergoes
degenerative changes to such a degree that by the end of
pregnancy merely remnants of it are present.* By the sixth
month the vera is intimately blended with the chorion.
 
THE PLACENTA.
 
The placenta, in certain groups of mammals, including
man, is the organ of nutrition for the fetus during about the
latter two-thirds of the period of gestation. In man, it is
a discoid structure, attached by one surface to the wall of
the womb, and connected on its opiH)site aspect with the
fetus through the medium of the umbilical cord.
 
The human placenta represents the highest specialization
of an apparatus for bringing the fetal blood into intimate
relation with the blood of the mother. In eggs that develop
otit^ide of the body of the mother, such as those of reptiles,
bink, and the lowest mammals, the Monotremata, the
^nviiuj ombrvo necessarily acquires no connection with
tl^* uterine mucosa, but draws upon its original dower of
outriuu^nt, the deutoplasm, until its development is compU»t\xL wht^n it bn»aks through the stiell and seeks its own
Kkh{ : in ilu>i** srr*>U[>s the chorion develops no villi. In
Uu> Miar^ipwl^ a srn>up of mammals higher than the monouvMuvx^ \\w ovum* although developing in the uterus, forms
IK* V U»^t> v^>nmvtJo« with it, but obtains its nourishment bv
viiMjsv utiMiMttort tr\mi the uterine mucosa. On the other
i,i.«xi, u u\ iKUtuuHls higher than monotremes and marsu>,;.^ \w iK'i-^oii is vlistinguished by the presence of villi
 
■ ii. vvHx.» »K *»uuM» pUwnta and the non-villous chorion
 
. iu t4,..^x4iS4*;v svi^tniit ^rn^lations exist; for example, in
 
■'^.^ ^va,<.v^ uKi '^i.Hm^ vnUvrs^ there is no proper placenta,
 
 
 
99
 
 
 
 
expiiUinii of the fetus. In the Cariiivora tbe pliiuenia
has tlie form of a zone or ring — placenta zonaria — while In
man anil wrtain iilliwl in.iinmals, as opes, rodents, and some
others, it is iliseoid in shape — placenta diacoidea.
 
The tLoman placenta is formed in the third month of pregnancy; since it rrsiilts from the union of the chorioa frondosum with tlic decidua serotina, it consists of a fetal and a
matemal part.
 
Our conceptions of the development of the placenta miiet
be modiHc<l to accord with recent investigations. The e
 
 
 
100
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
lx?<]<liii;rof thcnviim in the uterine mucosa and the modifications iM-curni)<; in the <-liiirion, including the };ru\vtli of iti>
villi and its ditli-ri'ntlution into the ehoriiiii fnmdusun) and
thf cliorion leve, have Ik-cii ci)nsidered nlwve. It will be
r<;(-:tlli.-d iliiit the uvuni e:its ii.s way, aH it were, into the
iiiwir-n, tliu-i <-aii>inf: th<' sii|terfieiiil layers of the latter to
di.s»|i|iear at ihe >\w of implantation. It is [tossihiy this
jiroeos of i-rosion n|>oii tlie jKirt of the fend trophoIilaKt
thai o|ieii~ u]) llie :di-<-:idy ililati'd caiiillarie!; of the s<.'n)tinu —
the siniiseB — and idlows the nialeriiiil hliMHl aeeesK to the
hlo.-1-hi.-iuia- of the li-oi.hol.lasl. wlure it thus Imthes the
jiriniiiive villi. Since the entire trojilmhlaiit is vaeiiolated,
 
 
 
Pn. TA— fthcnwIlF rrv
«j.. ■iidfithi'lliim :
 
 
 
 
lln- niati-mal hUfKl sit this time — the lirsl week — is hroiitrht
iiilo relation with the whole surface of the <'horii>n. lii the
lail-f l.ulf of llie first month the distiiuMiou li.twcen ehoiion
li'.i.'l'»-iitij and ehurion leve Itejiins to lie matiifcst, the villi
•/ lt» miKT irmdiiaUy retn^rading until, in the sixth week,
vui- i,Ti j^HAilv degenerated, many of iheni W\n\t without
 
'■ * : ii; .J*' M^ •■Ji'irion frondosnm inei-ease in si/e. nnmlier,
 
-.* ■■M.,)»n,.f wntu- of thorn acquiring jitlaelnucnt to the
 
- ■ •«• .re ui- tt.truW.mteringthei*<'n)tina] hhioil sinnti^'s.
 
— -«..... .' u*^ -^.li vtntinues tliroughont im'unaney.
 
_ -^v.,.« -5wil«-«* in ibe oecond week, these heing
 
 
 
THE PLACENTA.
 
 
 
101
 
 
 
extensions of the allantoic blood-vessels. The syncytium of
the chorionic laciinie, now the intervillous sp&cti.s, increases in
quantity, and not only tines the spaces, but exists in the form
of masses, some of which become attached to the semtina between the villi, while others penetrate into it, many l>eing found
at the fourth month in the serotinal connective-tissue sjiaces.
Tlie decidua serotina (basal decidual in the first month is
edematous ami hypercmic, presenting dilated capillaries and
blood-spaces, many of which communicate with the intervillous spaces of the chorion. By the sixth weelt its surface
epithelium is entirely lost, and the jiarts of its glanils contained in the compacta are to a great cxtutit tiblilerated. In
 
 
 
 
c leprvsenWdon of the dcvclopiui'iil <if Itii: placcnu. (altfir
iTopholilut: •'<!i,. lyncyllum; £>i., cndulhelium ; Va^
eUl cBplllariea; d.>., ileclilual septum; F.b.. Qbrln;
 
 
 
the fourth month it is thinner, more irregular in thickness,
contains less sinuses, and shows degeneration in the compacta, with many masses of syncytium. Toward the end
of pregnancy the sinuses increase in sine, and the irregularity in thickness and the degeneration are more marked.
The placenta at term is u discoid mass, in ttilu, but less
flattened after its expulsion from the uterus. Its diameter
is from 15 to 20 and its thickness from 3 to 4 centimeters.
The uterine surface is convex and irregular, and is imperfectly divided into tnfts or cotyledons. The somewliut
ooncave fetal surface, rather mottled, is covered by the
loosely adherent amnion, and (iresents, usually near its center,
 
 
 
I
 
 
 
102 TEXT-BOOK OF EMBRYOLOGY,
 
the attachment of the umbilical cord. The maternal part
of the placenta, the decidua serotma, is of varying thickness,
in some places being absent. Its compact layer shows
fibrinous degeneration, very few traces of glands, and no
epithelium. The blood-spaces, lined with endothelium and
representing greatly dilated capillaries, are in communication
with the intervillous spaces of the fetal placenta (Plate IV., a).
Scattered throughout the serotina are masses of syncytium.
In the shed placenta there is very little of the serotina, since
separation takes place through the compa^ta, the spongiosa
and a part of the compactii remaining upon the uterine wall.
 
The fetal part comprises almost the entire thickness of
the cast-off placenta. It is made up of villi* of all ages and
sizes springing from the chorion (some attaclied distally to
the serotina, others projecting free into the intervillous
spaces), and of masses of syncytium attached to both villi
and chorion (Plate IV., a). The intervillous spaces are a
system of intercommunicating cavities through which the
maternal blood circulates and which are in communication
with the blood-spaces of the serotina. Elevations of the
serotina between the villi constitute the septa placentae.
The so-called marginal sinus at the periphery of the
placenta is merely a system of intervillous spaces that
intercommunicate more freelv because of the relative
paucity and small size of the villi in this region.
 
The site of attachment of the placenta to the uterus is
usually the upper part of the posterior wall. Under certain
circumstances it may become attached lower down, even
extending partly or wholly over the mouth of the womb,
constituting then the condition known as placenta prsevia.
 
THE UMBILICAL CORD.
 
The blood-vessels through which the fetal blood finds its
way from the fetus to the placenta and back again to the
fetus, together with the atrophic vestiges of certain structures
associated with the development of these vessels, constitute
the structure known as the umbilical cord. In considering
the growth of the human allantois it was pointed out that the
 
* For structure of villi, see pages 93, 94.
 
 
 
 
Diagrammatic reprcsenlatlon or ri
hkltuf drat week; 2, ■ few dayi lalet;
dcBned (WeL»lcrj : n. feUI meiDblut, i
iolo Irophoblut (talks in I. nctual e
reduced In i and.eonnltatlng ben th
In l.enlirgad in
 
 
 
, a tew month
owing Indication
 
 
 
Ui diTidiia: I. in latter
 
 
 
. when pli
[>r beginning extension
 
3: b, trophciblaiit. being
larei or l.angh«nii; c. tmphoblul lacuna
■pace ; d. lyncytlum, §etin in its
 
 
 
e. dfcidua: /. maturnal blmdnlnua: g, cndotbcUain lining matumal
t. cplblMtlc covering of eord; f. amniotic eplbliut; J. nmbUieal
; meanbloal : m. extension a/ decldua an unde
Sf obarlon at edge ur piocvnta ; n, la
 
 
 
y
 
 
 
THE UMBILICAL CORD, 103
 
latter structure, as it grows from the ventral wall of the guttract into the so-called allantoic or alxloniinal stalk, bci'omcs
the seat of development of the two allantoic arteries and of
an equal number of allantoic veins. With the metamorphosis
of the chorion frondosum into the fetal placenta, the abdominal stalk becomes more slender and at the same time; much
elongated, and the allantoic blo<Ml-v(*ss(>ls are henceforth the
Tnnli11ic4il vasBels. The two umbilic^il veins fuse, so that, at
birth and for somc^ time before, there is but one vein, though
there are still two arteries. The nmbilical vein, entering the
body of the fetus through the umbilicus, passes diro<'tly to
the under surface of the liver, when^ it uuitt^s with the fetal
portal vein and gives oif* a bnineh of eoiumunieation, the
dnctos vanosiis, to the inferior vena <*ava, aft<*r wlii<*li it
enters the liver through the transverse fissure. The umbilical
arteries, whose intni-embryoni<' jxirtions are (^iIIcmI the hypogastric arteries, are the direct coutinuatious of tli<; su]MTior
vesical arteries of adult anatomy. They l(*avc the binly of
the fetus at the umbilicus.
 
The umbilical conl, while e<»nsisting essentially of the
three blood-vessels nu^ntioned, continus also the remnant of
the allantoic stalk and of the umbilical vesicle, these stnu^tures being surrounded an«l h(;l<l together by a <juantity of
embryonic connective tissue, tli(» jelly of Wharton, whi<*li
makes up the chief ])art of the mass of the eord ; upon the
surface is a Iay<'r of epitlu'lium, <M)ntiiiuous, at tlu^ <Iistal end
of the cord, with tli<; e])ithelium of tlu; amnion.
 
The umbiliejil eonl has an average l(»ngth of 5.") cni., or 22
inches, but varies Ix'tween the extremes of lo ('in. (<> in('hes)
and IGO em. (G4 inches); its thickness is about 1.5 em.
(J inch). The eord ])res(jnts tin* ap])earan(u^ of l)eing sj>irally twisted; it is prol)able, however, that the ap{>eanincc
of torsion is conferred l)y tlu^ sj>inil or coiled arrangement
of its arteries, due to their excessive growth, rather than by
a twist of its entire mass. There mav be one or more true
knots in the cord, produced by the slipping of the f(»tus
through a loop.
 
The position of attachment of the (^ord to the placenta is
 
 
 
104 TEXT-BOOK OF EMBRYOLOGY,
 
usually near, but seldom exactly in, tlie center of the fetal
surface of that organ ; rarely it may be found attached to
its edge, and still more rarely to the fetal membranes themselves at some little distance from the edge of the placenta,
with which, in the latter case, it is connected by its bloodvcwhjIh.
 
The great length of the human umbilical cord is thought
to Im5 du(i to the relatively large quantity of amniotic fluid
preH<*nt in the human subject.
 
Aflor birth, the portions of the hypogastric arteries extending from (h(? upper ])art of the lateral wall of the bladder to
the unibilic'UH undergo atrophy, becoming impervious fibrous
nirdn ; the intni-abdominal part of the umbilical vein likewine becomes atrophic and impervious, constituting the
M)*<«itlled round ligament of the liver.
 
KliLATIONS OP THE PETAL MEMBRANES AT BIRTH.
 
When the amniotic fluid attains its maximum bulk — at
about the end of the sixth month — it requires so much space
thiit it. preHM(>H the amniotic membrane closely against the
ehoHou, which IntttT, covered by the remnants of the reflexa, \n in turn f<»r<H'<l into intimate relation with the vera
(IMute V.)' At t(*rm the vem and chorion have become
pmctieully one menil)rane. The amnion, while adhering to
the inner nurfurc of the chorion, is so loosely associated with
the hitter that it may be peeled ott* from it. The membnines, which (M)nstitute a fluid-filled sac surrounding the
fetuH, are rupture«l by the contractions of the uterus at some
time during parturition. Through this rent the /;hild is
forc'ed chn'ing birth, the placentii and the membranes remaining behind. After the expulsion of the child, the vera
and the phieenta <h'tac'h themselves from the uterine wall,
and, with the <^horiou and the amnion, constitute the afterbirth, whieii is expelled shortly after the expulsion of the
child. The separation of the; after-birth takes place in the
compact layer resjM'etively of the dwidua vera and of the
utt»rine phieenta. The s|M)ngy layer and what remains of the
com|)acta serve for the regeneration of the uterine mucosa.
 
 
 
fl
 
!l
 
 
 
 
 
 
 
 
CHAPTER VII.
 
THE FURTHER DEVELOPMENT OF THE EXTERNAL
 
FORM OF THE BODY.
 
Having traced the growth of the germ to the time when
the body of the embryo becomes definitely diiferentiated from
the embryonic a})|)endages or fetal membranes, the development of the individual organs and tissues may be taken up.
The discussion of this latter subject, especially of that part
of it pertaining to the structures on the exterior of the body,
involves a consideration of the external form of the embryo
and fetus during the successive stages of growth.
 
In the preceding chapters it was pointed out that the cells
of the segmented ovum arranged themselves in such a manner as to form a hollow vesicle, the blastodermic vesicle
(Plate I.) ; that this vesicle, having at first a single-layered
wall, came to consist of two layers of cells, the ectoderm and
the entoderm ; and that, finally, a third, intervening layer,
the mesoderm, made its appearance. It was shown, further,
that the thickened portion of the vesicle wall, the embryonic
area, became more and more differentiated from the remainder, and that, by certain processes of folding, this area was
made to assume the definite form of the embryonic body,
while from the other parts of the vesicle-walls the fetal membranes were produced (Plate II.). It may be well to remind
the reader again that when the body of the embryo has become closed off from the fetal membranes, this body is an
irregularly tubular structure whose walls are the somatopleure and whose enclosed space is the body-cavity, and that
within it are two other tubes, a larger, the gut-tract, formed
by the splanchnopleure, and a smaller ectodermic tube, the
neural canal.
 
105
 
 
 
lOG TEXr-liOOK OF EMBRYOLOGY.
 
While, as a inattor of coiivcniciKjo, tho (]('scri]>tion of
the individual organs is taken up after tracing the course
of development to this stage, it should he borne in mind
that the rudiments of some of them are already distinguishable before the germ-layers become infolded to form
the IxKly-wall and the gut-tract. It will facilitate a comprehension of the gi'ueral principles concerne<l in the origin of
tlie different |>Jirts of the body to R»fer to the tabulated statement of the derivativi»s of the three primary germ-layers as
presenteil in Chapter III.
 
In ciMisidering the external form of the prcnluct of conception, one may adopt the classification of I lis, referred to in
the first chapter. This author divides the jHTiod of development into thn»i» stages, of which the ///W, the stage of the
onuiuor the blastodermic stage, comprises the first an<l second
we^ks of inira-uterine gri»vth ; the Htroiufy the stage of the
«BihiTO« extends frimi the seci>nd to the fifth wi»ek ; and the
nhtnL or tte fetal stage, includes the time bi^twwn the fifth
w<^k aud the end of gestation.
 
THE STAGE OF THE OVUM.
 
Durii^ :ht:* t'»rtnii:lit allotteil to this first staire of develop
invut nxMT chie various changi»< l\v whirh the impn»gnateil
 
•\'im H'»tui:>f^ th-.' fi^rm of a hollow <phen\ de<ignat«»<l the
 
fnoi-vtiiiii- 'i* ■^i:i>ciHK'rTnir vehicle. Tlu* M-rir^of tnin^torma
>Mi> wa^ u»'ii Fi"i*'rtb^il in Chapter II. In ihi> pla«v it will
 
'^ ^ttiKi:««iii I' r'vr c.» tht* external ch:inictrr< of the blasto
i«gHK ''«*»a» t> A'CVCcxl in Fisr^. 4!>, •"»♦», whi.*h ri*pre<tMit
 
:•■ » -III »■. ^»-i»%»i > KcIoh^Tt. and Fii:. ■'•■*•. tho t»vum of
 
" .,.,TK ■'' i * « »^ »vuru w:a< a vt>icU- iiu-:i««iinni; I mm.
 
aim.--. ii-.ri» iv ..•iiii>^.kU\t in thi- thi«k-nvd utrrine
 
-r- *.^x»\.»*«« -v iU»rott . l"j*.»n s*/"':!'!! :: -h-'W.il:iu
 
.^.,*,--.., m . • 1-^^ *. '•* ■"'"■ t'^ UniTtii. ::i r^la::-:! l-y i:«.
 
,1^^ . ^^, ttrt ♦* .iMi«oi'.i» oivitv. wti:!-- ':i>« :i :•- V- M
 
 
THE STAGE OF THE JiMliHYO.
 
 
 
107
 
 
 
was estimated tu be alitml twelve days old. Its fonn van
that of a spliore' snmewliut fijittf nod, its short and long diamot'.Ts inciisurins resiwcti vi'ly 3.3 mm. und 5.5 mm. T!ie
flattened Bur&ces were t^llloolll, ^vhile the equatorial sons wns
 
 
 
 
!.' ;-. hloiiii-iiLii Liiiij( nil ilif iiiiii-r iMiliir iwrlloii of the ftiMriimlB
 
l'.E. uledliG epiLhulluiii^ Oii., duUdiut reQeia: TV, Inipbobliul; (b, nuKriut
oaplllary: r)r, gland of uterine muMwa: /U.Jl.UciiaicIn (he traphoblail, conUlnIng malemil blood: K.A, nits' of embryil; Om/i. itvcldus rtuinparta : if, fbUI
mEdohlut : L'.Z. liiti!rgliiu<liil*r llMue ur mticcwi. In which enrly dccidunl n'lln are
 
 
 
beset with villi
istic of the hill
 
 
 
Tile I'arlv apiieiir.i
 
 
 
i is trharacter
 
 
THE STAGE OF THE EMBRYO.
 
It 13 during the early jKirt of the second stage, at about
the fourteenth day, that the somatopleuric layer of the lilastodermic vesicle becomes folded in to prodtiee the walls of the
 
 
 
10
 
 
 
TEXT-ROOK OF EMBRYOLOGY.
 
 
 
erabrvouic body. Fig, 67 allows a human etnbrvo of about
tlie tifteenth day, wbose form ia as yet imnerfectiy differentiate, the ventral wall of tlio bixly Iwing incomplete,
tiince the giit-trart Is tttitl in wmimimication with the iimbiliCftl viwioie throughout almost the entire lungth of the embryo.
The Iwck and sides of the embn-o arp enveloped by the
unnlon, and the dorsal outline is concave. The caudal pole
 
 
 
 
fM, M~HuHiiiii*itibrri>ifrKlii>ul (liu thltlvenlh asy (Rti) Tbemudiit
lll>WUll>r|ri>liouiiii*Dtvil trlUi Ihv tilutcidiinnlc! vmicle bj miianBof tlii! alid
ku wIIuhMi' lUUi Ui» ainnlan nl randy romplrtcly endtoiri the vtghryn.a
tvTtiH iltHlllnHiiiiii^iiminiinlrBli-i throuKhoUl th«it«lvt partofthi: mllnl i
 
 
 
U mvn t« \\f winni't'twl by means of tlie ullantoip sliilk with
ninu, which Intter Btnictiire, however, is not
 
 
 
th.i
 
 
 
iilllv.
 
 
 
i'fl>iv.(>nliil ill lliK rijrnre. The concavity of the dorsal ontliiu> in |Ht>nliiii- (.1 the hiiman embryo of this stage. The
(luvi<lopini<n( III' thiT Iicud i'h eUmcly iiHB0ciat*'d wilb the dilatalitui uf \\w .'cphaiic end of the neiinil hibe and the subse(picnt divi-i.iH of thin dilated extremity into the thrt^ primary
bmln-viwioh'M, tho forc-bmin, the mid-bruin, and the hindbmin. The oral pit, the Brut indication of ibe future mouth,
ia )>rvMii]t in the curly part of this stage ; it is a depression
 
 
 
110 TEXT-BOOK OF EMBKYOLOUY.
 
 
 
 
^
 
 
 
 
 
01
 
 
 
y„, -,., 1 ;,.:, nil.ry.n.
 
 
.ll.-iil«iB.'.!iil-iiill"ipn
 
 
1 n hair
 
 
 
 
'. il, fmin cightoi'iiili m
 
 
wri.ly-(iii
 
 
tram iwenly-iiiirU lo iwi'tity-iiitti <ii
 
 
y; 9-1*. from iwfnty-sfv
 
 
 
 
IS-17. (H,m Ihlrly-llrsi lo Ihlrly f..i.r
 
 
 
 
 
 
■rcbis: 0, aiMe \ftiMv. e>(, i^iic v
 
 
.Idp'i .1/, 'olfto|..ry pit:
 
 
/■''""
 
 
 
(Hl«):
 
 
 
 
 
 
THE STAGE OF THE EMBRYO. Ill
 
the rudiments respectively of the crystalline lens and of the
membranous internal ear; at this time also the visceral arches
and clefts first become distinguishable. On the twenty-first
day, the rudiments of the limbs appear as little bud-like
processes springing from the trunk. The conspicuous projection on the ventral surface between the now almost completed yolk-sac and the cephalic end of the body is produced
by the primitive heart (Fig. 59, 10, 11, and 12).
 
Until the twenty-first day the ombrvonic body is erect.
Between the twenty-first and twenty- third days a marked
alteration in the appearance of the germ is brought about by
a pronounced bending of the long axis of the embryonic
body (Fig. 59). The degree of curvature is such that the
caudal and cephalic extremities overlap. The flexion reaches
its maximum degree by the twenty-third day. The curved
dorsal outline is referable to four well-marked flexions, the
position of the most anterior, or cephalic flexure, corresponding to that of the future sella turcica and being indicated by
the projection of the mid-brain vesicle (Fig. (>2, III.) ; at this
point the anterior part of the head is bent almost sufficiently
to form a right angle with tho posterior half. A second or
cervical flexure is found in the future neck-region, while
further caudad are seen the less pronounced dorsal and coccygeal curves.
 
The fourth week marks the period of the most active
growth of the embryo. Afler the twenty-third day, the
body as a whole uncoils somewhat, although in the latter
half of the fourth week the individual flexures noted above
become more conspicuous.
 
The Visceral Arches and Clefts. — The visceral arches,
with the intervening visceral clefts, constitute a conspicuous
feature of the extemal appearance of the embryo during this
stage. These arches are a series of fivx» approximately
parallel ridges appearing upon each side of the future
neck-region and extending obliquely downward and forward toward the ventral surface of the embryo (Figs. f)0
and 62). Tho four furrows lying between the five visceral
arches are the visceral clefts. A coronal section of the neck
 
 
 
UTa)u||Ulur ui
prltntllvu l>sai
 
 
 
111* tu tkutUt: urtlii's (U^i ' "^- ">'■< uaxlltorr and mandlbuUr
'ml anh : o l-a IV, Bnl to Iburth wirlle anhiM : fv. cc. priuil*
liial vain*; dC. fliicl "f Cuvlur; al. r. ■trioai and ventricle of
iii'llin" •■a: ni. da. vtntral «nd donal anrtK: mi, oi, optic end
 
 
 
THE STAGE OF THE EMBRYO.
 
 
 
113
 
 
 
region (Fig. 61) — a section in a plane parallel with the ventral
surface — shows that the furrows seen on the Dctodemiie
surface correspond in position to a like number of deei>er
grooves on the inner or entodermic surface. The inner
furrows are out^ptK^ketiiigs of the entoderm lining tlie pliaryn^real region cif tiie furc-gut ; they are referred to as the
pharyngireal poucheB or throat-pockets to distinguish them from
the outer clefts. At the iiottoni of the clefts tiie ectoderm is
in contact with the entoilerra, the meaoderra being absent;
these two layers constitute the closliig membrane. The visceral arches or ridges consist of tliickened masses of niesodermic tissue covered outwardly and inwardly respectively
 
 
 
 
FlQ.
 
phsrynp'Bl end of gut-lrsot from behind (froi
 
eniljryo ot a.iinm.: B, of 4.ffi mm. (about 2510
 
ceni ftirrawa; I', bIdus privcerTlHlls, compriiing Ilifrd and tuurth oi
 
;, *. !l, (, rlMoral Bwhos. each witb lis »jB™™i-ureh vesasl : B. lubtrc
 
7, orinpo of lirj-Qi ; *, pulmonnry evagioatlou.
 
 
 
by the ectoderm anil the entoderm. Each arch contains an
artery, the visceral-arcli vesBel. Tliese five pairs of visceralarch vessels arise by :i common stem, the tnmcns arteriosus,
from the primitive Iieurt.'
 
The morphological significance of tlie visceral arcbes and
clefts may be a]ipreeiated by a comparison of the conditions
obtaining in lower types. While in birds and mammals the
 
' Fur U11 account of llii: luetaiiiorphusis of the viac!«rnl-Hrc]i vessels inlo
the udiilL arlcriee uf llie ihniit mid neck llie render in referred Lu Cliapler
 
 
 
114
 
 
 
TICXT-nuoK OF KMHRYOUMiY.
 
 
 
number of the lAetia 13 four, in reptiles, amphibiaiis, and
bony fishes, Hve clefts appear, and In some fi.shes (selachians)
the number is six. In alt aqnatiu verteiirates, the thio
epithelial closing membranes nipture, thus establishing communications between t!ie alimentary tract ami the exterior,
tlirough which ojienings water passes in and out. The margins of the cleibi — except the first or hyoraandibular cleft —
become the scat of a rich supplv i>f capillary blood-vessels,
the blood of which obtains oxygen fmm the water and yields
to the latter its carbon dioxid; while the visceral arches,
excluding the first and second, become known in these classes
as brancbial arches from their producing bony arches which
support the branchiEe or gilis. With the exceptions noted,
the viseenil arches and elerts with their capillary plexusea
therefore functionate in these classes as a respiratory ap»,J
pa rat us.
 
When, in the course of evolution, certain of the vert«-i
brates assume an aerial existenw, in consequence of whicbl
they acquire a breathing mechanism adapted to such a model
of life, the respiratory function of the clet^s or branchis^a
ceases, and they either disapjiear entirely or constitute merely.!
rudimentary structures of the adult. The so-called clefts in f
man aie never actual openings, the closing membrane always- 1
being present (His, KoUiker, Piersol, Born), To express the i
morphology of the visceral clefts* briefly, they are permanent J
structures in flshes and in tailed Amphibia; they are present '
during the larval stage of other Amphibia, while in bird» \
and mammals they are found only in embryonic life.
 
The growth of the visceral arches and clefts bears an intimate relation to the difTerentiatiou of tlie head- and the neckr^ions of the embrj'o. They first make their appearance at
about the twenty-third ilay and attain their greatest development by the end of the fourth we^k. Both the arches and
the clefts appear earliest and are best developed at the cephalic end of the scries, the fifth arch being exceedingly illdefinecl. During the fifth week the obliteration of the arches
and clefts as such begins, since certain of them become metamorphosed into permanent structure;^ wliile the 1
undergo regression.
 
 
 
THE STAGE OF THE EMBRYO, 115
 
The Metamorphosis of the Visceral Arches and Olefts. —
The first visceral arch becomes (livide<l into an upper i)art,
the maxillary arch, and a lower ])ortion, the mandibular or
jaw-arch (Fig. 62). The maxillary arches or processes of
the two sides unite^ at their anterior ends, with the intervening nasofrontal process (Fig. 67, and in tliis way is formed
the upper l)oundary of the mouth-cavity ; the mandibular
processes become joined with each other anteriorly and constitute the inferior boundary of this cavity. The maxillary
processes become the superior maxillie, while the mandibular
[)rocesses l)ecome the lower jaws. The mesodermic core of
the mass of tissue constituting the mandibular arch divides
into three sections, of which the two situated at the proximal
end of the arch are quite small and give rise respectively to
the incus and the greater part of the malleus ; the large distal
segment is a slender cartilaginous rod, Meckel's cartilage,
whose proximal extremity becomes the processus gracilis of
the malleus (see Chapter XVIII.).
 
The second visceral, or anterior hyoid arch becomes obliterated as such, although a bar of cartilage which it contains —
Beichert's cartilage — gives rise by its proximal extremity to
the stapes,^ while the remaining portion becomes metamorphosed into the styloid process, the stylohyoid ligament, and
the lesser cornu of the hyoid bone.
 
The third or posterior hyoid arch, which corresponds with
the first branchial arch of fishes, likewise loses its identity
as a surface marking, while the bar of cartilage it contains
becomes the body and greater cornu of the hyoid bone.
 
The fourth and fifth arches coalesce with the adjacent tissues, producing no special structures.
 
The first outer cleft, known as the hyomandibular cleft, suffers obliteration except at its dorsal extremity, where the
tissues forming its margins produce the external ear. The
remaining three outer clefts disappear in the following manner : the fourth outer cleft becomes covered and hidden by the
fourth arch, and the third and second clefts are successively
 
* Reichert, (iegenbaur, Ilertwig ; or to the ring of the stapes according
to Salensky, (jradenigo, and Rabl.
 
 
 
llfJ TEXT-HOOK iiF KMBRYOLOGY.
 
tiuried by tlie growth of the third and second arches. The
sinking-in of the lower arches and clefts (Fig. 61) results in
 
 
 
 
fc. »•.»». 1-^:1. J", limb*;. iJ.."11«nlolc »li.lk:rA. vil
 
 
« on the lateral surface of the
^ft^^i^^B*^«F^ 6*2, »p), ^v)lieh snUeqiienlly
 
 
 
THE STAGE OF THE EMBRYO. 117
 
is made to disappear by the coalescence of its edges. Occasionally this sinus, instead of becoming completely obliterate<l, persists, and the thin layer of tissue forming its bottom
ruptures — possibly spontaneously or perhaps more probably
as the result of exploratory probing — constituting the anomaly known as cervical fistula. Such a fistula establishes an
opening into the esophagus.
 
The first inner cleft or first pharyngeal pouch becomes metamorphosed into the middle ear and the Eustachian tube, the
closing membrane, which separates it from the outer cleft,
forming the membrana tympani. The second pharyngeal
pouches produce no special structures, but the adjacent tissues
give rise to the epithelial parts of the middle lobe of the thyroid body and to the posterior third of the tongue, in the
manner more fully indicated on pp. 143 and 226. The third
inner cleft produces the thymus body, while from the fourth
results the lateral lobes of the thyroid bod v.
 
The configuration of the face, depending as it does so largely
upon the development of the boundaries of the nose and of
the mouth, is closely associated with the growth of the first
pair of visceral arches. The earliest indication of the mouth,
the oral pit, appears at about the twelfth day as a shallow depression on the ventral surface of the embryonic body l)etween the fore-brain vesicle and the prominence caused by the
primitive heart (Fig. 59, 3 to 5). This depression is deepened
by the growth of the tissues surrounding it, as also by the
flexure of the head, which occurs at the twentv-first dav. In
the third week, therefore, the oral pit is a five-sided fossa,
being bounded above by the nasofrontal process, which has
grown down from the elevation of the fore-brain, laterally by
the maxillary processes, and below by the mandibular arches
(Fig. 67, ^1). The pharsmgeal membrane, which consists of opposed ectoderm and entoderm and which separates the primitive oral cavity from the gut-tract (Fig. 66, rA), ruptures at the
time of the appearance of the third branchial arch.
 
By the end of the third week, the communication between
the yolk-sac and the gut-tract has become reduced to the
relatively small vitelline duct. At the twenty-fifth day the
 
 
 
\\H TKXT-liOnK OF I'lMUnYOLoay.
 
enihrvo j»n'sc»iits a well-(lov(*l()i>o(l tail. By tli<» termination
of the fourth week the volk-sac has attained its maxiinum
size, and the presence nf the s<nnite.s is indieateil hy transverse ])ai*:inel lines on the dorsal snrfaee of tlu^ IxkIv.
 
THE STAGE OF THE FETUS.
 
This sta^e odinprises tin* time between the beginning of
the second inniith and the end of jircirnancy.
 
Dnrin^ the second month tiic rate of irrowth is far less
nipid than in the jm'ccdinj^ stap*. The marked enrvatnre
ol' the h»n^ axis (»f tlu* ImmIv jVradnMlly dimi>hes, the embryo
assnming a more (;re<a [)osture. Owin^r t-> the partial disaj)|M*aninc(^ of the cervical tU'xure, the iiead l)econie< raised.
 
I>nrin«^ \\w. fifth week tiie vitelline duct is >een to bo
lonjx iind slendei , the umbilical cord lias become longer and
nion* snind and mav contain u coil of intestine ; tlie abdomen is very |)i*ominent, and in the neck-region is a characteristic dorsjii concavity. At tiiis time al>o the nasal pits
Ikhmmuc conspicuous as depressions situated on either side of
llu* nasofrontal process (Fig. (57j. Th<* nasofrontal jnvx'css
ini':in\vldle undergtN'sdiiferentiation int«> the globular processes,
which constitute the inner boundaries of the na<al ]»its, and the
lateral frontal processes, which limit these tlepre^^ions exter
 
 
 
Vi« W llttmM»m\itytMiftl»ulrix weeks. cnlnrL'id ihr.-.- lii;., . \u.
 
\»\\\ a\u\ M'\»rAW\W'm fn>m the dcpn'ssicMi- tni- ih. . y..^.
Aw twM\ \wu ftw hl\\\ iu coramuuiiration WUw \\\\\\ the
 
 
 
THE STAGE OF THE FETUS. 119
 
primitive oral cavity. The lacrimal groove is well-markeil
at this stage, and tho external auditory meatus is indicated.
The mandibles become united mesially at about the thirtyfourth day. The third and fourth gill-clefts have by this
time disappeared in the cervical sinus. The paddle-like limbbuds have lengthened and present, at first, a division into two
segmcHits, of which the distal is destined to become the hand
or foot, while the proximal jwrtion undergoes segmentation
a little later into tlie arm and forearm or thigh and leg ; by
the thirty-second day, the hand, now showing differentiation
into a thicker proximal and a thinner terminal part, exhibits
the first traces of digitiition, in the form of parallel longitudinal markings which soon become grooves and, later,
clefts. The develoj)ment of the upper extremities precedes
that of the lower by twelve or fourteen days.
 
During the sixth week the head assumes more nearly its
normal [)()sition, and for this reason the apparent length of
the fetus is considerably increased, the dorsiil concavity in
the neck-region being ahnost obliterated ; the rudiments of
the eyelids and of the concha become recognizable, and the
various parts of the face assume more definite shape. By
the fortieth day the oral cavity has become separated from
the nasal pits by the union of the nasofrontal process with
the maxillary process(\s, and the external boundaries of the
nostrils have become marked out by the meeting of each
lat(^ral frontal j)rocess with the corresponding maxillary
])rocess. As a result of these changes, the nose, although
still very broad, begins to assume characteristic form.
During this week also the fingers are seen as separate outgrowths, while in the seventh week the rudiments of their
nails become evident.
 
Toward the end of the second month — about the fiftieth
to the fifty-third day — the toes are just beginning to separate, the protrusion of the intestine at the umbilicus is at its
maximum, the palpebral conjunctiva separates from tiie cornea, and the rudimentary tail begins to disiippear.
 
The eighth week witnesses the total disaj)pea ranee of
the free tail, the formation of the septum that divides the
 
 
 
120
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
cloaca info the rectum and the f^nito-urinarv passage, and
the presence of the project iiig genital tubercle with the accompanying genital folds and genital ridges. The external
genitals as yet show, no distinction of sex. Fnini the end
of the second month to the time of birtli, fetal growth is, in
great measure, merely the further develojmient of organs
already mapped out; it is held by many authoritifs, therefore, that if mal format ion.s are ever due to maternal impreasions, such impre.-isions could be oiierative only hi the event
 
 
 
 
of having been i-eceivcil prior to the eighth week of gestation.
 
Dnring the tJilrd month, the face, although definitely
formed, still presents thick lips, a pointed chin, and a rather
bniad and triangular nnse. At this time the Wtaha are wellformed and assume a eharaeteristie attitude, and the fiugiTs
and toes are provided with imperfect nails. The external
genitals, which, until the close of the second month, preserved the indifferent type, now begin to show sexual
distinction.
 
In the fourtli month, a growth of fine hair, the lanngo,
appear> npiwi llie sculp anfl some other parts of the body;
 
 
 
THE STACK OF THE FETUS. 121
 
the anus ojiens ; the intestine rece^Iea within the abdomen;
and the external generative organs present well-marked
sexual characteristics.
 
The fifth month marks the inauguration of active fetal
movements and the appearance of a more plentiful growth
of colorless hair.
 
In the sixth month the fetal bo(]y becomes coaled with
the Temix caseoaa, a modified sebaceous secretion whose func
 
 
 
tion is the protection of the epidermis fnmi maceration in the
amniotic fluid. The eyebrows and eyelashes also appear
about this time.
 
The aeventh month witnesses the appearance of the lanugo,
or embrj'onal down, upon practically the entire surface of
the body ; the testes of the male fetus are in the inguinal
canal or at the internal abdominal ring; and the nails
break through their epiilermal covering. Children born at
 
 
 
122 TEXT-BOOK OF EMUnVOLOUY.
 
tlie end of tlic seventli month niav survive, but usuallv tliev
do not.
 
In th(» eighth month the lanugo begins to disappear.
 
In the ninth month tlie testicles are found in the scrotum, while, in the ease of Xhv. femah*, the labia majora are in
contact with each other. The contents of the intestinal canal,
the meconium, consisting of intestinal and hepatic secretions
mingled with epidermal cells and hairs swallowed bv the
fditus, is now of a dark greenish color. The umbilicus is
almost exactlv in the middle of the bo<lv.
 
The weight of the fetus at full term is fnmi 3 to 3.5 kilograms (from to 7 ]K)unds), the average* weight of the male
child being about ten ounces greater than that of the female.
While variati(ms from these figures are not uncommon, statements of excessive weight are to be received with reservation,
since it has been found, ujxm careful observation by comjH?tent authorities, that the weight of a new-born infant rarely
excenls ten poimds. The weight of the chihl, besides dej>ending u|M)n the ])hysical condition of both parents, is influenced bv the age of the mother, young wcmien having the
smallest, and women between the ages of thirty and thirtyfive having the heaviest children ; by the nimiber of ])revious
pn-gnancies, the weight being gn»aler with each succeeding
iirt^nw»ev. pnividt^l the successive children are of the same
Ti^.x :.r:d ar\* not lK>rn at t(K» short intervals ; ami also by the
veir:.: Ga>>uiT' and height (FrankmhaiiMn) of the mother,
Mv •^.*-' >:n:r a dinrt (juc. Min(>t bcH<*V('s that these
vu''i'«i> T.f •iixx^ o|K*rate chiefly by prol(Hi<:iiiL^ or abbreviat-.-• T?r n:'"'» ti .:' 5:*Maiion, and that therefore the variati«»ns
T T^^-u iL i^^V. Jirv* rt»fenible to tw«. prineipal causes —
/,^..^.., ..^ ,1 '\^* s-^ ^\ birth, and variatit»n> in the rate of
 
Mggfc M lite li^ns Ht the time of birth ir* about oO
 
1^ jf ;u •jnbryo or fetus may l»e esti
liftir**-*:-^ 'pcvuliar to each >tai:e a- above
 
.iici. * Hi;. 'iK- nde tbnnulatetl by JIaa-e.
 
 
 
*.*-•
 
 
^^ *
 
 
 
_ :.-,..>.. |» '.' Me iiid of the liflh niMiitli, tin
 
 
TlIK STAGE OF THE FETUS. 123
 
square of the age in months equals the length in centimeters,
while after the fifth montli, the length expressed in centimeters equals the age in months multiplied by five. Thus
a fetus of four months would have a length of 16 centimeters; while one of six months would be 30 centimeters
long. Hence, the age in months is the square root of the
number expressing the length in centimeters; or, if the
length exceeds 30 centimeters, the age in months is one-fifth
of the length expressed in centimeters.
 
Reference has been made in Chapter I., page 40, to the
relation between conception and menstruation, and to the
manner of estimating the age of the product of gestation,
based upon this relation.
 
 
 
CHAPTER VIII.
 
THE DEVELOPMENT OF THE CONNECTIVE TISSUES
OF THE BODY AND OF THE LYMPHATIC SYSTEM.
 
THE CONNECTIVE TISSUES.
 
I'llK varioUHly iiKKlified forms of connective tissue distribiiti'd tlironj^hoiit the body, including such diversified tissues
iiH thi^ l>lood and the lymph, areolar tissue, fibrous and elastic
tiHHUr, adenoid tissue, tendon, cartilage, bone, and dentine,
an \v«'ll an i\n* ('onnective-tissue stroma of various organs, all
n^Mult from alt<»rations affecting the middle germ-layer or
meioderm. As pointed out elsewhere (Chapter III.), the
inner and the outer germ-layers are concerned in producing
the epithelial Htruetures of the body (with the exception of
the epithelium of the greater part of the genital apparatus
and t»f the kidney aixl ureter), the ectoderm giving rise not
only to the epithelium of the surface of the body, but also,
by prnerHHeK of infolding, to such important structures as
the eentrnl nervous system and the internal ear, while the
entoderm dillerentiates into the epithelial parts of the respiratory and digestive systems with their associate<l glandular
organs.
 
The proliferation of the cells of the mesoderm goes hand
in hand with thedifVerentiations of the inner and outer germlayers, so that even at an early stage of development the
middle germ-layer, besides having given rise to the mesothelium of the body-c^avity and to the primitive segments, C(mHtitutes a loose aggregation of cells that fill the spaces between the g<'rm-layers and spread about the deveh)ping
eml)ryoni(^ organs. This primitive relation of the mesoderm i<^ tissue foreshadows its future office as the supporting
fnunework not only of the bmly, but of the functionally
 
121
 
 
 
THE CONNECTIVE TISSUES. 125
 
active epithelial elements of the glands. Thus, the indifferent
mesodermic tissue that comes to surround the notochord and
the neural canal specializes into the spinal column and the
brain-case ; while the parts of this tissue into which protrude
the epithelial e vagi nations of the primitive alimentary canal
— as, for example, the cvaginations which are the beginnings
of its glandular organs, the liver and the pancreas — become
intimately associated with these epithelial sacs and tubes to
constitute the connective-tissue stroma and the vascular apparatus of the completed glands. All organs of the body,
therefore, that have a connective-tissue constituent obtain it
from the mesoderm. Owing to the varying degree of differentiation of the mesodermi(; elements in different localities
there are formed tissues of widely different character. The
most important factor in the production of these modifications is ihC' alteraiion of the intercellular svhstanccy as to
whether it remains soft and homogeneous, whether it acquires a fibrillar or an elastic structure, or whether it becomes dense and hard, as in the case of cartilage and bone.
The celh undergo comparatively little change, although,
according to the kind of tissue produced, they come to be
known respectively as connective-tissue cells, tendon-cells,
cartilage-cells, or bone-cells.
 
The slightest degree of specialization results in the production of mucous tissue. In this case a reticulum is formed
by the slender processes which the cells acquire, the spaces
of the meshwork beinj' filled with the semifluid or semigelatinous intercellular substance.
 
A further alteration in the intercellular substance, whereby
it acquires greater density and becomes permeated by bundles of fibers, some of which are highly elastic, results in the
formation of areolar tissue. Preponderan(;e of the non-elastic fibrous element produces white fibrous tissue, while elastic
tissue, such as predominates in the ligamentum nuchje, is
formed if the elastic fibers are in excess. Further increase
in the densitv of the intercellular material, with its accompanying conversion into bundles of non-elastic fibers having a
characteristic regularity of arrangement, produces the struct
 
 
r2« TKXr-IlOOK OF EMBRYOLOGY,
 
iin; of tendon. Wlion the intercellular substance gives rise
to a scant amount of fihrous material and the cells become
(li?*ten(le<l with oily or fatty matter, adipose tissue results.
 
A still greater degree of density of the intercellular subflt^incc givcffl the matrix of cartilage, the cells being enclosed
in H])aceH, the lacunse, as the cartilage-cells. Partial differentiation int^» either fibrous or elastic bundles confers the
r'hanicrter of eith<*r fibrous or elastic cartilage wy^n the
pHKlnct.
 
Great cond<;nsati(m of the intercellular substance and its
pennf.*ation with salts of lime, the cells being fixed in small
«pare*^, results in the prcKlucticm of osseous tissue (see Chapter XVIII.).
 
Blood and lymph may be hK)ked upon as forms of connective Xx^uii in w^hieh the interc<;llular substanc^e is fluid, con^titnting the plasma, the cellular elements thus remaining
free c<-II«, the blood- or the lymph-corpuscles. The development f>i br»th lymph and bl(KKl from the mesodermic elements
.J4*rv*^ to l>ear out the comparisf>n.
 
The cadotlielimn of the body is related with the connective
-••^les jzenetirally as well as anatomically. Reference has
ven iiaiie el?ewhere to the changes which occur in the
in*^Hi»»rji:»' rt-ll^ that bound the body-cavity — the fissure
 
vr««>v>i Tie two layers into which the parietal plate of the
iiK>!Miik«ni s?i:t> — to constitute the mesothelium of the body
atvTT*-^ r*iese change? consist in the flattening of the cells
 
091 :*v«r ts»!*in.'.oc>^n of the characters of endothelium.
^•WtTr;;ir-\, T-u»n 'cr.^T smaller clefts are formed in the meso
vi^iTTTX ->»«%.. ••\"^* which may l)e the beginnings of small
-r«iMr.>fwv%. r -r bUi»xl-vessels, or of bursal or articular
 
•• — ■*. -n xw^wr-ttc ^tfiL< of these cavities also assume the
 
^, .,^^. , 4ftMl^HMrt of the serous membranes and of
^^. ..r^^ ij^jBifcr Innalt and thecal sacs may be
 
^ n:., loc^ ">^tt Slid about the origin of the
 
.^„,„^. -iv N^itfti^'^tvv^isi^ie stroma of the mem
_ ^- H *nA»iiu'lmui rests, is simply a con
^^ . ^ .sv«v^«*»«^ :Min?Uifc vtf connective tissue.
 
 
 
THE DEVELOPMENT OF THE LYMPHATIC SYSTEM. 127
 
THE DEVELOPMENT OF THE LYMPHATIC SYSTEM.
 
The solid elements of the lymphatic system — the " Ijrmphglands," the lympli-follicles, and tlie diffuse adenoid tissue —
as well as the thsrmus body and the spleen, result from the
specialization of mesodermic cells, while the lymph-vessels
and the various Ijrmph-spaces of the economy — that is, the
s(»rous sacs, joint-cavities, bursid and thecal cavities, subarachnoid and sulKlural spaces of the brain and spinal cord —
are develo()ed by vacuolation or hollowing out of the mesoderm.
 
Definite knowledge is wanting as to many of the details
of the genesis of the lymphatic system. The various lymphspaces precede the vessels and the adenoid tissue in development.
 
The Isrmpli-spaces result from clefts in the mesoderm, the
earliest formed and most c<mspicuous space of this sort being
the body-cavity or coelom. This large fissure develops, even
before the differentiation of the body of the embryo, bv the
coalescence of numerous small cavities that appear within
the middle germ-layer. The body-cavity acquires more
definite boundaries by the alteration of the mesodermic
cells that border it into flattened endothelioid cells, the
mesothelium of the body-ca,vity. AVhen, in the progress of
development, the diaphragm and the pericardium are formed,
the body-cavity is divided into the peritoneal cavity, the
pleural sacs, and the pericardium. At a still later period,
a diverticulum of the peritoneum protrudes, in the male fetus,
through the inguinal canal into the scrotum to constitute the
tunica vaginalis testis. The stomata of serous membranes
are merely so many apertures of communication between
the serous cavities, which are enormous lymph-spaces, and
the lymphatic clefts contained within the stroma of the
serous membrane, the clefts themselves being the beginnings of lymph- vessels.
 
The large Isnnph-sacs surrounding the brain and spinal
cord, the subarachnoid and subdural spaces, as well as the
spaces within the capsule of Tenon and the sheath of the
 
 
 
DIH TEXT-nOOK OF KMiniYOLOdY,
 
o|)ti«t n<'r\w, jiihI tIm; p«*rilyiii])liati(' spaces of tlio internal
ear similarly ilevdop a-^ vaeiiolati<>ns <»f the nio.s<KU*rniio
ti^>ue. '1'Im> .siiiie is true of tlie joint-cavities, bursal sacs,
Hheaths of tendons, and the small lymph-clefts ioiinil in the
areiilar tissue and throughout nio>t orir»nis.
 
The lymphatic vessels fir>t lornie«l,a<v«)r<lin<!: toO. Schultze,
 
are the siilM'titaMeoii> v<*>r»(*N, whi<*h are present in a iinnian
 
ehihryo of 2 to .'» <'in.,' aii<l at a -onu'what later perio«] the
 
Jei'prr vrsH'ls apprar. From the stutlies <if Sal)in" upon
 
piir <'ml»rvos it appear^, liowever, that the lar<rer ves>els
 
preenh* th<' smalh't*. Tiii^i ol»erver found that at the jnne
tion t»f th<' sulielavian an<l jugular vc-ins of eaeh si<h» a sae
 
<»r lymph'hoart made ilsap|M*aranee, the oritiec In'ino: guarded
 
l»v a valve, and from th<'s<' >aes or hearts hninches arose
 
whieh passi'd t«»\vard th*- skin, from whieh hranehes a <reneral
 
hulieiitaneous net\\t»rk of ve-M-lr. aroH*. From eaeh lymj)h
Hie a vessi'l m'ow.s tailward, the vessel tui the left side
 
rt'Mehin^ \\\v a«>rta and divi<lin^ there to form two thoraeie
 
diU'l^, wliieli afterwani unite into a sinirh* duet. F^recpiently
 
ihU fi'lal condition oi* two thoracic duets is indicated in the
 
liinnMii adult hv a douMc condititin of the du<*t tor a greater
 
or h'HM extent, the diK't sometimes dividinir and reunitinir
 
two or three tiine>; sometime^ it i*^ «]oul)]e at its termination
 
ill the Mi1iclav!:in vein.
 
Tin* two llniraei<* «luctr* hefore fu-ioii dilate at their <-audal
 
ixtivmiti''^, ill the re^d«»n i»f the ki^imy, to form respeetively
 
twj /.ri/,/f»r»/f/ c/iy/, and a litth- fariher on miite with the
 
iw'i ywWlor lyinph-hearts or sa<'s, whieh iiave meanwhile
 
.1. m\u\^A ill th- jimctitin of the s.-iati<* veins with tl ardiual
 
^el1^.. TIm^*' IsiHcr sMcs MiliMMpicutly lo-c all coniieeiion
 
,.,^1. d,. v.in^ rrniii whidi tlK-y .irrew. ()uton>wih. frmn
 
h... .Wr v«'^-'U ^'i'the hmphatic system -erve f^r its
 
...M.-..M. nn., tlie vi-reni aiitl the skin. As the primary
 
' ..,.. ..„ '..■.iMri- iliT Kntwii-krluntf-niMliichti- .1.- .M.u^l.vn
 
. ..... \ ...,.,.1 ■ «HMlM-«»ri>:in«»f the Ly'»p''''t»'' ^>"»'''' '"'■•'''' ^'»''
 
..... ;,.... P^H.^1.' "f thf Kynn)h-lu*art< aiul Tlh.n. ir iMin in
 
 
 
THE DEVELOPMENT OF THE LYMPHATIC SYSTEM. 129
 
lymph-sacs increase in length, but fail to correspondingly
increase in calibre, they gradually become merged into the
vessels.
 
The lymphoid or adenoid tissue is produced at a later date
than the vessels. Observations upon the human lymph
nodes seem to have been confined to the inguinal and lumbar nodes. The first indication of an inguinal node is seen
in a 3 cm. embryo, in the shape of little aggregations of
lymphoid cells that have migrated from the lymphatic cords
or networks into a space hollowed out of the mesoderm.
This nodule of lymphoid cells is isolated from the surrounding mesodermic elements by a fissure or space except at one
point, the future hilom of the node, where strands of embryonal connective tissue connect it with the parent mesoderm. The retdcolum of the node appears later, as does also
the capsule, the latter of which results from the condensation
of the surrounding mesoderm.
 
The development of the spleen is considered with that of
the alimentary system because of its relation to the evolution of the peritoneum, while the account of the development
of the thymus will be found in the chapter on the respiratory
system.
 
9
 
 
 
CHAPTER IX.
 
THE DEVELOPMENT OF THE FACE AND OF THE
 
MOUTH-CAVITY.
 
The evolution of the £ace depends so largely upon the
growth of the parts concerned especially in the production
of the mouth and nose that any account of its development
must deal for the most part with the development of those
structures. In tracing the earliest stages of facial growtli,
it will be well to consider the face as a whole before proceeding to a detailed description of its several parts. If we
seek the principles underlying the conformation of the face,
we shall find that its apertures and chief cavities are merely
so many provisions for bringing the central nervous system
and the aliraentarv tract into relation with the outside world.
It will be seen, for example, that certain small depressions
apiH'ur U|H>n the surface ; that one of these, which is destined
to l¥H^>me the mouth and the respiratory part of the nasal
dviticss a-ssumes relationship with the alimentary tract and
with Its offshi^ot, the respiratory system ; tliat other depres^MtSs. whioh s«l>scquently develop into the olfactory parts
>^< th^ iwfcal ohanilH^rs, come into relation with (»utgrowths
•^•m tis^ hmiiu the olfactory bulbs ; and that still another
'*l^nl%^•^*tt\;j^acttwltum In^comes the lens-vesicle, which likewise
mvi>^ *»ttlk Att vHii^r\>wth from the brain to become a jmrt of
^ ^*-»f»4KMnt ^*ttp!*>H>npin* the eye.
 
^"N, ti-^ 'avp itt tht* ditferentiation of the face is the fort«i|tH>.r >4 iiKi <ml flilt» the earliest indication of the future
•t%Hu^. ^V' vHiJ i^iUit^ ;jip(H'ars on the twelfth day, and con* * >iH«ut ,^*vH of wtiHlerra and entoderm, the meso*^«4k|^.iU<ai« U icL ^tuatcd on the ventral surface of
 
 
 
DEVELOPMENT OF FACE AND OF MOUTH-CAVITY. 131
 
the head-end of the embryo, which already presents the
enlargement of the cerebral vesicles. The oral plate becomes
relatively depressed by the upgrowth of the surrounding
tissues, the fossa thus produced constituting the oial pit or
Btomodsnin (Fig. 57). The oral plate is now the pbatyngeal
» (Fig. 66). Reference to the sajpttal section will
 
 
 
 
Pro, W.— Medl«n setllon tlirough the henil of an emhryo rabbtt 6 mm. long
(arttr MlhslkovlcB): rA. laerobrene bulvtiin stomndsuni and 10 ru^t. pharyngeal
membrane (Rachenhauli; lip, place rrom which the hypophysla Is developed; A,
heart: kil, lumen or fare-gul: cH. ehorda; c, ventricle of (he cerebrum; i>). third
ventricle, ihat of the belween-braln ithalamenccphalonl ; e', fourth ventricle, that
or the hind'brain and after-brain tepeneephalan and tnctcncephHiun, or medulla
obloni^ta) ; ft, central canal of (he spinal cord.
 
show that the oral pit corresponds in position to the headend of the gut-tract. The formation of the pit is, in effect,
a piishin^-in of the surface ectoderm to meet the alimentary
entoderm.
 
A second important factor in the development of the face
is the appearance of the first and second visceral arches,
which occurs in the third week. As pointc<l out in a preceiling section, the flnt viBceral arch divides into the mandibular areh and the maxillary process (Fig. 62), the latter
being the smaller and appearing to spring from the mandibular areh. Both the maxillary processes and the mandibular arehes grow toward the mc<lian lino of the ventral
surface of the body. Owing to the growth of these struct
 
 
132 TEXT-BOOK OF EMBRYOLOOT.
 
itren and to the sharp flexion of the head and neck that
MM!iirH between the twenty-first and the twenty-third day,
the (inil pit becomes very much deeper and acquires more
definite boundarie.s. During the third week it is a iossa of
|H<nUif|^oul outline. Its upper boundary is ibrmed by the
unpaired nasobontal or nasal process (Fig. 67, A), which is
BAHentiully it thickening on the ventral wall of tlie forebrain vcHiele, brought int<> close relalion with the to^ea by
the flexion almve reierredto. The lower boundary is formed
by the tnundibular arches, while the lateral extent of the fosea
'i» limited liy the maxillary process of eaeh side.
 
KiHin atWr the appearance of the oml pit, the future nnre.s are
fiifeHhadowed by tile development of the two oUactorr plates,
flititiited one ou each side of the nasofrontal process, widely
iH'iHinited from each other. These epithelial areas, which
(HHin beeome dejiressions, the nasal pits, are closely united
witli llie wall of the fore-brain vesicle from the first; they
develop mibitefpiently into that [«irt of the nasal mucous
inemlii'Utie which Is concerned esjiecially wilh the sense of
Bmell. Thi.H fact Iwcomes very significant when it is rememberttd that the olfaotory bulbs, with which the olfaetorj- epithelium aNsumes intimate relationship, are outgrowths from
the bniiii.
 
The nasofrontal process, during the fifth week, becomes
mui-h ihii'ki'tu'd idoug ils lateral margins, forming thus the
Clobular pTOCosses (Fig. 07.^1), which constitute the inner
l)ouiKlnrii!.H of tlie nasal pits. At the same time, there grow
downward and forwanl from the nasofrontal process two
ridges, one on each t<tde, the lateral frontal processes, which
form the out«r Imundaries of the nasal pits (Fig. (i7, A).
In this manner the pitd become mmh increased in depth.
The lateral frontal process projects between the nasal pit
and the maxillary process, its line of contact wilh the latter
strui'tiirc being marked by a groove, the uaso-optic fttirow or
lacrimal groove. This groove later completely disappears;
it i;t of imixirtiuiee, however, as indicating the position of
the now developing nasal duct, which will l)e referred to
hereafter. The nasal pit-* are widely in commnniciition wilh
 
 
 
^ J
 
 
 
DEVELOPMEyT OF FACE AND OF MOVTH-CAriTY. 133
 
the cavity of the primitive mouth. About the fortieth day,
however, the extremities of the maxillary processes have
grown so far towani the median line that tlicy have met
 
 
 
 
 
Tie. ei — DeTeloproent of the Cuw ot the bummn cmbirn lllie) : A, emtiDo of
■bout twenly-nine dsyi, Tbe iiuaarrontai plalu difltrvuliBllnH Into pi-cicoiBUii
BlobDlana, tow md which tlio maxillary proeenBea of Ural viscenil mreh are cxlendiDg-. B. embiyo of atioul thiny-(>iur ilayB : the glubular. lal«nil fKinUl, and mailllai; processuK are In apposition : thu primitive npcnln; a now belter detlned. C,
embryo of about thu elvblh week. Immediate bnundarlei of moutb are more definite and the now.) iirincesnre partly (brmed. external ear appearing. D. embryo
at end of second month.
 
and united with the lateral frontal processes and with the
nasofrontal process (Fig. 67, B and C). In this manner
the nasal pits become separated from the oral fossa, each of
these openings aeqiiirinfi more definite lionndaries. It is
 
 
 
134 TEXT-BOOK OF EMBRYOLOGY.
 
apparent from this description that the upper boundary of
the primitive oral cavity is not identical with that of the
adult mouth. The nasofrontal process is the forerunner of
the intermaxillary portion of the upper jaw, including the
corresponding part of the upper lip and of the nasal septum and bridge of the nose.^ The lateral frontal process
becomes the wing of the nose. By the completion of the
changes here noted the face aajuires more distinctive form.
It will be seen that the upper jaw proper results from the
metamorphosis of the maxillary processes. The manner in
which its sinus, the antrum of Highmore, is added, as well
as the ossification of the jaw, will be considered hereafter.
 
The development of the eye will be described in connection with that of the sense-organs. In so far as the eyes
have relation to the external form of the face, it will be sufficient to say that the surface ectoderm is iuvaginated in the
fourth week to form the lens-vesicle, this sac, which gives
rise to the crystalline lens, being covered by two little folds
of iVtcKlerm, the primitive eyelids; that the organ is situated
on the siilo of the head, in marked contrast to its position in
tho matun* state ; and that the naso-optic ftirrow, previously
ivK^rnnl to» jmssrs from the inner angle of the eye toward
iW winvr ^*f the nosi*. The development of Ihe face having
Uv« ivMutinl t>ut in a giMieral way, the individual parts may
tv vXHwivlonxl j^»|Kirately.
 
THE MOUTH.
 
tV vx'x ik^w hrlotly, for the sake of convenience and clear*»%^s I'V s\<vli\^r history of the development of the mouth,
Hv 'UkI »{io tiiM stop to Ih» the appearance, at the twelfth
^b\;k »4 t^\^ ^^ y4»W« By the enlargement of the anterior
. .ui '4 ilv iKinnt tuU* to torm the cerebral vesicles, and bv
 
lu V 7' \ iiuv vk^K^tikii^v r«M«ultin^ frt>m failure of union l)etween the
 
■•a.<.i\»u*. u»*l \\s \kksK\\\\M\ |U\H>'«H«K. Since the musofrontal process is
 
».v ., vi'vJ *iu^»uic, \u ^\\w\\ ^tovolop the intermaxillary bones, and
 
•*•>..» \ '* s n » *tUs-4 v»<U* ^iih iN^ vMrre«|KUuUnj» maxillary |)r(K\»ss— the
 
»^-.\. >'»'*fc 'i»i xMviuuuv* v»l »ho up|HT maxilla proper— we have an
 
N^ V ♦ .!*.« i ;k i*;viul )Ki«itMik \4* h»rt» lip. This defect may be, of
 
 
 
THE MOUTH. 135
 
the development of the visceral arches^ this area becomes a
depression, the oral pit. The pit is at first bounded caudad
by the cardiac prominence and cephalad by the fore-brain
vesicle (Fig. 57). In the third week the oral pit becomes a
five-sided fossa, owing to the growth of several new structures. These are the unpaired nasofrontal process, which
bounds the fossa above, the mandibnlar arches, which bound
it below, and the maxillary processes, which form the lateral
boundaries (Fig. 67). The mandibular arches do not actually
unite with each other until the thirtv-fifth dav. A transverse groove appears on the outer surface of the united
mandibular process, the elevation in front of which is the
lip ridge, while behind the groove is the chin ridge; these
ridges respectively produce the lower lip and the chin. The
angle between the maxillary process and the mandibular
arch corresponds to the angle of tli(» future mouth. In the
sixth week — about the fortieth day — the oral fossa acquires a
new upper boundary, whi(!h separates it from the nasal pits,
by the growth of the maxillary and lateral nasal processes
The primitive oral cavity, as before mentioned, is at first
separated from the gut-tract by the pharyngeal membrane
(Fig. 6G). This structure ruptures at some time during the
fourth week, thus bringing the mouth into communication
with the upper end of the gut-tract. The exact location of
the pharyngeal membrane with reference to the adult pharynx
is somewhat difficult to define; it is certain, however, that
the primitive mouth includes more than the limits of the
adult oral cavity, comprising, in addition to the latter, the
anterior part of the adult pharynx. Reference to a sagittal
section, as in Fig. 66, shows the relation of the oropharyngeal
cavity to the brain-case ; in the tissue separating the two the
floor of the cranium is subsequently formed. A little evagination from a point (Ap, Fig. 66) in the back part of the
primitive oral cavity becomes the anterior portion of the
pituitary body or hsrpophysis, the posterior lobe of which
develops as an evagination from the floor of the primary
fore-brain vesicle. With the development of the floor of
the cranium, the hypophysis becomes entirely isolated from
the oral cavity. A little pouch or recess usually demonstrable
 
 
 
136 TEXT-BOOK OF EMBRYOLOGY.
 
ill the median line of the roof of the pharynx of the child,
though not always present in the adult, is the persistent
pharyngeal end of the diverticulum that forms the hypophysis; it is known as the pharyngeal bursa or Bathke's
pocket.
 
Very soon after the formation of the upper jaw in the
nuinnor above described, the oral surface of the jaw presents
two parallel ridges. Of these, the outer, which is the larger,
doV('h)pH into the upper lip, while the inner smaller ridge betH>iU(»H the gum. The lip and gum of the lower jaw are produiHHl Hiniihirly, at the same time or a little later. So far,
tho only d^Mnarcation between the mouth and the nasal
levity in furniHhed by the tissue representing the united
linmthHMital, hitenil nasal, and maxillary processes, the nares
ii|H'hihft widoly into the cavity of the mouth posterior to this
 
I^Mlttloh,
 
'V\\\> rorniation of the palate, however, effects a separation
Ih^Iw^h^U thr two that gives to each space its permanent limita\\\^\\^y i>u ihr inner or oral surface of the upper jaw two
«ih\'iritko phJiH^tiouH appear, one on each side, which are
\\s\^ y\y\\\\\\\'\\{n of the Aiture palate. These gradually grow
V\»n\«U'aI iHioh othrr, the tongue, which has meanwhile been
slv\\^l\»|»iu)^, p»H»hvting upward between them. In the eighth
VUH Ki Mhiou v»l* \\w^K' two lateral halves of the palate begins
^l' \\w^\ wwWvwA' <»\li*«»inities. By the ninth week union has
Uks^u I'lmn* a** lar back as the extent of the future hard
^H^iU^s mid. I\N llio t^h'venth week, the constituent halves of
ihiv joll \u\\\\W have uuiteil also. As these two halves ap^»u^^v^K \H\K'\\ \»tlirr the tougu<» recedes from between them,
v»\\ii4j^ lo \\\\^ v(iH»wth t»r the lower jaw, so that, when union
imui ', thai oi^au iHHMipies its normal position under the
^^^I»U^. ( Vv.Hum t\irmation within the soft tissue first formed
puulurr-' \\w }mlute piHuH^sses of the superior maxillae and of
tl*o juiluio Imiiert, wliioh prtH*esses collectively constitute the
hard palutr of the adult. The intermaxillary bones are
toiiiu'd wiihiu tlie primitive |mrtiti(m between the mouth and
the uau*.-.. The rompletiou of the palate definitely marks
otl tho uasil t'tiambers fixuu the mouth, thus dividing the
early oral luivity into a lower sjmkh*, the true mouth, and an
 
 
 
THE MOUTH. 137
 
upper region, which is essentially a part of the respiratory
system.
 
The uvula appears during the latter half of the third
month as a small protuberance on the posterior edge of the
soft palate.^
 
The Teeth. — The teeth, morphologically considered, are
calcified papillae of the skin, capped by a layer of peculiarly
modified and calcified cells of the epidermis. Although in
man and the higher mammals the teeth are found only upon
the gums, in certain lower types they have a much wider
distribution, occurring upon the roof and floor of the mouth
and in the pharynx, and also, in selachians, upon the general
skin-surface, in which latter case they are so modified as to
constitute scales.
 
The dentine and cementnm of the tooth, as well as its pnlp»
are derived from the mesoderm ; the enamel is a direct derivative of the overlying ectodermic epithelium. Mammals
are said to be diphyodonty since they develop two sets of
teeth ; while such groups as sharks, which continue to produce and lose new teeth throughout life, are denominated
polyphyodont
 
The development of the teeth is inaugurated in the sixth
week of embryonic life by the multiplication of the epidermal
cells covering the surface of the gums to form a linear ridge.
The growth of the ridge is away from the surface, so that
the new structure projects into the underlying mesoderm.
This horseshoe-shaped ridge, which corresponds in direction
and extent to the line of the gum, subdivides into two
parallel ridges, of which the outer marks the position of the
future groove between the gum and the lip ; the inner is the
dental shelf or dental ridge, which must be regarded as the
earliest indication of the future teeth. The dental shelf
extends into the underlying mesodermic tissue, not directly
 
* Deficiency of union of the halves of the palate, resulting in a median
fiuBure, constitutes the deformity, cleft palate. This deficiency may be
limited to the hard or to the soil palate, or it may affect both, or it may
be seen in the uvnla, either alone — deft or bifid uvula— or in conjunction
with cleft palate.
 
 
 
\m
 
 
 
TKXT-IiOOK OF E.lfBRYOLOOY.
 
 
 
iliiwiiwiinl l>iit in an oblique direction toward tbe inner op
llitKiitil wirfnoi- of tlie gum. While the dental slielf is growliiKt it" liiK' of L^ounectiuD with the surface ectoderm is
iiiui'ktKl liy tiw superficial dental groove, wliich at one time
wtifl hHiktKl iiiKiii as being thu first evidence of tooth-formation.
 
lI|Min lh« itidd of the dental shelf opposite the free or
oitil Niirfiici), individual protuberances develop, corresponding
ill niuiilHtr to that of the teeth of the temporary set — ten for
riioh Jiiw. ICach little projection consists of a mass of ectoili)i'inli^ I'clU. which soon becomes expanded at its deep exIroinity, iMTomiri)^ thus club-shaped and later Ha-sk-tihajied,
anil which in <ni]led llie enamel-sac or primitive enamel-Kenn,
nUlcv tJio eiitiiucl of the tooth is developed from it (fig. <JS),
 
 
 
 
rin. .Ut,-Tli™« i.iicFt.Hlv> ilaga hi the duvolop
 
 
mcnl or ■ tni>th-i;cnu ot
 
 
•mhtft («llor Vivs aixl TlilonH-IH; u. ft. e, Ujen
 
 
i.t Ihickencd onl epilhe
 
 
 
 
rc*n: /, dviital pa^dllft
 
 
Inlonial aixl i»li>ruid l«y»r> .>flhit taWM- walh
 
 
blood-v«Ml: *, m».ll
 
 
vptltmllkl InNrowlli. Hie xn'l xf whioh «xp«n<li> lnK>
 
 
h<! riii>mH-«u'
 
 
 
Mcntiwhile the continnity of the original dental shelf is
broken by the di»ip|K-uranoc of the t*Il3 in the intervals
between tlio in<)iviilual ennmel-germs, each germ iiccoming
thereby isiilnti-d from itn neighbors. The neck of the flask
 
 
THE MOUTH. 139
 
shaped enamel-germ becomes reduced to a slender strand of
cells and finally disappears, so that there is no longer any
connection between the enamel-sac and the ectodermic cells
of the free surface of the gum. While the enamel-sacs for
the temporary teeth are growing in this manner, the corresponding structures for the teeth of the permanent dentition
bud from the inner side of the dental shelf — that is, the side
looking toward the tongue — except those for the three permanent molars, w hich grow backward toward the articulation
of the jaw from the position of the second temporary molar.
 
As the enamel-germs grow downward into the mesodermic
tissue, the latter sends up a number of conical projections,
the dental papills, one for each enamel-organ. This dental
papilla, of mesodermic origin, is the parent of the dentine
and of the pulp of the tooth. When the dental papilla and
the enamel-sac meet, the sac becomes invaginated, its under
surface assuming a concave form. The enamel-sac at this
stage therefore is a double-walled cup which caps the dental
papilla. It is at about this time tliat the connection of the
enamel-organ with tlie surface ectoderm is lost.
 
The further evolution of the enamel-organ consists essentially in the arrangement of its constituent cells into three
layers and the formation, by the deepest of these three layers,
of the special elements of the fnlly-developed enamel — the
enamel-prisms. The most superficial stratum of the enamelorgan is composed of low columnar or polyhedral cells; the
deepest layer, that nearest the papilla, the so-called membrana adamantina, consists of beautifully regular columnar
cells, the ameloblasts or adamantoblasts ; between the two is
a group of less characteristic epithelial elements. The cells
of the deep layer, the enamel-cells, are alone con(?erned in
the production of the enamel. The enamel-organ for a time
covers the entire dental papilla. During the course of development, however, the growth of that part of it covering
the future root of the tooth aborts, leaving the crown alone
covered with the enamel.
 
The first step in the formation of the enamel-prisms by
the enamel-cells is that the protoplasm of the deep extremity
 
 
 
140
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
 
o[ each cell becnmex homogeneoii!>, and a tuft develops on
the end of the pell, projecting loward the papilla. By the
calcifieatiuD uf this tuft the formation of an enamel-prism is
l)Cgun (Fig. 69). The process
of caU'J Heat ion continues to advance from the deep or papillary
aspect of the enamel-org-aii toward
the surface. From this it comes
about tliat the newest enamel is
next to the enaniel-celis, or, in
other words, nearest the surface,
and also that the cnamel-prisma
are arranged in a directiun generally vertical to the free surface
cf tlie tooih. The formation of tlie
enamel of the milk-teeth begins in
rii. M -Himii-rtiiiKniniiuaik (Is- the latter part of tUe/uurih monlh.
iMieiiylng B.iiiiii«i.«rgnii (Tuur- The middle layer of the enamel<u>ii i,..i.iit™i™ii.r.ren«moi- Q^^p becomes greatly altered in
 
mil; J mill K.oi'Umf Inner larer ^ , , ° ■'
 
conatttution, owing to the accumulation of fluid and to the reduction of its colls to the form
of ihin plates, the appearance
being rather that of eonii«;tive
IImiic than of an rpitlielial structure. The sujierlicial layer
of w\U uiidi'rf{iii-M atrophy, their exact fate not being known.
The ritii'ithic ii>uinaut of the onaniel-organ is found upon
the IVee nuifac-c of (he tooth for a variable time after its
(iniplion, I'oimtilultMg (he membrane of Naamyth.
 
'I'lie dintaJ papilla hax beeu relVrrcd to as the structure
lliat K<veH rise to the dentine. It origitintes from active
milltiplicatiiiii of llie mesiMlermii- cells. The number of
|MpillH> cnrrcrtpondH to the number of enamel-organ^;. As
the [itipillu grows toward the enamel -organ it early acquires
vawnlurlty. The shu]ie of the papilla, whether that of an
ineinor, of a L^nine, or of a molar tooth, is determined by
the fthiipc which the eniiniel-orgiHi assnnics. Tlie eonneetive
 
 
I Iji'illK the r
 
 
 
THE MOUTH, 141
 
tissue cells upon the surface of the papilla assume distinctive
character, becoming large and branched, and constitute the
so-called odontoblasts (Fig. 69). They are virtually modified osteoblasts. Forming a continuous layer, they have
been styled the membrana eboris. Between this layer of
odontoblasts and the enamel-organ a layer of intercellular
substance appears, the membrana prsformatiya. The odontoblasts now send out processes toward the enamel-organ,
which are known as the dental processes. Calcification
begins upon the surface of the papilla and progresses toward
its center, but is not complete. Small uncalcified areas, corresponding to the globular spaces of the completed tooth,
remain next the enamel. The dental processes likewise fail
to become calcified, and these are the adult dentinal fibers
occupying the dentinal tubules of the finished dentine. The
odontoblasts continue the formation of dentine until the dental papilla is entirely surrounded by it. What remains of
the papilla, upon the completion of the tooth, constitutes the
pnlp, a highly vascular connective-tissue substance supporting upon its surface the odontoblasts. The deposition of
dentine begins in the latter part of the fourth month.
 
During the metamorphosis of the dental papilla the mesodermic tissue immediately surrounding it undergoes slight
condensation to form the follicle of the develoj)ing tooth.
As the enamel-organ recedes from the surface, the follicle
increases in extent to such a degree as to envelop the entire
rudimentary tooth. Only that part of the follicle which
covers the future root of the tooth is of subsequent importance, however ; undergoing partial transformation into true
bony tissue, it gives rise to the cementom or crosta petrosa,
while the unossified external fibrous layer constitutes the
lining periosteum of the alveolus (Fig. 68).
 
The development of the permanent teeth is precisely analogous to that of the milk-teeth. The enamel-germs for the
permanent teeth, with the exception of the molars, bud from
the lingual side of the dental shelf in the seventeenth week
(Fig. 70), the germ for the first permanent molar appearing
about a week earlier at the posterior extremity of the dental
 
 
 
142
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
shelf after llie manner of a milk-tooth, The germ for the
second molar XhvXa fr^ini the tieck of the first molar in
the tliir<l month after Itirth, while that of the third molar,
the wisdom tooth, springs from the neck of the second about
the third year. At birth, therefore, the gums contain the
 
 
 
 
;enua at o inl1k-t>i'>Ili and ul
, llilckenetl oral eplLLeUum: «i, i
•rown out at u rrom the dicIe (>) of
II. Uecki-raciLrtlligt^.
 
 
 
two sets of teetli except the second and third permanent
molars.
 
The eruption of the temporarr teeth begins usually at about
five and a half nioiilhs after birth wilh the appearance of the
central incisors, and is coniidete at from eighteen to thirtysix months, when thu sccniul molars are cut. The first t«eth
of the penuftnent dentition are the tirst mohii's, which are
eru)ited at ahoiil the sixth year. The accompanying table
shown the time and the onlir of eruption of the teeth :
 
 
 
THE MOUTH. 143
 
Temporary Dentition.
 
Central incisors 5} to 7 months.
 
Lateral incisors 7 to 10 months.
 
First molars 12 to 14 months.
 
Canines 14 to 20 months.
 
Second molars 18 to 36 montha
 
Permanent Dentition.
 
First molars 6th year.
 
Central incisors 7th year.
 
Lateral incisors 8th year.
 
First premolars 9th year.
 
Second premolars 10th year.
 
Canines 11th to 12th year.
 
Second molars 12th to 13th year.
 
Third molars (wisdom teeth) 17th to 21st year.
 
The Salivary Glands. — The salivary glands, which in
mammals consist of three pairs, the parotid, the submaxillary,
and the sublingual, develop as outgrowths of epithelium
from the lining mucous membrane of the mouth. The
epithelial elements of the glands are therefore of ectodermic
origin. The growth of the submaxillary gland begins in the
sixth week, that of the parotid in the eighth week. Each
epithelial outgrowth is at first a solid cylinder, which undergoes repeated branching and acquires a connective-tissue
framework and capsule from the surrounding mesoderm. It
is not until the middle of the fifth month that the lumen of
the gland appears. This is brought about by the moving
apart of the epithelial cells composing the cylinders and
their branches. The main duct of the gland first becomes
hollow, then its branches, and finally the lumina of the
alveoli make their appearance. The respective sites from
which the several glands grow correspond in a general way
to the |X)sitions at which the ducts of the adult glands open
into the mouth-cavity.
 
The Tongfue* — Although the tongue originates from
tissues belonging really to the walls of the pharynx, its development may be conveniently considered in connection
with that of the mouth because of its relations in the mature
organism. This organ, composed chiefly of muscular sub
 
 
144
 
 
 
TEXTBOOK OF EMBRYOLOOr.
 
 
 
stance, is formed from three originally separate parts, an
anterior unpaired fundament, and two posterior bilaterally
eymmetrical segments. The line of union of these three
parts is indicated approximately in the adult organ by the
V-shaped row of circumvallate [Htpilla; on the dorsum of the
tungtie. The anterior part of tlie tongue develops from a small
unpaired tubercle, tlio tu1)erculiim impar, which grows from j
the median line of the floor or anterior wall of the pharynx
between the first, or mandibular, and the second, or hyoid,
areh {Fig. 71, 6). Tlie posterior segment of the tongue results
 
 
 
 
., mm. (nbout 25 lo 30 linja). /, II, III, 1 , , uumt ,»oeral lurrow*; v, «inu» pr»pervi«all«, comprtaliig third and Fourth outer furrows:
l,S,3, 1., vlieeral archei, s>ch with lU v<8(vml-Arch vc«el; e, tubercalum Imparl
7, orlflce of larynx : S, pulmunary evaKiDalian.
 
from thegrowing together of two lateral halves, which develop
from the anterolatend walls of the pharynx at the position
of the second and third visceral arches, but not from the
i arches themselvos. These ridges arc sometimes described aa
' the fused anterior (ventral) extremities of the arches just
mentioned. The unpaired tubercle increases in size to such
an extent as to constitute the major part of the organ. In the
median line of the anterior wall of the pharynx, immediately
behind the tuberculum impar, the epithelial lining of this
cavity pouches forward and downward to develop later into
the middle lobe of the thyr()id body. As the ridges which
are to form the posterior [wrt of the tongue lie laterally and
 
 
 
THE DEVELOPMENT OF THE NOSE, 145
 
posteriorly to this median evagination, they completely enclose it in the process of fusing with each other and with the
anterior tubercle. In this manner a canal or duct is formed
leading from the surface of the tongue at the angle of junction of its three segments down to the middle lobe of the
thyroid body, the latter meanwhile having descended from
its original position. This canal is the thyroglossal duct or
canal of His. During the further progress of development,
the canal suffers obliteration, its only vestige beiug the orifice,
which is known as the foramen csecum of adult anatomy.
 
The papillae of the tongue are found exclusively on the
part derived from the tuberculum impar ; the line of union
between the anterior and posterior parts lies therefore behind
the row of circumvallatc papilla?. The papillae begin to
make their appearance as early as the beginning of the third
month.
 
Prior to the union of the two lateral halves of the hard
palate, by which the primitive oral cavity is divided into the
mouth proper and the nasal chambers, the tongue projects
upward between the palate-shelves, almost completely filling
the primitive mouth. As the palate-shelves approach each
other, however, the tongue gradually recedes to its subsequent
normal position.
 
THE DEVELOPMENT OF THE NOSE.
 
The nose being an organ of special sense, its development
is described in connection with that of the other special-sense
organs in Chapter XVI. Owing, however, to its important
relation to the other parts of the face, it is desirable to refer
to its evolution in this connection. For a more detailed
account, the reader is referred to Chapter XVI.
 
The first indication of the organ of smell is in the form
of the two patches of thickened ectoderm, the nasal areas or
olfactory plates, which appear on the head ward side of the
oral fossa in the third week of development. At the end of
the fourtli week the areas are depressed and constitute the
nasal pits (Fig. 67, A). The nasofrontal process, a mass of
thickened mesodermic tissue, lies between them. During the
 
10
 
 
 
146 TEXT-BOOK OF EMBRYOLOGY.
 
fifth week the lateral edges of this process become thick and
rounded, forming the two globular processes, while growing
outward and downward from the sides of its base are the two
lateral nasal or lateral frontal processes. Thus the nasal pits,
which corresiK)nd with the position of the future anterior
narcs, become bordered on the mesial side by the globular
processes and on the outer side by the lateral nasal processes.
Below, the pits arc continuous with the oral fossa. Owing
to the continued growth of these masses the pits gradually
become deeper. The lateral nasal process is separated externally from the maxillary process of the first visceral arch
by a groove, the naso-optic fturow. The lower extremities of
the maxillary and lateral nasal processes soon unite with each
other and advance toward the median line below the nasal
pit. In the latter part of the sixth week they unite with
the nasofrontal ])rocess and thus separate the nasal pits from
the oral fossa and furnish the basis of the up{>er lip. The
nasal pits are now the anterior nares, and the nose is represented by the irregular masses of tissue surrounding them.
While the orifices of the nares are separated from the orifice
of the primitive oral equity, their deeper parts are continuous
with the latter, there being as yet no hard or soft palate.
 
In the eighth week the nose first acquires definite form,
owing to the continued growth of the masses of tissue referred to above. The nasofrontal process forms the bridge
of the nose with the nasal septum, and also the intermaxillary
part of the superior maxiihe and the oonnoetive-tissue parts
of the upper lip. The lateral frontal process becomes the
ala of the nose. The nose is still verv broad and flat in the
third month, after which time it grailually assumes its characteristic form.
 
 
 
CHAPTER X.
 
THE DEVELOPMENT OF THE VASCULAR SYSTEM.
 
The vascular system, including the blood, the heart, and
the blood-vessels, begins its development very early in embryonic life.
 
While the heart is formed within the body of the embrj-o,
the blood and the earliest blood-vessels have their origin in
an extra-embryonic structure, the yolk-sac. It is noteworthy that all parts of the vascular system proceed from
mesodermic tissue, the heart and the vessels originating from
clefts within this structure, and being lined, therefore, with
endothelial cells.
 
In corres[)ondence with the varying relations which the
embryo sustains toward the fetal ap])endages at different
times, its circulatory system is distinguished successively
by certain special features. Thus, during the activity of
the yolk-sac as an organ of nutrition, the vitelline circulation
is present; following and supplanting this is the allantoic
circulation, which latter, in turn, gives place to, or, in fact,
becomes the placental system of vessels.
 
THE VITELLINE CIRCULATION AND THE ORIGIN
 
OF THE BLOOD.
 
The seat of the first formation of the blooil- vessels and
of the blood is the wall of the yolk-sac, entirely outside of
the body of the embryo. The wall of the yolk-sac, the
reader may be reminded, consists of the extra-embryonic
splanchnopleure covered with a part of the somatopleure.
The mesodermic layer of the sac exhibits — at the end of the
first day in the chick — a network made up of cords of cells,
the angioblast (Fig. 72). Interspersed throughout this network are groups of cells, the substance-islands, which lie
within the meshes of the network in relation with the cords
 
147
 
 
 
148
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
of cclU composing it (Fig. 72}. Bolli llie c«Us of the cords
and of the sulistauce-isbinds are meBenphvmal t-etls. The
superficial cells of the cell-cords become fiatttined iu each
case tu constitute a conliniions layer \?)iich eocloaes the renininiiig culls ortli^' cjLiI.and ihey thus fnrm the eiidutheliul
wall of the future b1m>d*- - vessel. The cells of the sub
-I stance-iiilunds move apart
 
^f^ '^i and acquire prolongation!)
W '' T or processes which intercom's , 1 mnnicate, while a gektin' ^ fl ous or f«raiHuid intercellular
 
ftnbslanee is formed, thus
~Bi pi^hicing an embryonal connective tissue in relation
with the network of developing vessels. The solid
"vessels" thus formed acquire luniina — on the second
day of inculution in the
chick — h_v the penetration
of fluid from the surrounding niesodernt, this fluid
cmwding the cells a|iart,
toward the vessel walls. The
channels of the vessels are
at fin*t quite irregular, being at some points entirely
blockeil, at others merely
*«iu, B. oncrouchcil upon, by masses
of splicniidal cells in connn'Clion with (he vessel walls.
arr ihe blood-iaUndB, the aggrt^itions
l^nvvd (h»> fetal nd blo-Hl-eells (Fig. 72).
' ■UpuJ-bdauib multiply by mitotic divi• l«rt, suci-eesively become detached and
.■•.■Ml-.stmuit. This process continues until
X\m*< ci'ILs the enrthioblasts, the first
 
 
 
 
TUE VITELLINE CIRCULATION. 149
 
corpuscular elements of the fetal blood, are at first colorless, but soon become pale yellow. Their formation goes
hand in hand with the formation of new blood-vessels.
Their color deepens somewhat, liemoglobin developing within
the cytoplasm. Their nuclei are hirge and reticular. The
majority of them acquire small dense nuclei and are then
called normoblasts. The erythroblasts continue to undergo
mitotic division in the bloo<l-stream just as they did in the
blood-islands, division being seen in the embryo chick up to
the sixth day. In man, multiplication of erythroblasts
occurs quite largely in early fetal life, particularly in regions
where the circulation is slow, as in the liver, the spleen, the
bone-marrow, and the lymph-nodes ; while in later fetal life
and after birth it takes place in the red bone-marrow only.
 
It is especially noteworthy that these early fetal bloodcells are nucleated in contradistinction to the adult nonnucleated red. blood-corpuscles ; and that the nucleated form
is present throughout life in all vertebrates but mammals.
 
Up to the end of the first month the nucleated red cells
are the only corpuscular elements found in the blooil. In
the second month the non-nucleated red blood -disks, the erythrocytes, make their appearance, and either in the third
month or very soon thereafter outnumber the nucleated cells.
Diflferences of opinion obtain as to the mode of origin of the
erythrocyte, but the prevailing view is that it results from
the normoblast by the loss of the nucleus of the latter. The
nucleus becomes globular and more dense, assuming in some
cases a dumb-bell shape, and is extruded from the cell, after
which it is thought to undergo partial disintegration and then
absorption by leukocytes. Some observers maintain that the
nuclei are dissolved within the cell. Nuclei in the process
of extrusion have been observed in cat-embryos. After extrusion of the nucleus the remaining cytoplasm of the cell
assumes the biconcave form of the adult red blood-corpuscle
or erythrocyte.
 
The origin of the leukocytes is a somewhat unsettled question. They are found in the blood of chick-embryos at the
eighth day and in the rabbit-embryo at the ninth day ; in the
 
 
 
150 TEXT-BOOK OF EMBRYOLOGY.
 
human embrj'o they are seen in the second month. It is
probable that they originate in the lymph-nodes, the bonemarrow, the liver, and the spleen during fetal life, but after
birth only in the bone-marrow, the lymph-nodes, and the
spleen. Their birthplace would be, therefore, lymphoid
tissue and their ultimate origin mesodermic. It has been
suggested that they may be derived from young ery throblasts ;
this is denied by Minot. Beard assigns them an entodermal
origin, claiming that they are produced by the entodermal
epithelium of the thymus and of the tonsil. From the investigations of Engel and of Florence Sabin it would appear
that they are first seen in the blood and the lymph-nodes at
the same time.
 
The blood-platelets have been variously interpreted as
small nucleated cells and as fragments of broken-down leuk<K?yt(»s. According to the recent work of Wright they are
fragments of the processes of the giant cells (myelocytes) of
bone-marrow.
 
Limiting the first network of vessels on the surface of the
yolk-sac is a circular vessel, the sinos termiiialis (Plate VI.).
Since the yolk-sac is relatively so large that the body of the
<'ml)ryo appeai*s to rest upon it, and since the surrounding
Homatopleure is translucent, a surface view of the ovum at
this stagt* shows a vascular zone encircling the embryonic
urea and the later Ixnly of the embryo. This zone is the
area ▼aaculosa, or vascular area, the seat of the earliest formation of bhxMl and of blood-vessels of the embrvo.
 
Th(» blood-yeBsels originate, as shown above, from the
angioblastio network of m<»senchymal cell-cords of the vascular an»a, the cords of cells, at first solid, gnulually becoming
hoUowed out to form the vessels. The vascular network at
first forni<»d extends by a pnK*ess of budding over the walls
of the yolk-sac an<l thence along the vitelline duct into the
ImkIv of tln^ embryo. The budding consists in the extension
of vessel sprouts or cell-cords — probably from proliferation
of the terminal cells of the vessels last forme<l — the sprouts
being solid at their ends, since the excavation of a sj^rout
alwavs occurs a little later than its forward extension.
 
 
 
THE VITELLINE CIRCULATION. 151
 
Neighboring sprouts communicate with each other to a
greater or less extent. In a human embryo of about eighteen
days the extension of the vessels — appearing macroscopically
as fine red threads — along the vitelline duct is well shown.
Having reached the bcxly of the embryo, the vessels take
their course to wanl the primitive heart, which has meanwhile
been d(;veloping. From the anterior and posterior and lateral
limits of the vascular area — using these terms with reference
to the axis of the embryonic body — four pairs of vitelline veins
converge toward the vitelline duct and unite to form the two
vitelline or omplialomesenteric veins. These veins, after entering the body of the embryo, pass headward along the wall of
the intestinal tube and empty into the lower or caudal end
of the primitive heart. The trunks, which are to constitute
the vitelline arteries, after entering the body with the vitelline duct, pass upward along the dorsal body-wall, within the
dorsal mesentery, to become continuous with large arterial
trunks that have i)roceeded from the primitive heart.
 
The large trunks referred to are the visceral-arch vessels,
which unite to form the primitive aorta?. The visceral-arch
vessels (see Fig. 60) are a series of five pairs of arteries that
arise by a common stem, the truncus arteriosus, from the
upper end of the primitive heart. They pass along the
respective visceral arches toward the dorsal surface of the
bo<lv where all the vessels of one side unite into a common
trunk, the primitive aorta. The two primitive aorta?, passing caudalward in the dorsal mesentery, give off, as their
largest branches, the two omphalomesenteric or vitelline
arteries above referred to. Tlie development and the regression of the visceral-arch vessels correspond with the
growtli and the decadence respectively of the visceral arches.
Not all tlie vessels are present in a fully-developed condition
at any one time, the first pair having begun to atrophy before
the fifth pair makes its appearance. The metamorphosis
into certain adult vessels of such of them as persist will be
considered in a later section.
 
This system of vessels constitutes the vitelline circulation,
the manifest function of which is to convey nutritive mate
 
 
152 TEXT-BOOK OF EMBRYOLOGY.
 
rial from the yolk-sac to the embryo. While the vitelline
circulation i.s of great importance in any ovum provided
with abundant nutritive yolk, such as that of the bird, it is
of (U)mpanitively slight consequence in man and the other
higher mauimals, and it must be regarded as a vestige of the
avian or reptilian ancestry of the mammalian ovum, or, at
Icanf, an a reminder that the mammalian ovum was originally
provided with an abundant yolk. It must be borne in mind,
how('Ver, that the mammalian blastodermic vesicle imbibes
frniM th(! walls of the uterus a richlv nutritive albuminous
Ituid, whieh may be taken up later and carried to the embryo by \\\i\ vitelline circulation. This system of yolk-sac
\v*^ni'\H diHuppears with the regression and disap])earance of the
jolt-HHc - -in the human embryo at about the fifth week. The
Inlni-i'inbryonic portion of the right vitelline artery persists,
hoWdViT, to bc<!onH! the Enipeiior mesenteric artery of the adult.
To riMidrr ilhi comprehension of the later phases of the
Viiw'nliM' HVMdMU more simple, their consideration is deferred
MMlil ihn dnvi'lopnient of the heart shall have been described.
 
I'lm DIJVKLOPMENT OF THE HEART.
 
Thn llt!lirt| when Htudied in the lower-type animals, is
Minn In bn niorpliologieally a dilatcil and specialized part of
a vunriiliir trunk enil>e<lded in the ventral mesentery. In
miiiii thi» Hint Inndunient of the heart appeai-s at a very early
 
Iii!»ioi| nunu'ly, brlore the splanehnopleure has folded in to
linn ihn pill t met. or. in other words, before the end of the
mM'tiiid sviM'k. Tlii" Inndanu'iit, in all higher vertebrates, is
bilalttntl, huviiiK thi^ lonn of two tubes prcKlucecl by vacuolation of tht) nplitnehnie ineHoderm an<l lying widely separated,
iin«« in ntifh half of th(« Htill spread out splanehnopleure
(Ki^. 7.1), .1). A tniiiHVi'rHe K(*etion through the future neckregion of a hiu'i'p wv nil»l»it-embryo shows tiie tubes cut
auritnn, hiiMv ihrir long ii\(*s ans [mnillel with that of the
binlv (Kig. 71). With tiu< li>lding in of the splanehnopleure
uiid the union of tli«' (ulgi^s of its folds, the tubes are carried
tiiward ea4^ii other, and hubse<|Uently, by the disappearance
 
 
 
THE DEVELOPMENT OF THE HEART. 153
 
of the tissue intervening b^etween them, their cavities become
one (Fig. 73, B ami C). After the formation of the guttract, therefore, and the simultaneous apj^aranco of the
ventral body-wall, the heart-fundament is a single straight
mesodennic tube, situated in the pharyngeal region, in close
relation with the ventral wall of the bmly, between the latter
and the fore-gut. Reference to Fig. 73, C, will show that
 
 
 
 
Fio, 7S.-SehPin«il
 
 
OM se on of abb n b
 
 
70 n show dcTelnpmcnt of
 
 
heart: A. embryonfc
 
 
 
 
■ > e[> ad oHl: fl. more
 
 
kdvanced bUrc, the h[
 
 
an hnnp euro p«rt y f d d
 
 
C Ep an hnoplourc folded
 
 
In to furra gul-tra«. th
 
 
 
 
e <ancr Slrahl).
 
 
 
the heart-tube is separated from the bwly -cavity (or coelom)
on each side by a layer of the mesoderm, and that these two
layers connect the heart dorsiilly with the gut-tract and
ventrally with the iMxly-wall, forming rcs|iectivoly the meso'
cardltun anterioa and the mesocardiam posteriua. These folds
temporarily divide the upper portion of the body-cavity into
two lateral jtarts.
 
The disap]>earance of the stratum of mesoderm immediately surrounding the heart-tube and tbo differentiation
of the tissue limiting peripherally the space thus formed,
results in the production of a second larger tube enclosing
the first. The cells of the outer tube become specialized
 
 
 
1^4
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
iiit'i miucle-ceUs, wliicli arc to contitilutc the fiituro heartmiixcle, while those of the inner cylinder Rutteii and assume
the endotheliotd type to become the endocardium. The gniwtli
of c«ntmlly projecting proceBaes from the muscular wall and
th« oiitpoi-kcting of the endothelial tube to cover these
proceHHCB ami line the spaces enclosed by thera foreshadow
thv tpongy churjctcr of the inner surface of the adult heart,
with it8 colnmne camen and omsciili pectinati. It is signifiCaiit, as lihowing tlic contractility of niidiff'erDiitiated proto
 
 
 
plasmic cells, that the heart l)^n9 to pulsate even before
the ap]>carance of any muscnlar tissue in its walls.
 
The upper end of the heart-tube ta(>ers away into the
truncus arteriosus (Fifi. 75, 4), a vessel which bifurcates into
the first jMiir of visceral-arch vessels, while its lower extremity receives llie vitelline veins above referred to. Excessive growth in length, each end of the tube Iwing more
or less fixed in position, necessitates flexion or folding, the
form which the heart-tul)e assumes in consequence being that
of the letter S placed obliquely {Fig. 70, A). The venous
 
 
 
156 TEXTBOOK OF EMBRYOLOGY,
 
the dorsal wall of the body, with the arterial portion ventral
to it, both being brought at the same time into practically
one transverse plane by the headward migration of the
venous, and the tailward migration of the arterial, moiety.
At this time the heart is relatively so large, and the ventral
body-wall covering it so thin, that the organ appears as if
situated outside of the embryo's body (Fig. 62, p. 116).
 
Simultaneously with these alterations in position, the arterial part of the heart is being marked off from the Tenons
segment by a transverse constriction, the former becoming the
ventricle, the latter the auricle or atrium (Fig. 76, A). The
narrow communication between the two is the anricnlar or
atrioventricular canal, which soon acquires the primitive
atrioventricular valves, or endocardial cushions, these being endoeanlial thickenings on the dorsal and ventral walls of the
Ciinal. Growing toward cacli other, the cushions meet and
unite, forming the septum intermedium (Fig. 78, B,/), which
now occupies the middle of the auricular canal, leaving only
its lateral portions jwtulous. The truncus arteriosus becomes
delimited from the ventricle bv a circular constriction, the
fretum Halleri, the proximal part of the truncus arteriosus
dilating somcwliat to constitute the bulbus arteriosus. The
truncus arteriosus divides into the visceral-arch vessels, as
pointed out in the last section.
 
The Metamorphosis of the Single into the Donble
 
Heart. — The heart with but one ventricle an<l one auricle or
atrium is found not only during the early periods of development in all air-breathing vertebrates, but is the permanent
conditi<m in fishes. In the development of the individual,
as in the evolution of the higher vertebrate type, the appearance of the lungs, which replace the branchiaj of fishes as an
aerating apparatus, is accompanied by a division of the heart
into right and left halves for the pulmonary and the general
systemic circulation respectively.
 
The division of the human atrium begins in iho fourth week
with the growth of a per|)endicular ridge from its dorsal and
cephalic walls (Fig. 78, B), this being indicjited externally by
a groove on the outer surface of the corresponding wall of
 
 
 
THE DEVELOPMKST OF THE HEART. 157
 
the auricle. The ridgt', gniwing downward, becomeB the
septum primiun or auricular septum and fuses with the upper
extremity of tiie septum intermediutii of the auricular caiiiil,
thus dividiDi; the atrium into the right aiul left auricles
(Fig. 77). The atrioventricnlar caual, it^ iiiiterior aud j>osterior cushions having united into the scpuim intermedium,
shares in this division, becoming thereby the right and the
left auriculo ventricular orifices. The division of the atrium,
however, is not as yet complete ; a hiatus, the foramen ovale,
exists ventral to tlie free ventral border of the unittnl si>ptum
 
 
 
 
tlonornlrlumcoTreipoailliieH'ith •uricuUr appcndagei r, iruneu* arterlasiu; d,
■nrlculitr canal; c, piimlUve Tcntricte. B, heart of buniBiU embryo of about tbe
flllh week (Hli): <i, left auricle: b, rigbt ■aricle: r. trunctu arteiioaiu; d. IntctOTc; e. rtglit veiitrido : /. led TeDtriplc.
 
 
 
primum and septum intermedium. A ridge grows downward from the roof of the atrium U|H)n the right side of the
septum pHmum and parallel with it; this is the septum
secundum (atrial crescent) and is very much thicker than the
primary septum. Its downward growth continues in such
manner that it comes t<t bound the foramen ovale vontrally
and below, its extremity uniting with the left extremity of
the fold which later becomes the Eustachian valve, and thus
forming the futuiv atmulus oralis. The psirt of the primary
eeptnm which is thus jiartially eiirronnded by the free margin
' the si.'ptuiu secundum [Mjuches into the left auricle to con
 
 
 
158
 
 
 
TEXTBOOK OF EMBRYOLOGY.
 
 
 
stitutc a fiort of valve for the prcvculioii of regurgitation.
At birtli or shortly ufler, the ventral edge of this valve-like
fold unites wiih tlie ventral margin of the foramen ovale,
thus obliterating the latter, the fohl beuoming thereby the
relatively thin floor tif the fossa ovalis of the adnlt heart.'
 
The diTiBion of the ventride, which follows that of the
auricle and whieh is completed by the seventh week, is first
 
 
 
 
Fio, 78.— A, BecllOD of liBarl n
SpUriiira ; 0. interaurlcular sopluni
t, left auricle: f. auricular Miiitl: [
 
 
 
 
ibryo of in mm. (Hts): ci, aeiiluin
c.moulh or Binm reunicnB; (/.right aurtclo;
right ventricle; A, Interventricular septami
t nr hamati ctnliryu of alKiiit Hie flflli week '
 
 
 
itidicate<l by a vertieat groove, the snlcns isterrentricalaris,
seen on both thedor!*!il uml the ventnil siiHiiee tl'ig. 77),
From the internal surface, corresponding to the position of
the sulcus, a median centrally projecting ridge appears aud
devehipg into a septum (Figs. 7K, /i, and 79, t*), whieli, however, i.s ineompkle above and in front. The deficiency thus
left, the ostdiuu interrentriculaie, is obliterated by the tlowngrowth of the aortic septum (Fig. 79, k), upon the coni])letion
' Occasioiiully the foramen ovnla remainit p«tuli)iiit for iicverul weeka ur
months nfter birtli or even thnniKhiHit life. Ab thin pondilion ullowx llie
venous bliHxl lu minifle willi tlie arterial, tlie miKiicu of tlie Ixily U bluish
or rynnntir, uii<) a chliil thus iiltedeil U om.\ I<> )« n - hiii<> baby."
 
 
 
THE DEVELOPMENT OF THE HEART 159
 
of which the original single ventricle is divided into the
right and the left ventricles. While the interventricular
septum of the completed heart is, for the most part, muscular,
that portion of it which is produced by the aortic septum
always remains membranous, constituting the pars membranacea septi of the adult heart. If this septum is incomplete,
as happens occasionally, there is an abnormal communication
between the two ventricles.
 
The tnmcus arteriosus, after having become somewhat flattened, is divided by the growth of a vertical septum, or partition (Fig. 79, «), into the aorta and the pulmonary artery.
The growth of the partition is initiated by the appearance of
two ridges on the opposite walls of the truncus, the ridges
growing toward each other and finally uniting to form the
aortic septum. Two longitudinal grooves which appear upon
the surface of the truncus, following the growth of the ridges
and corresponding in position with them, indicate the division of the vessel into the aorta and the pulmonary artery. The
septum grows downward to meet and unite with the ventricular sej)tum, as indicated above. Though the three septa referred to develop indejwndently of each other there is such
correspondence between them, as to position, that the effect
is as if they constituted one continuous structure.
 
Before the divisicm of the atrium into the auricles, its
walls pouch out on each side to form the auricular appendages, one of which belongs to each future auricle (Fig. 77).
While it is still a straight tube, the heart receives at its
venous extremity the two vitelline veins. Subsequently this
particular part of the atrium is distinguished as the sinus
venosus or sinus reuniens, this being a short thick trunk into
which empty, in addition to the vitelline veins, the ducts of
Cuvier and the umbilical veins. The mouth of the sinus
venosus is guarded by a valve composinl of two leaflets. The
right leaflet or fold is continuous above with a ridge on the
roof of the atrium, the septum spurium (Fig. 78, a). In the
division of the atrium the sinus venosus falls to the right
auricle, while emptying into the left auricle is the single pulmonary vein, which is formed by the union of the four pul
 
 
AradI W«« il^ •nd^si kaslk n— » nab
 
 
 
*f + f
 
 
 
IMJWM rrom iIm- fnjiH iif llie superior vena ca\-a to the (root
Iff llie infrrinr vciui cava.
 
The U-ft Irafict of iho valve at the month of the tdaiis
vcn'iMia hiH-imnii iHrophie, as does also the septuni spiirium ;
tl)'- richt ilrviiW into two partfl, one of which becomes the
EaitAchUn Yalr« iit tlie orifice of the inferior veoa ca\'a,
 
 
 
THE DEVELOPMENT OF THE HEART. 161
 
while the other forms the valve of Thebesius, or the coronary
valve, at the opening of the coronary sinus (the latter being
the persistent lower end of the left duct of Cuvier). The
Eustachian valve serves to direct the blood from the inferior
cava through the foramen ovale so long as that aperture is
present. The single pulmonary vein is in like manner incorporated in the wall of the left auricle, the four pulmonary
veins in consequence acquiring separate oj^enings into that
cavitv.
 
The Valves of the Heart. — Before the division of the
atrium and the ventricle into right and left halves, the atrioventricular canal has the form of a transverse fissure, each
lip of which is thickened into a ridge (Fig. 79, ^4). These
ridges or endocanlial cushions are the primitive valves.
When the atrial partition grows down and the ventricular
septum grows up, their free edges meet and unite with the
ridges, each ridge being thereby divided, on its atrial surface
by the atrial or interauricular septum, and on its ventricular
aspect by the ventricular septum, into a right and a left half
(Fig. 79, B). Since the ridges, at their points of union with
the septa, fuse likewise with each other, the original orifice
is bisected into the right and left auricnloventricular apertures, the only valves of which are the ridges or cushions in
question.
 
To trace the further development of the fully formed
valves, it will be necessary to consider the changes which
now take place in the walls of the heart. It has been seen
that the inner surface of the heart accjuires a spongy or
trabecular structure at a very early stage by the inward projection of nmscular processes from the outer tube and the
pouching out of the inner endothelial tube to cover these.
The wall of the ventricle in consequence is relatively very
thick and is made up largely of a network of fleshy columns,
the spaces of which network are lined with the endocardium
(Fig. 80, A). While the outer stratum of the ventricular
wall now becomes more compact by the thickening of the
trabeculae — and, to some extent, l)y their coalescence — the
 
trabeculae in the vicinity of the atrioventricular valves dill
 
 
 
162
 
 
 
TEXT-BOOK OF E.MBJiVOLOOr.
 
 
 
minisii iii thickness and lose their iiniacnlar cliaracter, Iwing
replaced by thin cutiucctive-tissiie cords (Fig. 80, B). That
port iif the vetitridilLir ividl which surround,* the iitrioventriciilur oriliee and to wlnt-h the onducardial enshions or
 
 
 
 
primitive valves are attached, likewise becomes deprived
of ninscle-cells, the remaining connective ti^Bue assuming
tlie form of thin plates. Thc.^TC plates, with the former
endocardial cushions attaehed to their edges, constitnte the
 
 
 
 
Via. tU.-OdieniD ■honing diviBlon ul
Into a'lfW and pnlmnniitT Brtery wUh thi
lateral leaQcla divldini; rtmpectlvely Inlo
 
 
 
permanent auricnlorentricalar valve-leafleta. The strands of
connective tissue mentioned above as remaining after the
degeneration of certain of the muacle-traheculffl arc the
chords tendineae of the adult heart. Attached at one end
 
 
 
ALLANTOIC ANJ) PLACENTAL CIRCULATION. 163
 
to the valve-leaflets, their other extremity is continuous
with trabeculae that have remained muscular, the adult musculi papillares.
 
The Bemilunar valves of the aorta and pulmonary artery
appear when the truncus arteriosus divides to form those
vessels. The orifice of the truncus arteriosus is provided
with a valve having four leaflets (Fig. 81, A). By the division of this vessel into the pulmonary artery and the aorta
(Fig. 81, £ and C), the lateral leaflets are bisected, the anterior half of each, with the anterior leaflet, going to the anterior vessel — the pulmonary' artery — while each posterior or
dorsal half, with the dorsal leaflet, falls within the orifice of
the aorta. The resulting disposition of the segments of the
aortic and pulmonary valves is such that, in the aorta, two
leaflets are situated anteriorly and one posteriorly, while in
the case of the pulmonary artery these conditions are reversed
(Fig. 81, C). In the fully developed heart, however, it is
found that the aorta has two posterior leaflets and one anterior, and that the pulmonary artery presents one posterior
and two anterior segments. In the division of the truncus
arteriosus, the anterior half, or the pulmonary artery, falls
to the right ventricle, and the posterior trunk, the aorta, to
the left ventricle, the two ventricles lying side by side. In
order, therefore, that the ventricles may accjuire the relative
positions which they hold in the adult there must be such a
rotation that the left ventricle comes to lie behind the right.
This rotation of the heart from right to left necessarily alters
the relation of the pulmonary artery, causing it to lie not
directly in front of the aorta, but in front and to the left.
If one conceives of a rotation of the two vessels from right
to left through an arc of 60 degrees around a vertical axis,
the altered relation of the pulmonary and aortic leaflets becomes at once intelligible (Fig. 81, C'and I)),
 
THE ALLANTOIC AND THE PLACENTAL CIRCULATION.
 
The development of the allantois and its accompanying
system of blood-vessels is simultaneous with the decline of
the yolk-sac and the vitelline circulation. Since the allan
 
 
164 TEXT-HOOK OF EMHUYOLOCY.
 
tois is an e vagi nut ion from the giit-trart (see p. 89), it is a
Bplanchiipleuric sac, its walls consisting tlierefore of an entodermic and a mesodermic layer. Blood-vessels develop
within the mesodermic stratum as extensions or branchea
of previously existing intra-embryonic tninks. These vessels are the aUantoic arteries and veins. The two allantoic
arteries are branches of the primitive aorta and leave the
body of the fetus, in compiuiy with the neck of the allantois,
at the nmbilicus. Having reached the peripheral jiart of the
allantois, they break np into a capillary plexus, tlie extension
of which into the villons processes of the false amnion completes the union of that strncture with the allantois to form
the trne chorion (Plate III.),
 
Tlie two allantoic Teina develop prxri passu with the arteries
and con\ey the blood from the chorion to the fetus. Entering the body of the fetus through the still lai^ umbilieiil aperture, they find their way along the intestinal tube
tn the septum transveisom — which structure may be regarded
as the jiriniitivc iliaphrngm — to the region of the heart,
where they <ii>en into the ducts of Cuvier. Each duct of
Cuvier (Fig. 84, A) is Ibrmed by the union of the primitiye
jugular vein with tbe cardinal vein of its own eidc, the cardinal and the jugular veins returning the blimd respectively
from the lower and upper [wirts of the trunk. Tliis system
of blood-vessels constitutes the allantoic circulation ; it is of
great importance in any ovum that is developed outside of
the body of the mother, as in the case of birds, reptiles, and
fishes, in which classes the allantois is the organ of nutrition
from the time that the yolk-sac ceases lo ]>erform that function until development is complete. In man, however, aa in
all other mammals except the monotremes and marsupials,
the aliant^iic circulation may be looked upon as, in a measure,
rudimentiry, since it serves to convey nutriment from the
chorion to the fetus only until the formation of the placenta.
 
The placental system of blood-yessels, apf>cHring
in the thiol month willi the ilcvclopment of the placenta,
includes th(^ principal trunks of the former allantoic system,
the allimtoic arteries and veins having Iwcimie the umbilical
 
 
 
THE FETAL ARTERIAL SYSTEM, 165
 
▼esaels. The two umbilical arteries convey impure blood
from the fetus to the placenta, where it circulates through
the capillaries of that organ and receives oxygen and nutriment
from the blood of the mother. As before stated, there is no
intermingling of the fetal and the maternal blood, the two
currents being separated by the very thin walls of the capillaries, through which osmosis occurs. The purified blood
returns to the fetus through the umbilical veins and reaches
the right auricle through the inferior vena cava, a portion of
it having passed through the liver. The two umbilical veins
which are present for a time fuse subsequently to form a
single vein. The complicated details of the arterial and the
venous trunks, and the relation of the latter to the development of the liver and its special system of vessels, may be
advantageously considered in separate sections.
 
THE FETAL ARTERIAL SYSTEM.
 
The truncus arteriosus, the large artery which arises from
the, as yet, undivided ventricle of the heart, bifurcates into
two trunks, the first pair of visceral-arch vessels (Fig. 82, 4).
These first visceral-arch vessels, also sometimes called the
first aortic arches, run from the ventral surface of the body
along the first visceral arches, toward the dorsum, where
they curve downward and pass caudalward, one on each side
of the median line, in front of the primitive vertebral column.
Very soon there arise from the truncus arteriosus below the
point of origin of the first vessels, four^ additional pairs of
visceral-arch vessels, which similarly pass dorsad along the
corresponding visceral arches, and which unite with the
dorsal part of the first pair to form the primitive aorta of
each side. Each primitive aorta results, therefore, from the
confluence of all the visceral-arch vessels of its own side
(Fig. 82). The two aortte afterward become merged into a
single trunk. At first the principal branches of the aorta
are the vitelline arteries. As these latter vessels become
 
^ It is sonietinies stated that there are six viscenil-arch vessels, the fifth
of which disappears, so that the vessel here designated the fifth would rei>resent the sixth-arch vessel of the earlv condition as well as of lower forms.
 
 
 
!(>«
 
 
 
TEXT-BOOK OF EMHRYOLOGY.
 
 
 
in«inspicuou8, the allantoic or umbiliciil arteries come into
prominence as the chief branches. Iinleod, thp iimhilical
arteries may be said to be tlie coiitiniintion of the aorta, since
the largest part of the hlooH-stream is ilivcrt«l into them.
The nurta profxr continues in the median line as the caudal
aorta, which latter is represented in the adult by the middle
sacral artery. A branch from the fiftli arch goes to tlie lungs.
So I'lir till' arterial f-ystem of the fetu.s presents an absolutely symmetrical armngenient (Fig. 82). Changes very
soon occur, however, which lead to the asymmetrical conditioD
 
 
 
 
Fto. KL— DlwraTiis illiistmliriE arranm
 
 
nil-Ill nf prtnimvi)
 
 
lieBit Knd aorUa
 
 
•Tch« Imodlfled rrom Allfd Thoinsonl t
 
 
vIMIllne Ttln* reii.
 
 
rntng blood from
 
 
vucular area; 2, ieni>ii« AesmeDt of hvart
 
Qbe: 3, primitive te
 
 
lricle;«, irunciw
 
 
 
 
 
 
iHlon of double
 
 
Aiirtie M veHcli to csudul pole ol embrro :
 
 
. vllolllno ■rttTl™ r
 
 
 
 
VUG alar «ra».
 
 
 
 
 
 
 
found in the adult. These changes are due to the atrophy
of sonic trunks and the preponderance of others. From the
point where the dorsal extremity of the fourth arch joina
the fifth, a branch pa.sses to the rudimentary arm (Fig. 83).
The first and second arches, except their ventral and dorsal
limb.s. undergo ati-opliy. The ventral limbs of the first and
eecond arches persist and become tlie external carotid artery,
while their dorsal extremities, with ttic tliird viscend-arch
vessel, become tlic Internal carotid artery. The vcnlnil stem
of the third arch coic^titiitcs the common carotid. The right
 
 
 
THE FETAL ARTEIIIAL SVSTE.V.
 
 
 
167
 
 
 
fonrtli-arch vessel hcroTncs tlic liglit BubclaTian, its stre&ni of
Iilowl hc'uv;; .■onvcycd in the arm Ity ttio hrancli which Ims
takcu its origin from tlie [xniit of junction of thi; dorsal t'DtU
of the fourth and fifth arches. This latter branch is therefore the continuation of the subclavian. The ventral segment of the right fourth arcli would be represented in the
 
 
 
 
The foorth arch of the left
• into tile thorax, itbe
 
 
1 adult by the innominate artery.
B assuraea a lower position ;
I comes the arch of the aorta. Sinee therightflftharclibecomes
I atrophic beyond the point of origin of the right pulmonary
[ artery, the dorsil end of the right fourth-arch vessel — tlie
[ future right subclavian artery — loses its connection with the
k primitive aorta, and the latter now a]>)>ears as the continuaF tion of the left fourth arch. The ventral stem of the left
L third arch, which becomes the future left common carotid, Mi;d
 
 
 
M-^ -.L
 
 
 
Jj
 
 
 
168 TEXTBOOK OF EMBRYOLOGY.
 
alwo the left subclavian, ivhicti arises from the posterior or
tiomil onil of thi' left fmirth ardi, nn? noM- branches of the
arcti of llie nwlu. \\\wn llic tniiicus arteriosus becomes
divkleil into the aoria anil the piilmnniiry artery, the left fiftharch vessel and the right pnlnionarv- artery are the only
branchou of the trnncus that fall ti> the pulmonary artery,
all the other viscer.il-arcii vessels W-'m^ connected wilh the
aorta. Tlie left ftflli Tisceral-arcli vessel, therefore, is represontinl in the adult hy the pnlmonaTy artery and the ductus
arterioaua. The fchd Kings lieing impervious, only a very
sniiill piirt of the lilood of the pahnonury artery is sent to
them. Tiie larger ]Kirtioii of the hiiMxl passes fnmi the pnlnionarv artery to the aorla through a communicating trunk,
the ductus arterioauB, wliich represents the greater part of the
left fifth iireh and whiih lM?eomes imjwrvioiis after birth with
the establishmeut of the proper pidmonarv circulation.
 
These tranafiiimations afford an explanation of the different
relations of the recurrent laryngeal nerves of the two sides.
At first they are symmetrically arranged. The pnenmogastric nerve, as it crosses the ftinrlh visceral-arch vessel,
gives off the recurrent laryngeal nerve, the latter winding
around the artery from before backward on its way to the
larynx. When the left fourth areh becomes the arch of the
aorta and sinks Into the cheat, the nerve is carried with it ;
hence after this time, the left nerve is found winding around
the arch of the aorta.
 
Anomsloua airanEements of the branches of the aortic
arch, as well .ns of the areh itself, are referable to anomalous
development of the original system of visceral-arch vessels.
For example, if the right fourth arch, which usually becomes
liie right snholavian artery, be suppressed from its origin to
the point where the anery for the right upper extremity is
given off, the bliKMl must find its way into the latter vessel
through the dorsal stem of the fourth arch, and this doi-sal
stem will then become the right snbclavian artery. In such
case, the right subclavian of the adult will be found to arise
from the left extremity of the artih of the aorta and to pass
obliquely upward to the right side of the neck behind the
tracliea and the esophagus.
 
 
 
THE FETAL VENOUS SYSTEM. 169
 
THE FETAL VENOUS SYSTEM.
 
The venotlS Sjrstem of the embryo presents several successive phases, corresponding in part with the various stages
in the evolution of the arterial system. The first trunks to
appear are the vitelline veins. These vessels have their origin
in the vascular area on the wall of the yolk-sac in the manner
already described in connection with the vitelline circulation.
The two vitelline or omphalomesenteric veins, which result
from the convergence of all the venous trunks of the vascular area, follow the vitelline duct into the body of the
embryo through the still widely open umbilical aperture and
take their course head ward along the intestinal canal to open
into the caudal end of the primitive heart-tube (Fig. 82,1, 1).
At a later period they open into the sinus venosus of the heart,
and still later, when the sinus venosus becomes a part of the
general atrial cavity, into the atrium itself. Near their termination these veins communicate with each other by anastomosing trunks that encircle the future duodenal region of the intestinal tube. As the yolk-sac diminishes in size and importance, the vitelline veins decrease in caliber, and the umbilical
veins, conveying blood from the allantois and subsequently
from the placenta, functionally replace them. The proximal
parts of the vitelline veins have an important connection
with the circulation of the liver, as will be seen hereafter.
 
The umbilical veins, which are developed in the mes(Klermic
tissue of the allantois, pass from the placenta along the
umbilical cord and, entering the fetal body at the umbilicus,
run at first along the lateml, and later along the ventral,
wall of the abdomen toward the heart. Meanwhile there
liave been established a pair of venous trunks, the primitive
jugular or anterior cardinal veins (Fig. 84, A), to return the
blood from the head and the upper part of the trunk ; and a
second pair, the posterior cardinal veins, which bring the
blood from the lower part of the trunk, and especially from
the primitive kidneys. The primitive jugular vein — which
represents the external jugular^ of the adult — passing down
^ According to Salza (ol)scr nations on guinea-pig) and MaU (observations
on human embryo) the external jugular is a secondary vein and the primitive jugular becomes the adult internal jugular vein.
 
 
 
170
 
 
 
TKXT-BOOK OF KMHUYQLOGY.
 
 
 
ward along the dorsal region of the neck, nipets the cardinal
vein of it» own aide and unites with it near the heart, tlie
short thick trunk thus formed i>eing the duct of Cnvier. The
right and left ducts of Ciivier converge and open tt^ther
into the sinuB veuoans (siiiiis reunieus) of tlie heart, which
nlao now receives the vitelline veins and the nnihilical veins.
Upon tlie development of Ihc upper and the lower limhs, the
(posterior) cardinal vein appears as if formed by the confluence of the internal and external iliap voin.s, while the primitive jugular below the entrance of the subelaviiin vein is
designated, with the duct of Cuvicr, the superior vena cava,
since, owing to the preponderance of the jugular over the
cardinal vein, the Cuvierian duct appears to be a direct continuation of the jugular. At this time, then, there are two
superior venre cava;, the terminal parts of wliieh, however,
are not exactly symmetrieid, since the left passes around the
dorsal or posterior wall of tlie atrium, owing to the rotatioQ
of the heart from right to left.
 
The lower venous tnmks likewise jjresent a symmetrical
arrangement. The bilateral symmetry of this stage of the
vonuns system, while permanent in fisliec, becomes modified
in man to produce the familiar asymmetrical condition of the
adult venous trunks by two factors principally — first, the
development of an unpaired vessel which is to constitute a
part of the inferior vena cava, and second, the atrophy of
certain vessels and parts of vessels with a consequent diversion of the major part of their blood-stream into other channels. Associated with these alterations is the evolution of a
special set of bIo< id -vessels, the portal venous sfBtem, for the
supply of the deveh)ping liver. The development of the
portal system, however, may he deferred for separate consideration (see [tage 177).
 
Wiien the sinus venosns becomes a part of the atrium —
constituting tliat jiart uf the wall of the adult auricle whieb
is destitute of musculi pectinati — the two ducts of Cuvier, or
the superior cavse, as well as the veins from the atKloniinal
viscera, open by separate orifices into the atrial cavity. An
nnpaiied vessel now develops below the heart in the tissue be
 
 
THE FETAL VENOUS SYSTEM,
 
 
 
171
 
 
 
tween the primitive kidneys (Fig. 84, A, 1). This vessel is
described as growing downward from the ductus venosusnear
the point where the latter vessel is joined by the right
hejmtic veins (p. 180). It is also described (Lewis) as being
formed by the enlargement of the right subcardinal vein, the
subcardinal veins being themselves produced by longitudinal
anastomoses between veins on their wav from the mesentery
to open into the respective cardinal veins. Tiie vessel in
 
 
 
 
Fig. 84.— Schematic representation of the human venous system, with three
successive stages of development (after Hertwig): 1, vena cava inferior; '2, cardinal veins; 3, vena azygos major; 4, vena azypos minor; .">, renal veins; 6, external
iliac vein; 7, internal iliac vein; H and 9, common iliac veins; 10, early superior
vena* cava?; 11, ducts of f'uvier; IJ, primitive jugular vein; 13, internal jugular ;
14, subclavian vein; l."> and IG, right and Irft innominate veins: 17, vena cava superior; 18, coronary vein; ID, duct of Arantius; 20, hepatic veins.
 
question constitutes the upper or cardiac .segment of the
inferior vena cava. The lower extremitv of this trunk
anastomoses by two transverse branches with the right
and the left cardinal veins (Fig, 84, 7?). The cardinal
veins of the two sides are furtlu^r connected bv a transverse trunk at their lower extremities and bv one that
passes across the vertebral column just below the heart.
In like manner the two su{)erior vena) cavje commu
 
 
17:
 
 
 
TKXT-BOOK OF EMBRYOLOGY.
 
 
 
nicatc with each other by a transverse vessel, the tranflverae
jugular vein, at the upper jmrt of" the thorax, above the arch
of the aorta. With the exception of the unpaired trunk
which is destined to constitute the upper port of the inferior
veua cava, the arrangement of the veins at this time is absolutely symmetrical. The apparently meaningless asymmetry
of the adult venous trunks is easily accounted for if one
notes the alterations in the course of the blood-current wliich
now occur.
 
The blocKl-strcam of the left superior vena cava gradually
becomes entirely divertetl into the right cava through the
transverse jugular vein, and the part of the left cava below
this point, being now functionless, shrivels to an impervious
cord (Fig, 84, C). This cord or strand of tissue, the remnant of the left superior cava, is found in postnutjd life, in
front of the root of the left lung, embedded in a fohl of the
eerous layer of the pericardium, the so-ealletl Testigial fold
of Marshall, Since the left superior vena cava receives, near
its terminalion in the auricle, the lai^e coronary vein, which
returns the greater part of the blond from the heart-wall,
this ])roximal extremity of the left cava persists as the
coronary sinua of the heart. The transverse communicating
trunk — the transverse jugular vein — and the part of the left
cava above it now constitute the left imiominate vein, the
course of which from left to right is thus exphiincd. The
left superior vena cava of tlie fetus is represented in the adult,
therefore, by the sinus coronarius, by the atrophic impervious
cord lying in Marshall's vestigial fold, by the vertical part
of the left innominate vein and by a part of the left superior
intercostal vein.
 
The lowest connecting branch between the cardinal veins
enlarges and conveys to the right enrtiinal vein the blood
from the left internal and external iliac veins (Fig, S4j, in
consc<|uennp of which the part of the left cardinal vein
bctiiw ihu kidney undergoes atrophy and, finally, complete
ohliloralitiu. The newly-formed transverse trunk is the left
common iliac vein. The part of each cardinal vein above
the renal region suffbi's an arrest in growth, in consequence
 
 
 
THE FETAL VENOUS SYSTEM. 173
 
of which the blood is diverted from these veins into the
transverse anastomosing branches before mentioned as connecting the respective cardinal veins with the lower end
of the unpaired caval trunk (Fig. 84, B and C> 5). As
a result^ the lower half of the right cardinal vein, now
receiving at its distal end the two common iliac veins, becomes directly continuous with the unpaire<l caval trunk, and
with it constitutes the inferior vena cava. The inferior vena
cava, therefore, is partly an independently formed structure
and is partly the greatly developed lower half of the right
cardinal vein. The upper half of the right cardinal vein,
conveying now a relatively small part of the blood-stream,
becomes the vena azygos major, the termination of which in
the superior vena cava is explicable when it is borne in mind
that the cardinal and the primitive jugular veins, by their
confluence, form the duct of Cuvier.
 
While no part of the right cjirdinal vein suffers complete
effacement, the left one, in a part of its course, entirely disappears. All the blood of the left external and internal iliac
veins being transported to the right side of the body through
the lowest transverse^ trunk — that is, the newly-formed left
common iliac vein — the part of the left cardinal vein Ix'low
the kidney retrogrades and disappears. The part of the left
cardinal above the renal region lagging behind in growth,
the blood from the left kidney is conveyed to the inferior
vena cava bv the transverse trunk that (M)nnects the cardinal
veins in the renal region ; this transverse trunk becomes, therefore, the left renal vein. Sinee the spermatic veins originally
emptied into the cardinal veins, it is found, after these transformations, that the right spermatic opens into the inferior
vena cava, while the left spermatic is a tributary of the left
renal vein. Some anatcmiists, indeed, regard the left spermatic vein as the representative of the lower part of the left
cardinal vein of the fetus.
 
As the left renal vein develops into the channel for the
major part of the blood from the left kidney, the portion of
the left cardinal vein above this {)oint remains an inconspicuous vessel, and that part of it intervening between the
 
 
 
174
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
duct of Ciivicr and tlie cross branch (Fig. 84, C, 4) situated
immediately below the heart undergoes total obliteration. The
blood ascending through the persisting part of the left cardinal vein must therefore pass across to the upper part of the
right cardinal vein, now the vena azrgos major; and the
pervious portion of the left cardinal vein, with the transverse trunk referred to, constitutes the vena asygos minor.
 
 
 
The development of the pericardium is so mtimately related with that of the pleuree ind of the diaphragm that an
account of it invohc- a de-icription if the c\olution of tho»e
structures. By wa\ of fatilitatuig a cc m prehension of tlie
rather complicated details of tlie procc ^ the reader is reminded that the tube which constitute-, the primitive heart
is formed by the coalescence of the t«o tubes prodiced
within the splanchnic mesoderm and that this tube and also,
for a time, the heart resulting trom it ire embedded vMthin
the ventral mesenterj and further that the part ot the
ventral mesentery connecting the heart mth the central
 
 
 
 
body-wall in the mesocardium anterius \tliile the fdd passing
from the heart td the jut tnct is the mesocardium posterius
(Fig. 8.'i, A, and F jr 73 () Tht '•| ice betw n the he irt
and tlie Ixxly-wall is a part of the body-cavity or cwloin
(throat-cavity of Xcllliker, parietal cavity of His). The
 
 
 
THE FORMATION OF THE PERICARDIUM. 175
 
first indication of the separation of this space from the future
abdominal cavity is furnished by the appearance of a transverse ridge of tissue growing from the ventral and lateral
aspects of the body-wall. This mass is the septum transversum. It bears an important relation to the course of the
vitelline and the umbilical veins. As the veins diverge
from the bodv-wall to reach the heart, thev carrv with them,
as it were, the parietal layer of the mesoderm in w-hich they
are embedded, forming on each side a fold that projects mesial ly and dorsal ly (Fig. 85, B and C), the two folds approaching and finally meeting witli the ventral mesentery
in the median plane. The septum transversum thus formed
contains in the region nearer tlie intestine a mass of embryonal connective tissue which is called the liver-ridge or
prehepaticus from the fact that tlie developing liver grows
into it. Since the septum transversum, exclusive of the
so-called liver-ridge, is the primitive diaphragm, it will be
seen that the liver, in the early stages of its growth, is intimately associated with the anlage ^ of the diaphragm. The
septum transversum partially divides the body-cavity into a
pericardiothoracic and an abdominal part, as shown in Fig.
85, J5 and C Near the dorsal wall of the trunk, on each
side of the intestine and its mesentery, the septum is wanting, and thus the two spaces communicate with each other
by openings that are known as the thoracic prolongations of
the abdominal cavity. At this stage, then, the four great
serous sacs of the body, the two pleural, the pericardial, and
the abdominal, are indicated, but are still in free communication with each other.
 
The pericardial cavity is the first one of these to be closed
off; subsequently the pleural sacs are delimited from the
abdominal space. Just as the transverse septum, which
partly forms the floor of the thoracic cavity, holds an important relation to the course of the vitelline and the umbilical veins on their way to the heart, so is a vertical septum
 
* Anlage, a German word signifying groundwork, or, in embryology, the
first crude outline of an organ or part, has come into use in English writings upon the subject because there is no exact English equivalent for it
 
 
 
I7»; ii.xr-nooK of kmi:ryology.
 
'\i'/. 'M, .1. f-, -nl,]'}, -'-fftrriK— il,.- yn-rii'apiKil *[«'•<;- iV'-m xi.r
pir-iiral -fiai-i- H"i<'r.HU-i\ «ith th% i<ii~itv/n of a lan;'^ vtin.
'Ilii" ■.•■ill, tt.': duct of Cuvier, fortu^ in the ujip«-r pan c>t'
(111- td'iNix !.y ttn- (■/.nrtm-ri'-'r "f tli*: (ranJJDal anJ tli(- jiij:tilar
■,<i(i-, li'-. at fir-: w.ir rii" 'I'lrsil Ic-ly-wall aii<l tln-n aL.ng
(I-- liiii-ral u-fi'if. In tii<: hiti'-r [Hi^iiiMii it t-ii'-n fli-lit- uimii
lli«- |>l"tir',|.f-ri";ir'lial •fr.n— ari'l i^ oi>vt-re<J bv tlit- M<tuatii- f^r
iiiil i(i—*rfl'-rrii 'Kij^, iff;, ,1.. It ir this inwanlly j)r.>it.it
 
 
 
ii>K!
 
 
 
P
 
 
 
vi'i'lii-iil r<il<l •'!' M-roiiH ini-mbnin': 'Xint;tiiiin<r the duct uf
t'iiT whii'li ('•iij'-liliit''-i rlii- plenropericardial fold iiiul the
niinirii'c "I' wliifh iiiiti:ilr- the liivj-ion of tht- ihi-nicic rav
liilo IW" "IMii-fr-, "til- I'"!- ihi- h'-;irt iiinl "iif fl.r tiit- )iiiitr:i.
,.■ j,l.-iii'<i|M'rl<iiri!iiil ("hi <-..iiiiriii'- to yi""* t'.wani tlu- nie,1, |,l.it... ..I" Ih.' 1..-1.V until il in.-i- Til.: t.i.-i.<Tii-.iiiim iv>^
i»> (Fin. «". ")■ "''■' "'■'''' ■' *''""'"• ''"' "I'l'tin^^ 'lie
 
,i.»itlinl MM* (/.) iiii'l iH.hilinj.' it li-....! ih.. phnnil >|.!icf. (y).
 
Thr Ik'iii'I i« "lill ivliilivi-ly v.ry hir-.' an.l .,.rii|.i..s the
 
,i,UT i«.tl "1" <li" III"""'"' '"'vily, l.iiviiij: .1
 
,..U .11 ,,M..-.-. Hlimt<-1 .i.trMilly. f-r the ri.v.,i,iiii.,.
 
ii« ,i..v..i..i.1m^ liiiir- 'I'l''" '"*"•'" ^1'=""- "" '"■'''"
 
^.,.i...w.l «W...- r-r « 1<'I1K ""»■ '» •" ■'"""'""" «"''
 
.^ i..,.l.ri...iHi1 .iivilv 1.V th." tw.. tlK.ni.ir |.ri.l.ms:!.u,.ii-^ ..f
I... «i-.. wl.lrli li.- '.II.'- 'til »«''!' f*'''*""*" <l'fi'il'>'ii'ii'l tiihc
 
"[. I. «'. i;. *ilUh..w llmt l!i..s..liil..-lik..-^|,;..-..-iir.'
.l-.v> .^.,».:iM-' tw ^fnmt menilTJiii' ini.i th;it llny iir.'
 
 
 
miKii-n11 1 ion
 
imslv
 
 
 
THE PORTAL CIRCULATION. 177
 
entirely distinct from each other. It is evident also, that the
mesial wall of each sjjace is constituted by the mesocardium
posterius and the dorsal mesentery. The hings first appear
as two little sacs, connected by a common pedicle, the future
trachea, with the upper end of the esophagus. As they grow
downward in front of the esophagus and in contact with it,
they push the serous membrane before them carrying it away
from the esophagus (Fig. 86, B)y and thus they acquire an
investment of serous membrane, which is the visceral layer
of the pleura. The layer of serous membrane in contact
with the body-wall is the parietal layer of the pleura. The
lower extremities of the lungs at length come into relation
with the upper surface of the liver, from which organ they
are finally se|>arat(Ki by the growth of two folds, the pillars
of Uskow, from the dorsolateral n»gion of the body-wall.
These folds or ridges projecit forward and unite with the
earlier formed septum transvcrsum to complete the diaphragm. So far, however, the diaphragm is merely connective
tissue, the muscular condition being acquired later by the
ingrowth of muscular substance from the trunk. Occjisionally the dorsal or younger part of the diaphragm fails to
unite with the ventral or older fundament on one side of the
body, leaving an aperture through which a jx)rtion of the
intestine may pass into the thoracic cavity. Such a condition
constitutes a congenital diaphragmatic hernia.
 
The heart and its pericardial sac occupy the greater part
of the thoracic cavity, while the lungs arc merely narrow
elongated organs lying in the dorsal part of this space as
shown in Fig. 86, B. As the lungs increase in diameter,
they spread out ventrally and gradually displace the parietal
layer of the pericardium (Fig. 86, B) from the lateral wall
of the chest, (Towding the pericardium forward and toward
the median plane of the body (see Fig. 86, C) until finally
the adult relationship of these structures is established.
 
THE PORTAL CIRCULATION.
 
The circulation of the adult liver is peculiar in that the
organ is supplied not only with arterial bloo<l for its nutrition
 
12
 
 
 
TEXT-ISOOK OF EMBHVOLOaY.
 
but receives also venous blood laden witli certain products of
digestion obtained frtmi the aliiuentarj' tract, tlie spleen, and
the pancreas. This venoua blood enters the liver thrtj
tin- portal vein ami is designed tfi supply to the gland the
tnat«;riats for the jwrforinance of its fjiecial functiuas.
 
 
 
 
THE PORTAL CIRCULATION. 179
 
duodenum by trunks that encircle the bowel, these connecting vessels collectively constituting the anniilar sinns (Fig.
87, B and C). The liver originates from a small diverticulum which is evaginated from the ventral wall of the intestinal canal. Growing forward between the folds of the
ventral mesentery, this little tubular sac divides and subdivides so as to produce a gland of the compound tubular
type. The developing liver is fn)m the first in close relation
with the vitelline veins and their ring-like anastomosing
branches, and receives its blood-supply from the latter through
vessels that are known as the ven» hepaticsB advehentes
(Fig. 87, 10, 10). These afferent vessels break up within
the liver into a system of capillaries, from which the blood
passes through the efferent vessels, the venae hepaticse revehentes, into the terminal parts of the vitelline veins. Thus
a part of the blood of the vitelline veins is diverted to the
liver and, after circulating through that organ, is returned to
them further on to be conveyed to the heart. As the liver,
with its increasing development, requires more and more
blood, the entire blood-stream of the vitelline veins passes to
it, and the parts of these veins l)ctween tlie vena? hepaticw
advehentes and the vena; hepaticK) revehentes become obliterated (Fig. 87, B and (,■), The vitelline veins, therefore,
leave the intestinal canal at the duodenal region and traverse
the liver on their way to the heart. In this early dage of
the development of the livery then, it receives its nutrition from
the yolk-saCy through the vitelline veins.
 
When the yolk-sack undergoes retrogression, as it does
about the fifth we(;k, the liver must draw upon the allantoic
and the placental vessels for its nutrition. To do this it
must acquire connection with the umbilical veins. The latter
vessels j)ass upward from the umbilicus along the ventral
wall of the body and empty into the sinus venosus of the
heart above the site of the liver (Fig. 87, A, 4, 4). The
umbilical veins effect communications beneath the liver with
the vente hepaticaj advehentes from the vitelline veins. At
about this time the left umbilical vein begins to predominate
over the right one, the latter retrograding until, in the umbilical
 
 
 
/ ^
 
 
 
 
 
 
FINAL STAGE OF THE FETAL VASCULAR SYSTEM. 181
 
the cava, whereby its tributaries, the vena) hepatiese revehentes, come to empty into the cava, the downwanl growth
of the latter carrying downward likewise the terminations of
these veins to their normal position as the hepatic veins
emerging from the dorsal surface of the liver. Meanwhile
the volume of blood flowing through the umbilical vein has
increased to such an extent that the liver is no longer able to
transmit it to the inferior vena cava, and consequently a part
of this blood passes through the ductus venosus, which extends from the portal fissure, along the dorsal surface of the
liver. The blood of the umbilical vein is divided, therefore,
into two streams — one that enters the inferior vena cava directly through the ductus venosus and one that traverses the
liver on its way to the cava.
 
The portal vein results from the persistence of a part of
the vitelline veins. The vitelline veins, as we have seen,
anastomose with each other by two ring-like branches that
encircle the duodenum. The right half of the lower ring
and the left half of the up|>er one atrophy, so that the blood
of the vitelline veins makes its way to the liver through the
left half of the lower ring and the right half of the upper
one (Fig. 87, D). The left half of the lower ring and the
now united portions of the right and left vitelline veins immediately below constitute the superior mesenteric vein, which
passes in front of the third part of the duodenum, as in the
adult^ and which is later joined by the splenic vein ; while the
anastomosing portion of the loop and the right half of the
upper loop become the portal vein. So long as the yolksac is present, the vein receives blood both from it and
from the walls of the intestine. After the disappearance of
the yolk-sac, the intestinal and the visceral veins are the sole
tributaries of the portal vein.
 
THE FINAL STAGE OF THE FETAL VASCULAR SYSTEM.
 
The circulation of the fetus at birth and the changes ensuing immediately thereafter may now be easily understood.
The fetal blood being sent to the placenta through the hypogastric or umbilical arteries, receives oxygen there and is
 
 
 
FINAL STAGE OF THE FETAL VASCULAR SYSTEM. 183
 
returned to the body of the fetus through the umbilical vein.
The latter vessel takes its course upward along the ventral
wall of the abdomen to the under surface of the liver, lying
here in the anterior part of the longitudinal fissure. In this
position the blood-stream of the umbilical vein is divided
into two parts, one of which unites w^th the fetal portal
vein to enter the liver, while the other passes through the
ductus venosus directly to the inferior vena cava. The
blood which enters the liver, after traversing that organ,
reaches the inferior vena cava through the hepatic veins.
Thus, in the one case directly, in the other case by passing
through the liver, all the placental blood reaches the inferior
vena cava and passes on to the right auricle of the heart.
 
From the right auricle the blood passes through the foramen ovale to the left auricle, and thence, through the mitral
orifice, to the left ventricle. Being driven from the left ventricle into the aorta, it is conveyed through the branches of
the aortic arch to the head and the upper extremities. Finding its way into the veins of these parts, it is returned,
through the superior vena cava, to the right auricle, from
which cavity it passes, through the tricuspid orifice, into
the right ventricle. From the right ventricle it goes into
the pulmonary artery. Since the lungs are not as yet pervious, or but very slightly so, the current is deflected almost
entirely through the ductus arteriosus to the descending aorta
instead of going to the lungs. Some of the blood of the
descending aorta is distributed to the various parts of the
body below the position of the heart, while some of it is
sent through the hypogastric or umbilical arteries to the placenta for aeration! It is evident that no part of the fetal
blood, except that in the umbilical vein, is entirely pure, the
venous and the arterial blood being always more or less
mixed.
 
With the detachment of the placenta at birth, several
marked alterations occur. The circulation through the
umbilical vein ceases, that part of this vessel which intervenes between the umbilicus and the portal fissure of the
liver becoming, in consequence, an impervious fibrous cord,
 
 
 
184 TEXT-BOOK OF EMBRYOLOGY.
 
the round ligament of the liver. The ductus venosus likewise suffers obliteration, becoming the ligamentum venosum
Arantii. Since the lungs now assume their proper function
of respiration, the communication between the right and the
left sides of the heart and also that between the pulmonary
artery and the aorta cease. Hence, the respective avenues
for these communications, the foramen ovale and the ductus
arteriosus, become obsolete. There being no further need for
the hypogastric (umbilical) arteries, the circulation through
them ceases, and they become mere cords of fibrous tissue,
whose ])resence is evidenced by two ridges in the j>eritoneum
on the inner surface of the anterior wall of the abdomen.
The proximal parts of these arteries persist, however, as the
superior vesical arteries.
 
 
 
CHAPTER XI.
THE DEVELOPMENT OF THE DIGESTIVE SYSTEM.
 
The adult digestive system consists of the mouth with its
accessory organs, the teeth, the tongne, and the salivary glands ;
of the pharsmx, the esophagus, the stomach, and the small and
the large intestine, including also the important glandular
organs, the liver and the pancreas. Notwithstanding the
apparent complexity of its structure, the alimentary tract
may be regarded as a tube, certain regions of which have
become specialized in order to adapt them to the performance of their respective functions, tiie salivary glands, the
liver, and the pancreas being highly differentiated evaginations of its walls. While in man and in the higher vertebrates the tube is thrown into coils by reason of its excessive
length, in the lower-type animals it is much more simple in
its arrangement. For example, in certain fishes and in some
amphibians the alimentary tract has the form of a slightly
flexuous tube, the deviations from the simple straight canal
being few and insignificant, and the stomach being represented by a local dilatation of the tube.
 
The simple condition obtaining in the representatives of
the animal kingdom referred to above suggests the likewise
simple fundamental plan of the human embryonic gut-tract.
There is, in fact, a period in development when the gut-tract
of the human embryo has the form of a simple straight tube.
The processes incident to the formation of this tube mark
the earliest stages of the development of the alimentary system, the tube itself acquiring definite form simultaneously
with the production of the body of the embryo.
 
The first indication of the alimentary canal appears at a
very early period of development, being inaugurated in fact
by those important alterations that serve to differentiate the
 
185
 
 
 
186 TEXT-BOOK OF EMBRYOLOGY.
 
blastodermic vesicle into the body of the embryo and the
embryonic appendages. It will be remembered that, after
the splitting of the parietal plate of the mesoderm into its
two lamellae, and the union of the outer of the layers with
the ectoderm and of the inner with the entoderm to form
respectively the somatopleure and the splanchnopleure, these
two double-layered sheets undergo folding in different directions. Before the folding occurs, the germ is a hollow
sphere whose cavity is the archenteron and whose walls
are the somatopleure and the si)lanchnopleure.^ While the
somatopleure in a zone corresponding with the margin of the
embryonic area becomes depressed and is carried under that
area to form the lateral and ventral body- wall of the embryo
(Plate II., Figs. 2, 3, and 4), and also more distally folds
up over the arcji to produce the amnion and the false amnion,
the splanchnopleure, likewise in a line corresponding with
the ivriphery of the embryonic area, is depressed and carried
in^-^r^i fr^>m all sides toward the position of the future
wimWlk**!?^ This folding in of the splanchnopleure effects
t>K^ <^iviMi>n of the archenteron into two parts, a smaller
v^x-itv t^Uiuj; within the body of the embryo, which latter
iv "SNJnvi^nf «it x\\o same time, and a larger extra-embryonic
^\s*^Yv^itiiw'^t. whioh is the yolk-sac or umbilical vesicle. The
Jk-mj -^H*^*;>Nn \>s^^u> \>rtvity is the gut-tract. The constricted
,ss4*,**w*s>ji^v»\^^ Uiw^MMi the two is the vitelline duct. While
»K s;',v''''^^ v^^^'' is still a nither wide aperture, the anterior
k^isi 'Ns<*v.-«NSi jv^rts of its intestinal orifice are designated
vv^NNv*\v^\ vKs^ i^Wrtor and the posterior intestinal portals.
 
Vv'*K^ ^NWiMvv^vKsm* oK>st*s in around the vitelline duct, it
v^^a. iW NV.4H wt' iho abdomen, the opening left, which is
• *.o V <nnI '^^ » ^K^ vi\K U Iviutf the umbilical aperture.
 
U X . \i\Wii\ \\w\x(\>xy> timt the lining of the gut-tract is
 
v.. i.s.AxI N> ihs* »uhonmv»t giTm-layer, the entoderm, and
 
*Iw * i ^\ - V jmlu l»ul clcnunits art* consequently of entodermic
 
M^ »4 Mk loKliu^ ill of the splanchnopleure begins at
 
I'v'n iii^ . uvl vU ihv^ <%HHMul week, and is so far advanced
 
•M'^'i.s 4>. A.»«^. iho -«k44»Hl\«|4%nm« nml the splanchnopleure are not
(\> .: . .1 \>- , vU« ix*UU«(^ NS\ut«k bul lU«» |mK*v88es go on at the same time.
 
 
 
 
no. W.—RHUtisl ructions nf humaq emhrro of ibuut MvuuWen dayi (Ula) ; dd,
optic and u(, otic veakloi: ne, nr', nolochord : Mg. hcod-gut; g. mid-gut ; A0. hludKUt; It, Titclllne aBi:; 1. liver: V, lu, ijrlmUlve vcnlrlde and truncu* >HeHcwu>;
ni, da. TCDtral and diiraal aortie; on. Hnrlic aruhes; jv, prluitlva jugular vein; cv,
cardinal vein; dc. duct of CuvIef; iii'. h>i, umbilical vein and artery: ol, allanUli:
 
 
 
before the end of the third week that the arehenteron
nitcly divided int^i the gut-tract and the yolk
 
 
M
 
 
 
1«6 TEXT-liUUK OF EMBRYOLOGT.
 
blastodermic vesicle into the body of the embryo and the
embrj'onic apiwndages. It ^v■i^ be remembere<l that, after
the splitling of the parietal plate of the mes<xiemi into its
two lamellje, and the union of the outer of the layers with
the ectoderm and of the inner with the entoderm to form
respectively the somatoplenre and the spUnchnopleure, these
two double-layered sheets undergo folding in different directions. Before the folding occurs, the germ is a hollow
sphere whose cavity is the archeutcrun and whose walls
are the somatopleure and the splanchnopleure.' AVhilc the
somatoplenre in a zone corresponding with the margin of the
embryonic area becomes depressed and is carrie<l under that
area to form the lateral and ventral body-wall of the embryo
(Plate II., Figs. 2, 3, and 4), and also more distally folds
up over the area to produi-e the amnion and the false amnion,
the aplanchnoplenrc, likewise in a line corresponding with
the periphery of the embryonic area, is (iepressed and I'arried
inward from all sides toward the position of the futnre
umbilicus. This folding in of the splauchnopleure effects
tlie division of the archenteron into two jjarte, a smaller
cavity falling within the body of the embryo, which latter
is forming at the same time, and a larger extra-cnibryonic
compartment, which is the yolk-sac or nmliilical vesicle. The
intra-embryonic cavity is the gut- tract. The constricted
commnnioatiou between the two is the vitelline duct. While
the vitelline duct is still a rather wide aperture, the anterior
and posterior parts of its intestinal orifice are designated
respectively the anterior and the posterior intestinal portals.
 
As the somatopleure closes in artjund the vitelline duet, it
forms the wall nf the abdomen, the opening left, which is
traversed by the dnct, Iwirig the mnhllical aperture.
 
It is evident therefore that the lining of the gut-tract is
constituted by the innermost genn-layer, the entoderm, and
that all its epithelial clementA are consequently of entodermic
origin. The folding in of the splanehnopleurc begins at
about the end of the second week, and is so far advanced
 
' Strictly Hi«nkin(t. llie somntopleurc and tlie splanchnopieiire ire not
formed bf/ore Ibe foldmg ocrurti, but the proceues ){o on M Ihe »aaic time.
 
 
 
188
 
 
 
TEXT-BOOK UF EMBRYOLOGY.
 
 
 
In its earliest definite form, then, the ^t-tract is a tube
extundiug from one end of the embryonic body to the otiier,
whii;h o[)ens widely at the middle uf its ventral aspect into
the vitelline duct, but which is closed at both ends. It is
nsuul U} speak of the primitive gnt-tract as consisting morphihlogically of three jmrts, the head-gut, which is the region
on tlie headward side of the orilice of the vitelline duct;
the hind-pit, wliich is the part near the tail-end of tlie
embryo; and the mid-gut or interveniug thinl portion
(Fig.' 90).
 
The closed head-end of the gtit-tuhe corresponds with the
floor of the primitive moutb-cavity, the two spaces being
separated by a thin veil of tissue, which consists of the
enfotlern) and the ectoderm and is called the pharysKeal
ffiembrane (Fig. 91). A considerable pniportioii of the so
 
 
 
Pm. tl —Median ncUon Hiroueh Ihe haad nf an embryo nbbit S mm. loDC
(kfUtr Mlbklkovical ; rh, membrane bclween alomiKliEUm and rnrc-eut. pharfngeal
membrane (Baaheiihiiiit) ; hp, place bam whicb the b^papbysis in iJcvelotidl : A.
heart: M, lumen of fore-gut; th. ehonla; v, ventricle or the eerobruui: r", tbird
Tenlriele, tliatoflhebclween-braiii (ihalamencephalou) : r<. n>nrlli vcntritle, Ihat
of the hlnd-braln and afler-braln (epencephalon aniJ ineteowphalon, or medulla
(iblongala): (*, cential canal or the iplnat cord.
 
called head-gilt constitutes the primitive pharynx. This
region of the tube has a relatively large enliher. and presents on its lateral and ventral walls the serios of recesses
or evaginations known as the throat-pockets or pttarTngeal
pouches (Fig. 71).
 
 
 
 
THE DKVELOPMEyr OF TUE VIOKSTIVE SYSTE.V. 1S9
 
While llic inner, entodermic layer of tlic gut-tiilie l)ecomes
tlie inteatinal mucosa, tli<? outer, mesodermic stratum |inulu(
tlie muscular and tlie connecttTe-tiBBne p:u-t^ of tlii! Imwelwall, the must superficial layer uf tliL- latt*;r with ila mesothelial or ciidotholial ooIIh furmiiig the visceral l&yer of the
peritoneum. Since the lueHodermic layer of the gplnnclmupleurc of each side ia coiitiDuoua with the corresponding
mcsodemiic layer of the tiomatoplrure on either side of the
embryonic axis, the primitive intestinal canal has a broad
area of attachment with the dorsal wall of the body-cavity
(Fig. 92). Tiio ventral wall is Iikewirie connected with the
 
 
 
 
Fta. Hi— TtanBvcras seollon of
 
 
 
ventral body-wall throughout the anterior or upper part of
its exteut by the continuity of the splnnchnopleuric mesoderm of each side with the somatopleuriu rucsoderm of the
fjame side. As development advances, the body-cavity increases in caliber more rapidly than does the intestinal tube,
BO that the interval between the two is augmented, in consequence of which the masses of connective tissue uniting the
dorsal and the ventral surfaces of the gut with the
 
 
 
;he I
 
i I
 
 
 
190
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
Hpouiling woUs of the bod>-caMh become drawn out so as to
oontttitute m each case a median vertical foM consibting of
two closely approximated lasers of serous membrane with a
little toiinective tissue between them These folds are the
dorwl and the vsntral msBenteries (Fig 93) While the
 
 
 
 
Frii. Ml.— Uluinunmallr crofis-sectlons of the body of the embrro In the region
of lliu linart nl IbvcI of fliture diaphrtgni : a. eaophagenl Be^nnent of KUt-lnct ; b,
(liirHl inuHntury: r, mt'Bocardliiin poaterlua ; d. meioordluiu BnleriuB: c. be^nnliiii iif H-iiliiiii truiiavvrtnni. conlalnlnu vlt«11tne and allamotc veins ; /, aeptum
traiiavnnuiii ; a- tlioraclv proliiagation n( abdominal cavity ; nc, neural canal,
 
(loroul ni(!ri<-nt<^ry extends throughout the entire length of the
(Hiiml, till- vt'tirral fold in present only at its anterior or upper
IHirt, iiiirn-HjHinding in the extent of its attachment to the
dijii'^livc tiilie to that jwrtion representing the future stomach
and iip]KT jHirl of the duodenum (Fig. 94). The ventral
niirwnU-rv iil tirNi in ]ir('sent throughout the entire extent of
the caiinl, Init very early undergoes obliteration except in the
Hituiiliiiii iibovi' imU'd. ( '(infeniiiig the reason and the method
of ilH diHiippi'iii-iini'i' uotliing is definitely known.
 
'I'hi- intcMliiiid lulu-, iit a romiKiratively early stage, pre(K'ntH on iN ventral Mirliice near the posterior or cuudat end a
smatl evaginiitioii that eidni^s to form the aU&ntolB (see p.
89). While a jKirt of the intra-cnibryonic portion of the
allaiitriis diliilon luid develo[>K into the bladder, the part between this latter anil the intestine is known as the urogenital
alnuB. Tli(' jtiirt <)f the jrut-tiibe posterior to, caiidad of, the
origin of the allantois, is a blind ]ioneli known as the cloa,ca.
The latter is, tlicrefori-, the (Mnnnion termination of the urinan,and the intestinal tnicis.
 
To repeat, we have now, in the thini week of development,
the alimentary canal repn'scntwi by a single straight tube
 
 
 
THE ItEVELOPMEyr OF TUK DIGESTIVE SYSTEM. 191
 
(compure Fig. 90 J, idoried at each eiiJ, bin willi moiitli-cavity
an<l anus bolh Judicated, the tube lying within a larger tnhe,
the body-cavity, with the walla of which latter it is connected
by the dorsal and ventral mesenteries. Along the dorsal wall
 
 
 
 
FlO. !M,~-R«oi>iutrucUuii iif baiiinn embryo of ■ bout leventeon days (sRer Hli]:
00. optic and nf.otic valcles; ne, uotaehori: hdg, head-gut: p, mid-gut: Aj7. hiadgot: vt, vKollIiio hut; <, liver; e, prInilllTe Tenlrtcle: va, da. renlral and donal
«orl«! Jir, primitive Jugblar vein; n. canlinal vein; 'IC. duot of Cuvlcr; iin, ho.
umbilical vein aud artery: a', allanloii: Hi. umbillca) cord: dn. dureal mesGUlery;
tm, ventral miiaentvry (modified from HIb).
 
of the body-cavity, dorsad to the parietal peritoneum, pass
the two primitive aortic, and later, the single aorta which
results from the fueinn of these two. Between the two folds
of the dorsal mesentery pass the blood- vessels that nourish
the walls of the gut. Within the ventral mesentery are the
 
 
 
192 TEXT-BOOK OF EMBRYOLOGY.
 
vitelline veins, which bring the blood from the yolk-sac and
VAnwi^y it to the primitive heart. On the ventral wall of the
gilt in the wide aperture of the vitelline duct. Farther
(JiUHlud, uIho on the ventral surface of the bowel, is the orifice
of the iillantoiH. These conditions may be better understood
by reference to Figs. 90 and 94. Before tracing the further
diivrlopiiicnt of the abdominal part of the alimentary system.
It will be pro[)er to note certain very important processes
|Htrtiiining to its anterior or head-extremity, and also to conNldor the formation of the anus.
 
THE MOUTH.
 
The development of the mouth, the tongue, the teeth, and
ilio lUiUvAnr glands has been fully described on pages 134llil. Ill thin connection, therefore, it will be necessary to
mil iit((*iiii()ii to only a few of the salient features of their
itvoliifloii,
 
Tim oral oaylty is produced by a folding in of the surfaceiMftodnnn, the fortsa thus formed becoming deeper until it
liMMifN llin hcad-itiid of the gut-tract. From the walls of this
limMii (Im« NAlivAry glands are developed as evaginations, in the
liMiiiiMM' iilnuidy <h»Hcribed, while the teeth are specialized
gi'owfliM of itH iM'toderinal lining and of the underlying mesoiJMriii (vlih' p. I.*J7). The first intimation of this infolding
U ii|ipiir(Mif at (ho twelfth <lay in the form of a localized
ihlitkiMiliiK <»'' the HiirfjMuwells on the ventral surface of the
ImmIv oI* \\\\\ oiiibrvo nrar the head-end. The thickened area
in the orftl plato, whieli Hp<»edily becomes depressed, producing tint oral pit or fossa. By the third week, the oral fossa
or Nteiuodaum Ih a well-marked pit of pentagonal outline, its
buiiiidiirMiM brin^ the niiHolVontal process above, the maxillary
proneHHort hitenilly, and the mandibular arches below. The
origiiiul oral plate, having n»<M»d(Hl farther and farther from
the Hiiriiieo and ioriiiiiig the posterior limit of the mouthmvity, iinw H««piiniteH that cavity from the pharyngeal region
of the gut-tube aiui eoines into contact with the anterior wall
ot* tlhi hitter. It \h (Milh*d the phanrngeal membrane (Fig. 91).
ItH dimippeaniiKu* tM'ciiiN at Home time during the fourth week.
 
 
 
THE PHARYNX. 193
 
by which event the gut-tube is brought into communication
with the mouth.
 
The exact position of the pharyngeal membrane is not
easily definable. It is certain, however, that it falls farther
back than the posterior limit of the adult oral cavity,
since the primitive mouth includes the anterior part of the
adult pharynx. For example, the diverticulum that gives
rise to the anterior lobe of the pituitary body belongs to
the primitive mouth, yet its vestige, the pharyngeal bursa ^
or Rathkd's [K)cket, is found in the pharynx of the adult.
The primitive oral cavity, by the growth of the palate, becomes divided into the adult mouth and the nasal cavities.
The hard palate is completed in the ninth week and the soft
palate in the eleventh week.
 
THE PHARYNX.
 
The pharynx is represented in the embryo by the expanded
cephalic end of the primitive gut-tract. It is of greater relative length in the earlier stages of development than later,
including as it does, almost half the length of the gut-tube
in the fourth and fifth weeks. The primitive pharyngeal
cavity is widest at its anterior or cephalic extremity and
narrowest at the opposite end, tapering here into the esophagus. Until the breaking down of the pharj^igeal membrane, which takes place in the fourth week, this structure
marks the anterior limit of the pharynx and separates it
from the oral cavity.
 
The pharsrngeal poncheB or tliroat-pockets have been referred to in connection with the visceral arches on page 111.
They are out-pocketings or evaginations of the entodermal
lining of the pharynx, there being four furrows on each
lateral wall, and they pass from the ventral toward the
dorsal wall of the cavity, each pouch lying between two
adjacent visceral arches. The entoderm of the pouches
comes into close relation with the ectoderm of the outer
visceral furrows (Fig. 71). The mesodermic stratum being
 
^ It has been shown recently. (Killian) that the pharyngeal bursa is not
identical with Rathk^'s pocket, but is an independently formed evagination.
13
 
 
 
j ,l*\
 
 
 
 
 
 
i. i"
 
 
 
 
,.'j.« m" 1 -•
 
 
1
 
 
t .1 -. ♦• f '
 
 
«
 
 
lit"- "i
 
 
-' .
 
,. .., w ■•
 
 
. m ''
 
 
I If.-.
 
 
. - .
 
 
• ■•/••»•*' '
 
 
■
 
 
,.., ■*•••-.
 
 
■
 
 
■».. Jb«t
 
 
Xii
 
 
• "»T ■" '■
 
 
• •
 
 
-tyi0»«ju: »
 
 
rni
 
 
•
 
 
22S
 
 
^ . • 1 ■ ■
■ ^ ■■ ■ ■
 
 
 
 
 
 
 
 
L«^ -T'-iersL Jia ne
 
 
 
-?-..^ Iri-l "~:.r 'TDlUr^e 3.
 
 
 
: r lij
 
 
 
'•■'1
 
 
 
::::;::■■' rrn—
 
 
 
4^ I
 
 
 
t^st • -pgy— ^T»^- •onm
 
 
 
-_-:a * t-r
 
 
-• ! X
 
 
 
• I
 
 
 
l
 
 
L=
 
 
I. - ».
 
 
 
•\
 
 
 
 
 
 
I.
 
 
 
- "ilT ■ 1 1":'" — ■
 
 
 
.yniL*? iiicrj
 
 
 
• • «
 
 
 
 
 
 
r'-^^a. ini"-X^
 
 
 
JLT&irV^ ^13«£S
 
 
 
.^-rnu ?l!lt7
 
 
 
"11" -'-•
 
 
 
,.: V
 
 
 
 
 
 
....:. ^ . ■ .
 
 
 
jL2a«
 
 
 
-■ -. .- •a.TTiui :«:»i7
 
 
 
t-\»:-C'*v:
 
 
 
iitar
 
 
 
r:i:r
 
 
 
*■- ••s^tit;
 
 
^ 5x=:
 
 
v^ ■
 
 
 
* ., «^^.«tv".>
 
 
 
- ..- y.'srjsrjjT third • :" •:.
 
 
. ^
 
 
 
\*%l^>i* V .■ »
 
 
 
■^ •
 
 
 
^ :^ v.^*-*-'-: lvii»j;lioi«l ti— ij«- afi-iiji
 
 
 
THE ASUS.
 
 
 
195
 
 
 
 
^=*L_
 
 
 
Fin. Wi. Section Ihrougb *Dlage of
tonsil of a human fetus (Tourneui) : 1,
tonsillar pit, continuous with mouthcsvlly: 2. BeoondBry diverticula: 3. Bolld
epltbelial buds ; 4, striped muscular flber.
 
 
 
an evagination of the lateral wall of the pharynx. In the
third month the lateral pharyngeal wall jwuches out to form
a little foasa (Fig. 95, 1)
which is i^itiinted Iwtween the
second and thir<l visceral
arches, the fossa Ixiing lined
witli .''tratificd pqiiamous epithelium continnons with that
of llie i>lmryngeal cavity.
Little solid epithelial buds
(Fig. 95) proceed from this
diverticulum into the surrounding connective tissue,
the buds subsequently becoming hollowed out. Wandering lenkocytes from the
neighboring blood-vessels —
or, according to some authorities, fnim the mesenchyme cells or from epithelial sources —
infiltrate the connective tissue around the young crypts, and
these cells becoming aggregated into condensed and isolated
groups give rise to the iTmphoidfoUicleB peculiar to the tonsil.
The separate and well-differentiated condition of the follicles
is not attained until some months after birth. The place of
origin of the tonsil lietween the second and third visceral
arches explains the position of the adult organ between the
anterior and posterior palatine arches, since the latt«r structures represent the deep extremities of the former,
 
THE ANUS.
The early stages of the development of the anus are similar to those of the mouth. The so-called anal membrane is
produced by the growing together of the ectoderm and the
entoderm, the mesoderm being crowded aside. The site of
the aual membrane, or anal plate, is in the median line of the
dorsal surface of the embryonic body, at its posterior or
caudal extremity. It makes its appearance in the third
week. Since the tissue immediately in front — that is, head- ■
ward, of the anal plate projects and develops into the primi
 
 
;i«
 
 
 
SJij:: -nuitT :
 
 
 
2:i33.'2 -.C^frl
 
 
 
ijT^ tmL. Mil "ixu** -iut i:ry» ii* lit* i«k^ i»--tiiu*= vouaJ^
«un"«:- 'a« uuu jwn* i» tii^r»-< uniuuL -inut'va^ iKfturc
 
Tii^ ^'.i^r^'.n. ;^ 'u> uuu itr b>>r^ ii>t
 
Tt-vrjrKM. -.■. "utt «t'. n"' "i« nrrr^m-, iiir ii i mine jdiiB
 
rf i*. ; •-di'r r-K"- ^A«r*ivi-*. *':r,-niu- ■,•■■■ 'lut rxf iti-snua cc Tut
 
W;,1a till tia. ;■:: j- j.imLUE,. "ii* «"*'■■-"""* ir rriT-inr
 
 
 
 
IrHiiEt'inri'-'l '-liii'fly ifil'* t\u: iiriiinry lilmlilt-r. Imt it ^ivc:i
r)w iil«i, liy it" |»rf>«imal <-xtnmit_v, to :i i-hort wide duct.
lli« iiroKanlt^I nlnuii, wliidi !■> iiti avi-iiiic of 'ffiiiiiiiiiiitntiDn
ttllli III'- 1«>«.-I. 'Dm- )rtir1 of tliff (;iil on tlit- caudal side ot'
lli>- ii)H'rliir<' Iff rill' iiroff'-iiitiil kIiiiim i^ llio closca, whieli is
 
||ii. i-oirir I I'Tiiiiiiittiori, X\>i-TfU<ri; of tlie p>tiit<Mirinur\
nyiilriii iind of lln- iiili-xtiiiid 'iiriul.
 
'I'll.' ■iirliiir ilcjir<*«iofi n-fi-rri-il In atiuvft an llic; anal pit in
 
 
 
DIFFERENTIATION OF THE ALIMENTARY CANAL, 197
 
often called the doacal depression during the time that the
cloaca is present. In the lowest mammals, the monotremes,
as also in the Amphibia, in reptiles, and in birds, the cloaca
is a permanent structure. By the breaking down of the membrane between it and the cloacal depression, it acquires an outlet, through which the feces, the urine, and the genital products
find egress. In all higher mammals, however, including man,
the cloaca suffers division into an anterior or ventral passage-way, the urogenital sinus, and a posterior canal, the rectum and canal of the anus. This division is eff*ected by the
growth of three ridges or folds, of which one grows from the
point of union of the urogenital sinus and the gut, while the
other two proceed, one from each lateral wall of the cloaca.
The three folds coalesce to form a perfect septum. The
division is complete at about the end of the second month
(or, according to Minot, at the fourteenth week). The
cloacal depression or anal pit shares in this division, so that
at about the tenth week, it is separated into the anal pit
proper, or the proctodeum, and the orifice of the urogenital
sinus. The newly-formed septum continues to thicken,
especially near the surface of the body, until it constitutes
the pyramidal mass of tissue known as the perineal body, or
perineum.
 
The anal pit deepens, the anal membrane being thereby
approximated to the end of the bowel, and in the fourth
month the anal membrane breaks down and disappears.
Persistence of the anal membrane after birth constitutes the
anomaly known as imperforate anus.
 
THE DIFFERENTIATION OP THE ALIMENTARY CANAL
 
INTO SEPARATE REGIONS.
 
The fourth week marks the beginning of certain important changes in the simple straight alimentary tube. The
reader is s^in reminded that this tube is connected with
the dorsal body-w^all by the dorsal mesentery and with the
ventral wall, for a part of its extent, by the ventral mesentery ; that the canal is, as yet, without communication with
the exterior ; and also that the vitelline duct and the allan
 
 
198
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
tois are connected with its ventnil surface (Fig. 94). The
umbilical vesicle having reached the limit of its development
in the fourth week and having begun to shrink, the vitelline
duct likewise begins to retrograde and very soon becomes
an inconspicuous structure.
 
The dorsal wall of the tube at a point nearer the head-end
begins to bulge toward the dorsal body-wall, forming a somewhat spindle-shaped enlargement (Figs. 97, 98). This di
 
 
Afidd/e lobe
 
of thyroid gland.
 
Thymus gland.
 
Lateral lobe
 
of thyroid gland.
 
Trachea.
Lung.
 
 
 
Right lobe of liver.
 
 
 
Vitelline duct.
 
 
 
 
Pharyngeal
pouches.
 
 
 
Stomach.
 
Pancreas.
 
Left lobe of liver.
 
 
 
Small intestine.
 
 
 
• Large intestine.
 
 
 
Fio. 97.— Scheme of the alimentary canal and its acceiisory orj^nB (Bonnet).
 
latation is the beginning of the future stomach. The part
of the mnal on the cephalic side of the stomach lags behind
somewhat in growth, corresponding in this respect with the
relatively smaller size of the adult esophagus. The esophagus begins to lengthen in the fourth week. At this time,
also, the beginning of the liver is indicateil by a small diver
 
 
DIFFERENTIATION OF THE ALIMENTARY CANAL. 199
 
ticulum which pouches out from the ventral wall of the
intestine just posterior to (below) the stomach — the future
duodenal region therefore — and which grows into the ventral
mesentery. Very soon after the appearance of the hepatic
evagination, a similar out-pouching from the dorsal wall of
 
 
 
 
Fig. 98.— Outline of alimentary canal of human embryo of twenty-eight days
(His) : p5, pituitary fossa ; tg^ tongue ; Ix^ primitive larynx ; o, esophagus ; tr, trachea;
Ig, lung ; «, stomach ; p, pancreas ; Ad, hepatic duct ; rd, vitelline duct ; a/, allantois ;
hg, hind-gut ; W'd, Wolffian duct ; t, kidney.
 
the future duodenal region of the intestine indicates the
beginning of the development of the pancreas.
 
In the latter part of the third week or in the beginning of
the fourth, the esophagus presents a longitudinal groove on
the inner face of its ventral wall. This groove increases in
depth and caliber and finally becomes constricted off from
the esophagus, with which it retains connection only at its
pharyngeal end. The tube or tubular sac thus formed is the
first step in the development of the lungs and the trachea.
 
 
 
200 TEXT-BOOK OF EM BRYOLOGY.
 
It may be said then that the gut-tract has now, in the
fourth week, reached the stage of differentiation into the pharTux, the esophagus, the Btomach, and the intesttne, with the
liver, the pancreaB, the respliatoiy sTstem, and the allsntoia
fairly begun.
 
As lieretofore pointed out (p. 90), the allantois — which
grows directly from the primitive gut-tract, and which con
 
 
 
alimentury caiial nf huTTinn pmbryo or thIrty-flTe dayi
(HIh) : Jib, picuibiry fiiwu: tg. tOQKUv: U. priDiitivv larynx: ", eeo|ib(>KUB : Ir,
(rachea; Iq, liing; «, Jlomai-h; ji, pancri.-B*; ftif. hcpalk duel ; e. conira; c(, cloaca;
It, kldnuy ; a, anus ; -Tp, genlUt ominonco ; I. cauilal piocesB.
 
sists therefore of the entoderm and the visceral meaodenn —
althoiigli destinetl to produce in part the permanent bladder,
functionates for a time, after its union with the false amnion
to form the chorion, an an organ of respiration ; while the
permanent respiraU)ry pystem, as we have seen, likewise
developH from the entodermal epithelium of the guttract The entoderm, therefore, sustains an important re
 
 
DIFFERENTIATION OF THE ALIMENTAHY CANAL. 201
 
lation to the nutrition of both the embryonic and the adult
oi^nism.
Increase in I^ength and Further Subdivision. —
 
The intestinal canal grows in lengtli much more rapidly than
<loes the embryonic body. It is in consequence of this disproportionate growth that the tube becomes bent and thrown
into ooiU or convolutions. During the fifth and sixth weeks
a conspicuous flexure appears at some distance below the
stomach. Here the bowel assumes the form of a U-shaped
tube, the claseil end of the U projecting toward the ventral
body-wall (Fig. 100), In other words, the redundant portion
 
 
 
 
Fia. lOCb— iDtmtniil omi] at Iiuman embryo
 
 
 
of the gut is pulle<] away, as it were, from the dorsal wall of
the body-cavity and, as a consequence, the dorsal mesentery is
Icngthenetl in this region to a corresponding extent (Fig. 100).
The vitelline duct is attached to the piirt of the bend nearest the ventral wall (Fig. 98). At a point on the lower limb
of the U the bowel abruptly acquires increased caliber. This
dilated part is the beginning of the cacnm or head of the
colon, and its appearance initiates the distinction between the
large and the small intestine, since the part' of the bowel on
the distal side of the point in question becomes also of lai^r
caliber and forms the colon.
 
During the succeeding week or fortnight, the character of
the colon and of the csecum becomes better established. The
remaining part of the lower limb of the U-loop, with all of
 
 
 
202
 
 
 
TEXT-BOOK OF KMliRYOLOGY.
 
 
 
the tube included between the loop and the stomacb, is the
small intestine, wliicli presents a (iltght dorual flexure at it£
proximal extremity. The stomach meanwhile ha» increatied
in size and has almost attaineil its characteristic shape. By
the end of the sixth week, then, the alimentary canal has not
only increased in length but lias so far dilfereulialed as to
have aci)uireil stomacli, dnodetiiun, Bmall intestiiie, cscmn, and
Tectnin,
 
Alteration in the Relative Position of Parts, and
Further Development. — The nmst iniportsuit ntudiliciition
of llie uliinfutary tube a^ it exists at the end uf the sixth
week is effected by certain changes of position of some of its
parts. The stomach and the large intestine are the jjortions
of the tract most conspicuously affected. The lower limb of
tlie U-segment of bowel, which consists chiefly of the radimentary ctecum and a [Mirt of the colon, is lifted, as it were,
over the npfwr limb and comes to occupy a position above it
(Fig, 101, ,1], the wecum assuming a posilion in the right
 
 
 
 
CFm. 101,— Three sucreulre staires ■hnwlng tln> fli'velopnieiil ol the dlgcatlTe
tnboand Ihe meaeutcrietln the human IbIiiR{modincdIhnnTounivUii: I.buiduihi l,i1uiH]enum; S, anikll 1ntral1ne:4, colon; 5, vlleUine dnol: B,CKOiim; 7, great
Dmentum : 8. mnioiluodcnuni : 9, mrw nt^r? : >", meaocolun. The nrraw polols to
the oiiflcc of \h>r nmcntHl buna. The veDlral mespntory In not shown,
hyjK
vers
L
 
 
 
hypochondriac region, and the colon paasinf; thence transversely acros-s the abdomen ventrad to the diiwlenum.
This shifting of position on the part of the colon brings
 
 
 
DIFFERENTIATION OF THE ALIMENTARY CANAL. 203
 
about important complications in the arrangement of the mesentery, since the part of the dorsal mesentery that pertains to
the upper part of the colon correspondingly alters its position
and line of attachment, becoming adherent to the i)eritoneum
on the ventral surface of the duodenum. The part of the
mesentery in question becomes the transverse mesocolon
(Fig. 101, B), The large intestine, after this change of position, presents csecum, transverse colon, descending colon, and
rectum, the ascending colon being still absent.
 
The vermiform appendix in the third month has already
acquired the form of a slender curved tube projecting from
the cflecum. At the time of its first appearance and for some
weeks afterward, the appendix has the same caliber as the
caecum. Subsequently the c«ecum outstrips the appendix in
growth, the latter appearing in the adult state as a relatively
very small tube attached to the much larger csecum.
 
The caecum soon begins again to change its position, gradually moving downward toward the right iliac fossa (Fig. 101).
The downward migration of the caecum is (hie to the growth
of the colon in the same direction. In this manner the ascending colon is gradually produced, it having developed to such an
extent in the seventh month that the caecum lies below the
right kidney, while in the eighth month it passes the crest of
the ilium.^ Corresponding with the growth of the ascending
colon, the mesentery shifts its parietal attachment and increases in extent until the ascending mesocolon is produced ;
and with the descent of the caecum, the terminal part of the
small intestine necessarily alters its position to a like degree.
 
The stomach, up to the third month, is a localized dilatation of the intestinal tube, bulging most in the dorsal direction and having its long axis parallel with that of the body
(Fig. 100). In the third month, however, it undergoes an
important alteration in position, rotating about two axes.
First, it turns about a longitudinal axis, whereby the left
side comes to face toward the ventral surface of the body
(anteriorly) and the right surface looks toward the spinal
column. In addition to tlie longitudinal rotation, the stom
* According to Treves, the caecum lies under the liver until the fourth
month after birth.
 
 
 
204 TEXT-BOOK OF EMBRYOLOGY.
 
aoh also rotatt^ ui>oii a dorsoventral (anteroposterior) axis,
bv which the lower or pyloric extremity moves somewhat
iipwTinl ami to the rijrht, and the cardiac end goes tailward
^dowHx^nnh and to the left (Fig. 101). By this double rotatuMi tho stoniavh is made to assume approximately its adult
jHv^iuoiu Tlu^ lonjritudinal rotation of the stomach, in which
«bo U^xwr jK^rtion of the esophagus takes |>art, explains the
t^lMi «t ^« TaffUB nenres in the adult. The nerves, before
U^^* rx^i^li^MK Ho one on each side of the esophagus and stom?^^^, bul ^in*v tlu» loft surfaces of both turn forward and the
\k\^\\\ MUlJuHw turn Imokward, the left vagus lies on the
t^t^v tivM' ^urfa*^* of the esophagus and of the stomach, while
vKo M.^hl uorvo Is in rt^lation with their posterior surfaces.
 
V\w x^dkUimtL of the mesogastrium are influenced in an im|K«tiiOi( uuuuior by the rotation of the stomach. As long as
{\w ^\\^\\us\'\\ itiains its original position and relations, with
W^ \S,^\^sWv \'\{V\i\U\\v facing dorsad (or |)osteriorly), the mesoyiua^uiu i« a voriiivil mesial fold of peritoneum (Fig. 100),
wlulv* iho wntnd mesoutory similarly connects the future
\\ y»\'\ \'\\v\'i\\\\\v or vent ml surface of the stomach with the
w hii.il b\Hl\ wall At the very beginning of the process of
ua-Uhm, tho nir«ogt»ster luHMmies somewhat redundant and
 
uU'. » t^'waid llio Irl^ O'^K* ^^^y -^)* -^^ ^'^'^ increases in
\ slrm, \U\\v \^ ronnod, between the stomach and the dorsal
U»mI\ Nsall, a |u»ni^h or |MH*ke(, the omental bursa, whose o|)enu\i\ ti u»\\anl tlh' v\^\\i ( Kig. 101). In the third and fourth
iuuulli"« llir ori^riniil unv*4i>gjister, lengthening more and more,
i\\\\\ lu'iiu? JilVo*'hHl by the inertMising torsion of the stomach,
Hui|i»l"i lu tho \\m\\\ {\f a sae considerably below the level of
Clii. ^liiumoli, in fwwxt of (ventral to) the small intestine and
{\w l»iiu-.\ri>ir oolt»n. It ultimately becomes the great omeniuur riio nirio^MMiriunu fn)m having been a vertical mesial
told, i^ miN\ become a tnmsvers** fold, so nnlundant as to be
Ibldrd M|iou it.nrir and to constitute a bag.
 
\\\ \\\\v nuiuht^r (he ventral mesentery (Figs. 94 and 102),
whirli roiuimli llh« anterior or ventral surface of the stomach
with tlu' Nrnind iMulv-wall, and in which the liver develops,
i^ ulUTfil tVoiu a medial ft)ld to a transverse fold by the rotatiou of the. rttiunai'h. As the liver migrates to a position
 
 
 
DIFFERENTIATION OF THE ALIMENTARY CANAL. 205
 
above the stomach, the part of the ventral mesentery which
connects the liver with the body-wall becomes its falciform
ligament and coronary ligament, while that portion of this
mesentery that connects the originally ventral surface of the
stomach, now its lesser curvature, with tlie liver is the lesser
or gastrohepatic omentum. The lesser omentum, therefore, is
the anterior or ventral boundary of the orifice of the omental
bursa referred to above.
 
The small intestine begins to exhibit flexures as early as
the fifth week, and by the end of the sixth w^eek the duodenum is well indicated as a segment of the gut-tube passing
from the pyloric end of the stomach toward the dorsal bodywall. From this time the development of the small intestine, aside from its liistological characters, consists chiefly in
increase in length with consequent modification of its mesentery. A striking feature of human development is that,
with the growth in length of the small bowel, it is gradually
extruded from the abdominal cavity into the tissues of the
umbilical cord. The extent to which extrusion takes place
increases until the tenth week, after which period the intestine is gradually withdrawn into the abdomen. In the
fourth month it lies entirely within the abdominal cavity.
Failure of complete restoration of the gut to the cavity of
the abdomen constitutes congenital umbilical hernia.
 
The histolog^ical alterations incident to tiie development of the alimentary tube, from the beginning of the
esophagus to the end of the rectum, consist in the differentiation of the constituent elements of its walls from the two
strata, the entoderm and the visceral mesoderm, which compose the walls of the early gut-tube. As an initial step in
the process, the cells of the mesodermic stratum undergo multiplication and arrange themselves in a narrow loose inner
zone and a thicker outer lamella. The inner layer subsequently becomes the submucosa of the fully formed state,
while the cells of the outer layer undergo differentiation into
unstriped muscular tissue, and constitute the muscular coat
of the canal. In the case of the esophagus and stomach, at
least, this muscular tunic, in the fourth month, exhibits the
 
 
 
206 TEXT-IIOOK OF EMBRYOLOOY.
 
distinction between inner circular, and outer longitudinal,
layers. The surface-cells oC the mc.':Jodennic stnituiu of tlio
primitive stonuich and bowel become the endothelium of the
serous coat.
 
The glands of tlie entire canal are products of the inner,
entoilermic stratuni, and tlierefore they are intimately related
genetically, as well as histologically, with the mucous membrane.
 
The glands of the Btomach, both the peptic and the pyloric,
originate from small cylindrical cell-masses that have been
produced by local multiplication and aggregaii<jn of entodeniial cells. By the hollowing out of the cylinders and the
branching of the tubes thereby formed, the two varieties of
gastric glands are evolvetl. Both sets make their appearance
in the tenth week. Until the fourth month the peptic glands
contain cells of but one tyi>e ; at this [leriod, however, certain cells of these glands becurae altered bv the gradual
accumulation of griinules within their protoplasm, by which
they arc trnnsfurmcd into the ehurac (eristic acid or parietal
calls of these glands.
 
The glands and villi of the intestine are likewise products
of the entodermal lining f>f the gut. Their evolution begins
in the second month, and they are fairly well formed by the
tenth week. As in the case of the gastric glands, the glands
of the bowel develop from cylindrical masi^es of entodermal
cells which are at first solid, hut which later become hollowed
out to form tubular depressions or follicles. In the regioii
corresponding to the upper part of the small intestine many
of these follicles branch to give rise to the gl&nds of Bmnner,
while tmbranched, simple, tubular depressions distributed
throughout the entire length of the bowel become the glands
of LieberklUui. While the surface entoderm is thus growing
into the underlying mesodermic tissue to form the glands, it
becomes elevate<l into nnnnte projections between the months
of the gland-ducts, forming the villi of the intestinal mucosa.
The connective-tissue core of the villus is derived from the
»ndprlyini» mcsmicrmic tissue, the cells of which, proliferate
tng, grow forth into the entoderm. The villi at first are
 
 
 
THE DEVELOPMENT OF THE LIVER. 207
 
present throughout the large and the small intestine alike,
being well developed by the fourth month. While the villi
of the small bowel continue their development, those of the
large intestine, after the fourth month, begin to retrograde.
At the time of birth they are still discernible, but at the end
of the first month after birth they are completely obliterated.
 
Meckel's Diverticulum.— The vitelline duct, it will
 
be remembered, is the avenue of communication between
the early gut-tube and the umbilical vesicle. In the sixth
week the umbilical vesicle has already begun to retrograde,
and the vitelline duct is attached to the ventral extremity
of the U-loop of the bowel present at this stage. The vitelline duct in most cases suffers complete obliteration in the
later stages of fetal life. In some instances, however, its
proximal extremity persists in the form of a small blind tube
varying in length from one to several inches, which is known
as Meckel's diverticnlunu Since the site of attachment of the
vitelline duct is not far from the termination of the small
intestine,- Meckel's diverticulum, when present, is connected
with the lower part of the ileum, at a point from one to three
feet from its termination. Should this tube remain attached
to the umbilical aperture and retain a patulous orifice, there
would result a congenital fecal fistula.^
 
THE DEVELOPMENT OF THE LIVER.
 
The essential features of the dev^elopment of the liver will
be more easily apprehended if the reader will not lose sight
of the fact that the organ is a compound tubular gland, and
if, further, he will recall the method by which glands in
general are developed — that is, as evaginations of the wall
of the cavity or organ to which they pertain.
 
The first step in the evolution of the liver is the growth
of a diverticulum from the ventral wall of the gut-tube at a
point corresponding to the region of the future duodenum.
This occurs in the third week, since His found the diver
* Meckel* 8 diverticulum is of interest clinically, since by contracting
adhesions to adjacent coils of intestine or by entanglement, it may produce
acute obstruction of the bowel.
 
 
 
THE DEVELOPMENT OF THE LIVER, 209
 
abundant cell-proliferation. The numerous branches into
which they divide are not tubes, but solid cylinders of
cells, the hepatic cylinders. The secondary branches of
these cylinders unite with corresponding branches of
adjacent systems, producing thereby a network of inosculating cell-cords, the meshes of which are occupied by
young connective-tissue cells and the developing bloodvessels. The connective and vascular tissue of the liverridge, thus surrounding and permeating the epithelial cellcords, produces all the connective-tissne parts of the liver,
while the liver parenchyma — the proper hepatic cells — and
the epithelium of the bile-dncts originate from the primitive
entodermic evagination. The cords of cells are in part hollowed out to form the bile-ducts and bile-capillaries, and in
part become the cells of the lobules. The cylinders that are
to produce the bile-ducts acquire their lumen by the fourth
week.
 
Until the middle of the fourth month, the right and left
lobes of the liver are of equal size, but after this period
the right lobe outstrips the left in growth. The liver grows
very rapidly and is relatively of much greater size in the
fetus than in the adult, almost filling the body-cavity at the
third month. In the later months of pregnancy it reaches
almost to the umbilicus, while at birth it makes up oneeighteenth of the body- weight.
 
The £fall-bladder develops as an evagination from the
original diverticulum. It is present in the second month.
The pedicle of this evagination lengthens somewhat and
becomes the cystic duct. The stalk of the hepatic evagination itself becomes the ductus communis choledochus.
 
The ligaments of the liver, save the round ligament,
are simply folds of the peritoneum which connect the organ
with the abdominal wall. Falling into the same category,
though not usually designated a ligament, is the gastrohepatic
omentum, which connects the liver with the stomach. These
various peritoneal folds may be looked upon as parts of the
ventral mesentery. Since the liver evagination grows between the two layers of the ventral mesentery to reach the
 
14
 
 
 
210
 
 
 
TEXTBOOK OF KMBRYOLOGY.
 
 
 
septum transversum, the liver will be found, iti tlie early
stages of its development, embedded between the lamellte
of tliis mesentery, which in a riiedian vertical fiild of jxritoneum (Fig. 102). The liver is therefore enclosed iu the
peritoneum and is connected below, by a part of the ventral
mesentery, with the lesser curvature of the stumiich, which
still lies in the median plane of the body, and above and in
front, with the diaphragm and the ventral body-wall by the
upper and anterior part of the same structure. The latter
fold is somewliat modified by the intimate asi^ociation of the
early stage of the liver with the primitive diaphragm, the
liver having develi)i>ed within a ]H>rtiori of the septum transversum, the liver ridge. As development advances, a partial separation of the liver and the diaphragm is effected, the
peritoneum, as it were, growing between the two from both
the ventral and the dorsal edges of the liver. The region
which is not invaded by the peritoneum represents the nonperitoneal surface of the adult liver between the lines of reflection of the two layers of the coronary ligament. Since the
peritoneum on the under surface of the diaphragm is reflected
from that muscle, both in front of and behind this area of
contact, to liecome continuous with the peritoneum on the
convex surface of the liver, there are formed two transverse,
parallel, but separated, folds which constitute the coronary
ligament of adult anatomy. The lateral prolongations of
these foldp to the lateral wall of the abdomen constitute the
lateral ligaments of the liver.
 
The nitation of the stomach to assume it^ i)ermanent relations alters the position of the fold that ci>nnect8 its lesser
curvature with the liver, bringing this fold into a plane parallel, approximately, with the ventral wall of the abdomen.
This fold is now the lesser or gastrohepatlc omentmn.
 
The ronnd ligament of tlic mlult represrnls the impervious
vestige of the umbiliiid vein. This vessel, entering the fetal
body at the umbilicus and pa.ssing to the under surface of the
liver, diverges from the abdominal wall to reach that organ
and. in doing so, carries with it the piirietal iK'ritoneiim,
The fold thus formed is the falciform or suspensory ligament.
 
 
 
THE DEVELOPMENT OF THE PANCREAS,
 
 
 
211
 
 
 
The special system of blood-vessels belonging to the liver
is described in the chapter on the Vascular System, p. 177.
 
THE DEVELOPME^a' OF THE PANCREAS.
 
Until recently it was believed that the pancreas developed
from an ovagination of the dorsjil wall of the gut-tube in
the region of the future duodenum, opposite the site of the
hepatic diverticulum. Later investigations have shown,
however (Stoss, Hamburger, Brachet, and others), that three
evaginationSy one dorsal and two ventral, appear upon the
wall of the duodenal region of the gut-tube, the method of
development being strikingly similar in mammals, birds,
fishes and ampliibia.
 
 
 
 
 
Fig. 103.— Reconstruction of duodenum with pancreatic diverticula (after
Uamburger) : A^ Five weeks' embryo ; It, six weeks' embryo ; D, duodenum ; D.chol.,
common bile duct; V.P, ventral pancreas; D.P, dorsal pancreas; JT, point of
fusion of the two ; .S, stomach.
 
In the sheep a dorsal evagination appears in a 4-mm. embryo (Stoss), and somewhat later two ventral outpouchings
apj)ear in close proximity to, if not in actual connection with,
the hepatic diverticulum, the stalk of which latter becomes
the common bile-duct. The dorsal diverticulum penetrates
between the two layers of the mesogastrium (Fig. 103) and
gives off lateral branches, remaining attached to the dorsal
wall of the duodenum by its stalk or duct. Eventually tliis
system of branching epitlielial tubes, the dorsal pancreas,
becomes the body and tail of the adult pancreas.
 
The rigid and left ventral evaginations become confluent and
form the vential pancreas. According to some authorities
the left diverticulum atrophies, the right alone persisting to
form the ventral |)ancreas. In either case the stalk or duct
 
 
 
212
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
of this ventral fundament bw;oinc^ tnorgfxl into the eommoJCQ
bile-duot — if not jireviously eouaected with it — so tliat it i^
in effect, a bramli of that dnct. The ventral pancreas grow
to the left, in front of the portiil vein, this change beinj
favored by the rotation of the duotlcniim on its long i
penotmtos between the kvei-a of the nicsogastritini and fuses
with the dorsal pancreas (Fig. 103), hecomiiig the head of the
adult oi^n. Thin onioo occurs in the sixth week in man
(Hamburger). Willi the union of the two |iortions their respeetive ducts— the dorsal duet or duct of Santorini and the
ventral or duct of Wiranng — acquire auaslomosefi with each
other, after wliieh event the terminal or intestinal part of tl
duct of Santorini atrophies and disappcais, the duct of Wi:
suug being heneefiirth the avcune by whieh the later-estate
]ishe<l .'ieeretinn enters the dmiflenurn. Occasionally
entire duct of Santorini jiersisls to adnlt life, entering t
duodenum upon its dorsal wall. In the cow and pig I
ventral duct atrophies, the duct of Santorini alone per&istin^a
while in the horse and dog both duets persist.
 
What has been said above applies to the origin of thftl
epithelial ])ai'ts of the gland ; the coimectlTe-tissne and i
lar elements are of mesnderniic origin.
 
At six weeks the long axis of the pancreas nearly c
sponds with that of the body of the fetus. With the rota-^
tion and change of position of the stomach and the alti
tionft in the mesogastrium, it moves to the left, acquiring its 1
permanent relations with the lert kidney and the spleen. It ]
continues to be an intraperitoneal organ until the fifth month,
when, by the disappearance of the tlorsal ]iart nf its investment, it becomes retr(i[>eritoneui (Fig. lOfi, ,1 and H).
 
 
 
THE DEVELOPMENT OF THE SPLEEN.
 
Although the sjileen dm-s not belong to the digestive system, it may convenienliy lie eonsidcrid here because of its
position and relations.
 
This organ is differentiated from the meaodermic tissue
(me.se nchy ma) found between tlie layers of the mesogastrium
in close proximity to tlie developing pancreas (Fig. 102).
 
 
 
THE DEVELOPMENT OF THE SPLEEN. 213
 
Primitively, therefore, it is situated behind the stomach. The
first step in its development, recognizable at about the end
of the second month, is the accumiilation of numerous lymphoid cells with large granular nuclei. The origin of these
cells has been a matter of dispute. It has been asserted
(Maurer, Kupfer) that they come from the epithelium of the
gut-tube, but this is denied by most authorities. The findings of Laguesse in fish-embryos, demonstrating tlie origin of
the spleen aniage from mesencliyma in close relation with
the branches of the later portal vein, are possibly significant
in view of the relationship between the spleen and one of
the largest tributaries of the porta\ vein, namely, the splenic
vein. Tonkoff's observations on birds and mammals (1900),
confirmed by Hochstetter, reaffirm the mesenchymal origin
of the spleen.
 
The mass of cells is augmented by the addition of cells immediately beneath the peritoneal surfaces of the mesogastrium, which cells elongate imtil they are spindle-shaped
and then become aggregated into fusiform masses. Bloodvessels penetrate the fundament in the third month and
become surrounded by cells of the same spindle-shaped
type. From both the cells surrounding the blood-vessels and from those of the fusiform aggregations, processes grow out and unite with each other, and from the
network thus formed the trabecular framework of the organ
is ultimatelv evolved. Accumulations of small nucleated
cells, forming dense masses along the arteries, furnish
the chief constituent of the pulp. The delicate intercellular
substance which makes up the remainder of the pulp is filled
with blood-corpuscles. The Malpighian corpuscles appear
before the end of the fourth month. By the sixth month,
the spleen attains its characteristic shape and the fibrous capsule is clearly indicated.
 
The spleen undergoes a change of location coincident with
the rotation of the stomach and the alteration of the mesogastrium. The organ being from the first embedded within
the mesogastrium, it follows that peritoneal fold to the left
side of the abdominal cavity. Here it lies close to the
 
 
 
214 TEXT-BOOK OF EMBRYOLOGY.
 
cardiac end of the stomachy between the two layers of the
mesogastrium, but projecting toward the left. The part of
th(» mcsogastrium which intervenes between the spleen and
th(* stomach is the gastrosplenic omentum ; while the part that
pass<;H from tlie spleen to the posterior wall of the abdomen,
r(»prcs<»iiting the parietal attachment of the mesogastrium,
(^onstitiitcH the phrenicosplenic omentum.
 
THE EVOLUTION OF THE PERITONEUM.
 
The arrangement of the peritoneum being subordinate to
thc! pcmition and relations of the vis(»era contained within the
iilidonion, th(! development of this complex membrane can be
pro|M?rly <lcwTib(Hl only by tracing the growth of the digestive
HyHlnii. Am the formation of tlie early gut-tul>e by the infolding of tlu; splanchnopleure has been pointed out (pp.
I HO iiiid IH8), w<» may begin at once with the i)eriod when
fhi' IriM'l luiH ilir form of a straight tube connected with the
doi'.-iil niid iIm' vontPid IxKly-wall respectively by the dorsal
iiiid I III' ventral mesentery (Fig. 104). Covering the tube as a
roih'shtiiriit purl of its wall, is the splanchnic or visceral layer
111' llir iiiiv<^od('rm, while the somatopleuric or parietal layer
ol' ilii« lallrr lificM th(» wall of the bodv. Obviously these
Iwo liiiiirllip of llic incsfMlcrm are continuous with each other
tliroiipfli \\\i* iiM'diiim of tlu* mesenteries mentioned above
(l''l^i. lO/i, .1 Mild //). The space thus enclosed by the mesodrniih* nlnitn Im the body-cavity or co»loin or pleuroperitoneal
nu\M\. TIm' ^iirfiUM'-cells of both strata flatten and assume
llin rliiinirtiT of mcsothclial, the later endothelial, cells. If,
at thin ntit^e, one iN'gins at any point to trace the mesothelial
lining, of the body-<'avity, that lining is found to form praclirallv oiiti ctMitinuous sheet.
 
Tliin hiiiiple nrrangenuMit of the primitive peritoneum is
(ninntonnrd into the (Mimpli(*ated membrane of the adult,
prinmrily, by tin* inerejiHe in length and cons(»quent tortuosity
id' tht) ulinii'iitiirv tube; and, sec<mdarilv, bv the fact that
iH^i'tain oppoM'd portions of the s<tous membrane, which have
been brought int<» eontaet by the altered relations of the
bowel and tlu* stomach, undergo con<»reseence or fusion
 
 
 
THK KVOLUTION OF THE PERITONEUM. 215
 
with each (itlior. Siiiiiihancoiialy wilh ihcsc alterations, [he
original pIeuroperi1»neal cavity sufl'ci's division into tlic abdominal or peritoneal cavity anil the thoracic part of the
body-cavity by the devclupinent of the (lia[»luiigni. This is
described on p. 175.
 
 
 
 
Fin, im —Recoiislruplion of Immun embryo of »boiilaeTtnteen(l»yB(«flerHW:
OP, opl[c and of, otic vinlclcs: nr. imtuchord : Mg, head-gut: g. mid-gut: lig, hindgut; T,, vitelline aac; I, liver; v, primitive venlrtcle; m. da. ventral and doraal
«i)iniE:Jp. primitive Jugular vein; ™, cardlual vein; rfC, dnct of CuvlBr; m, no,
umlilUcal vein and artery; at, allaniaii; ua. umbiilcal eord; dm. doiMl meKDtery;
vn, vcntnl meneiitvry Imudilled (mm Hlsi.
 
Tho first modification of the original arrdngemeDt is effected
by tiie development of the stomach a^ a spiiidlfi-shaped <lilatation of lh{' gul-tiil>e, ditferentiatiiig the tube int*) the
stomach and the intestine, and the c-omniuD dorsal mesentery
 
 
 
216
 
 
 
TEXr-BOOK (IF EMBRYOLOGY.
 
 
 
into the meBoeastrinin and lite iatestmal mesentei?. The
drawing out of the U-shu])cd liwp of ihc inti'.-^tiiic from.the
dorsal body-wall, which is the prolirniiiary siep to the distinctioD between the small iulcstinp and the colon, iiiL-reaspa
the length of the iiitcHtinal raesenlerv to a corresjwnding
extent (Fig, 10'>, C). As heretofore indicated, the lower
limb of the liKip presents an enlargement which is the beginning of the development of the lai^e intestine.
 
 
 
 
Km. los -A. B. two tniuvene sectlona, A Ihniiii
Intl regjao: C aogiltal aecKon (Toumcux) : l, clone
lerr; H, mewKardlum poalerlui: 4, inesocanlliiia anterliu: !>, lumer omentum ^ 6,
■UBpensory ligament uF tlvcr; 7. eaophague; 8. lungs: H. bi»rli ID, puncnu; 11,
itnmnch: 12, llrer; l:i, apleen: 11, loop of lulcatlno vilh Tltclllue duct: IS.ciccum;
IB. trecbcL
 
An important stage in the evolution of the jieritoneum is
tnarkod by the rotation of the stomach and by the migration
of the proximal part of the largo intestine to a new location.
The eliange of position on the part of the colon may perhaps
be best expressed by saying that the U-Ioop of intestine
rotates upon an obllqne dorsoventral axis, whereby the lower
limb of the loop, in other words, the termination of the small
bowel and the l>eginning of the colon, is carried to a positioD
above, cephaliid to, the nppor limb (Fig. 101, A), This rotation brings the beginning of the colon into tlie riglit hyjK)
 
 
THE EVOLUTION OF THE PERITONEUM. 217
 
chondriac region of the abdomen, from which point the
transverse colon passes across the abdominal cavity, ventrad
to the proximal end of the small intestine or duodenum. As
a consequence of the altered situation of the transverse part
of the colon, its mesentery shifts its area of attachment by
fusing with the peritoneum of the dorsal wall along a horizontal line and also with that of the ventral surface of the
duodenum. The descending colon having meanwhile moved
to the left, its mesentery likewise acquires a new area of
attachment by concrescence with the parietal peritoneum of
the dorsal wall of the abdomen on the left side. During the
progress of these alteraticms, the small intestine increases in
length, and its mesentery becomes correspondingly more
voluminous both in the extent of its intestinal border and
in length. The convolutions of the small intestine now
occupy the space below the transverse colon and its mesentery.
 
The duodenum, which in the early stage shares with the
gastro-intestinal tube in the possession of the common dorsal
mesentery, loses its mesenterial connection with the abdominal wall and becomes thereby a fixed part of the intestine.
Mention was made above of the fusion of the transverse
mesocolon with the peritoneum of the ventral surface of the
duodenum. At about the same time, the duodenal mesentery
(Fig. 101, A) fuses with the parietal peritoneum of the posterior abdominal wall, the result being that the lower layer of
the transverse mesocolon, as it passes downward, is now continuous with the parietal peritoneum, there being no longer
any serous membrane between the transverse part of the
duodenum and the abdominal wall (Fig. 106, J5). This part
of the duodenum therefore becomes retroperitoneal, there
being an investment of serous membrane only on its anterior
or ventral surface.
 
The second modifying DEietor in the complication of the
peritoneum, the rotation of the stomach, initiates alterations
in its mesogastrium. The latter membrane, it will be remembered, is a vertical median fold of peritoneum continuous with the mesentery of the duodenum (Fig. 105, C).
 
 
 
21.S TEXT-BOOK OF RMBRYOLOGY.
 
As the stomach tcovca about its two axes of rotation, the
mesogttstriuni begins to eaj^ toward the left (Fig. 101), sotliat
now it constitutes a pouch or foasa, the omental bursa, situaated between the stomach and tlie dorsal body-wall, the
opening of wliich looks towartl the right side of tlie body
(Figs. 100, ^1 and 107). Wilh the rapidly increasing rednndaney of the niesogastriTnii, the omental bun-a become.s
 
 
 
 
Fra. V».—A, B. Tw
 
(■ctiernaUc repnwwiitatii „ „.
 
■nd Hertwig}: l.aiiiinach: 2, duodeuu]
G, pniicnaa; fi. liver; T. leuvr omi'iitum;
9, tranavKrae mesopolon: 11, mcseulcry;
lly of (luiontal bursa or leBatr jierltonual
 
 
 
more and more capacious. In correspondence with the progressive rotation of the stomach, what was at first the left
surface of the mc>sogastriiim comes Into contact with the
peritoneum of the dorsal abdominal wall and fuses with it,
thus changing its area uf parietal attachment fnim a'median
vertical Hue to a transverse one. This change is (Mimpleted
by the time the :«tomach has attained its normal adult jmsition. The omental bursa now has the position and relations
shown in Fig. 106, A, 8. A still further increase in the size
of the bursa results in its protrusiim downwanl in front of,
ventrad to, the transverse colon and the small intestine. Kefence to Fig. 106, B will show that the dependent part of
the lnirs;i very nearly corresponds with the fully formed
 
 
 
THE EVOLUTION OF THE PERITONEUM. 219
 
great omeDttiro. It will be seen, however, that the deeper
layer of the bursa, the layer nearer the intestines, may be
traced above the transverse colon and its mesentery to the
dorsal wall of the abdomen, where its two lamellie separate
to enclose the pancreas, one lamina passing over the ventral
surface of the pancreas to become continuous with the parietal
peritoneum, while the other layer passes between the pancreas
and the abdominal wall. The latter layer is in continuity
here with the i»arietal peritoneum, which almost immediately
leaves the abdominal wall to form the upper layer of the
transverse mesocolon.
 
 
 
 
The further alterations necessary for the attainment of
the completed condition consist in the coD<3«Bcence of
certain opposed peritoneal snrfaceB. As a conspicuous example of such concrewcnce, the deeper lamella of the layer
of the omental bursa just dcicribed fuses with the ventral
jxritoneal surface of the transverse colon and with the
upper layer of the transverse mesocolon (Fig. 106, A), after
v^hich event this deeper lamella is practically continuous
with the lower layer of the mesocolon, while the superficial
lamella is in continuity with the upper layer of the mesocolon (Fig. 106, B), Thus the transverse colon appears as if
enclosed between the two lamcllic of the deeper layer of the
great omentum, while its mesocolon is constituted by a part
of the same structure. In other words, the adult transverse
mesocolon includes not only the primitive membrane of that
name but also a part of the early mesogastrium. Similarly,
the opposed surfaces of peritoneum between the pancreas and
 
 
 
J^20 TEXT-BOOK OF EMBRYOLOGY.
 
iUt*iltirm\ iilHlcmiinalwall undergo fusion (Fig. 106), the effect
of wlii<*|i, after the concrescence of the mesocolon with the
<l<tif|Mfr lay^'r of the omental bursa, is to make the lower layer
of iUt*, in(*M<K*f)lon continuous, over the transverse part of the
<iM#HliffMitii, with the parietal peritoneum.
 
I ii« ffreat omentum of descriptive anatomy, resulting from
Mi<< downwardly projecting process of the omental bursa,
i'^malMn originally of two layers of membrane, each one
liuvin^ two wTous surfaces. At the time of birth these
iwtf UtyovH uro, ntill separate — the permanent condition in
^*ini' tMuiMfiialH— l)ut during the first year or two after birth
imy \u*iuniu» adherent, the great omentum thus coming to
iunii\ir\tui but a ningle layer.
 
it rttitHiuiti to note the metamorphosis of the ventral mesen
tWVi H bii'h, prior to the rotation of the stomach, is a vertical
 
liMfdiaii fold ronnecting the lesser curvature of that viscus
 
with i\m vt'Ofnil ulHlominal wall. Since the evagination of
 
tUii |iiif-(iibi; (hut given rise to the liver grows between the
 
liiytii'D of \\w. \'i*ulvii\ ni(?sentery to reach the septum trans
Vi^reMnii (Ih^ liv<*r in not only enclosed by the mesentery, but
 
}H roiiiH'rh'd by it with the stomach and with the ventral
 
wall of the lilNlonii'ii and also with the primitive diaphragm
 
(Kij/. lorj). l^y tin* rotation of the stomach, the vortical
 
nM'diaii (old whirh connrrts that orgjin with the liver becomes
 
ho allrnd in pohitinn an to \\r in a plane approximately par
alhJ with lh<^ vi'nlrnl hurfarr of the body. This fold is now
 
till' gatttrohapatic or lesser omentum. As referenc'c to Fig.
 
lOtI will hhow, il iri (hf anterior boundary, above the position
 
of thii ht4»nnu*h, of tint nac deH<Tilwd above as the omental
 
burrtu.
 
That pari of thi^ ventral incHentery that connects the liver
with the abdominal wall antl with the diaphragm, while
originally orfupyiiig the inetlian plane, is modified by the
relation of the <h»v<«loping liver to the primitive diaphragm.
Tliene organs are intiniati*ly united with each other (p. 175)
in the early stjige <»f tlifir growth, l)Ut with their completion
a Hi'pamtion takrs place. V\h)u the two separated surfaces,
except in a region near the dorstd wall, the cells assume the
 
 
 
,THE EVOLUTION OF THE PERITONEUM, 221
 
endothelial type, the opposed surfaces thus acquiring the
characters of serous membrane. The peritoneum on the
under surface of the diaphragm is continuous with that on
the upper surface of the liver, both in front of and behind
the non-peritoneal area of contact. Therefore, in the completed condition of the liver and the diaphragm, these two
structures are connected by two layers of peritoneum separated from each other by a region containing only areolar
tissue. These layers constitute the coronary ligament of the
liver. If now Fig. 106 is inspected, it will be seen that the
posterior layer of the lesser omtMitum, and the upper layer
of the transverse mesocolon, together with that part of the
peritoneum with which they are in direcjt continuity, enclose
a sac which is the so-<^alled lesser bag of the peritoneum or
the lesser peritoneal cavity. All other parts of the peritoneum taken together constitute the greater peritoneal cavity.
The communication between the two, the foramen of Winslow,
situated behind the free right border of the lesser omentum^
is the constricted orifice of the early omental bursa.
 
The position of the kidneys and the ureters as retroperitoneal structures and the relations of the bladder and of the
uterus to the peritoneum, encroaching as they do upon the
parietal layer of this membrane, and being, therefore, invested by it to a greater or less extent, are easily accounted
for when it is recalled that all these organs develop from the
somatic or outer layer of the mesoderm.
 
The peritoneum does not acquire all the characteristic
features of a serous membrane until about the third month.
The histological alterations begin in the fourth week, from
which time until the sixth week the superficial cells, the
mesothelium, pass through various phases of transition to
reach the condition of somewhat flattened elements. By the
eighth week they have acquired the form of true endothelium. It is not, however, until the third month that the
subjacent tissue has attained to the condition of a fullyformed basement membrane.
 
 
 
CHAPTER XII.
 
THE DEVELOPMENT OF THE RESPIRATORY
 
SYSTEM.
 
Although the Dasal chambers and the pharyDgcal cavity
contribute to the formation of the respiratory system, these
 
 
 
Middle lobe
 
of thyroid gland.
 
Thymus gland.
 
Lateral lobe
of thyroid gland.
 
Trachea,
Lung.
 
 
 
Right lobe of liver.
 
 
 
Vitelline duct.
 
 
 
 
Pharyngeal
pouches.
 
 
 
Stomach,
 
Pancreas.
 
Left lobe of liver.
 
 
 
Small intestine.
 
 
 
Large intestine.
 
 
 
Fig. los.— Scheme of the alimeiiUry canal and its accessory organs (Bonnet).
 
jiarts will not be considered here, since they are described
elsewhere.
 
222
 
 
 
DEVELOPMENT OF THE RESPIRATORY SYSTEM. 223
 
 
 
 
Anatomically and according to their mode of development^
the lungs might be looked upon as a pair of glands having a
common dnct, the trachea, which latter, through the medium
of its dilated proximal extremity, the larynx, opens into the
pharyngeal cavity. In point
of fact, these organs are developed as an ontgrowth from the
entodermal alimentary canal in
a manner similar to the development of the liver and the
pancreas.
 
The first step in the development of the lungs is the outpouching of the ventral wall
of the esophagus throughout
its entire length. The longitudinal median groove thus
formed is the pulmonary groove.
It makes its appearance when
 
the embryo has a length of 3.2 mm. (0.128 inch) or probably early in the third week. The groove is more pronounced at its lower or gastric extremity. As the groove
deepens, its edges approach and finally meet and fuse
^vith each other. In this manner the groove is converted
into a tube, wliich gradually separates from the esophagus,
the separation beginning at the end toward the stomach and
progressing toward the pharynx. The separation, however,
is not complete, stopping short of the upper end of the
groove, so that the tube retains communication with the
pharyngeal end of the esophagus. Even l>efore the constricting oiT of this tube or pulmonary diverticulum is completed, its free end bifurcates. The pulmonary anlage consists, then, at this stage, of two short wide pouches connected
by a common |>edicle with the primitive pharynx (Figs. 108
and 109), and this condition is present in the fourth week.
 
A'^ery soon after the end of the first month each of the
pouches undergoes division, the right one into three branches,
 
 
 
Fkj. 109.— Transverse section to show
outgrowth of pulmonary anlage from
gut-tube (alter Tourneux): 1, dorsal
mesentery; 2, ventral mesentery ineluding 3, mesocardlum posterius ; 4,
mesocardlum antcrius; 7, esophagus;
 
8, diverticulum which becomes the
lungs, the trachea, and the larynx;
 
9, heart.
 
 
 
rfcVrJKWA" Of EMBRYOLOGy.
 
tW Wft vm lnh> *w»», whilp at the same time tiiev increase
(u fW (V^^ t l(*V Tlio riirther eteps toward the attainnieDt
 
 
 
 
VloW u( > BxiiuiHrHi'lli'ii of llie fUndsnipnl of Ihn lungsof a bnmui
|. i>l |)U> |UiMUi'l'>li|.nei'ltmouurcincnt(alWr HIa); Ir.trad^ea; br.rlgbt
lili<U"l>iUi •!'. <«>-|-li<M|ilai Vi uitnuuPlIve-Iiooe «iivelopv anil seiuos membTane
llitiiuitti (ut" wlilwli Hi« »|iltlit>1lal fiindninent iif Ihe lung grows; O, M, L', ninda(IlkllUul IllWItWv'i lulilillt-, ■IKllDwrrlobMuniierJgbt lung: C. r>. ftrndamenti
u{ lb* H|i|>ir( kUil tiiWHr liilMH ut Iho lull lung,
 
mC lltv mim|i|«"li'tl tiiintlitioii consist lai^ely in the continued
lM)ii'llliua ill' lllli imMwwi of dichotomous division (Fig, 111),
 
 
 
 
fl» Kl - VWw lif rtiuiliilriliilliin nf th« nindiuni'nt of the lungs of a hnmkn
•tnliryo iN, ■'( lll*)<>Mi>r tliNii tliadifPli llo^ariur HIb, maKulned .w dlammin):
A», Mrturla |iul|ii»ii«lla t H. Inuiliaa i ni, vnorihaiiui ; lb, pulniuiii,ry vesicle in procen
(ililltttlHUi <l. ii|i|it>r liilMur lliu rlcht luni wltb an oiWTterlal bmnchua leading
tu U i H. t\ IHlillllo ami liiWBt l»hM nflho righl lune; 0<, upper lobe of tbe ledluDs
WKh li|)«ilvrlnl liniiinlmi liia>tlti|rl"ltl t". lower lobe of Ibe IcRlung.
 
Wllinh luIltM- K<><'- > "Ill Ihc ."ixlli mouth. The original
 
iivut(<ii»tii>ii, luitmiKliii^ 111' ontodermal epithelium, gives rise
only til till' ftpltbeUtl pirtii nC the hiiigs mid iiir- passages.
 
.VII ihn iil)ii-r i'<in»lit tH, thi' I'oiini'ctive tissue, the mua
riiliir, viirMHiliii', iiiiii I'lirtihtgiiiuiiri i-lements, are prodmttM of
ihi' msBodermle Umuo into which the diverticuhim gmws.
I 'pun llii'ir lii-i. iippi'iiriinci- the "tubes" are alwiiys so'.id
 
 
 
THE THYROID AND THE THYMUS BODIES,
 
 
 
225
 
 
 
epithelial cylinders, the lumina being acquired later. At
first, the lining entodermal cells of the primitive tubes are
tall and cylindrical, the tubes themselves having a relatively
small lumen. In the fourth month the cells acquire cilia.
From the anatomical standpoint, the lungs now present the
characters of compound saccular glands.
 
From the sixth month to the end of gestation occur the
changes which give to the organs their essential characteristics. Upon the dilated extremity of each terminal tube
numerous little evaginations develop. These are the air-sacs,
or pulmonary alveoli, the terminal tubes from which they are
evaginated being the alveolar passages and the inftmdibnla.
Their walls remain very thin and their lining epithelium
flattens to such a degree as to closely resemble endothelium.
The trachea is simply the elongated stalk of the pulmonary
diverticulum. Its incomplete cartilaginous rings first appear
in the eighth or ninth week.
 
The larsrnx is the dilated proximal extremity of the stalk
of the pulmonary diverticulum specially modified to serve as
an orgjin of plionation. It is first indicateil at the end of
the fifth week (or, according to KalHus, in the fourth week).
One of the earliest changes is
the appearance of two dorsoventral ridges at the junction
of the primitive tnichea with
the esophagus. They are close
together in front, ventral ly,
but separated dorsally. They
are the first indications of the
true vocal cords. At this time
the pharyngeal aperture of the
primitive larynx is at about
the level of the fourth viscenil
furrow, behind the three segments of the developing tongue
(p. 144), and is separated from them by the Aircnla, a horseshoe-shaped ridge which bounds the aperture in front and
laterally and which represents apparently the ventral parts
 
15
 
 
 
 
com
 
 
Fio. 112.— Entrance to larynx in a
forty- to forty-two-<luy human embryo
(from KalliusK /, tuberculum impar;
p, pharynjro-epiglottic fold; e, epiglottic fold : l.e, lateral part of epiglottis ;
ni. cuneiform tubercle; com, cornicular tubercle.
 
 
 
226 TEXT-BOOK OF EMBRYOLOGY.
 
of the third visceral arches (Fig. 71, A, 3, p. 144). A little
later the furcula difTerentiates into a median eleratioii, which
is the anlage of the epiglottis, and into the two lateral arytenoid ridges, each of which latter presents two little elevations^ the comicnlar and cnneiform tabercles respectively (Fig.
112). The arytenoid cartilages are thus well indicated by
the sixth week. The lateral p)rtions of the furcula also produce the aryteno-epiglottidean folds.
 
The thsrroid cartilage develops in two lateral halves from
corresponding masses of mesenchyme which chondrify from
two distinct centers for each mass. It is regarde<l as representing the cartilages of the fourth and fifth branchial arches.
The two alie fuse with each other ventrally as development
advances. Failure of cartilaginous iniion between the two
alo) constitutes the malformation, /oramr/i thyroidcum. The
cricoid cartilage is regarded as being an independent cartilaginous formation in scries with the rings of the trachea.
Tiie chondrification of these various elements of the larvnx
begins in the eighth or ninth week.
 
The development of the pleurae has been descril)ed in connection with that of the ]x»ricardium and of the diaphragm
(p. 175).
 
THE THYROID, THE PARATHYROID, AND THE THYMUS
 
BODIES.
 
Th(\*<e organs may be considered in this connection as a
matter of convenience and because of their embrvological
relationship to the* respiratory system, being developed, like
the latter, from the epithelimn of the gut-tract.
 
Th(» thyroid body, an organ common to all vertebrates,
genetieallv consists of two parts, a median and two lateral
portions, or lateral thyroids.
 
The median portion originates from an evagi nation of the ventral wall of the pharynx, in the median line, posterior, caudad^
to the tuberculum imi)ar, and between the ventral extremities
of the first and second visceral arches. This median diverticulum is present in the human embryo of 5 nmi. It soon
pouches out on either side, assuming thereby the form of an
 
 
 
THYROID, PARATHYROID, AyD THYMUS BODIES. 227
 
epithelial vesicle connected by the constricted pedicle of the
diverticulum with the ventral wall of the pharynx (Fig. 113,
3). From the situation of the original point of evagination
behind the tuberculum impar and vcntromesial to the two
halves of the posterior segment of the tongue, the orifice of
the pedicle corresponds to the line of junction of the three
parts of the tongue. As a consequence, when these parts
 
 
 
 
Fig. 113.— DiagTammatip representation of pharynx of human embryo seen from
in front (after Tonrnenx): I, II, first and second pharyngeal pouches ; 1, tuberculum
irapar; 2, course of thy mglossal duct leading from 3, median lobe of thyroid gland;
•I. laryngotracheal tube: a, esophagus: 6, thymus: 7, epithelial body [parathyroid];
8, lateral thyroid ; 9, postbranchial body [parathyroid ?].
 
unite, the pedicle or duct is prolonged upward and comes to
open upon the surface of the tongue. The canal is known
as the thyrogloBsal duct or canal of His. In the fifth week
it begins to atrophy, and usually by the eighth week has become obliterated. Occasionally it persists throughout life.
The foramen csecnm on the dorsum of the tongue is the
vestige of the orifice of the duct. Other vestiges of the
thyroglossal duct are sometimes present. For example, the
lower part of the duct may persist as a short tube, the
tlisrroid duct, leading upward from the median lobe to the
hyoid bone; and again, according to His, isolated persistent
segments of the duct constitute the little vesicles in the
neighborhood of the hyoid bone which are known resjx?ctively as the accessory thyroid and the suprahyoid and prehyoid glands. According to some recent observations the
 
 
 
22S
 
 
 
Tl-:XT-nOOK OF ESinRYOLOGY.
 
 
 
lower piirt of what His calls tlie tliyroglossal duct gives rise
ti> the pyramidal process of thn thyroid, whirh extends ujw
waixl towiml the hyoiil bone, usually a litllc to the lefl of
the mid-line. Tiiis impaired median anlage gives rise to
tile isthmus of the adult oi^aii and abo, to a considerable
jMirt at least, of eai-h lateral lobe.
 
The lateral thjrroida begin their development somewhat
later than dues iho niodlan ]xirtion. In the embryo of 10
mm., the fourth inner visceral ftirroT or throat-noeket of each
Bide pouches om to form a. vesicle {Fig. 113, S). As the
vesitrle grows, its iH-dieli- W'TOmes attenuated and finally disappears. After their isolation from the thniat-jKwkets, the
 
 
 
 
Fio, lU.-Seml-dlaenmnuitlc iniutrallam (o i>how Ww iilUmnlu |>oaliliiii of (be
ttifiniu, Ih^rold gUntl, *nd pcMibranoliUl txid]' on thu niTk of tlii' vlilpk {A) and
the K«1f(B). niter de Mciimn: ■■(, Ihjrfiid Elnnd ; p, postbranrhlal bodjr; «. Ihymxa: r, epithelliltiody [pumlhjniW]; ;r,lrBcbBfl; A.hrort; t^, vena Jugulmrls i eo.
eiRHid Ti-ln.
 
vesicles give out small bud-like processes after the usual
manner of the development tif glantl- and gradually approach
the median lobe {Fig. 1 14, B), fusing with its posterior surface. The three parts unite probably in the seventh week.
In the vertebrates below mammals the lateral parts of the
thvmid d<i not unile with the median segment, and in certain
animals they remain wiilely Bei>arated from it as the suprapericardial bodies. According to the older view of His, the
lateral thyroids produce all of the lateral lobes of the adult
thvroid; laler i-eBoarehes have shown that they do not, but
 
 
 
THYROID, PABATHYROIDy AND THYMUS BODIES. 229
 
authorities are not in harmony as to whether they produce a
large part or only a small portion of the adult lateral lobes.
The more recent view of His is that the adult lateral lobes
develop only in part from the lateral anlages. Verdun, the
most recent worker in this field, maintains that the entire
thyroid body of mammals and man is developed from the
median anlage and that the podbranchial bodies (Figs. 113
and 114), by which name he designates the structures referred
to above as the lateral thyroids, atrophy.
 
After the union of the three portions of the gland, the latter
consists of a network of cords of cells, the meshes of which
reticulum are occupied by embryonal connective tissue. Subsequently the cords of cells become hollowed out and exhibit
alternating enlargements and constrictions. By the increase
of the constrictions the continuity of the cell-cords is interrupted at short intervals, and so the network is converted
into numerous closed follicles lined with epithelium, the formation of follicles beginning in the eighth week. The follicles
later undergo considerable increase in size on account of the
secretion by their epithelial cells of a peculiar colloid material,
characteristic of the thyroid body. The embryonal comiective
tissue, made up necessarily of mesodermic elements, furnishes
the comiective-tissue framework and the blood-vessels of the
organ, while the epithelium originates in the manner indicated
from the entoderm of the gut-tract.
 
The Parathyroid Bodies. — The parathyroid bodies,
usually two in number on each side, were discovered by
Sandstrom in 1880. The lower pair lie upon the trachea in
close relation with the thyroid body, while the upper pair
lie at the level of the lower border of the cricoid cartilage, in
relation with the dorsal surface of the lateral lobes of the
thyroid body.
 
Their origin is still somewhat obscure. Apparently they
are outpouchings respectively from the third and fourth
visceral furrows, being composed, therefore, of entodermal
epithelium. These epithelial bodies develop in a manner
similar to the development of the thyroid body, but the fact
that the cell-gi'oups are not broken up by the invading embryo
 
 
 
230
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
anal connfdive timue to the «(me extent as in the ease of the
thyroid rendei'a them hixlolagicaUy distinfjuhhabh from the
laltrr ; moreover, it is stated by Maiirer that tliey never form
colloid Hiibstjiiicc.
 
The Thymus. — What remains of the thymus after the
second year of life is in;i(io up chiefly of Ijrmplioid and connective tissue, (.'nilniiilrd in wliicli are c ha racl eristic little
epithelial bodies, the corpascles of Hasaall.
 
Tlie epithelial parts of tlic thymns, in all vertebrate aniinalij, are derived from tlie entodennal lining if the pharyngeal region of the p;ut-trart. In tiio lower gronps, such as
reptiles, amphibians, and bony fishes, the epithelinni of all
the inner visceral clefts or throat-pouches shares in the development; while in birds, only two or three clefts bike
part. In mammals, however, including man, the thymus
body is derived probably from but one throat-pocket, the
third.
 
The entodennal epithelium of the third inner pouch becomes evRginated (Fig. 113) to form an epithelial sac whose
connection with the pharyngeal cavity is subsequently lost.
The isolated and elongateil sac soon gives out small lateral
buds or processes at the distal extremity. While the original
sac has from the first a cavity, the bud-like branches are
solid masses of epithelinm. The liranching continues and
affecl^ not only the lower or distal extremity of the thymus
sac but also the proximal end, the structure now resembling
an acinous gland (Fig. 115). While this growth is taking
place, the epithelial muss is being invaded by lymphocytes
and young conneetive tissue with developing blood-vessels.
(According to some recent studies by E. T. Bell, the lymphocytes are dcrivetl from the epithelium of the originul Ihymna
anlage; but this is denied byStohr.) The encroachment by
these elements continues to such an extent that lymphoid
tissno — including leukocytes and ervthroblasts — becomes the
predominant constitnent of the thymus, the epithelial parts
sutferitig rc<luction, relatively, and liecoming fiiiiilly broken
up into isolated maswa which are the corptucleB of Hassall of
 
 
 
THYROID, PARATHYROID, AND THYMUS BODIES. 231
 
the mature gland. The breaking down of the epithelial cords
is probably res[>onsible also for the irregular cavities of the
thymus. Not until after birth do
the glands of the two sides of the
hcKly unite to form a single vnpaiied
strnctore, and the development of the
thymus is not completed until tiie
end of the second yi'ar of life. Having attained its full development, the
organ l>egins to retrograde, and ut tlie
time of puberty has almost disappeared. Although sometimes persistent throughout life, it is usually
represented by an insigniticant vest^
ige. (It has recentlybeen said that the
thymus increast^s in size and weight
up to puberty, and that it is an active
organ until the fortieth year, after
which time it atrophies.) While the
epitheli&I parts of the thymus body,
represented in the fully developed
oi^n by the corpuscles of Hassali,
are derived from the entodennal epithelium of the third inner visceral
furrow, all other parts, the Irmpboid
tisBoe, conoectlTe tissue, and bloodvessels, are products of the surrounding mesodenn.
 
 
 
 
Fni. lis.— ThyiDua of ao
embrro nbblt or ■liteen
da;B taRer K&Ulker), magnlfled: a, canal of the tb^mus;
b, npper, c, lower end of Ihe
 
 
 
(MIAPTER XIII.
 
IIIIJ UUVRLOPMENT OF THE QENITO-URINARY
 
SYSTEM.
 
OwiNU to tlio intimate anatomical and functional associa\{\\\\ \\( llio ^viiomlivo organs with the urinaiy apparatus, it
U M^HH^*iM'v to iliseuHH the development of these two systems
 
MU'« lUiVlilOPMUNT OF THE KIDNEY AND URETER.
 
V\\y \*M^iu ol* {\\v ki(hu'y and ureter of the higher vcrte^^^^U^-^ U rtwn^oiat^^d with the development of certain fetal
^»V^M\-UM\'^^ \\\\' VHW^jfiiTOU and the mesonephros, which represent
Sv |Hv ^uvl\ \\\s' kidiiry t»f hirval amphibians and the perma\\\M\ ki\\\\\\\ \»r llfthrw. In man and other allied types, the
U^ui^ 4^^^^^^o i« of litthM)r no importance functionally,
\\luK ihv' Iwwv t\iiu»lioimteH (hiring a i)art of fetal life as the
'«»<.. ^^ \a uuu.M\ *'\oi'vti(»n, prior to the development of the
|i\Muu^ui lvidiu\\.
 
\\u ^uvu^i^k^vui \»r ht»A(l kidney constitutes the most primiVvw ^I'U^UiAW I.VM^ pr)unc(*hiinism for the excretion of urine,
iln iui«(\iir mii',mi*Uvi iu the foUowing manner: When
Ok pu.ivtil iiu .mUiiu, Nshioh Hnbsc<jncntly divides into the
 
MMU, \ .\\iM\\\ (o riAjuMuto Ironi the lateral plate of mesoU i»u \Ux i\\,i |tvit t lU'o oouneoted for a time by an interven(M. )m.( I .>( K air, ibc middU plate or intermediate cell-mass
\U. u». I hi ihii'kndiig of this intermediate cell-mass
y\ ' l»i . 1 1*. Wu^UiiiU iidgt». whioh projects into the ctelom or
I- ■ U M'iv ri»*' uu'^iuU'iiual i»r mesenchymal eh»ments of
I'l \\ i'Uli in udm* brroiiu^ i^i'ouprd into cords of cells which
it ui iiu. I liMU .u \'i'ii;iiu points with the mesothelial cells
 
I \U ■ s liMii I hr iU'iv'iu of (hcs(« et»ll-cords of the Wolffian
\.A^,. \i\ hiiij^; biiu u Uk;Uici' ot' dispute, some authorities
 
 
 
DEVELOPMENT OF THE KIDNEY AND URETER. 233
 
maiiituiniiig that they come from the mesothelium of the
body-wivity, while others believe that they are of ectotlerraic
origin. Further cimiiges bring about the hollowing out of
the cell-cords so that there results a long tube, the pronephtla
or Besmental duct, which has several siiort tranaverre tnbtilen
— ill some vertebrates, sis ; in man, two — ojwiiing into it ami
communicating by their opposite oi>cn extremities, the nephridial fonnels or nephrostomata, with the cojlom (Fig. 117).
In the human embryo the pronephric tubules have been
found with oi»cn nephridial funnels, but without connection
with the pronephric duct. The mesothelium in imnifxliato
proximity to the open end of each short tubule is invaginated
 
Axia/trnr. , IflanU canml.
 
 
 
'1^.
 
 
 
Lttttal fUuti/sr
 
 
 
 
by a tuft of capillary blood-vessels from the adjacent primitive aortro to constitute a glomemlna (Fig. 117, hh). The
pronephric duct passes tailward and opens into the cloaca,- a
receptacle which receives, in common, the terminal orifice of
the primitive bladder and that of the primitive intestine. It is
apparent, therefore, that the pronephros or head-kidney is anatomically adapted to the function of removing certain substances from the blood by virtue of the action of the cells surrounding the glomeruli or tufta of capillary blood-vessels, and
that these substances may be conveyed away through the duct
into the cloaca and thence evacuated from the body. This
organ is functionally active, however, only in certain lower
 
 
 
234
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
classes of vertebrates, as in the Amphibia during the larval
stage and in bony fishes. In mammals it is exceedingly
rudimentary and very soon gives place to a more important
organ, the mesonepliros.
 
The MesonepliroB or Wolffian Body. — As in the case of the
pronephros, the origin of the mesonephros is to be found in
the Wolffian ridge. Reference has been made, in treating
of the primitive segments, page 77, to the middle plate (Fig.
116) as a tract of mesodermic tissue connecting the paraxial
tract with the parietal plate. When the paraxial mesoderm
 
 
 
 
 
Fk;. llT.^Diagnim of pronephros (F) and proncphric duct
(Prf): Al, allantols; G, gut; a,
cloaca; W>, glomeruli.
 
 
 
Fig. 118.— Diagram of Wolffian
body and duct : AU allantols ; (?.
gut ; (jy, cloaca ; K. kidney evagination.
 
 
 
segments to form the somites the middle plate likewise
undergoes segmetation, each segment being designated a
nephrotome. Each nephrotomo, in the lower vertebrates, contains a cavity which communicates with the general bodycavity and which is, therefore, in effect, an evagination of tlie
mesotheliuni of this space. In mammals, however, as well
as in reptiles and binls, the nephrotome is a solid cord of
cells. I5y the hollowing out of these cell-cords or nephrotomes a scries of transversely directed tubules is formed,
each nephrotome, in fact, becoming converted into a short
 
 
 
DEVELOPMENT OF THE KIDNEY AND URETER. 235
 
 
 
 
Frii, 119.— TmiiJivcrso •cction of ■evenWen-day sheep ombryo iBonnetl; n«,
■nnlnn: on, amniotic atic-. n. nearal canal; >. ■omlte dUTerenllatecl Into miucleplate: H'd, Walfflau dut^l: R'b, Wulffliiii body: pm. psHvCal meioderm; vm. visceral lueaadenn; a, a, tUiIng primlllre aortas ; I, InlcsMne.
 
canal. These tubes acquire oonnection by their deeper ends
with the previously formed proncphric duct (Fig. ] 1 7), which
 
 
 
 
Fro. lai.-DtepnaUlo
 
 
1/'
 
or Che M
 
 
,;:?*2i^-Jy
 
 
"^
 
 
6.6 cm (12 In.l long (To
 
 
 
 
, Wolffian body :
 
 
^ m-ary : a. InBUlnal llgamtnl:
 
 
*, diBphrBpn^llo IlKBine
 
 
nl:5,«OTi
 
 
ach ; 0. tnwstinp
 
 
7, bU.lder: s. unibilitiil Httery.
 
 
 
is known hereafter, therefore, as the mesonephric or Wolffian
duct (Fig. 118). The latter duct and the short transveraa
 
 
 
&aa I
 
:raa ^^^^
 
 
 
236
 
 
 
TEXT-BOOK OF EMBRYOLOCY.
 
 
 
liibulow which open into it constitute the Wolffian body or
meaoaephros (Figs. 119aiid 120, 1). Tlie tissue of the intermediate eell-nias!) from whicii the W'ollflan tiihiil<'s develop
is designated, by Sedgewick. the Wolffian blastema, and by
Rabl and by Schreiner, the nephrogenic tissue. At tliii* stage
of its development the Wolffian bmiy consists of a tube or
duct lying behind the parietal layer of the mesoderm, parallel
with, and lateral to, the primitive vertebrid column, and
opening at the caudal end of the embrj-o into the cloaca ; and
of a series of transverse Wolffian tubules opening into the
duct and abutting by their opposite ends upon the bodycavity. At the bead-end of the Wolffian duct the now atrophic pronephric tubes are still in connection with it.
 
As a farther step in the development of an organ adapted
to the function of the secretion of urine, each Wolffian tubule
becomes somewhat saccular midway between its two extremities, and this dilated ptirt of the tubule is invaginated by the
capillary branches of an artery from the aorta. In this
manner the cells that line the tubules are brought into relation with the bltK)d of the fetns and acquire at the same time
the charaotors of secreting epithelinra. Such an invaginating
tuft of capillaries, known as a glomernlns, with its enveloping
capsule of Bowman, which latter is the invaginated saccular
part of the tubule, constitutes a primitive Malpighian cor*
puBcle, a ritructure analogous to the Malpigiiian corpuscle of
ihe permanent kidney. Thi.t simple form of the mesonephros
is seen as a permanent structure only in some of the lowest
vertebrates. In all higher vertebrates it attains to a more
complex degree of development, reaching its maximum in
man in the seventh week of fetal lii'c. Its complexity ia
increased by the dcvelopmont of secondary tubules and
Malpighian corpuscles conneoled with those first forme<l.
White at first the number of tubules corresponds with the
number of nephnitomes, tlus corre^^pondenee is soon lost by
the appearance of the secondary tubules.
 
The horizontal or transverse tnbnleB of (be Wolffian body
arc divisible into an anterior or iipf)or scries, diatiiiguislied
us the sexual aegmeut, anil a lowir set of atrophic tubules —
 
 
 
DEVELOPMENT OF THE KIDNEY AND URETER. 237
 
 
 
atrophic for reasons that will appear hereafter. In certain
vertebrates that are of higher type than those in which the
pronephros functionates, such as adult amphibians and fishes,
the Wolffian body persists throughout life as an organ of
urinary secretion. In binls and mammals, however, its
functional activity is but temporary, since it is supplanted,
before the end of fetal life, by the permanent kidney. In
man it disappears relatively early, retrogression beginning in
the eighth week and the Malpighian bodies having almost
disappeared by the fifth month. The presence of the mesonephros as a temporarily functionating organ in birds and
mammals, while it is a permanent structure in certain lower
members of tlie vertebrate series, exemplifies the embryological principle elsewhere referred to, that the higher types
pass through stages during their development that are permanent in some of the forms below them in the scale of
evolution.
 
The Metanephros or Permanent Kidney. — While the Wolffian
body is temporarily functionating as a kidney, a structure is
developing from the lower, caudal end of the AVolffian duct
which is to form the permanent
organ. It has been stated that
the Wolffian duct opens into the
cloaca. From the dorsal asjiect
of this duct, near its cloacal end,
a small diverticulum, the kidney
evagination (Fig. 1 1 8 and Plate
VII., 1), grows forth and soon
lengthens into a tube which
grows head ward, dorsomesial
to the Wolffian duct, penetrating
into the nephrogenic tissue or
mesonephric blastema (p. 236).
 
The cephalic end of the tube dilates somewhat to form the
primary renal pelvis, the anlage of the adult pelvis of the
kidney, while the duct itself becomes in time the ureter.
From the primary renal pelvis several small divertictila
 
 
 
 
Fig. 121.— Diagram to show extension and branching of kidney evagination and separation of its stalk
from the Wolffian duct: u,primitiYe
ureter; ;>, pelvis of ureter; WD,
Wolffian duct ; Bl^ bladder ; u*, urogenital sinus ; CI, cloaca ; G, gut.
 
 
 
238 TEXT-BOOK OF EMBRYOLOGY,
 
pouch out (Fig. 121), while the surrounding blastema becomes condensed and vascular.
 
The development of the kidney from this stage onward
has been for some years a disputed question. According
to the older conception, still maintained by Golgi and by
Minot, the small tubes which branch from the primary renal
pelvis become the collecting tubules of the kidney and themselves give off branches which, increasing in length and
acquiring tortuosity, become the secreting tubules (proximal
and distal convoluted tubules, loops of Henle, etc.). The
blind end of each convoluted tubule, becoming dilated and
saccular, is invaginated by a tuft of capillary blood-vessels,
thus being converted into a capsule of Bowman. The invaginating mass of blood-vessels constitutes a glomerulus, and
 
Mesodermic tissue.
 
 
 
 
i Wter.
 
V*.. V > \Mi^v<«m^^MMo n>)xiriiontAtton of the development of the kidney (after
 
Ui'Kvubaur).
 
^KiiMs'MiUu ausi s^i|wulo of Bowman together make up a
llH^VA^t^UH sSMl^UiK^Wx Thus the entire system of tubules
i^y K I hi I w \\\\ \\w i^lvin aiul the ureter have a common origin
lU'iu \\u^ ^A\\^^\\ \'\\\\ \»t* thv Woltlian duct, while the blood\v V I \\\x\ Ks^wwwMW' {\^<\\K\ as well as the capsule, originate
u villi ()i. uiuMtiubu^ Huvvnvhymo.
 
\. V I'lvliu^' lo \\\\^ K^\\wy viow — S»mi>er, Sedgewick, Balfour,
»u.l ill. I \\\\\\\ iasUHiiiuhI bv the n»searches of Schreiner,
wU- \y nil. li»\*' tnvh v^mtlmuHl for the most jwirt by
UnJ». I. »li. « ^uixmIiu^hI uibuKvH and the capsule of Bowman
..ULUint ux.i \ *\U II .u^h.'t v»t* ilivertieula from the primitive
\K\\.\\ |ul\i . tiui iiuU^ik^ikU^uIv tViMU the nephrogenic tissue,
 
 
 
DEVELOPMENT OF THE KIDSEY AND URETER. 239
 
and in the folluwiag inanoer : The nephrogenic tissue into
which the kidney evagination peaetrates shows a differentiation into two zones — an mTtefOfif,immediatelysurrounding the
primitive renal jielvisij consisting of epithelioid cells, and an
oiUer zone of less differentiated mesenchyme. Soon after the
appearance of the small diverticula which evaginate from the
primitive renal pelvis and which arc designated the primary
collecting tubules (which lattercorresjxtnd with the " primitive
renal vesicles" of Haycraft), the nephrogenic tissue breaks up
into smaller cell-masses, each snch mass surrounding a primary
collecting tubule (Fig. 123, mk). This part of the nephro
 
 
 
FiO. 1Z3.— Section throuRh the kldn«]r tiT human TeIus <if Eeven monlhs (from
Felii. sfterSchreinert: Sr, collt-cllnu tubules of wlilrh three nre ehuwn. each with
itfl cap of metauepbTogenlc tiHue, mt; in relation with each is un early UTiniferoua
tubule, the three latler. n, b, c, cuch nbovlnif a dlHerent itBgc nf development— a,
showing beglnnltiK of eipanslon; b, evaclnatlon at At; r, S-sbapeil aiage, M Indicating development of Bouniana capsule.
 
genie tissue Schreiner calls the metanephrogenic tiasoe by way
of distinction from the remaining part of this mesenchymal
aggregation which, from its relation to the development of
the mesonephros, is called the meBonephiogenic tissue. Each
primary collecting tubule, after becoming bulbous at its end,
divides into two tubules, each one of whicli in ttirn divides
into two, this process of division Iwing repeated several times.
These tubules become the adult straight collectinK taboles. The
branching of (be primary collecting tubules continue'^ to the
time of birth ; or until the fifth fetal month, according to
Hamburger.
 
In the development of the secreting tubnles the inner zone
of nephrogenic or metanephrogenio tissue alone is concerned.
 
 
 
240
 
 
 
TEXT-BOOK OF EMBRYOLOQY.
 
 
 
 
This tissue preseDts little btid-like prulongatioiis, each such
little bud of i-ells later acquiring u luiuen and M'jurating from
the pureiit tissue (Fig. 12^,1, 6,c}. The buds are now small
sacs, the renal veaicleB (Emery), of which there are at least
two for each collecting tubule. Each vesicle now elongates and assumes an S-shaped form, the concavity of the
up]»er part of the looking toward the collecting tubule,
the vesicle on the right of
the tnliule being, therefore, a
reverse.1 S (Fig. 124). The
lower limb of the^ S, from
being simply tubular, becomes expanded, its upper
wall being indented (Fig.
124, x), no Ihat it acquires
the shape of a double- 1 aye red
saucer, the space between its
two layers being continuone
with the remaining part of
the lumen of tiie S tube. In
the concavity of the saucer,
 
'"■" " """^■'""'^ "' - jyg^ beneath the middle piece
 
of the S, a strand of mesenchyme makes its appcamnce
and into this tissue blooil-veKscls penetrate, hi that it
finally becomes the glomemlns of the Malpighian corpuscle.
The saucer-shaiied lower limb of the S becomes the capsnl*
of Bowmaa; the lumen of the saucer, the apace of fiowm&n.
Meanwhilii the upjwr limb of the S acquires continuity with
the collecting tubule in close relation with which it has
developed, and the two extremes of the S l)eing thus relatively fixed points, the ensuing elongation of the intervening portion neces^ii tales the formation of curvatures. A small
part of the h)wer limb of the S, not being concerned in the
formation of the saucer-shnjied anlage of Bowman's capsule,
Wcomes a jmrl of the proximal convoluted tubule, the remaining portion of the latter, and tlic suc<vt'ding loop of Henle, the
distal convoluted tubule anil the arched collecting tubule, being
develojied respectively from the succeeding parts of the S.
The outer zone of the metanephrogenic tissue gives rise to
 
 
 
Iftnm Felix, after Bloofkl; Jir. ■mpulla
af cnllvcUoR tubule ; oB. upper limb nf
S; hB, lower IJmb of S (Bowmmn's cap(Die); X, pmllinn occupied by glomeru
 
 
THE SUPRARENAL BODIES, 241
 
the capsnle of the kidney and the supporting connective tissue,
ineludiDg the columns of Bertini.
 
The kidney acquires its characteristic features by the end
of the second month of fetal life, and it reaches its permanent
position by the third month.
 
THE SUPRARENAL BODIES.
 
The development of these structures has been the subject of much discussion. It has been maintained, on
the one hand, that the cortex of the organ develops
either directlv or indirectlv from the mesothelium of the
body-cavity and that the medulla has its origin in outgrowths from the sympathetic ganglia ; on the other hand,
that the entire organ is a product of the mesenchyme — indirectly, therefore, of the mesothelium. Thus Minot, Human
Embryology, 1892, remarks, "That both the cortex and the
medulla of the adult organ are formed in man from the
mesenchymal cells, as Gottschau showed was the case in several mammals, is, I think, beyond question "; but in his
Laboratory Text-book of Embryology, 1902, p. 267, the
same authority, in describing pig-embryos, says : "The sympathetic tissue gives rise to the so-called medulla of the
adult organ." Again, Aichel, 1900, from his investigations
concluded that both medulla and cortex arose from the mesenchyme. O. Hertwig, in his Lehrbiich, 1906, expresses his
conviction of the correctness of Poll's ^ conclusions as to the
double origin of the organ.
 
The cortex, according to Poll, whose work reaffirms in
many particulars the conclusions of some earlier investigators, arises from small bud-like masses of cells that come
from the mesothelium of the coelom in close relation with the
genital gland and the mesonephros, but distinct from them.
These buds lose their connection with the coelom ic epithelium
by the fifteenth day in the chick. They are situated on each
side of the root of the dorsal mesentery and all those of one
side unite to form a single organ. This occurs in man by
 
* H. Poll, Die Entwicklung der Nebennieren Sjrsteme, Handbuch der
vergleich. und experim. Entwicklungslehre d. Wirbeltiere, Bd. III., Abt
1, 1905.
 
16
 
 
 
242 TEXT-BOOK OF EMBRYOLOGY.
 
the twenty-eighth day (Souli6). In the lower vertebrates
this organ fails to unite with the anlage which represents the
medulla of the higher vertebrate suprarenal body and constitutes the separate interrenal organ of the lower vertebrates.
In some cases — e. g., in sharks — the anlages of the two sides
unite with each other, forming a single unpaired interrenal
organ, which lies l)etween the primitive kidneys. Failure of
union of some of the buds, it is believed, is responsible for
the accessory suprarenal organs of Marchand which are occasionally found between the layers of the broad ligament of
the female or in relation with the epididymis of the male>
these having followed the descent of the testis or ovary
respectively.
 
The medulla of the organ, still following Poll's account,
originates at a later stage than the cortical anlage from
chains of cells that grow forth from the sympathetic ganglia
and form groups which for a time retain their connection
with the ganglia. The cells show a differentiation into two
classes, tlie sympathoblasts and tiie phjeochromoblasts, the
latter gradually becoming the phseochronie cells, so called
from their staining darkly by chromium salts. In many
vertebrates these cell-masses remain separate from the interrenal anlage and constitute the phseochronie bodies or suprarenal bodies of lower vertebrates. In birds and rej)tiles the
union of the phfeochrome and interrenal anlages occurs by a
mutual intergrowth of their cells, the result being an irregularly stratified organ ; in mammals and man, however, the
cells of the sympathetic anlage gradually {wnetrate in the
form of cell-coixls into the interior of the interrenal anlage
to occupy their adult position as the medulla of the organ.
This process of intergrowth continues in man until the time
of birth.
 
In the early stages of fetal life the suprarenal body is relatively much larger than in the adult condition, and is situated
chiefly on the ventral surface of the kidnev. At about the
thinl month it Iwgins to assume more nearly its normal position.
 
The account of the development of the bladder and of
the urethra mav be deferred until the evolution of the internal sexual system shall have been consider(»d.
 
 
 
THE INTERNAL GENERATIVE ORGANS. 243
 
THE DEVELOPMENT OF THE INTERNAL GENERATIVE
 
ORGANS.
 
The Indifferent Type. — The internal generative organs of
both sexes, in the course of their development, pass through a
stage in which there is to be found no distinction of sex.
This stage is designated, therefore, the indifferent type of
sexual apparatus.
 
While the Wolffian body is attaining its full development,
there appears in its vicinity a tube, the duct of MUUer (Plate
VII., Fig. 1), which lies parallel with, and to the outer side
of, the Wolffian duct. In non-amniotic vertebrates the duct
of Miiller arises by fission or longitudinal division of the
mesonephric duct. Its exact mode of origin in the amniotic
vertebrates is not as yet definitely settled. According to one
view its upper or cephalic portion is produced by an evagination of the mesothelium of the bwly-cavity, while the
remaining lower segment results from fission of the mesonephric or Wolffian duct. According to another view the
lower or caudal portion is produced by the direct extension
of the upper portion in the caudal direction by the proliferation of its own cells. In whatever way the duct may be
formed, its lower or caudal end opens into the cloaca, which
receptacle receives also the termination of the Wolffian duct.
The upper end of the duct maintains a communication with
the body-cavity or coelom by means of an expanded funnelshaped mouth. Its lower segment is closely associated with
its fellow and with the Wolffian ducts, forming thus the genital cord. The function of this canal in lowly organized animals — that of receiving from the body-cavity the female genital products, the ova, and evacuating them from the body —
foreshadows its subsequent metamorphosis in most vertebrates.
 
While the duct of Mfiller is forming, the mesothelial
cells overlying that part of the free surface of the Wolffian body which looks toward the median plane and
somewhat forward, its ventro-mesial aspect, undergo multiplication and thickening (Fig. 125, a), forming an elongated
swelling or ridge. This is known as the genital ridge, which
produces a projection upon the wall of the body-cavity. The
genital ridge is still further thickened by the proliferation of
 
 
 
244
 
 
 
TEXT-BOOK OF EMBBYOLOOY.
 
 
 
llio niPstKlermid tissue (£') beneath tlie niosothelial cells. The
genital rulgos ol' the liiimaii fetus apijesir in the fifth week.
 
Further differentiation of tlie genital ridge results in ita
tninRformatiun into the so-cilUciI indiffeient sexual gl&nd (Plate
VII., Fig. 1), a structure common to Iwth sexe.s at this stage.
The essential feature of this process is that the thickened
raesothelial cells overlying the genital ridge become modified
in character and penetrate the ridge in the form of conls or
Btrands of cells. These mesothelial e]eioent.s were called by
 
 
 
 
Fin. 1».— Crou-fcrtlnn Uirnugh
IcrlBn duct, and the leiual gland of ■ vMcV nr i
mrunlnud 100 dliiineMn: m, iDeacDti>ry: L, Mimatopleun
germinal epllheliiim tnim wlilob the Milllprlan dtict (i) hi
Uilckcncd pari of the Rcrminal eplih.lium, iii wlilch the
 
 
 
lifun
 
 
 
nviilii
 
 
 
 
 
 
r the UUlilsy (afttr WaldcyOT),
a', [he nglon of the
been invaglnated ; a,
rlRiary sexual cvlli. C
 
 
 
Waldeyer the genninal epithelium, because, after their extenaion into the interior of the ridge or gland, they give ri.se to
the genn-cells, namely, the ova or the spermatozoa as the case
may lie. The eelt-cnrda include two kinds of elements, the
smaller mesothelial cells and the primitive sexual cells, which
latter art! laryir und less numerous than the mesothelial cells
 
 
 
 
I>ia?ramm«llc reprewnliillnn of the ilcmlopmeiil of [be genllim
WolIBiMi bod; and Its derlvittlTeg belni; colored red, the MUUerlBn
rtTBtlves, green: I. ludHRri-nt type; i ilnHBerent type, Islet Btage,
HfllleTlan ducta and tlic piimillve areter now openlog into ibe iirc^eultal ilnn*
e type, lower ooda of MaUerlMi dnole ftued lu futm Ibe ilniu pocuUirli: ~
 
 
 
B type.
 
 
 
Xom, the
 
.,»«,. I
 
nkn and ^M
 
 
 
 
awl kive hr^ niicK^»lati*tl iiurl<M. Tlir tirlmlHv«i m^^^At
 
or primitiye ota, tin' mi imiIIihI iM^Hiiiat' H Ihi
that thoy ilovolop olllirr inli» \\u' n\ii tit 0\t*
1 filaments accimliii^ tt) \\\v **v\ nl iIh' i'ImI«i\i« t'lu*
ceU-cords have boon so<mi in \\\v iiHlillrrtnl iiImmiI i«I tltr
hmun embn'o a:!: oarlv as (In* lil'tli wv^U \\ \\s\^ \\\s\\
althoagfa thero are no ^ross st^xiiitl tliiliiiiliitn i ii t>i«iMti"*ti«ii
it ia po^iblo to ilotonninr iVnni tlir lii^tolnpitiil i)fn*ti'ii \ M
the organ whether it is tn Im* n Itwti^j m tiM«M*n\. \\\* l'M«it>
oells bein^ far loss iniinrniiM itliiti\i>l\ in \\\* t^Mni t
 
se than in the hittor (Nii^rl).
 
The indiBbront soxnal ^IiiihI iMiinr : imIh 'Hi i-^pn i'lllx .In .•
rehitiun with tho n|)|NT or si*\niil •:i'iii> nl tin- nii i^nipln*
orWolffianb<Kly (IMato V 1 1., \'"\\\. 1 1. llM«'i|»niMi -Mh . .m \\!\\. U
fact will a|>|)oar hit or.
 
The olomunts tif tlir inililli'tonl >>i!ifif m| sU,- . om\ ^)
panitus arc, tlioroinro, tho InililTpiniti m>^tmt p\*\\\\\ \\\r \\\s\^»^
dnct, and tho duct of Miillnr r ritiii \' 1 1 !(.. I •>« ) . «^^
this ai«exual sta^o, ritlirr the mnli m Hi. {.«««. d, n | . ; ^ >
duced by tin* inot:nnoi'|iliii:i : nl lln iM.liil. » «m .,)«.• J . •
the testicles or tlir oviiii*- Mini Hn i.mmi»i; ^ » i.. .
provide for tli(* osoa I H' (iT tin •:• • umI i L m. n» iii | .i.,«t
or the ova, |)nMlnr(Ml \ty tlnni
 
The Male T3rpe of Hnminl FtriHiMM 1 1,. .WtV « ^^m.^;. m . «
the indifforoiit soxiinl ■^■:fMu iiii< «li.. m.j. •«)«, ; .0;.»..l
 
by the furtlior ili-vrlnjiim nl >•! ."»i». i-m »m.I J«, .tn>'|*lu
 
or the arrostoil |»rii\vlli •.! mHi. ♦
 
The teSticlo llil-: IL <ImiiIi|i. Ill ik:iM ini. llii |«tiipti qiii'titiotV
part of tho orpin i -. iniiiliniil In dti im ('ihiiii|ilin t - •«! ilio
 
indifToroiit scxiimI ^'IiiimI. \Jiili ii - :\ dm ni pfYptonl iturU i-i
furnisli(;(I liv ilw Wnllli'in Imih Miiifmn h'l > liri-n nciilr
 
■
 
of tho O4;ll-r.onl"- of I III' iiiilillrtrill :r\llMl ^JmihI = Mini ot' (lirir
origin from tin* nn- ■nlliilnihi nl (hr IumIx rii\il\. <unl mUo <»('
the fact tliMt llii'v ron-M-l nf tlir iintilliM' inrMotln'lijil orlU
and tho lar^^rr and Irrvi ninnrroiiM priinitivo srvnal (*olls.
The mesothelial coIIn inrrraso in nninhor and Uooonio so
grouped as to form (^ylindrioal masses known as sexual cords,
each of which inolndos sonu* of tlu* primitive seminal or
 
 
 
246 TEXT-BOOK OF EMBRYOLOOY.
 
sexual cells. By the ingrowth of connective tissue from the
surrounding mesoderm, tlie timica albnginea is formed and
the sexual cords are divided into roundish masses, each of
whicli is made up of many of the smaller elements and a less
number of the large seminal cells. These follicle-like masses
become hollowed out to form the seminal ampulla, which
afterward, undergoing great increase in length, are transformed into the seminiferous tubules. During fetal life, however, and even to the period of puberty the "tubules"
remain solid cords of cells. The small mesothelial or epithelial cells give rise to Sertolli's columns, while the primitive
seminal cells produce the spermatogonia.
 
Spermatogenesis, or the development of the spermatozoa
from the cells tliat line the seminiferous tubules of the functionating testicle, has been considered in Chapter I.
 
While the sexual cords are bein": transformed into the
cylinders that become the seminiferous tubules, the surrounding mesodermic tissue ix^netrates the genital gland and forms
the connective-tissue septa that constitute tlie stroma of the
organ and divide it into lobules. At the same time, also,
marked chanj^es occur in the Wolffian bodv. From certain
of the Malpighian corpuscles of this structure, cords of cells,
the medullary cords, grow forth and penetrate the genital
gland, their ends fusing with the primitive seminiferous
tubules. The conversion of these cell-cords into tubes
furnishes the initial part of the system of excretory ducts
of the testicle, namely, the vasa recta and the rete testis.
Somewliat later, in the twelfth week, tlie rete testis is extended to form the vasa efferentia, and still later, in the
fourth and fifth months, the efferent vessels lengthen and
become tortuous, producing thereby the coni vasculosi or
head of the epididymis (Plate YII., Fig. 3; Fig. 126).
The upper part of the Wolffian duct develops into a convoluted tube which constitutes the body and tail of the epididymis, while the lower |X)rtion becomes the vas deferens, thus
completing the system of canals provided for tlie escajx* of
the spermatozoa. Near the caudal end of the Wolffian duct
a little pouch-like evagination grows from its wall and becomes
 
 
 
THE ISTBRNAL OEHERATIVE ORGASS.
 
 
 
247
 
 
 
 
Fia. 126— Inlemal generative o
EBni of ■ m&le fetua of about fouruc
wiiekB (Waldeyer); (. testicle; t, ej
dldymla: f. Wolffian duct: u>. lowi
jlfflan body; j, guberoac
 
 
 
the Beminal vesicle, the lower end of the duct, below the orifice
of the semiDal vesiule, being the sjacnlatory duct. Since the
"Wolffian duct terminates in the
cloaca, and since the anterior
part of tbc cloaca corresponds
to a portion of the later urethra, the termination of the
ejaculatory duct in tlie prostatic part of the urethra ig
accounted for. Thus it will
be seen that while the secreting part of the testicle results
from the transformation of the
indifferent genital gland, the
secretorr cells having their
origin in the germinal epitlieliam, the complicated system
 
of dnctd with which it is provided is fumbhed by the mesonephros or Wolffian body.
 
The series of tubules connected with the upper extremity
of the Wolffian duct, the remnant of the pnmspbros or headkidney, frequently persists as a little pedunculated sac attached to the upper part of the epididymis ; it is known as
the stalked liydatid and sometimes also as the hydatid of
MorgagnL The posterior or lower set of Wolffian tubules
likewise give rise to an atrophic structure, the paradidymis or
organ of Oirald^a, which consists of a series of short tubes
closed at both ends, lying among the convolutions of the tail
of the adult epididymis (Plate VII., Fig, 3), while a lateral
evaginalion from that i>art of the Wolffian duct which forms
the tail of the epididymis becomes thevaa abeirans.
 
The dnct of Uttller remains atrophic, in the male, throughout its entire extent, and in fact, with the exception of its
two extremities, it usually altt^ther disappears. Its upper
extremity persists as a small vesicle, the nnstalked or sessile
hydatid, attached to the upper aspect of the testicle. The
lower extremity of the duct, uniting with its fellow, becomes
converted into the sinus pocnlarls or ntenu mascnlintiB of the
prostate gland (Plate VII., Fig. 3). If the intervening part
 
 
 
248 TEXT-BOOK OF EMBRYOLOGY.
 
of the tube persists to post-natal life and remains patulous^
it is known as the duct of Rathke.
 
Tlie change of location whicii the testicle undergoes is a
conspicuous feature of its development. To understand this
clearly, it is necessary to recall the relation of the mesonephros and the genital gland to the peritoneum. Since
both of these bodies originate from the cells of the outer
wall of the body-cavity, or, in other words, from what becomes the parietal peritoneum, necessarily they lie between
the body-wall and the parietal peritoneum — that is, behind
the peritoneal cavity. With the increase in size of these
structures, they project toward the peritoneal cavity, the
peritoneum passing over them and forming a "mesentery,"
which anchors them to the posterior wall of the abdomen.
In the case of the testicle, this peritoneal fold or " mesentery "
is called the mesorchium ; in the case of the ovary, the mesovarium. It is prolonged upward to the diaphragm as the
diaphragmatic ligament of the primitive kidney, and downward to the inguinal region as the inguinal ligament of the
])rimitive kidney (Fig. 120), since this latter organ is the
largest constituent of the projecting mass. When the primitive kidney has disiippeared as such, the inguinal ligament
nientiontul seems to connet^t the ovarv or testicle with the
inguinal region of the abdominal wall.
 
The inguinal ligament eontiiins between its folds connective ti>«^ue and unstriped muscular fibers. These become the
gub«»)iUACulum testis in the male or the round ligament of the
uteiHH in tl»e female. As the bodv of the fetus continues to
)i}\^\\ while the tissues of the ligament remain stationary or
i;ro\N lr.-»-' rapidlv, the testicle is gradually displaced from its
pmiiiioM al iho side of the lumbar spine, and by the third
uioutii Uiuhr.-x I he false pelvis. In th(» fifth and sixth months
it it in roui.u'l willi the anterior abdominal wall, near the
iniu'i' iJuloioiiial ring. In th(» eighth month it enters the
in>;ninal lanal, and \\\^\v the end of th(> ninth month, shortly
bi-fou' Iniih, it K'a\e« the inguinal canal and enters the
 
* .\i>M «Uv.ivui .>i \\w Uviiis'U>M. \^llli (H>nstM)Ucnt emptiness and flabbinesH
 
 
 
THE INTERNAL GENERATIVE ORGANS. 249
 
Before the testicle leaves the abdominal cavity, the parietal
peritoneum pouches through the inguinal canal into the
scrotum, the protruded part being the processus vaginalis.
Since the testicle is from the first behind the parietal peritoneum, it passes into the scrotum behind the vaginal process,
the latter then folding around it as a shut sac of two layers.
Subsequently the connection of the sac, now the tunica vaginaJis testis, with the abdominal peritoneum is reduced to a
slender strand of tissue lying in front of the spermatic cord.^
 
The testicle necessarily carries witli it, in its descent, its
blood-vessels, the spermatic artery and vein ; its duct, the
vas deferens ; as well as its nerves and lymphatic vessels ;
and these structures collectively constitute the spermatic cord.
 
The Female Type of Sexual System. — While the indifferent
sexual gland, in the development of the male generative system, undergoes metamorphosis into the testicle, it becomes,
in the evolution of the female type, so altered as to constitute the ovary ; and while the Wolffian tubules and the
Wolffian body become in the male the system of excretory
ducts of the testicle, they produce in the female merely
atrophic structures. On the other hand, the duct of Miiller,
which gives rise in the male, to atrophic appendages, forms in
the female type 'the Fallopian tube and, by fusing with its
fellow of the opposite side, the uterus and the vagina.
 
The ovaiy results from alterations in the structure of the
genital gland analogous to those that occur in the evolution
of the testicle. The special features of these changes are
better understood, however, than arc those of the testicle.
As in the case of the development of the testicle, the mesothelial cells on the peritoneal surface of the genital ridge
become thickened, these altered cells constituting the germinal
 
of the scrotum, is designated cryptorchism (hidden testes). The presence
of but one testicle in the scrotum is called monorchism,
 
* Occasionally it happens that the funicular process of the tunica vaginalis — that is, the stalk of the sac, remains patulous throughout its entire
extent, a condition which allows of the easy and sudden protrusion of a
segment of the bowel into the cavity of the tunica vaginalis, constituting
the so-called congenital hernia. Or the funicular process may close only
at one or the other end, givitig rise to other varieties of hernia.
 
 
 
 
2W TEXT-BOOK OF KMBRYOLOGY,
 
epithelium (Fig. 125). Coiuciden tally, (lie primitive connective tissue — iiiesodermic tii^ue — u ml e Hying the germinal
epitlieliiim pmliferates, contributing to the thickness of the
genital ridge. By the sixth
or seventh ^vcek, tlie germinal epithelium consi!^t»of
several strata of cells, gruu{>3
of which begin to penetrate
the umierlying niesodermio
tissue in the form of cordlike procesBSB (Fig, 128,
f, sch). The indifferent
mcsodermic tissue at the
same time increases in quantity, in turn penetrating
between the groiijis of advancing cells, so that what
takes place might be described as a mutual intergriiwth. The presence of
the growing connective tissue acceniuatfis the grouping of the cells into cylindrical
masses. These latter are the sexual cords or egg-colunma
(Pfluger's egfi-tulie,-). They ccntyin twit special kinds of
cells, the large sexual cells or primitive ova (Fig. 128, xie),
and the smaller hnt more numenins mesothelial cells.
The connection of the sexual cords with the germinal epithelium is much more obvious in this case than in the
case of the developing testicle, and the primitive sexual cells
are much more abundant. The e^-colnmns, surrounded by
young connective tissue, constitute the nucleus of the cortical
part of the future ovary. This mass is later sharply marked
off from the free or peritoneal aspect of the gland, the region
of the germinal epithelium, by a zone of proliferating mesodermic cells which become the tunica albuginea of the ovar^'.
An important change now takes place in the egg-columns ;
the primitive ova, or large sexual cells, increase in size, their
nuclei l>ecoming espi-cially well ilevelojKH], while the small
 
 
 
FiO. 127.— Internal or^ni of ft remale
Ibtiuof sbtiut fourtti'ii werks (WnMeyer):
0, ov»ry; *, epoSphoron or ptrovnriiim :
u-.WulfflKD duct: m. MUllerlftD duct; v.
iQwcr put uf (he Wulfflaii body.
 
 
 
THE INTERNAL GENERATIVE ORGANS. 251
 
mesothelial cells become smaller and less conspicuous. It
frequently happens that several of the large cells fuse into a
single mass of protoplasm, while one of the nuclei outstrips
the others in growth and, with the surrounding zone of
protoplasm, becomes the ovtun. Each egg-column is now
broken up into several groups of cells by the penetration
of connective tissue, each group (Fig. 128, c, «;/(') containing a single ovum, but many of the smaller cells.
 
 
 
c'^\^-i^
 
 
 
Fro. 128— PBrt of safflltal aeclfon of ati iivarx of a ehlld jujt born (after W«ldeyor). Hlfdily matniifled: te, Berminal epilhi-llum ; t. acft. PflQger's egg-lubes;
ue, primitive ova iylng on the germinal epilhclium ; e, kK, long PflOgert tubes. In
process of being coiiverled Into folll«les ; el, b. L-gK-balla (nests), likewise in process
of being resolved Into follicles: /, youngest follicle already Isolated: gu. bloodveBsels. In the tubes and egg-nests the primordial eggs are dlstingofshable from
tbe smaller epithelial cells, the future follicular epltheUum.
 
These groups are the young Qraafian folllclas of the ovary
(/). The enveloping zone of connective tissue becomes
the tbeca of the fotliote, while the single large cell constitutes
the omm, and the smaller cells are the membrana granulosa.
At first the graniitosa cells surround the ovum as a single
layer of flattened cells which gradually assume the columnar
type and become so numerous as to form many layers. They
secrete a fluid, the liquor folliciili, which crowds the ovum to
one side of the follicle where it is enveloped by a special
group of graniilosa-cells, the discns proUgenu (Fig. 129).
 
 
 
252
 
 
 
TEXTBOOK OF EMBRYOLOGY.
 
 
 
The question of the origin of the lolliouhir cells ia still an
unsettled one, though it seenib probable, thut the> are derived
from the cells of the egg <. Itimiis jnd Minot believes that
thej are probabjv ikscendt i from the prmiitue o\a.
 
Till formation of new Qraaflan follicles iiid consequently
of o\!i, begins in the decjitr pirt of thi o\ar\ and advances
toward tlic iiurface TIk production ot o\a and follicles is
 
 
 
 
Fin. tiB,— Section of human OTary. including cortex . a, geriolnal epiltiellum
or ft^e aurdicv: b, lunica Bltjuglnea; c, peripheral aironm canUlnlng loimBluTe
Gnuflnii follicles Cf I -. c. WHtl-advonced fullicle IWim whcsc wall memljninil granaloBii hu partialis Mimrsleil :/. cavity of liquor folUcull: g, ovum surruunded by
cell-miut coiistliullng illaciii proligenis (Pis reol).
 
limited to the fetal stage and to the early part of post-natal
life, their formation not occurring, according to Waldeyer,
after the second year.
 
What has been said above refers to the development of the
cortex of the ovary. The medulla is produced by the growth
toward the egg-columns of cord-like processes, the medollaiy
cords, fi^m the epithelial walls of the Miilpighian corpuscles
of the primitive kidney or Wolffian body, the cords l)e('oniing
surrounded by connective tissue and forming a network. The
fetal medullary cords are reprcsentwl in botli the cortex and
the medulla of the mature ovary by the groups of interstitial
cells di'-]>o«ed between the bundles of the stroma-tissue.
 
 
 
THE INTERNAL GENERATIVE ORGANS. 253
 
The Oviducts, the Uterus, the Vagina. — The system of passage-ways that constitute the outlets for the ova and the
means of nourishing them and evacuating the product of
gestation from the body in the event of impregnation —
namely, the Fallopian tubes, the uterus, and the vagina — result
from the metamorphosis of the ducts of Miiller. These ducts,
as stated above, lie along the dorsal aspect of the body -cavity,
separated from it by the parietal peritoneum, and parallel
with the primitive spinal column (Plate VII.). The probable
method of their formation has been pointed out (p. 243).
Near the lower (caudal) end of the body they approach each
other, and finally unite about the second month to form a
single duct for the rest of their extent (Plate VII., Fig. 4). The
upper, ununited parts of the ducts become the Fallopian tubes
or oviducts, while the lower portions, now fused into one,
become the uterus and the vagina. The upper end of each
single duct expands trumpet-like to form the fimbriated extremity of the Fallopian tube.
 
Until the fifth month there is no distinction between the
vagina and the uterus, the two being represented by a
single sac-like structure. The development of a circular
ridge in the wall of the sac marks the division between the
two organs, the part above the ridge acquiring thick
muscular walls, while the part below it, the future vagina,
remains thin-walled and more capacious. In the third
month the uterus is bifid at its upper extremity, a condition
which is permanent in some animals and occasionally in the
human subject.^
 
The Wolffian duct, which, in the male, becomes metamorphosed into a part of the epididymis and the vas deferens,
remains undeveloped in the female, producing merely atrophic
or vestigial structures (Plate VII., Fig. 4). The upper series of
Wolflian tubules, the remnant of the pronephros, frequently
 
' The formation of the uterus and of the vagina by the coalescence of two
parallel tubes affords an explanation of the uterus bicomis or bifid uterus and
of the condition of double uterus sometimes met with, as also of the presence
of a median septum in the vagina^ since by the failure of union of the two
tubes in greater or less degree one or other of these anomalies would result
 
 
 
254 TEXT-BOOK OF EMBRYOLOGY.
 
persists, as in the male, in the form of a small pedunculated sac,
the stalked hydatid or hydatid of Morgagni. When present, it
is to be found in the broad ligament, in the neighborhood of
the outer extremity of the ovary. The middle or sexnal
series of the Wolffian tubules with the adjacent part of the
Wolffian duct, which, in the male type, develop into the
epididymis, become in the female, an atrophic structure
known as the epodphoron or parovarium, or organ of Bosenmiiller (Fig. 127). This structure, which is almost constantly found between the layers of the broad ligament in
close proximity to the ovary, consists of a larger horizontal
tube representing a segment of the Wolffian duct, and of
shorter vertical tubes joining this at a right angle and representing the transverse Wolffian tubules. The lower set
of small Wolffian tubules, those which, in the male become
the paradidymis, give rise in this case to a similar atrophic
body, the parodphoron. This is also situated in the broad
ligament, usually to the inner side of the ovary. The
Wolffian duct, with the exception of that portion of it that
assists in the formation of the parovarium, usually entirely
disappears. Occasionally, however, it persists as a small
canal traversing the broad ligament close to the uterus and
passing on the dorsal side of the upper part of the vagina to
be lost upon the wall of the latter or, more rarely, to open
near the urinary meatus. When thus persistent, it is known
as the duct of Gartner.
 
The change of position of the ovaries is similar to, though
less marked than, that of the testes. The inguinal ligament in
the female (Plate VII.) extends from the primitive position
of the ovaries in the lumbar region of the abdominal cavity
to the groin, where it pas>5es through the abdominal wall,
traversing the inguinal canal, to terminate in the labium
majus. The upper part of this ligament, coiitiiining involuntary muscular substance, firmly unites with the ovary. In
the third month the ovary descends to the lower part of the
abdominal cavity and is now connected, by the succeeding
portion of the inguinal ligament, with the uterus. This connection may be a factor in the fiiial change of position of the
 
 
 
THE BLADDER AND THE PROSTATE GLAND. 255
 
ovary — that is, its descent into the true pelvis. The part of
the inguinal ligament that passes from tlie ovary to the uterus is the permanent ligament of the ovary, ivhile the remaining portion, which passes from the uterus through the inguinal canal to the labium majus of tlie vulva, is the round ligament of the uterus. As the inguinal ligament perforates the
abdominal wall, a small diverticulum of peritoneum goes
with it. Normally this peritoneal pouch subsequently becomes obliterated. Occasionally, however, it persists and
then constitutes the canal of Nuck. Should tlie canal of
Nuck be present, the ovary may pass into or through it,
thus reaching the labium majus. A patulous canal of Nuck,
as in the case of a patulous funicular process of the tunica
vaginalis of the male, may j)ermit the sudden occurrence of
an inguinal hernia in the female. The mesovarium or
" mesentery '' of the ovary accompanies the ovary in its
descent and constitutes a fold of peritoneum which envelops
not only the ovary but also the adjacent part of the duct of
Miiller and the remnants of the Wolffian body. Upon the
uniting of the lower parts of the Miillerian ducts to form
the uterus and the vagina these mesovaria unite with each
other mesially to become the broad ligament of the uterus.
Thus it comes about that the uterus, the ovaries and their
ligaments, the epoophoron, and other fetal remnants are contained between the layers of the broad ligament.
 
The account of the development of the external genital
organs will be deferred until after the consideration of the
formation of the urinary bladder and of that part of the
urethra that originates from the same embryonic structure.
 
 
 
THE BLADDER AND THE PROSTATE QLAND.
 
As stated in Chapter V., the urinary bladder and a part of
the urethra are derived from the intra-embryonic portion of
the allantois. In the same chapter the allantois was described
as a sac which develo|3ed as a pouching-out of the ventral
wall of the gut-tract near its caudal end (Plate II.,
5 and 6). The sac protrudes from the still widely open
 
 
 
256 textb^mjk of embryology.
 
abdominal cavity, enters the extra-embryonic pAit of the
bodv-cavit*-, and reaches the inner >arface nf the fidse
amnion, with which structure it intimatelv unites to form
the true chorion (Plate III.L A? tht- «-all> of the abd<4DeD
gradually Q\i^\ leaving only the umbilical aperture, it is,
neceRsarilyy through tlii< aperture that the allantois prxH
trudes.
 
We have seen ''p. 91) what become? of the extra-abdominal part of the allantoic — in what dejrri'C* it contributes to
the formation of the placenta and of the umbilical cord.
Obviously, with the j^evering of the umbilical cord after
birthy all this extra-embiyonic ]Kirt of the allantois disappearsy giving rise to no adult organ.
 
Its intra-enibiyonic ]K>rtion consists of a tube extending
from the caudal end of the intestine to the umbilicus (Plate
II., 5 and Gj. As early as the second month, the middle
segment of this tube dilates and assumes the form of a spindleslia(KHl sac, which becomes the minary Uadder (Plate VII.).
The part of the tube connecting the summit of this sac with
the umbilicus remains small, gradually loses its limien, and
constitutes in the adult the (usually) impervious cord known
as the urachns. Should the cavity of the urachus persist in
its entirety, and should there l>e at the same time an external
o{>eniug at the umbilicus, the condition would constitute an
imibilical minary fistula. The proximal {>art of the allantois
— that is, the i)orti*)n intervening l>etween the bladder and
the intestine — is designat(f<l the sinns nrogenitalis, while the
caudal end of the intestine, which is, in (fffi'ct, a jH>nch in
which lH)th the allantois and the intestine terminate, is known
as the cloaca (Fig. 96). The urogenital sinus receives the
terminations of both the Mullerian and the Wolffian ducts
(Plate VII.).
 
In the sixth week or slightly earli<»r, there api)ears uix)n the
surface of th(i IxhIv, in the region corres|M Hiding to the iK>siti(m
of th<» <'loaca, a (l(>pr(»ssion, the cloacal depression (Fig. 96),
which lat<T, except in man and the higher mammals, meets
the cloaca, and thus establishes a communication l)etween it
and the exterior. In the Amphibia, in reptiles, and in birds.
 
 
 
THE BLADDER AND THE PROSTATE GLAND, 257
 
as also in the lowest mammals, the monotremes, the cloaca
is a permanent structure, and through it, in these groups of
animals, not only the fecal matters and the urine, but also
the genital products, the spermatozoa and the ova, are evacuated from the body. In all mammals, however, with the
exception of the monotremes, the cloaca undergoes division
into a posterior part or anal canal and an anterior urogenital
aperture. This division is brought about by the growth of
three ridges or folds, of which one springs from each side
of the cloaca and one from the point of union of the urogenital sinus and the intestine. These folds coalesce about
the eighth * week to form a complete septum, which continues
to thicken antero-posteriorly up to the time of birth and
constitutes the perineum.
 
It will be remembered that the ureters originally spring
from the terminal parts of the Wolffian or mesonephric ducts
(Fig. 118), Owing to alterations brought about by processes
of unequal growth, the orifices of the ureters subsequently
change their position so as to open into the urogenital sinus
(Fig. 121), and still later, by the further operation of the
same agency, they come to open into the bladder on its dorsal wall, thus gradually assuming their permanent relations
(PL VII.). After the division of the cloaca the urogenital
sinus, as stated above, opens independently upon the surface
of the body. In the female it is transformed into a short
tube, the urethra, and an expanded terminal recess or fossa,
the vestibule of the vulva (PI. VII.). In the male it becomes the first or prostatic part of the urethra.
 
In the twelfth or thirteenth week, the future prostatic urethra acquires very thick muscular walls, and the original
epithelial tube pouches out into the muscular tissue in the
form of little sacs, the lining cells of which assume the characters of secreting epithelium. In this way is produced the
aggregation of muscular and glandular tissue known as the
prostate gland. This is a well-developed structure by the
fourth or fifth month (Tourneux). The recess in the floor
of this part of the urethra, the sinus pocularis or uterus mas
* Fourteenth week, according to Miuot
17
 
 
 
258 TEXTBOOK OF EXBItYOLOGY.
 
enlintu, Iia« l>een previously referred to as the homolc^e of
the atcrus, being the persirtent caudal extremities of the
ducts of Miiller (Plate VII.).
 
THE EXTERNAL ORGANS OF QENERATION.
 
In the early stages of the development of the external
f^nital oi^ns no ^xual distinotions are apparent.
 
Reference has been made to the eloacal depression as a
superficial fossa which makes its appearance at the caudal end
of the body of the embryo iu the sixth week (Fig. 96). At
 
 
 
 
Pio. 1:10.— Pi:ur iiut'i:vulvi' lUufoa titAe\e\n\iiaenltit tliv i;!!!'^!!!! gciiical organr
(IndlR^rcnl typcl of Iho hurnun felui iif Sito M mm. {0.ie> to I J5 inuhl ITourneux) :
1, MEnlUl etnlneucG or lubcrck; 1, eliin<: 3. geoilal groorc; 4, gcnllal ridge; 5i
 
olilMBl ilupTWlxD : fi. cncr^Rcil emlaeace.
 
about the same period an ciieireling elevation, the genital ridge
(Fig. 130, -"1,4), is seen to surround this depression. Within
the gciiitul ridge, at the anterior part of the cloncal fbcsa, a
email tubercle, the genital eminence, appears at the same time.
On the under asjH'ct uf the tjonilal eminence there is soon
distiiiguishftlile tin; genital groove (Fig. 130,3), which apjieare
as if a continuation of the fis,sure-Iikc cloacsil dcpi-ession iji)^
 
 
 
THE EXTERNAL GROANS OF OENERATION, 259
 
and the groove very shortly becomes flanked by two ridges,
the genital folds, one on each side.
 
The genital eminence becomes the penis or the clitoris^
according to the sex of the fetus. It very early acquires a
knob-like extremity (2) which is the beginning of the glans
penis or of the glans clitoridis, as the case may be. Further
development of the glans is brought about by the appearance
of a partially encircling gr(X)ve which serves to differentiate
it from the body of the organ.
 
At this stage of development, the rudimentary organs, as
described above, are precisely alike in the two sexes. Early
in the third month — about the ninth week — sexual distinctions begin to become manifest. Since the female organs
exhibit the less degree of deviation from the early indifferent
form, they will be first considered.
 
The External Genital Organs of the Female. — The sexually
indifferent genital eminence which, as we have seen, presents
even by the end of the second month a rudimentary glans and
an indicaton of a prepuce, elongates somewhat and becomes
the clitoris. The genital folds bounding the genital groove
on the under surface of the genital eminence (Figs. 130 and
131, A) never unite with each other as they do in the male,
but become prolonged in the direction of the future anus and
constitute, by the fourth month, the lateral boundaries of the
orifice of the urogenital sinus, or, in other words, of the vestibule of the vagina (Fig. 131, 3). These folds, continuous
over the dorsum of the clitoris with its rudimentary prepuce,
are the nympha or labia minora (Fig. 131, 2), 7) of the fullyformed state. The masses of erectile tissue in close
relation with each labium minus, the pars intermedialis
and the bnlbns vestibuli, are the homologues respectively
of a lateral half of the male corpus spongiosum and its
bulb. The genital ridge, which, from the first, encircles
the genital eminence and the cloacal depression, and, consequently, the later clitoris and the aperture of the sinus
urogonitalis, increases greatly in thickness. The part of it
situated on the ventral side of the clitoris becomes the mons
veneris, while the lateral parts of the ridge become the labia
 
 
 
260 TEXT-nnOK OF EMBRYOLOGY.
 
ntajora of the vulva. The several jiarls of the female g«niJ
talia develop lo such a degree durhig the fourth month tlmn
their aexual characters at this time are well marked.
 
 
 
 
Fre, ISI.— FouKUCcesalvt aUR*Tiiprilovlip|.iiii 'i' .r im. ■ m. iim! jii-uital
Of the human remsle fisliis {Tuuruvuxi^ 1, clHi.tiK. •;, tluii.s clllDriilli; \
genlMlflnun:: 4, labia majors: S. anui; 0, rwc jgeHl emlncDPC : 7. Uljlami
 
The reader is again reminded that in tlie stage when the 1
cloaca is present, the Miilleriaii ducts terminate in the siniia |
I un^nitalis (Plate YII.). As previously stated, the stnus u
gsnitalia becomes the female urethra, ita terminal portion
expanding into the vestlbulum vagins. The openings of the
Miillerian ducts fall widiin tliis latter veatihular region of
the sinus. Tlie lower purtidu of the two ducts by this time,
however, have fused to form the uf«ru3 and the vagina, and
 
 
 
262 TEXT-BOOK OF EMBRYOLOGY.
 
the glans penis, wliilc the integuinentary fold that ])artially
encircles the latter assumes more distinctive character as the
prepuce. This fold gradually advances over the glans and
adheres to it, the adhesion j)ersisting until, or shortly after,
birth. All of the rudimentary penis, exclusive of the glans
and of the genital folds, becomes the corpora cavernosa of
the adult organ. The characteristic structure of the corpora
cavernosa is forcshadowcKl as early as the third month by the
appearance in the ])enis of capillary blood-vessels, which, in
the sixth month, undergo marked dilatation.
 
The groove on the under surface of the penis becomes
dee{>er, and the genital folds, which bound the groove laterally,
increase in size. This groove extends from the orifice of the
urogenital sinus to the glans penis. The genital folds, which
in the female, remain distinct and l>ecome the nymphie, unite
with each other in the male and convert the groove into a
canal, which latter is practically an extension of the urogenital sinus along the entire length of the j)enis to the
glans. The e^uial thus formcnl is the anterior part of the
male urethra, or, in other words, it includes all of the urethra
except its prostatic i)ortion, whicli represents the urogenital
sinus. The orifice of this newlv-formtHl canal, situated in
the glans, is the meatus urinarius. Failure of union of the
genital folds, either wholly or in part, results in total or in
partial deficiency of the floor of the urethra, tliis anomaly
being known as hypospadias. If the defective closure involves only the glans, the condition is denominated glandular
hypospadias.
 
The genital folds form not only the si<les and the floor of
the penile urethra, but by an extension of their growth, also
its roof, thus completely surrounding it. Upcm the acquisition, by the now united genital folds, of bhwKl-vessels and
cavernous spa(!es, they become the corpus spongiosum of the
j)enis, and thus is established the p<?nnanent or adult relation
of these partes.
 
The genital ridge be(^omes differentiated into two prominent folds or pouches placed one on either side of the root of
the penis. In the fourth month thes<' unite to form the
 
 
 
THE O LANDS OF COW PER. 263
 
scrotum, the line of union being indicated by the raphe.
Pailure of union of the two halves of the scrotum is one of
the features of certain forms of so-called hermapliroditism.
 
The glands of Cowper, which correspond to the glands of
Bartholin of the female, are developed, like the latter, as
evaginations of the terminal part of the urogenital sinus.
 
The accompanying tabulation exhibits a comparison of the
organs of the two sexes on the basis of their common origin.
Male and female parts that develop from the same fetal
structure are said to be homologous with each other.
 
Homologies op the Sexual System.
 
Fetal Structure. Female Organs. Male Organs.
 
Indifferent sexual gland. Ovary. Testis.
 
Wolffian body —
Its middle series of tu- Short tubules of par- Vasaefierentia^rete testis
 
bules and ovarium and and coni yasculosi.
 
Corresponding part of Horizontal or long tube Tube of epididymis.
 
Wolffian duct. of parovarium.
 
Keinainder of Wolffian Usually altogether dis- Yas deferens, seminal
 
duct. appears ; if persistent, vesicle, and ejacula
Gartner's duct tory duct
 
Upper serin of short Stalked hydatid of Mor- Stalked hydatid of Mor
tubules (pronephros). * gagni. g^^i
Lovoer series of tubules. Paroophoron. Paradidymis (org^an of
 
Girald^).
 
Duct of Miiller —
Its upper extremity. Fimbria of oviduct. Sessile hydatid.
 
Succeeding portion. Oviduct Usually disappears ; if
 
persistent, duct of
Bathke.
Remaining portion, by Uterus and vagina. Uterus masculinua.
 
fusion with its fellow.
 
External Organs.
 
Fetal Structure. Female Organs. Male Organs.
 
Genital eminence. Clitoris. Penis.
 
Genital folds. Nymphae and bulbi ves- Corpus spongiosum, en
tibiili. closing spongy part of
 
urethra.
Genital ridge. Labia majora. Scrotum.
 
Urogenital sinus. Urethra and vestibule. Prostatic urethra, mem
Glands of Bartholin. branous urethra, pros
tate, Cowper's glands.
 
 
 
2(54 TEXT-BOOK OF EMBRYOLOGY.
 
SUMMARY.
 
1 . Th(» male and the female internal generatiye organs, as
well as the kidney and the ureter, originate from the mesothelial Hning of the b(Kly-cavity, being directly produced
from the Wolffian IxhIv and the duct of Miiller.
 
»
 
2. The bladder and the female urethra, but in the male
only th(» prostatic urethra, result from the metamorphosis
of the intrn-ombryonic jmrt of the allantois, and are therefore
to be repirded as of entodermic oripn.
 
W. Ikfore the estiiblishment of the j)ermanent kidney, a
temponirily functionating organ, the mesonephros, performs
the oflice of a kidney during a jxirt of fetal life, and this
latter is preceded i)y tlie pronephros, an organ which, though
represented in the higher vertebrates by a vestigial remnant^
is functionally active only in larval Amphibia and in bony
fislu's.
 
L The Pronephros. — The mesothelial cells of the outer or
pari(>tal wall of the body-cavity become invaginated in a line
parallel with the axis of the body, and the cord of cells thus
Ibrined bectnnes hollowed out to constitute the pronephric or
s('j;;iiiciitnl du<'t. At several points this duct retains its connection with the surface-cells by nieiins of cell-cords, which
latliT beeonie tubes and acquire glomeruli. The long tube
anid the shorter tubules with their glomeruli constitute the
pronephros.
 
Ti. The Mesonephros. — The transverse segmentation of the
middle plate, whi<'h connects the ]>araxial mesoderm Avith
tlit^ parietal plate, results in the formation o( a series of
ccll-mass<'s, llu' nephrotomes. Each nephrotome becomes
a tube and a<M|uircs one or more glomeruli. The decjwr
eniU of the tubes bcc<)me c<»imected with the pronephric
*ir M«n;iiicntal dnct, which latter is known henceforth as
the nicsonephric or Wolflian duct. These tubes, with the
atljacent part of the Wolffian duct, constitute the mesonephros, which fnnctii»nat<'s, for a time, even in the human fetus,
as an organ of urinary ex<»retion. The entire AVolffian duct,
with the pronephric tubules and th(» mesonephric tubules,
eonstitntes the Woltlian body. The AVolffian duct ojK»ns at
its lower or caudal extremitv into the cloaca.
 
 
 
DUCT OF MULLER, 265
 
6. The metanephros or pennanent kidney develops in part
from a small diverticulum that pouches out from the lower
or caudal end of the Wolffian duct. The straight collecting
tubules and the pelvis of the kidnev correspond to the
dilated and subdivided fundus of this diverticulum, while
the ureter represents its stalk. The secretory tubules of the
kidney develop from the inner zone of the metanephrogenic
tissue. The surrounding mesodermic tissue furnishes all the
component elements of the ureter-walls and of the kidney
except the epithelial parts, as noted above.
 
7. The supraxenal bodies probably are derived, in part,
from epithelial outgrowths which proceed from the mesonephros to form the cortical part of the organ, and, in part,
from chains of small cells that bud forth from the embryonic
sympathetic ganglia to form its medulla. The surrounding
mesodermic tissue contributes the connective-tissue parts of
the suprarenal body.
 
8. The sexual apparatus in its earlier stages presents no
distinctions of sex. The elements of this earlv indifferent
type are the indifferent genital gland, the Wolffian duct, and
the duct of Miiller.
 
9. The indifferent genital gland originates in the mesothelium of the body-cavity. The mesothelial cells overlying
the ventromesial aspect of the Wolffian body undergo multiplication in the fifth week and thereby produce an elongated
elevation, the genital ridge. Further multiplication of its
cells and the addition of other elements bring about the
transformation of this ridge into the well-defined genital
gland, which now lies in close relation with the Wolffian
tubules. The mesothelial cells are the "germinal epithelium '' of AValdeyer, the cells that produce the ova or the
spermatozoa, according to the future sex.
 
10. The duct of Miiller makes its appearance soon after
the AVolffian duct. It lies parallel with and to the outer
side of the Wolffian duct and also terminates in the cloaca.
It is of mesodermic origin, being produced either by evagination of the mesothelial cells of the body-cavity, or by a
splitting off from the Wolffian duct.
 
 
 
266 TEXT-BOOK OF EMBRYOLOGY.
 
m
 
11. The generative systems of both sexes result from the
metamorphosis of the three structures making up the early
indifferent sexual apparatus — namely, the indifiei'ent sexual
gland, the Wolffian body, and the duct of MuUer.
 
12. The male sexual system is ])roduced by the transformation of the indifferent gland into the testicle, and the conversion of tlie Wolffian tubules and the Wolffian duct into
the system of excretory ducts for that gland, the short tubules
becoming the va.sa effcrc^ntia and coni vasculosi, while the
Wolffian duct itself furnishes the body and the globus minor
of the epididymis, the vas deferens, the vesicula seminalis,
and the ejaculatory duct. The duct of Muller remains undeveloped and is represented in the adult by the atrophic
sessile hydatid and the uterus masculiuus.
 
13. The female sexual apparatus is brought about by the
development of the indifferent gland into the ovary, and by
the metamorphosis of the upper segments of the ducts of
Muller into the Fallopian tubes, and the fusion of the remaining portions of the two ducts to form the uterus and
the vagina. The Wolffian duct and tubules give rise to
atrophic structures in the female, tlie most conspicuous of
which is the parovarium or ej)oophoron.
 
14. Both the male and the female external genitalia are
developed from fetal structures conmion to the two sexes,
the genital eminence, the genital ridge, and the genital folds.
The genital eminence is situated at the anterior or ventral
part of the eloaeal depression. The genital ridge is an elevation surrounding this pit and the genital eminence, while the
genital folds are on the under surface of the genital eminence,
on(» on each side of a longitudinal groove.
 
15. The Wolffian ducts and the ducts of Muller open into
the cloaca, but when that a|>erture becomes differentiated into
the anus and the urogenital sinus, as it does at the fourteenth
week, these ducts fall to the latter apartment. The orifice
of the urogenital sinus being at the base of the genital eminence, the sinus comes into continuity with th(» groove on the
under surface of the eminence.
 
16. The female external genitalia are produced by the
further development of the three structures mentioned above.
 
 
 
HERMAPHRODITISM, 267
 
The genital eminence becomes the clitoris. The genital folds
on the under surface of the clitoris become somewhat prolonged to constitute the labia minora. The genital ridge
becomes, anteriorly, the mons veneris and laterally the labia
majora. The orifice of the urogenital sinus is represented by
the vestibule, and since the Miillerian ducts near their termination in the urogenital sinus fuse to form the vagina, the
latter passage opens in the adult into the vestibule. Since,
also, the urogenital sinus receives the termination of the
allantois, which becomes the female urethra, the latter canal
likewise opens into the adult vestibule.
 
17. The male external genitals represent a further development of the embryonic genital eminence, genital folds,
and genital ridge than do the female organs. The genital eminence becomes the penis, the genital folds, uniting
with each other so as to surround the groove, producing
the corpus spongiosum. The groove itself, being thus
converted into a canal which extends the now closed urogenital sinus to the end of the penis, constitutes all of
the male urethra except the first or prostatic portion. The
prostatic urethra represents the proximal extremity of the
allantois. Since the Wolffian ducts open into the urogenital
sinus after the division of the cloaca, the terminations of those
ducts, represented now by the ejaculatory ducts, open into
the prostatic urethra ; and since the Miillerian ducts also
open into the urogenital sinus, the uterus masculinus, which
is the representative in the male of the terminal parts of the
Miillerian ducts, is found likewise in the prostatic urethra.
The lateral parts of the genital ridge, which, in the female,
become the labia majora, fuse with each other in the male to
form the scrotum.
 
18. The condition of so-called hermaphroditism may be
produced either by an unusual degree of development of the
female external genitals, resulting in a clitoris resembling a
penis and in labia majora which simulate a cleft scrotum ; or
by the arrested development of male organs, whereby the
genital folds and the genital ridges fail to unite, the urethra
in consequence opening at the base of the penis.
 
 
 
CHAPTER XIV.
 
THE DEVELOPMENT OF THE SKIN AND ITS
APPENDAGES.
 
Thk a]))iGiidatroH of the Kkin include the BebaceooB and
sweat glands, tlio manunoi; glanda, the nails, and the halis.
 
THE SKIN.
 
isiiitiiig of tlic epiderniU or cuticle and of the
lorm, or coritim, is derived from two sources,
the epitEiclial epidermis being a
prtKhict of tlie ectoderm, and the
corinm originating from themesoderm. The nuils and hairs are
outgrowths of the epithelial
layer, wliile tlio various glands
Are lierived from infoldings or
in vagi nations of the same
stratum.
 
The corinm, the connectivetissue i'"m|Mment n( the skin, is
an i>nt}:nhwtli of the cntis plates
tif the primitive segments or
somites (Fig. l;'.;!). If iirst appeju-s ill (Tilde P)rm in the
s^'i'inid niontii lis a layer of
spindle-cells liemtfitli the ectoderm. In till' third month, the
more siipcrlicial imrt of this
layer ac<iiiires more definite and
distinctive eharaeter, the nilher
loose aggn'gation of cells having
diilerentiate<l inti> a tissue whieh
is a niesh-work of handles of
 
 
 
 
THE SKIN. 269
 
white fibrous connective tissue with some intermingled elastic
and muscular fibers ; this constitutes the corium proper. The
deeper layer of cells becomes a loose, subcutaneous areolar
tissue containing a few scattered fat-cells. About a month
later the external surface of the primitive corium loses its
smooth character and presents numerous little elevations, the
villi, which project into the overlying epidermis. The villi,
being highly vascular, play an important i)art in the nutrition
of the epidermis and being also freely supplied with nerves
they sustain an equally important relation to the sensitiveness
of the skin.
 
From the middle of fetal life onward, the fat-cells in the
subcutaneous tissue increase in number to such extent that
there is formed a continuous and well-marked subcutaneous
layer of fat, the panniculus adiposus.
 
Certain of the cells of the primitive corium differentiate
into unstriated muscular tissue, forming thus the muscles
of the liair-follicles, the airectores pilonun, as well as the
subcutaneous muscular tissue of the dart(»s of the scrotum
and penis, and that of the nipple and of the perineum.
 
The epidermis, consisting of the suj>erficial horny layer
and the deeper mucous layer or stratum Malpighii, is entirely
an epithelial structure. Its element* are simply the descendants of the early ectodermic cells specially modified to afford
the necessary protection to the more sensitive and delicate
corium.
 
The division into the two strata of the epidermis is indicated a? early as the latter part of the first month, at which
time the cells of the ectoderm have become arranged into
two single layers, a superficial layer of rather large flattened
cells and an underlying stratum of smaller elements. The
cells of the outer layer, or epitrichiiun, which probably represents the future stratum corneum, successively undergo
degeneration and descpiamation, the places of those lost
being supplied by the formation of new ones from the deei)er
layer. As time goes on, both layers increase in thickness
and the hairs and the glands of the skin are gradually
formed. With increased proliferation there is increasingly
 
 
 
270 TEXT-BOOK OF EMBRYOLOGY.
 
active desquamation of superficial cells, and as the degenerate
and cast-off cells become mixed with tlie products of the
sebaceous glands, there is formed a sort of cheesy coating
of the skin, the vemix caseosa or smegma embryonmn. This
is first easily recognizable in the sixth month, and first covers
the entire surface of the body in the eighth month. It serves
to protect the epidermis of the fetus from maceration in the
amniotic fluid.
 
The completion of the epidermis, aside from the development of its accessory parts, consists simply in further increase
in thickness and in the modification of the superficial cells
to produce the characteristic scale-like elements of the corneous layer of the skin, accompanied by the differentiation
of the deeper cells into those of the rete mucosum or stratom
Malpiglm. The extent to which these modifications are carried varies in different regions of the Ixnly.
 
THE DEVELOPMENT OF THE APPENDAGES OF THE SKIN.
 
The Nails. — The nails have their beginning in little clawlike projections, the primitive nails, that appear upon the
tips of the still imperfect fingers and toes in the seventh
week.* These result from localized proliferation of the cells
of the epidermis, Ix'ing entirely epithelial structures. The
rudimentary nails project from the tips -of the digits, instead
of o<!eupying the dorsal position of the completed structures.
The claw-like primitive nail, between the ninth and twelfth
"weeks, becomes surrounded l)v a groove, which starves to separate it from the ij:eneral eetodermic
surface. Th(»se claw-like rudiments
of the human nails are (juite similar
to the primitive chiws of many mamY\r.. iw- Longitudinal sec- mals, the ])rinntive nail in ea(^li case
 
tion throuKh the toe of a <'«'r- •it i i i .1 •!
 
copitiu<ussoiftir<ieKrni.aur^: inchi(hnir a (lorsa! part, the nailn,>.naiH.iato:^A, plantar horn plate, and a portion which belonjrs
 
(Sohlenhorn) ; 7IM', nail-wall. * , . ,. .
 
t<) the ventral surra<*e ol the <ligit,
called the plantar horn (Fig. l.'U). The striking difference
between the nails of the human a<hilt and the claws and
* Or ninth week, Miiiot. ' A j^eniis of lon^MaikMl African monkeys.
 
 
 
 
DEVELOPMENT OF APPENDAGES OF THE SKIN. 271
 
hoofs of many animals is due in great measure to the
degree of development to which this ventrally situated
plantar horn attains. In the hoofed mammals (Ungulata)
and the clawed mammals (Unguiculata), the plantar horn
undergoes very great development, whereas in man it retrogrades and leaves no trace except the nail-welt, or the narrow
line of thickened epidermis where the distal end of the nailbed merges into the ordinary skin. After the atrophy of the
plantar horn, the dorsally situated nail-plate being alone
present, the rudimentary nail bears a greater resemblance
to the adult condition.
 
As the nail-plate gradually acquires more distinctive character, the deeper layers of the skin specialize into a structure
adapted to its nutrition. This is the nail-bed, a highly vascular and sensitive cushion consisting of the corium and of
the stratum Malpighii of the epidermis. It is especially
from the proximal part of the nail-bed, representing the
matrix of the fully-formed condition, that the nail grows.
The rate of growth is such that- the ends of the nails protrude beyond the tips of the digits in the eighth month.
 
The tissue of the folly-formed nail corresponds to the
stratum lucidum of the typical epidermis, developed to an
unusual degree. The epitrichium or future stratum comeum,
the most superficial layer of the epidermis, does not form a
part of the nail, but constitutes a thin covering, the eponychium ; this is lost in the seventh month, with the exception
of a small band over the root of the nail, which persists for
a short time as the perionyx.
 
The nails of the toes are always somewhat behind those of
the fingers in development.
 
To repeat, the claw-like rudimentarj- nails appear in the
seventh week, the nails are perfectly formed about the twelfth
week, and break through their epidermal covering in the
seventh month, reaching to or beyond the finger-tips in the
eighth month.
 
The Hair. — Each hair consists of the projecting shaft and
the embedded root, with its expanded deep extremity, the
hair-bulb, the root being embraced by the hair-foUicle. The
 
 
 
272 TEXT-BOOK OF ICMBriYOLOGY.
 
hair is entirely of ectodermic origlu, being ileriveii from the
epidermal Isij-lt of fhi.' Hkin, while tlie liair-follicle is partly
derived from tlie epidermis and in jjart is a product of the
coriiim. The hairs are homologous with the feathers and
scales of the lower animals.
 
The development of the hair is initiated in the third fetal
month hy the appearance of small solid masses of epithelium
in the stratum Mulpighii of the epidermis. The epithelial
plugs or hair-germs grow into the underlying corium and
are met by outgrowths or papiUn of the latter, which develop
almost simultanoonsly. The papilla; are very vascular and
serve for the nutrition of the developing hair.
 
The root and the shaft of the rudimentary hair result from
the specialization of tlie axial or central cells of the hairgerm. These eells lengthen iu the direction of the long axis
of the hair-germ and become hard and corneous, thus constituting the ro'it and the shaft, the cells of the deepest part
of the hair-eenn forming the bulb. Tin- irrowtli of the hair
 
 
 
 
b«Jr; te, liuUiof hBtr; Aa, young hair;
of Ihelialt: W, balr-n>nide: M.wbBG<
 
 
 
Lir (Hertwlg): ^ and B,
Ir-pnpms : U, ireriD of
 
 
 
in length is due to the proliferation and specialization of the
cells of the bulb. The papilla of the niiderlying corinm
indents the deep surface of the hair-bulb, this close relation
of the two structures enabling the {>apilla the better to fulfil
its function of providing nourishment t« the bulb.
 
 
 
THE SEBACEOUS AND SWEAT-GLANDS. 273
 
The hair-follicle, consisting of an outer connective-tissue
portion or fibrous layer and an inner epithelial part, the
inner and outer root-sheaths, is partly of mesoderniic and
partly of ectodermic origin. The inner and outer rootsheaths are produced by the peripheral cells of the hairgerra augmented by cells contributed directly by the stratum
Malpighii of the epidermis. The outer fibrous constituent
of the follicle results from the mesoderniic cells of the corium
that immediately surround the hair-germ.
 
Gradually increasing in length by the addition of new
cells from the hair-bulb, the primitive hair at length protrudes from the follicle as free hair. This first growth of
hair is unpigmented and is extremely fine and soft, being
known as the lanugo or embryonal down. This appears upon
the scalp and some other parts of the body in the fourth
month, gradually extending over the entire surface in the
succeeding months. In the eighth month the lanugo begins
to disappear, but is not lost as a whole until after birth, when
the permanent growth of hair takes its i)lace. Upon the face,
in fact, the lanugo j)ersists throughout life.
 
The development of the secondary hair is still a disputed
point. It is claimed by some authorities (Stieda, Feiertag)
that they develop from entirely new hair-germs. Most authorities hold, however, that the secondary hair develops from the
same papilla that produced the hair just lost. According to
this view, the empty root-sheath of the cast-oflP hair closes so
as to form a cell-cord which represents a hair-germ for the
new hair. The cells of this germ in most intimate relation
with the underlying papilla produce the new hair in the same
manner that hair is produced by the original hair-germs.
As the new hair grows toward the surface the old one is
gradually crowded out.
 
The Sebaceus and Sweat-glands. — The sweat-glands, including not only the sweat-glands proper but the ceruminous
glands of the external auditory meatus and the glands of Moll
of the eyelids, are derived from the ectodermic epithelium.
The glands are of the simple tubular type. Each gland
develops from a small accumulation of epidermal cells that
 
18
 
 
 
274 TEXT-BOOK OF EMBRYOLOGY.
 
grows, in the fifth month, from the Malpighian or mucous
layer of the epidermis into the underlying corium. The
solid epithelial plugs l)ecome tubes in the seventh month by
the degeneration and final disappearance of the central cells.
The deeper part of the tube becomes coiled and its lining
epithelium takes on the (characteristics of secreting cells.
Some of the cells of the original epithelial plug undergo
specialization into muscular tissue, thus producing the inyolnntary muscles of the sweat-glands.
 
The sebaceous glands arc dcvelopeci from solid epithelial
processes that originate from the deep layer or retc inucosum
of the epidermis in a manner similar to that of the development of the sweat-glands. There is the difference, however,
that the ej)ithelial plugs acquire lateral branches and thus
usually produce glands of the compound saccular or acinous
variety. There is the further difference that the epithelial
outgrowths generally develop from the ectodcrmic cells of
the outer sheath of the root of the hair near the orifice of
the follicle (Fig. 136, id), in consecpience of which the ducts
of the finished glands usually open into the hair-follicles. In
some regions, however — regions devoid of hair, as the prepuce
and the glans penis, the labia minora, and the lips — the
growth is directly from the stratiini Malpighii, as in the case
 
of the sweat-glands.
 
The Mammary Gland. — The mammary pflnnd represents a
number of highly specialized p:lan<ls of the skin, so associated as to constitute the sinjrie adult structure. Its origin,
therefore, is to be sought in the cells of the epidermis in
common with that of the ordinary glaiuls of the skin.
 
It is claimed by many authorities, by (ie^enbauer especially, that the mamnue are modified sebaceous glands ; others
assert that they are to be elass<'d with the sweat-glands,
Ilaidenhain having shown that in the development of the
milk-glands there is no fatty metamor])hosis of the central
cells as in the sebaceous glands, and Minot enijihasiziiig tin*
fact that their mcwle of development closely resembles that
of the sweat-glan<ls.
 
The development of the milk-glands is begun as early as
 
 
 
THE MAMMARY GLASD.
 
 
 
275
 
 
 
\
 
 
 
the second month. At this time the deep layer of the
epidermis, in the bites of the future gknds, becomes thickened by the multiplication of its cells, the thickened patch
encroaching upon the underlying coriuni (Fig. 136, A, b).
Thia thickened area enlarges somewhat peripherally and its
mai^ins become elevated, owing to wliich latter circumstance
the piitch appears relatively depressed [B). The depression
is known as the Klaadular area., and it corresponds with the
fatnre areola and nipple. In many mammals the development of the milk-glands is initiated by the appearance of a
 
 
 
 
Fio. IM,— Secllunii representing (hree luceeislvc 8t»Be« of dBTclopmont of tha
human DiiimmB (ToiirDeui|; .1, fetiaof SlWmm. (l.S la): fi. of 10.16 eni. (4lii,):
C, of '^4.3% GtD. (9.A In.); u. epidermis; A. kggregnlloii of epldcmtKl veils funnlDg
■nUgo of gtand : e, gBlnctophoroiu dui^ts; d, groove limiting glamliitsr area; i,
(treat pectoral muscle; /, unslrlalcd muBLnilar tisaue of arvolu; g, subCDtlDeoiu
adipose tisaue.
 
pair of linear thickening;s of the epidermis on the ventrolateral aspect of the bfjdy, called the milk -ridges or milklines, from localized thickenings of which the multiple
mammary glands of such animals develop. These railklines have also been observed in the human embryo, but the
constancy of their occurrence in man has not as yet been
established.
 
From the bottom of the glandular area, numerous small
masses or bad-like processes of cells ^row down into the
corinm. Some of the buds acquire lateral branches. By
the hollowing out of these cell-buds the latter are transformed
 
 
 
276 TEXT-BOOK OF EMBRYOLOGY.
 
into tubes (c), which open upon the glandular area. The branching of the cords begins in the seventh month and is carried
on to such a degree that each original cell-cord gives rise
to a tubo-racemose gland. The hollowing out of the solid
processes begins shortly before birth, but is not completed
until after that event. Each cell-cord becomes, in the strict
sense, a complete gland, each such individual structure forming a lobe of the mature organ.
 
This stage of the human mammary gland — that is, a depressed gland-area upon which open individual glands, the
nipple being absent^ — is the permanent condition in some of
the lowest mammals, as in the echidna, one of the monotremes. In all higher mammals, however, further metamorphoses occur in the tis.sites of the glandular area, and in
the human fetus these tissues become the nipple and the surrounding areola.
 
The nipple is partly formed before birth, but does not
become protuberant until post-fetal life. The depressed
glandular area rises to the level of the surrounding parts,
and its central region, which includes the orifices of the
already formed or just forming ducts, swells out into a little
prominence, the nipple. This prominence is a protrusion of
the epidermis and includes the terminal extremities of the
milk-ducts as well as the blood-vessels and connective- tissue
elements which surround tli(» ducts. In the dermal constituent of the rudimentary nipple unstriated muscular tissue
develops. The region of the glandular area not concerned
in the formation of the nipple becomes the areola.
 
At birth, as above intimated, the manmiary gland is still
rudimentary, since many of the ducts have not yet acquired
their lumina nor their full degree of c(mij)lexity. Shortly
after birth a small quantity of milky secretion, the so-called
witches' milk, may be ex])ressed from the glands — in the male
and female infant alike. This is true milk according to
Rein and Barfruth, but according to Kolliker, it is merely a
milky fluid continuing the debris of the degenerated central
cells of those rudimentarv ducts that were still solid at birth.
 
So far, the milk-glands are alike in the two sexes, but
 
 
 
THE MAMMARY GLAND. 277
 
while in the male they remain rudimentary structures, they
continue to increase both in size and in complexity in the
female. The increase aflfeets not only the glandular tissue
proper but the connective-tissue stroma as well. At the time
of puberty the growth of the glands receives a new impetus,
which is very materially augmented upon the occurrence of
pregnancy. There may be said, therefore, to be several distinct phases in the development of the milk-glands, first, the
embryonic stage ; second, the infantile stage ; third, the stage
of maturity beginning at the time of puberty ; and finally,
the stage of fhU fanctional maturity consequent upon preg->
nancy and parturition.
 
 
 
CHAPTER XV.
THE DEVELOPMENT OF THE NERVOUS SYSTEM.
 
The nervous system of the adult, including the cerebrospinal axis and nerves, and the sympathetic system of ganglia
and nerves, is made up of the essential neural elements, the
neurons, together with the supporting framework or stroma.^
 
The neurons and a part of the stroma result from the
specialization of the ectodermic layer of the embryo. The
ecto<lermic origin of the nervous system acquires certain
interest in view of the conditions that obtain in some of the
lowest and simplest organisms. For example, in the ameba,
the single protoplasmic cell which constitutes the entire individual possesses the several fundamental vital properties of
protoplasm, such as resi)iration, metabolism, contractility,
motility, etc, in ecpial degree, no single property being more
highly developed than the others, and no particular part of
the cell exliibiting greater s])ecialization than the other parts.
In other words, the j)n)t()plasmic substance of the animal is
at once a respiratory mechanism, a nervous apparatus, and
an organ for the execution of the various other vital functions.
 
In somewhat more highly developed creatures, as the
infusoria, although there is no differentiation into separate
tissues and probably not even into separate cells, there is seen
some progress toward the sj)ecialization of certain parts of
the orgimism for the performance respectively (»f the different
functions of life. For example, the central part of the ani
' The neuronti are the units of which tlic nervous Bvstem is made up.
Each neuron consists of a nerve-cell with everything belonging to it -that
is, with its various processes, including tlie nxis-rylinder process or iieuritf
which becomes the axis-cvlindcr of a nerve-ri))er.
278
 
 
 
THE DEVELOPMENT OF THE NERVOUS SYSTEM. 279
 
mal has digestive functions, while it is by the superficial
portion alone that the creature is brought into relation with
the outside world, the sensitiveness or irritability of the
surface, by which the animal is made responsive to external
impressions, being the nearest approach to the function of a
nervous system that it possesses.
 
This primitive function of the surface of the organism
is suggestive as to the origin of the nervous system of
higher type creatures. It will be seen, indeed, that not only
is the nervous system proper derived from the ectoderm ic
cells of the embryo but that the peripheral parts of the
organs of special sense, as the olfactory epithelium, the organ
of Oorti, and the retina, have the same origin.
 
The alteration of those cells of the ectodermic stratum
that are to specialize into nervous elements begins prior to
the fourteenth day in the human embryo, in the stage of
the blastodermic vesicle. The change consists in a gradual
modification of the form of the cells, the cells common
to the general surface of the germ assuming the columnar type. The process affects the cells of the median
line of the embryonic area in advance of the primitive streak,
resulting in the production of a thickened longitudinal median
zone. This thickened area is the medullary plate (Fig. 41,
p. 70). On each side of the plate — which is apparent at the
fourteenth day — the adjoining ectodermic cells become heaped
up to form the medullary folds, which latter therefore bound
the medullary plate laterally. The medullary plate becomes
concave on the surface, forming the medullary groove (Fig.
137). By the deepening of the groove, the lateral edges of
the plate approach each other (Fig. 138), and finally they meet
and unite, thus producing a tube, the neural tube or canal.
 
Since the medullary folds similarly meet and unite with
each other — their union slightly preceding that of the edges
of the plate — the neural tube comes to lie entirely beneath
the surface-ectoderm and soon loses all connection with it.
The closing of the tube and the union of the medullary folds
occur first near the anterior end of the embryonic area, in a
position that corresponds with the region of the future neck,
 
 
 
2S*I TEXT-ROOK fiF EMUnYOLOGY.
 
flnil fnim this jwint it proceeds Iwlh cephalad and caudad.
Sinw the nie»!ullar>' fnUis at their caiidal extremity embrace
 
 
 
 
ITctKkarJ, Scmitr. Cnl /•laJrrm.
 
Fid. is;.— TrBDirene lecllon of > ■Ixteen-Rnd'a-l
 
 
 
mbryo poaMctlOK
 
 
 
the primitive streak {Y\g. -11, p. 70), the latter structure i»
inchidefl within the cjuidiil end of the neural tube by the \
 
 
 
 
isHTie BBcUiiii of II fitl<;i;n-mnl a-hslf-dnj sheap embryo
Kvca lomltci (Bonnet).
 
coming together of the folds, and thus the blastopore, which
was previously the external aperture of the archonteron.
 
 
 
THE DEVELOPMENT OF THE SPINAL CORD. 281
 
comes to constitute the neurenteric canal, or an avenue of
communication between the neural canal and the primitive
intestine.
 
The neural canal then is a tube composed of columnar
cells, which is formed by the folding in of the ectoderm and
which occupies the median longitudinal axis of the embryonic
area and consequently of the future embryonic body. From
this simple epithelial canal the entire adult nervous system is
evolved.
 
The evolution of the highly complex cerebrospinal axis
from such a simple structure as the neural canal is referable
both to the principle of unequal growth — the walls of the
tube becoming thickened by the multiplication of the cells —
and to the formation of folds.
 
The portion of the neural canal — approximately one-half —
that is devoted to the formation of the brain is delimited
from the part that produces the spinal cord by the dilatation
of the anterior or head-end of the tube, and the subsequent
division of this dilated sac-like portion into three communicating sacs called respectively the fore-brain, mid-brain, and
hind-brain vesicles (Fig. 142). These three vesicles give
rise to the brain, while the remaining part of the neural canal
forms the spinal cord.
 
THE DEVELOPMENT OF THE SPINAL CORD.
 
In the growth of the spinal cord from the spinal portion
of the neural canal we have to consider the evolution of a
cylindrical mass of nerve-cells and nerve-fibers with the
supporting stroma from a simple epithelial tube.
 
The wall of the neural tube, although consisting at first
of a single layer of epithelial cells, is not of uniform thickness throughout its circumference. While the external outline is oval, the lumen of the tube is a narrow dorsoventral
fissure (Fig. 45, p. 73). The cavity is therefore bounded on
the sides by thickened lateral columns, while the dorsal and
ventral walls, which connect the lateral columns with each
other, are thinner and are called respectively the roof-plate
and the floor-plate.
 
 
 
 
282 TEXT-BUOK UF EMBRYOLOGY.
 
After a short time, the walls of the tube having thickened
by the multiplication of the cells, the shape of the lumen
alters, two laterally projecting
angles being addetl (Fig. 139).
The effect of tiiis change is to
partiallydivide each lateral half
into a dorsal and a ventral
region. The neural canal at
tlii,-i stage may be said to consist of six columnri of cells, the
two dorsal zones connected with
each other liy the roof-plate, and
the two ventral zones united by
the floor-plate. These regions
are also distinguishable, with .
certain characteristic modifi<a- I
tions, in the head-region of tlw I
tnl>e. They are important in
their bearing upon the further
development of the atrnctnre,
since the dorsal and ventral
zones are related respectively to the dorsal or sensory and
the ventral or motor roots of the spinal nerves.
 
The differentiation of the cells of the neural tube into two ^
kinds of elements, one of which gives rise to susteutative
tissue or neuroglia wliile the other produces the nerve-cells, is (
observed at about the end of the third week. The single ,
layer of columnar cells which at first comjMises the wall of
the tube, the long axes of the cells being radially arranged,
soon exhibits near the lumen a row of roond cells, jirobably
the first offspring of the columnar cells. The round cella
are the genn-cells or Kerminating cells, from which are develoiied ihe neuroblasts or young nerve-colls as well as the
neuroglia cells. All the other cells, known as the spongioblfista or ependymal cells, are concerned in producing susten
 
 
36<rWim Kaillkcr): c.ccDtraJ
t, its eplthdUI nntng: Me'
ortyl, Iho original placu of
ot (he cuul ; a, tlie wblte lU
of the anterior coluouu : a. g
atance of BnlerolUiiral born
terlar column ; or. miterlui
pr, iMatarior roola.
 
 
 
tati\
 
 
 
ssue.
 
 
 
Tiie stroma of the central nervous system includes two
constituents — a connective-tiBsue element, and a part, the
neuroglia, which is of epithelial origin, and which is not to
 
 
 
THE DEVELOPMENT OF THE SPINAL CORD. 283
 
be regarded, therefore, as connective tissue. The connectivetissue portion of the stroma is produced by the ingrowth of
the pial processes from the pia mater, and is hence of mesodermic origin.
 
The neuroglia is derived from the spongiobhtsts, which
result from the specialization of the large colnmnar cells of
which the wall of the neural canal is composed. These cells,
whose length comprises the entire thickness of the wall of
the tube in the earliest stages, undergo partial absorption and
disintegration, each cell being transformed into an elongated
system of slender processes or trabecule, and each such system
 
 
 
 
no. IW.-CrOBB-Bectlon through the ipinal cord of ■ Tertebnite embryo {after
His}: a. outer nmltlng membrane; b, outer DeurogUa layer. re«lon of future white
matter; e, germ^cells ; d, central canal: e. Inner UmitlnE membrane or ependymal
t>rer 1 /, spongioblaiU : a, neuroblaata (mantle tajer) : A, anterior root-flben.
 
being a completed spongieblaet or ependymal cell (Fig. 140).
Thp inner ends of the spongioblasts coalesce with each other,
forming thus the Inteinal limiting membrane, while the peri])heral extremities interlace with each other to form a close
network, the marginal velum. As the walls of the neural
tube increase in thickness, the spongioblasts become mors
and more broken up to form the delicate neurogliar networf
 
 
 
284 TEXT-BOOK OF EMBRYOLOGY.
 
with interspersed nucleated glia cells, which latter are derived
from some of the round cells noted above as lying near the
hmien of the neural tube and which have taken a position in
the marginal velum. Such of the spongioblasts as border
the cavity of the neuml tube become the cells of the later
ependyma of the central canal of the spinal cord and of the
ventricles of the brain. The cells of the ejwndyma become
ciliated in the human fetus in the fifth week.
 
The nerve-cells of the spinal cord — as also of the brain —
are tlie specialized descendants of the germ-cells referred to
above. The proliferation of the germ-cells produces the
neuroblasts, or young nerve-cells (Fig. 140). The latter elements move away from the primitive position of the germcells near the lumen of the tube and, taking up a position
between the bodies of the ei)endymal cells and the periphery
of the neural tube, develop into the nerve-cells. The transition is effected by the accumulation of the cell's protoplasm on
the distal side of the nucleus and its elongation into a process.
This ])rocess is a neurit or axon or axis-cylinder process and
is the beginning of a nerve-fiber. The dendrites or protoplasmic processes apj>ear considerably later. Some of the
fibers thus prcHluced grow out from tlie neural tube to constitute the eiferent filxTs of the jxTiphcral nerves, that is, the
ventral roots of the spinal nerves, while others contribute to
the formation of the fiber-tracts of the cord.
 
After the appearance of tlie neuroblasts and developing
nerve-cells, the wall (►f the neural tube is divisible into three
layers (Fig. 140): an inner or ependsrmal layer, next the
lumen of the tube; adjoining this, the mantle layer, made
up of neuroblasts ; and a peripherally situated neuroglia layer
or marginal velum, which occupies the position of the future
tracts of white fibers (►f the cord.
 
The alterations in the form and size of the sj)inal cord go
hand in hand with the histological changes noted above.
While th(»se areas that have been mentioned a> the dorsal and
ventral zones in{.Teas<» greatly in thi<'kness, the floor-plate and
the roof-plate — the ventral and dorsal walls of the neural
tube — remain thin i Fig. 141). They are n<'ver invaded by
the nerve-cells, but consist of thin layers of neuroglia which
 
 
 
TUE DEVELOPMENT OF THE SPINAL CoRD.
 
 
 
285
 
 
 
later become penetrated by nerve-fibt-ra tliat pro"' fnim oiif
side to the other. They thus represent the anterior and posterior white commiBsmes of the cord. Thetie plates remain
relatively fixed in position because of their failure to expand,
while tlie liitcral walls iif the tulie undergo great expansion,
in both the ventral and dorsal directions, as well as laterally.
In this way a median longitudinal cleft is produced on the
ventral wall of the spinal c<>r<l and a similar one on the
dorsal wall. These are the anterior and posterior median
fissures. Since the so-called jKisterlor median tissiiit is not a
true fissure but merely a eeptiim, it differs from the anterior
fissure, and It is held by some authorities that this septum is
 
 
 
 
 
 
 
formed by the gmwing togetiier of the walla of the dorsal
part of the central canal.
 
The flber-tracts or white matter of tlie spinal cord develop
in the outer or neuroglia layer, each filjer being the elongated
neurit of a nerve-cell. Some of the fibers originate from the
nerve-cells of the cord while others grow into the c«rd from
other sources. As examples of the former method may be
cited the direct cerebellar tract, composed of the axons of
the cells of the vesicular column of Clark, and the tract of ,*
 
 
 
286
 
 
 
TEXT-BOOK OF KMBRYOLOVY.
 
 
 
GrowtT, made up of the axons oi' cells t>f tlie dorsal gray
horn ; while the direct und crosat-d |iyniniiilul tracts ai-e the
axons of cells in the cortex of the ct-rebrum. and the tracta
of Goll and of Biinlach are composed largely of the axons >]
of the cells of the Hpinal ganglia (see p. 318), The devcl- 1
opmont of these filwr-tracts is not complete until the tilwrs
ap(|iiirc their niyeltn-sheaths (see p. 414), The myelination
of the tracts of Biirdach and of Goll occurs In the latter
part of the fourth month and in the tit\h month ; of the :
direct cerehellar tract, in the seventh month ; of ihe jiyramidal tracts, at or soon after birth.
 
As the walls of the neural canal thicken through the mul-^
tipliciition of the cells, the cavity of the tube is gradually]
encroached upon almost to obliteration. Whea development f
is complete, all that remains of the cavity is the sm&U ceatral f
canal of the spinal cord.
 
The lengtb of the spinal cord in the fourth fetal month \
corresponds with that of the spinal column, Its lower termi- J
nation being opposite the last coccygeal vertebra. From this 1
time forward, however, the cord grows less rapidly than does J
the spinal column, so that at birth, the cord terminates at the4
last lumbar vertebra, and in adult life at the second lumbar 1
vertebra. This gradually acquired disproportion in thai
length of the two structures explains the more oblique 1
direction of the lower spinal nerves as comi«ired with those g
 
E higher up. In the early condition of the cord, each pair of '
Uerves passes almost horizontally outward to the cprrespoading intervertebral foramina, but us the spinal column gradn- ]
ally outstrips the cord in growth, the lower nerves necessarily ^
pursue a successively more oblique course to reach their j
foramina, the lower nerves being almost vertical in direction
and constituting, collectively, the canda equina.
-J
(lev
dih
ves
nei
 
 
 
THE DEVELOPMENT OF THE BRAIN.
 
The encephalic portion of the neuml iuIk, — that part
devoted to the production of the bniin — after undci^ing
dilatation, becomes marked "ff into ihe thnc ])riman' brainvesicle«, the fore-brain or prosencephalon, the mid-brain or
mesencephalon, and llie hind-brain or rhombencephalon, by
 
 
 
THE DEVELOPMENT OF THE BRAIN,
 
 
 
287
 
 
 
constrictions in the lateral walls of the tube (Fig. 142).
Tlie constricted part of the hind-brain that adjoins the midbrain is the isthmns. This division occurs at an early stage,
before the closure of the tube is everywhere complete. The
vesicles communicate with each other by rather wide openings. As in the spinal part of the neural canal, the walls of
the primary brain- vesicles consist of epithelial cells, and it
is by the. muliiplicalion of these cells in unequal degree in different regions^ and by the fo)*r)iation of folds in certain localities,
that the various parts of the adult brain are developed from
these simple epithelial sacs.
 
The stage of three vesicles is soon succeeded by the fivevesicle stage, the primary fore-brain vesicle undergoing division into two, the secondary fore-brain (telencephalon) and the
inter-brain (thalamencepalon) or diencephalon, and the primary
hind-brain vesicle likewise dividing, a little later, into the secondary hind-brain (meteneephalon) and the after-brain (myelencephalon).
 
The division of the primary
fore-brain is preceded by the
appearance upon each of its
lateral walls of a small bulgedout area which soon assumes the
form of a distinct diverticulum.
This is the optic vesicle, the earliest indication of the development of the eye (Fig. 142). In
the further process of growth
the base of attachment of the
optic vesicle becomes lengthened
out into a relatively slender pedicle, which remains in connection with the lower }>art of the
hiteral wall of the brain- vesicle.
Following the appearance of the optic vesicle, the anterior
wall of the primary fore-brain vesicle projects as a small
ovagination, which latter is then distinctly marked off from
 
 
 
 
 
Anterwr hrtUn-wtieU.
 
Middle brain-vesicle.
P0Uerior breun-vesicle.
 
Fare-brain.
 
Primary optic vesicle.
 
Simik ^ optic vesicle.
Inter-brain.
Mid-brain.
Hind-brain.
 
After-brain.
Fore-brain.
 
Primary optic vesicle.
 
Jnier-brain.
Mid-breun.
 
Hind-breun.
 
J^er-brain.
 
Fio. 142.~Diagrram8 illustrating
the primary and secondary segmentation of the brain-tube (Bonnet).
 
 
 
 
288
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
the parent vesicle by a groove on either side. This anterior
divertieuhim is the secondary fore-brain vesicle or the vesicle
of the cerebrum, and the original or ]>riniary fore-brain vesicle is now the vesicle of the inter-brain.
 
The division of the primary hind-brain is eflTected by the
development of a constriction of its lateral wall, this resulting
in the production of the secondary hind-brain or the vesicle
of the cerebellum, and the after-brain or the vesicle of the
medulla oblongata.
 
While the three primary vesicles at first lie in the same
straight line, they begin to alter their relative positions
shortly before division. The change of position is coincident
with the flexures of the body of the embryo that occur at this
time. Three well-marked flexures appear, the result being
 
 
 
InUr-brain.
 
 
 
Fore-brain.
 
 
 
Cephalic flexure.
 
 
 
Mid-brain.
 
 
 
Olfactory lobe
 
 
 
Optic stalk.
 
 
 
 
I I I
 
Cerebral portion of Pontine
pituitary body. fltxure.
 
Fig. 143.— Diagram shuwin^ relations t)f l)raiii-vesiclvs ami flexures (Bonnet\
 
that the fore-brain is bent over ventrad to a marked degree.
The most anterior of these flexures, and the first t4) develop,
is the so-called cephalic flexure (Fig. 143), the primary forebrain, in the advanced stiite of tlie curvature, being bent
around the termination of the chorda dorsiUis so as to form a
right angle, and later, after its division, an acute angle with the
floor of the mid-brain. This curvature makes th(» mid-brain
very prominent as regards the surface of the embryonic body,
producing the parietal elevation or the prominence of mid-brain.
In the region of tlie future pons Varolii, on the floor or
 
 
 
THE DEVELOPMEST OF THE BRAiy. 289
 
ventral wall of tlie secondary liind-brain, is a second wullmarked an^^iilarity. This is the pontal flexure. Its convexity projects forward.
 
A third bond, the nuchal flexure, is a less pronounced
cnrvature at the jniicture of the after-brain with the spinal
part (if the neural tube.
 
The Metamorphosis of the Fifth Brain-vesicle.—
The fifth brain-vesicle, the caudal division of the primary
hind-brain, (lifferentiates into the strnctnres whioh surnmnd
the lower half of the fourth ventricle, these stnictures con
 
 
 
F\a. 144.— Diagram of > sinrlttal section or the brain of ■ mammal, sbowlng
tbe trpc of stroctiirc and tlic pailB that develnji from the Beveral bialn-TeBlclei
(modified rrnm Edltim^r).
 
stituting the mTelenceplialoii (Pig. 144). The Ustological
clianges eorresjjond essentially with those that occur in tlie
spinal segment of the neural tube, the aerve-cells and fibers
and the neuroglia resulting from the diilbreiitiation of the
original ectoderniic epithelium of which the wall of the tube
is composed, and tlie coimective-tiflaue stroma growing into
these from the surrounding mesoderm.
 
There is a marked disproportion between the rate of growth
of the tube in different parts of its circumference. The
great thickening of the ventral and lateral walls produces the
several parts uf the mednlla oblongata. In the dorsal wall
 
 
 
290 TEXT-BOOK OF EMBRYOLOGY.
 
growth occurs to such slight extent that the wall in this
region remains a thin layer of epithelium. As a consequence,
the cavity of the neural tube is not encroached upon on its
dorsal side and the central canal of the spinal cord therefore
expands in the myelencephalon into a much larger space, the
lower half of the future fourth ventricle. This relative expansion of the central canal begins to be apparent in the third
week in the human embryo, from which period it continues
to increase. A cross-section through the lower part of the
developing medulla shows a cavity which is narrow laterally
but which has a considerable anteroposterior extent. A section at a higher level disclosc^s a triangular space, the base of
the triangle being the dorsal wall of the cavity.
 
At the time when the cavity of the after-brain acquires a
distinctly triangular shape — about the third week — each thickened lateral half of the tube is divisible into a ventral and &
dorsal segment, these being known respectively as the basal
lamina and the alar lamina (Fig. 145).
 
The first indication of the longitudinal fiber-tracts of the
medulla is presented by two bands of fibers which appear upon
 
the surface of the alar lamina and which
constitute the ascending root of the
fifth nerve and the ascending root (funiculus solitarius) of the vagus and glossopharyngeal nerves. These are later covorcil in by the folding over of the dorsal part of the alar lamina (Fiff. 146) and
thnaighupiHT part core- tlius coiiic to ()crn)>y tlicir permanent
bviiar rtri<m. of tho po^jticm ill tlic interior of the medulla.
 
fourlh venlrielo <.f an * /» i i i •
 
omhryo (His): r, roof of 1 hc parts of the alar lamina^ that are
 
mv.rai ;a""i : "/. "i"r f^^i^j^.^j ^,^,^.,. j^^ ^\^^ manner referred
 
Iniiiina ; W, basal luiiiina ;
 
r. ventral ixjrdiT. to diiliTeiitiatc for the most part
 
into the restiform bodies or inferior
peduncles. These are distinguishable in the third month.
The anterior pyramidal tracts develop from the ventral parts
of the basal lamiiue and are recognizjible in the fifth month.
CoiiK*i<leiitally with the f\)rmation of the fibers, the gray
matter of th(* medulla assumes its prrmancnt form and arrangement. This gray matter, although in part ])eculiar to the
 
 
 
 
THE DEVELOPMEyX OF THE BRAIN. 291
 
nie<1iilla, is in great measure hut the continuation of the gray
matter of the spinal cor*l rearranged and differently related
because of the motor and sensory decussations and of the dorsal expansion of the central canal. A notable feature of this
 
 
 
 
■t/ {Ills) : V, ventml border : (, tenls : ot, otic vesicle ;
 
 
 
rearrangement is the presence of masses of gray matter immediately beneath the floor or ventral wall of the now expanded cavity or fourth ventricle.
 
As stated above, the dorsal \vall of the aftcr-brain vesicle
remains an extremely thin epithelial lamina, and the cavity
in consequence expands toward the dorsal surface. Owing
to the excessive delicacy of this dorsal wall of the cavity, it
ia easily destroyed in dissection, with the effect of disclosing
a triangular fossa (Fig. 151) on the dorsal surface of the
medulla, which in connection with a similar depression on
the dorsal surface of the pons, constitutes the rbomboidal fossa,
or the foortli Tentride of the brain.
 
It is often stated in descriptions of the medulla and fourth
ventricle that the latter is produced by the opening out of
the central canal of the cord to the dorsal surface. It should
be borne in mind, however, that the central canal does not,
in reality, open out to the surface, although it may appear to
do so l)ecause of the attenuated condition of its dorsal boundary. The thin epithelial roof or dorsal wall of the aflerbrain l>ecomes adherent to the investing layer of pia mater,
thus forming the tela choroidsa inferior, which roofs over
the lower half of the fourth ventricle (Fig. 144). The pia
 
 
 
292 TEXT-BOOK OF EMBRYOLOGY.
 
mater invnj^inUcs ilu» epithelial layer to form the choroid
plexuses of the fourth ventricle. Although apparently
within the cavity of the ventricle, the choroid plexuses
are excluded from it hy the layer of epithelium, the morphological roof of the after-hrain, which they have pushed
before them.
 
AVhile, for the most part, the roof of the after-brain consists of the thin epithelial layer referred to above, there are
slight linear thickenings, the ligulsB, along its latei'al margins,
and at its lower angle, the obex. At the up|)er margin of the
roof, at the place of junction with the hind-brain, there is
also a thicken<>d area, the inferior medullary velum. These
regions of thick<T tissue serve to eife<'t the transition from the
thin epithelial layer that helps to form the inferior choroidal
tela to the more massive boundaries of the rhomboidal fossa.
 
The Hind-brain Vesicle or Metencephalon. — The
 
metencephalon <*onsists of the pons, the cerebellum with its
suj)erior and mid<lle jx^duncles, and the valve (valve of
A"ieuss(Mis). Tlie>e structures are j)r()duced by the thickening of the walls of the fourth or hind-brain vesicle.
 
Th(» pons is forni<Ml by the thickening of the ventnd wall
of the vesi(rlc. Its tnmsverse libers become recognizable
durintr the fourth mouth.
 
The cerebellum grows iVom tin? posterior part of the nK)f
or dorsid wall of the vesicle (Fig. \A\). The lirst indication
of its development is seen as a thick transverse ridge or
fold on th(; po>terior extremity of the <lorsid wall (Fig^*.
147, 1 4S). In tli<* tliinl mouth the <M'ntral j)art of this
ridge, now grown larger, )>r('S('uts iour deep transverse
grooves with the n'sult oi* dividing the originid eminence
into five transverse ridges. The grooved ))art of the ridge
is the ]>ortion that subse<|ueutly becomes the vermiform
process or median lobe of the cerebelluui, while the smooth
lateral ])ortious becom<' the lateral hemispheres. As the
vermiform process increases in bulk, two of the ridges come
to lie ujM)n its upper surface and three? on the inferior aspi»ct.
These ridg(»s and furrows j)ersist throughout lifi^ as the
principal convolutions and fissures of the vermiform process (Figs. 14J», 1 :»<)).
 
 
 
THK IIISDBRMS VESICLE OR METKNCEPIfAl.OX. 293
 
The lateral parts of the primary ridge inercaso in size and
eventually, in the hiiin&n bmin, outstrip the niwlian lobe in
pritwlh. They acquire their chief transverse fifisures in the
fourth or fifth iiKinth, and the smaller sulci later.
 
The thickened cerebellar ridge nn the roof of the hindbrain vesicle being continuous with the lateml walls, the
continuity of the cerebellar hemispheres with the jHins
through the middle and superior cereliellnr |>eiliincles and
with the medulla by means of the inferior pcdnnrles. is easily
 
 
 
 
294 TEXT-BOOK OF EMBRYOLOGY.
 
thickens and dcvelojxs into the cerebellum, all the remaining
part of this roof remains relatively thin and becomes the
anterior medullary velum or the valve of Vieussens (Fig. 144).
The relations of this structure in the mature brain, stretching across, as it does, from one sujHjrior cerebellar })eduncle
to the other and l)eing continuous posteriorly with .the white
matter of the cerebellum, ixw. easily explained in the light of
the fact that all these parts are but the specialized dorsal and
lateral walls of the hind-brain vesicle. Since the roof of the
hind-brain vesicle is continuous with that of the after-brain
or fifth vesicle, it will be seen that the cerebellum must be in
continuity with the roof of the medullary part of the fourth
ventricle. The transition from the cerebellum to the epithelium of the tela choroidea inferior is eifected by a pair of
thin crescent-shaped bands of white nerve-matter which i>ass
downward from the central white-matter of the cerebellum,
and which are collectively known as the inferior or posterior
medullary velum. Thus, as the result of unecpial growth,
there are ])ro<luced from the continuous dorsid walls of the
fourth and fifth vesicles the thin laminar medullary velum
or valve, the massive cerebellar lob(»s, the thin bands known
as the infi^rior niedullarv velum, and the single layer of epithelimn which, with a layer of pia mater, constitutes the
inferior choroidal tela.
 
Although the fourth and fifth brain-vesicles are at first
delimited from each other by a constriction, this constriction,
as development goes on, <Iisappears, the cavity of the fourth
vesicle and that of the iifth together constituting the fourth
ventricle of the brain.
 
The walls of th<» fourth or hind-brain vesicle then give
rise vent rally to the j)ons, latendly to the superior and middle cerebellar peduncles, and dorsally to the valve* and the
cerebellum, while its cavitv beciunes the anterior half of the
fourth ventricle.
 
The Mid-brain Vesicle. — The third brain-vesieh? or
the vesicle of the mi<l-l)rain or mesencephalon gives rise to
the structures surroun<ling the acpieduct of Sylvius, the ])ersistent part of the cavity constituting the aqueduct itself.
 
The thickeninir of the ventral wall of the vesicle results in
 
 
 
THE 2d ID-BRA IN VESICLE. 295
 
the formatioD of the crura cerebri and tlie poaterior peribrated
lamina nr space included between them. The crura first
become apparent in the third month as a j»air of rounded
longitudinal ridges on the ventral siirface of the vesicle.
These remain relatively small until the fifth month, when
the longitudinal fibers of the pons begin to grow into them.
After this occurrence their increase in size is comparatively
rapid, their ventral parts or cmsta becoming separated from
each other ami iiidndiiig between tiieni the posterior perforated lamina.
 
The toof or doisal wall of the mid-brain vesicle nndergoes considerable thickening (Fig, 147), especially in the
Sauropsida (birds, reptiles, fishes). In the fifth week a longitudinal ridge appears upon the dorsal wall, which in the third
month is replaced by a furrow. The expansion of the wall
on each side of the furrow produces a pair of rounded eminences (Figs. 148-151), which, in birds, attain to a much
 
 
 
 
Fid. UK.— Brain
aire; f 6, foro-brafu ; lb,
brain: P, ruliln uf pLa
 
greater development than in mammals and constitute the
corpora bigemina or optic lobes. In the human emhr}'o, each
of these elevations is divided into two by an oblique groove,
and thus arc formed the coipora qtiadiigemina, which are
peculiar to man and other mammals.
 
The jiart of the dorsal wall of the vesicle that underlies
the corpora quadrigemina is the lamina qnadrigemina.
 
The thickening which the walls of the vesicle undergo to
 
 
 
29fa TEXT-BOOK OF EMBRYOLOGY.
 
produce the several parts of the micl-bmin encroaches so
miicli iiiK)n its cavity tliat an I'.xceedinj^'ly small cjiual, tlie
aqueduct of Sylvius, remains. It is scarcely necessary to
piHiit out llijtt llii>; canal is a part of the ventricular system
of the iiniii), ost:ihli'-hiu<;a ctminiunicatiou l>etweeu the fourth
ventricle ami the thini ventricle or tyivity of the intcr-braiu.
The Metamorphosis of the Inter -brain "Vesicle. —
The inter-limiii vesicle results fn.m tlie division of the primary fore-brain vehicle, comprising what in lell of the latter
after the outgrowth from it uf the diverticulum that l>ecome8
the secondary fore-brain. The thickening of the walls of
the inter-brain vesicle produces the sirueture-s which surround
the third ventricle in the mature eoudition, and which constitute collectively the thalomencephalon or inter-brain, the cavity
of tJie vesicle persisting as the adult third ventricle. These
Btructures are the optic thalajoi, which iirc iorincd from the
lateral walls; the velum interpoBitum and the pineal bod7>
which develop from the roof; and the lamina cinerea, the
 
 
 
 
■>'/
 
Fio. ]4S,~A. mualsl icrtiDii IhroURh bniiii <i[ a huiunii rclUB of two-Bud-a-bfttf
months (Hla): cA. cerebral lii'mlnphi-'ru ; o. ituWc UinliiiDue:/Hi. ri>niin«n of Monro;
o{f, olOwtory tobc: p, pllultar; body ; no, minlulU (iblongaU: eq, corpora quadrlpmlDai tb, eerebetlum, B, brain of human Celui of (hrve montbi (HI*): olf,
olftntory Inlie; rM, rnrpUB sltUlum; eq, corpora quadriffemlna ; eft, eerebBllnrnj
inn, mcdiinn obloiiKOU.
 
tuber cinereum, the infiindibuluin, the posterior lobe of the
pituitary body and the corpora albicantia, which are differentiatoil fiiuii the floor of the vesicle.
 
The lateral walls of the vesicle undergo the most marked
 
 
 
METAJUOBPHOSIS OF THE ISTER-BRAIX VESICLE. 297
 
thickening. The cell-multiplication here is so r.ipid that
each lateral wall is converted into a large ovoiil inaris of
cells with iiiterminglctl bands of fibers, the optic thalamus.
 
The roof of the inter-brain vesicle, in nutahle coiitra.st
with the lateral walls, remains extremely thin throiighnnt
the greater part of its extent (Fig. 144). Fmm ihe Ijack
part of the roof, at a point immediately in front of the
lamina iiiiadrigeiuina of the mid-brain, a diverticulum grows
otit and becomes metamorphosed into the pineal tody. With
this e.\ception, the roof of the vesicle reinains a single layer
of epithelium, just aa in the ease of tlie roof of the afterbrain. This epithelial layer adheres closely to the pia mater,
which covers it in common with the other parts of the hrain.
As the fore-brain expands, it covers the inter-brain, the
under surface of the cerebral hemispheres of the former
l>eing closely applied to the roof of tlie latter. As a consetjucnce, the pia mater on the under surface of the fore
 
 
 
Fin. i:iO.—itra<n of A^tm of ihreemnnihs, enlarged. Tbc outer wall of the light
 
hcmlipheru hu been lemoveil ; LH, left bemlHpliere ; Ca, part of corpiu ■Irialum;
FS, site of fossa of Kylviiis; I', vascular fold of pia mater which has been InvagInalcd ihniuRh the mesial wall of the hemisphere: Mb, miil-broln; C.ceiebellum;
Jf , medulla oblongala,
 
brain is brought into contact with and adheres to the pia
covering the roof of the inter-brain. Thus the thin epithelial
roof of the inter-brain becomes closely united with the two
layers of the pia luater to form the velnm iutarpodtam or
tela choroidea anterior or superior of adult anatomy. Obviously, the edges of the velum interpositum rest upon the
optic thalami, and its piamatral layers are continued into the
 
 
 
298 TEXT-BOOK OF EMBRYOLOGY.
 
cavities of the lateral ventricles (Fig. 150). The space occupied by tlie velum is designated the transyerse fissure of the
brain, and it is often stated that the pia mater is pushed in
from behind, between the optic thalami and the cerebral hemispheres. As will be seen from the above description^ however, its development really begins in front.
 
The pineal gland or conarium develops from the back part
of the roof of the inter-brain at its point of junction with
the mid-brain (Fig. 144). This body is found in all vertebrate animals except the amphioxus, but its form varies
greatly in difierent groups. In all cases it begins as a small
pouch-like evagination from the roof of the inter-brain, the
diverticulum being directed forward. In the human brain
alone the structure is subsequently directed backward, so
that it conies to occupy a position just over the corpora
quadrigemina. This peculiarity of location is due probably
to the greater development of the human corpus callosum,
by whicli tlie conarium is crowded backward.
 
In selachians (sharks and dog-fish), the enlarged vesicular
end of the diverticulum, which is lined with ciliated columnar
cells, lies outside tlie cranial capsule and is connected with
the inter-bruin by the lonij: hollow stalk which perforates
the roof of the (M-aiiiuni. In many reptiles, the conarium is
more liighly specialized. In the chameleon, for example, the
peripli(;ral extremity has the form of a small closed vesicle
which lies outside the roof of the cranium and which is
covered by a trans|)aroiit pat(*li of skin. The stalk in this
case is ])artly a solid cord and ]>artly a hollow canal, which
latter oj)ens into the cavity of the inter-brain. The solid
portion lies within a foramen in the pjirietal bone, the parietal
foramen. A farther modification of the conarium is jiresented
in lizards, blind- worms, and some other reptiles. In these
the vesicle underji^oes a marked specializjition, its peripheral
wall being so nicxlilled as to become trans))arent and to resemble the crystalline lens of the eye, while the opposite
deeper wall comes to consist of several layers of cells — some
of which become piirmente<l — ainl ac(juire«* a striking resemblance to the retina. The stalk of the body, which perforates
the roof of the skull and is attacheil to the roof of the inter
 
 
METAMORPHOSIS OF THE INTER-BRAIN VESICLE, 299
 
brain, bears a certain likeness to the optic nerve, being solid
and composed of fibers and elongated cells. The presence
of the transparent epidermal plate which covers the vesicle
serves to complete the similarity of this particular type of
pineal body to the eye of vertebrate animals. It is for this
reason that it is often designated the pineal or parietal eye
and that it has been looked upon as a third or unpaired
organ of vision.
 
In man and other mammals and in birds the pineal diverticulum does not reach the degree of development that is
attained in certain of the Reptilia. The evagination from
the roof of the inter-brain begins in the sixth week in the
human embryo. The peripheral end of the process enlarges
somewhat and small masses of cells project from it into the
surrounding mesodermic tissue. These cellular outgrowths,
giving off secondary projections, become converted into small
closed follicles lined with columnar ciliated cells. The follicles in the case of mammals very soon become solid or nearly
so by the accumulation of cells in their interior. Solid concretions of calcareous matter, the so-called brain-sand (acervulus cerebri) are found in the follicles in the adult. By
these alterations the pineal body of birds and mammals
acquires a structure resembling that of a glandular organ.
Since it is onlv the end of the diverticulum that becomes
thus altered, the remaining part constitutes the relatively
slender stalk of the pineal body, the stalk being solid at
maturity except at its point of attachment to the inter-brain,
where a portion of the cavity persists as the pineal recess of
the third ventricle.
 
The pineal body of man and the higher vertebrates is therefore a rudimentary structure and is the representative of an
organ that is much more highly developed in some of the
lower members of the same series. Its true significance is
still a matter of conjecture. Although resembling the eye in
its structure, and although regarded by some on that account
as primitively an organ of vision, it is considered probable by
others that in its most highly developed condition it is an
organ of heat perception.
 
The floor of the inter-brain vesicle presents several interesting
 
 
 
;]{){) TKXT-BOOK OF JJMBRYOLOGV.
 
iiu'tainorphosos. Tlui anterior ])art of the floor n?mains quite
thill an<l Ix'coines the lamina cinerea of th<^ niatinv hraiu (Fig.
111). Iiuiiiediately posterior to this region, the floor of the
vesiele poiK^hes out, this evagination developing into a slender
IhIm', (he inftmdibuluin. Behind the [XHut of origin of the
ini'undilMihiin a sc>eond protnheranee indicrates the beginning
nf* (lie tuber cinereum. By subse(|uent altenitions, the tuber
eiurreiini enlarmnir in ('ircuniferenee so as to include the
point ol'ori<::in of the infundibnlnm, the base of attachment
III" the infnn(lil)nluin eonies to be the center of the tuber
riniTruni, so that tlie cavity of the former is a continuation of
thr eavilv of the latter. Tlie end of the infundibulum
limiMies tiie posterior lobe of tiie pituitary body or hsrpoMliynlH ( Vi\r>, 144 and 140). Posterior to the tulxjr cinertiiiiii a small evagination of the floor of the vesicle
.ippiMi'i an<l berimes divide<l in the early part of the fourth
iiiniilh iiiln two lateral halves bv a median furrow. The
Iwii bllh' bodies thus forme<l become, after further developiiH lit. ihr corpora albicantia.
 
I hr hypophysis or pituitary body briefly referred to above
iii|iiiir: iimrr <'.\tende(l consideration because of its morlihuiti^firid liiipnriancc. The posterior lobe of this body is the
• iil.ii^fiil nid of the infiindihulnm, which is an evagination of
I hi iImiii 111' I he inlcr-brain. The cells in the lower end of
I hi iiitiiiiihbiihnu specialize into nerve-cells, and ncrvelilii I < .il:ii drv<'h»p. In some lower vertebrates these eleiiii 111 < .111* i-i'iiiiiird throu(rhout lii'<\ but in man and the
hi;'. hi I l\pi- niiiiiial> the distinctively nervous character of
ihi II- 111 • I- -ooii lost, and the cavity of this part of the
ihiiiiiilihiihiiii iill'ris oblitenition. The bmnched ])igmentiilh •iiiir(iiiir-i nM'oiTiii/jihh' in the j)osterior K)b(» of the
hiiiiiaii piiiiitiir\ body an* the only remnant of tiie early
III I \ »• ii'lU.
 
I hi' Hiitttiior lobn of the hypophysis is essentially different
III Hiii'.iii ii^ wi'll MM in structure from tin* ]>ost<»rior h»be. It
i- piodiii-nl b> nil cviiixination from the pcisterior wall of the
piiiiiili\c phar\n\,l»ut from that region of the ])harvnx which
i- anterior to the |»haryngcal membrane and which therefore
bcloii^> to the primitiv(^ mouth-cavity (Fig. GO, p. l;ilj. The
 
 
 
METAMORPHOSIS OF THE INTER-BRAIN VESICLE. 301
 
out-pocketing of the pharyngeal wall begins in the fourth
week, shortly after the rupture of the pharyngeal membrane.
The little pouch is the pocket of Bathke. The pouch grows
upward and backward toward the floor of the inter-brain and
meets the end of the infundibulum. As tlie pliaryngeal
diverticulum lengthens, its stalk becomes a slender duct,
which for some time retains its connection with the pharynx.
As the membranous base of the skull becomes cartilaginous,
the duct begins to atrophy, and finally entirely disappears.
In selachians, however, it is retained permanently, establishing thus a connection between the hypophysis and the pharyngeal cavity. AVith the disappearance of the duct the enlarged
extremity of the diverticulum becomes a closed vesicle lying
now within the cavity of the brain-case, in contact with the
end of the infundibulum. From the wall of the vesicle numerous little tubular projections grow out into the enveloping
mesodermic tissue, and these, by detachment from the parent
vesicle, become closed tubes or follicles. The entire structure
becomes converted in this manner into a mass of closed follicles held together by connective tissue, after which event
this mass acquires intimate union with the infundibular lobe.
 
Thus the pituitary body consists of two genetically distinct
parts, the anterior lobe being derived from the ectoderm of
the primitive pharyngeal or buccal cavity, and the posterior
lobe from the ectoderm of the central nervous svstem. The
posterior lobe, developing as it does as an evagination from
the floor of the inter-brain, is to be regarded as a small outlying lobe of the brain.
 
AVHiat remains of the cavity of the inter-brain, after its
walls have thus developed into the several structures described, is the third ventricle of the adult brain, and the
aperture of communication with the secondary fore-brain
vesicles becomes the foramen of Monro. Since the lateral
walls become the massive optic thalami, while the dorsal and
ventral walls give rise to much thinner structures, the cavity
of the vesicle is encroached up(m to a greater extent on the
sides than from above and below, and hence the form of the
third ventricle in the mature condition is that of a narrow
vertical fissure between the thalami.
 
 
 
302 TEXT-BOOK OF EMBRYOLOGY.
 
The Metamorphosis of the Fore-brain Vesicle. —
 
The secondary lore-brain vesicle gives rise to the telencephalon, which includes the cerebral hemispheres and the
structures belonging directly to them. As above indicated,
this vesicle grows from the anterior wall of the primary forebntin vesicle as a diverticulum which is at first single, but
which sfK)n becomes divided into two lateral halves by the
formation of a cleft in the median plane (Fig. 147, /6). This
cleft or interpallial fissure is the early representative of the
longitudinal fissure of the adult cerebrum. The two vesicles
remain attached at their bases or stalks with the parent vesicle
and communicate by a common orifice with its cavity. The
vesicles of tiie secondarj' fore-brain grow in an upward and
backward direction as well as laterally, and their develo])ment is so much more raj)id than that of the other vesicles
that they soon spread over them and partially hide them
from view. It is for this reason that the mass resulting
from the fore-bniin vesicles, except their basal ganglia, is
known in comparative anatomy as tiie pallium or mantle
(Fig. 144).
 
The relative rate of growth of the cerebral hemispheres is
such that in the third month th(»y completely overlie the
inter-bniin and bv the sixth month thev have extended so
far back as to hide the corpora rjuadrigcniina.
 
The mesodermic tissue surrounding the developing brain
becomes ditlerentiated into the three brain-membranes, which
penetrate into the fissure and tliereforc invest the vesicles
on their mesial surfaces as well as elsewhere. The invaginating layers of the dura mater constitute the ])rimitive
falx cerebri.
 
The metamorphosis of this pair of sacs into the cerebral
hemis])heres is broujrht about by three important processes :
first, the multiplication of the cells whicli compose its walls
to form the masses <»f nerve-cells and fibers of the hemispheres ; second, the formation of folds in the wall whereby
are pr<Khiced the fissures which divide the hemispheres into
lobes and convolutions ; and third, the development of adhesions within certain areas between the mesial walls of the
 
 
 
METAMORPHOSIS OF THE FORE-BRAIN VESICLE, 303
 
two vesicles, by which the system of commissures of the
hemispheres is produced.
 
The walls of the cerebral vesicles are at first very thin,
consisting merely of several layers of spindle-shaped cells.
By the rapid multiplication of these cells, the walls are thick•ened and the cavity of the vesicle is gradually encroached
upon until the mature condition of the brain is attained,
when the cavity is relatively very much smaller than in the
fetus and constitutes the ventricle of the hemisphere or the
lateral ventricle. The nerve-cells develop processes or polar
prolongations, of which the most conspicuous, the axis-cylinder processes, lengthen out to form the axis cylinders of
nerve-fibers. The fibers thus formed are directed away from
the surface and make up the white medullary matter of the
hemispheres, while the more superficially placed layers of
cells constitute the gray matter of the cortex of the brain.
 
In addition to the cortical or superficial gray matter there
are masses of gray matter within the hemisphere, the basal
ganglia, which are likewise collections of nerve-cells. Witliin
a limited area on the lateral wall of each cerebral vesicle,
near the lower margin, the cells undergo excessive proliferation resulting in the production of a large ganglionic mass,
the corpus striatum, and of two smaller aggregations of cells,
the claustrum and the nucleus amygdala. These basal ganglia
are in reality an infolded part of the cortex.
 
Inasmuch as the cortical matter develops more rapidly, as
regards superficial extent, than does the medullary substance,
the cortex becomes thrown into folds, forming thus the convolutions and fissures of the hemispheres.
 
Some of the fissures of the brain are produced by an infolding of the entire thickness of the vesicle-wall so that
their presence is indicated by corresponding projections in
the walls of the ventricles. Such fissures are distinguished
as total fissures. Included in this category are the fissure
of Sylvius, which is represented in the wall of the lateral
ventricle by the corpus striatum; the calcarine fissure, the
dentate fissure, and the collateral fissure, which are responsible
respectively for the calcar avis, the hippocampus major, and
 
 
 
304
 
 
 
TEXT-BOOK OF EMBRYOLOOY.
 
 
 
the collateral eminence of the lateral ventricle ; and the gnst
transrerse flseure of the brain, the infolded wall in thia case
being very thin and consisting merely of the layer of epithelium which covers the choroid plexus.
 
The flssnre of Sylvins is the earliest fissure formed and one
of the most imjmrlant. At an early period in the history
of the secondary fore-brain, there is a region in the lower
part of the lateral wall of the vesicle where expansion is
loss rapid than elsewhere, this area, as it were, remaining
fixed. As the vesicle-wall innnediatoly surrounding thb
 
 
 
 
8))ot etmtiniies to expaml, n dimpling of the wall is produced,
(he depression bcinfj (li'sii;imtc(l the fossa of Sylvius (Fig. 152,
S'V The jKirt of the vcsiclo-wall In-hind the fos.'ia advances
forwanl and downward to form the future temporal lobe, and
thus till- fiwHJi ronu's ti» Ih' siirroiindwl hy a convolution
having the form of an incfiniplctt' rinfr, i>|M'n in front — the
ring lobe. The llcmr of tlii^ fossi undcinocs very eonsidorable thiekeninj; to form ilie basal ganglia — that is, the corpus
striatum, the amygdaloid niielens, and lln" ehuistnim. These
structures, most conspicuously tli<' i-orpns striafmn, cniToiioh
ujKin the cjivily of iho vesiili', the nucleus caudatus of the
 
 
 
METAMORPHOSIS OF THE FORE-BRAIS VESICLE. !!<J5
 
corpus Btriatum bulging intu the floor anJ outer wall of the
adult lateral ventricle.
 
 
 
 
The cortical matter of the floor of the fossa of Sylvius,
beiug circumscribed by a groove or sulcus, constitutes tbe
 
 
 
 
bi. wllh right half of fore-braJn.
Dved: lb, CBvilf of inter-brHln; hy, stle of b^p. : Mbr. mid-brain roof; mv, nilil-braliL cuvily ; C wrvbi-lliiin : M. medulla
 
 
 
central lobe or island of Reil, which is subsequently brokea
up, by seiMjndnry fissures, into from five to seven email convolutions.
 
 
 
306 TEXT-BOOK OF EMBRYOLOGY.
 
By the extension of the fossa of Sylvius backward, and by
the increased gi^owth of the vesicle-wall above and below it,
the fossil is converted into the flssnre of Sylvius (Fig. 156, B)y
and the island of Iteil is hidden from view. Subsequently
the ascending and anterior limbs are added to the chief or
horizontal part of the Kssure.
 
The anterior part of the ring lol>e corresponds with the
future frontal lobe, the ]K>sterior part represents the parietal
lobe while the lower part of the ring becomes the temporal
lobe. A backward extension of the ring lobe produces the
occipital lobe.
 
The cavity of the vesicle is mcKlifiiMl in form and extent eoincidentally with the formation of the corpus striatum and
the alterations in the ring lobe. Just as the ring lobe partially encircles the fossa of Sylvius, so does the cavity of
i\w. ventricle partially encircle the corpus striatum. An
anterior prolongation of the cavity extends into the com|>l<»tc(I frontal lobe as the anterior comu of the ventricle, and
iin (»xteusion downward and forward into the apex of the
temporal lobe constitutes the descending comu, while the
posterior horn is ^nulually protruded into the occipital lobe as
ihr latter dcvc](>])s. From the earliest stage, therefore, until
I he eoiii])lete(l condition is attained, the cavity of the ventrieli' eoiilonns in a general way to the shape of the henii■'?ph«Te. The a])ertiires of (Mummiuieation between the vesirli-j (>r thi* cerebrum and the eavitv of the inter-brain are the
lithr Y shaped foramen commune anterius or the foramen of
M«»iito.
 
The numial surfaces of the hemispheres are much modified
ht ehintieirr by the (levelopnicnt here of two total fissures,
(hr tiiruato flKHure and the choroid fissure. TIksc ap|)ear in
Hie lillh week while the ve^i<*les are >till separate fnun each
iiilnr ilnNMi III (heir Ntall\< of attaehnient to the inter-brain,
pii«ii it» the development, th(M*efor(', of tile eorpiis callosuni
.Old the Cnriiiv. The two lis<ni*e^ lie ejox' to<rother, pandlel
Willi iiiili othri* an«l with the niarLrin of the riuir lobe, their
r»»iir-.i' ront'nnniiiL: lo ihiit ot'ihe eaviiv t»t'the ventricle. Ik»'jiiiiiMiv ne;ir the anlrrioi* evtreniitv of the brain, ahove the
 
 
 
METAMORPHOSIS OF THE FORE-BRAIN VESICLE. 307
 
level of the corpus striatum, they pass backward and then
downward and afterward forward to terminate near the anterior extremity of the temporal lobe, thus incompletely encircling the striate body.
 
The arcuate flssnre is the more peripherally placed of the
two. Its anterior portion lies just above the region throughout which adhesions subsequently develop between the two
hemispheres, or in other words, above the position of the
future corpus callosum (Fig. 154, a./.). This part of the arcu
 
 
 
Fi«. 154.— Mesial surface of left fore-brain vesicle of brain shown in Fig. 148 (F6) :
/.3/, foramen of Monro, or opening into inter-brain ; o/, arcuate fissure : chj, choroid fissure ; r," randbogen," corresponding to future corpus callosum and fornix;
o^f, olfactory lobe.
 
ate fissure is the sulcuB of the corpus callosum of the mature
brain. The posterior segment, that which belongs to the
temporal lobe (not present at this stage), is the future bippocampal or dentate Assure. The hippocampal fissure is represented ujion the mesial wall of the descending horn of the
lateral ventricle by the prominence known as the hippocampus
major.
 
The choroid fissure or fissure of the choroid plexus, forming
an incompI<?te ring within, and parallel with, that described
by the arcuate fissure, encircles the corpus striatum more
closely (Figs. 154, 155). It begins at the foramen of Monro,
and its anterior part lies under the position of the body of
the future fornix. It then sweeps around into the tem])oral
lobe and terminates near the anterior part of the latter. The
fissure of the chon)id plexus, like other total fissures, is an
infoMing of the wall of the cerebral vesicle. It presents the
l>eculiarity, however, that the infolded part of the wall is
extremely thin, consisting of but a single layer of e])ithelial
 
 
 
308 TEXT-BOOK OF EMBRYOLOGY.
 
cells. The pia mater, which everywhere closely invests the
surface of the bniin, is infolded with the vesicle-wall, the infolded part becoming very vascular and constituting the
choroid plexus of the lateral ventricle. The choroid plexus,
although within the limits of the ventricle, is excluded,
strictly sjKjaking, from its cavity by the layer of epithelium
which still covers it and which has been simply pushed before
it into that cavity. Since the epithelial layer is very thin
and easily ruptured, the choroid fissure is apjiarently an
opening into the cavity of the ventricle through which the
pia enters ; in the adult it is called the great transverse fissnre
of the brain.
 
The calcarine Assure, another of the total fissures, develops
in the latter part of the third month as a branch of the
arcuate fissure. It bulges into the mesial wall of the posterior horn of the ventricle, i)r()ducing the elevation known as
the calcar avis or hippocampus minor. Since the posterior
horn of the ventricle is developed as an extension of the cavity into the backward prolongation of the ring lobe which
forms the occipital lobe, the calcarine fissure necessarily is
later in appearing than the fissures above described.
 
The parieto-occipital fissure is added in the fourth month
as a branch of the calcarine, ellecting the definite demarcation between the parietal and occi])ital lobes.
 
The fissure of Rolando develo]is in the latter part of the
fifth month in two ])arts. The two furrows are at first
entirely se])arat('(l from each other by an intervening area of
cortex. Subsecjuently this part of the cortex sinks l>eneath the surface, as it were, sinec it expands less rajndly
than the adjacent regions, and in this way the upi>er and
lower limbs of the fissure become continuous. The sunken
cortical area is to Ix* found even in the adult brain as a deep
anneetant gyrus embedded in the Kolandic fissure at the position of its superior genu. TIk^ development of the fissure
of Kolando effects the division betw(HMi the fnmtal and jwirietal lobes.
 
The collateral fissure appears in the sixth month as a
longitudinal infolding of the mesial wall of the hemisphere
 
 
 
METAMORPHOSIS OF THE FORE-BRAIN VESICLE, 309
 
below and parallel with the hippocarapal fissure. Being a
total fissure, its presence affects the wall of the cavity of the
vesicle, producing the eminentia collateralis. At about the
same time the calloso-marginal Assure . makes its appearance,
and this is morphologically continuous, through the medium
of the post-limbic sulcus, with the collateral fissure (Fig. 157).
These three fissures constitute the peripheral boundary of a
region of the mesial wall which is known in morphology
as the falciform or limbic lobe.
 
The longitudinal Assure in the early stage of the growth of
the cerebrum separates the two vesicles from each other except at the place where they are attached to the inter-brain ;
here the two sacs are united by that part of their common
anterior wall which is immediately in front of the apertures
of communication with the inter-brain and which is called
the lamina terminalis.
 
The development of adhesions between the mesial surfaces
of the hemisphere vesicles throughout certain definite areas
marks the beginning of the corpus callosnm and the fornix.
The fusion of these areas begins in the third month in the
region corresponding to the anterior pillars of the fornix, the
septum lucidum and the genu of the corpus callosum ; in
the fifth and sixth months adhesion occurs in the position of
the body of the fornix and of the body and splenium of the
corpus callosum.
 
Although the central white medullary matter of the cerebral hemisphere is covered almost universally by the cortical
gray matter, there is a limited area of the mesial surface from
which the gray matter is absent, leaving the white matter
ex]>osed. The area of uncovered white matter has the form
of a narrow band, which begins at the base of the hemisphere,
in front of the opening into the inter-brain, extends upward
along the anterior wall of the inter-brain, then passes backward along its roof and curves downward and outward behind,
and then forward under it, to terminate at the front part of
the temporal lobe. Thus this white band, which is known as
the fimbria, and which represents the lower mesial edge of
the hemisphere, almost encircles the inter-brain. The fimbria
 
 
 
310 TEXT-BOOK OF EMBRYOLOai'.
 
runs between the arcuate fit^sureand the fissure of the choroid
plexus (Fig. 155, /). It holds such a close relation to the lat
 
 
 
F[o. IM.— Mu'sl«l Bnrftcc of left hcmlsphcp;, hmln of fi'tim of three months
(ciilurRiid) : /.. fiirnii: r.r.. beginning of u>rpus cHlluiam; c.rl., partot vuriiui atrtstiiiii iirelilnB uruiind fmra of Sylvius ; a/., unttrinr. mill nj./i.. |HiitvrU'r parti of
urrnauj Huiin? ; rhj., i-l]i>ri>id Usrun.', the coni'ui li.v lH-tH'i'i.'ti u'liii'li und the corpni
klrlatnin ucComniudatLii the luUT-linin, which hna Ik'vii ivmiirtil. The Itiaare U
■icL'Upiiil by the pla luutuc.
 
tor fissure, Iwiiig placet! on its ]>erii)licr.il side, that it constitutes the e<lge of the apjtaroiit o|Htniiig into the cavity of the
vesicle thi-ough wliiuh the piji niiiter, iKiiriiig bloofl- vessels, is
reflctrted iuto the interior, and which, as pointed out above,
is the tniiisverM' fissure of the Itniin. The <i|ieuing is only
a])piir(^nt, however, since tlif wall is still iinlirokfii, although
reduced to ii single layer of ejntltflinin. The pia mater, forming, with its blood-vessels, the cliDniiil plexus of tlic lateral
ventri.-Ie, pushes tlie layer "f cpirhellnin before it, and althoM^di the |)lexns is s;ii«l to be within the eiivily of the ventricle, it is still covtirod by the layer of epitliclinm, the ependrma, whifh lines that oiivity.
 
Thi' part of (he (iuibria that ini mediately overlies the roof
of the inter-hrnin Iieeonii's iiitinmtely uniteil, as noted alx)ve,
with the eorri'sponiling jiarl of the titiibria of the other heniis|ihen>, these fused portions of the two limhri;e forming a flat
tnangular sheet, the body of the fornix. Tlie anterior and
IKisti'i'ior portions of the fimbria, whieh diverge from the
moilian plane, represent res|Mitiv<'ly the anterior ami posterior limbs of tlie fiiniix.
 
Noting the relation ->r tlu^ anterior part of the fimbria to
till- a]>or(ur(Mif eonniinnii'ation between the inler-brain and
the cereijnil vesicles, it becomes apparent that the anterior
pillar of the fornix forms the anterior and n|»per Iwuiidarj' of
 
 
 
ilKTAMURPHOSlS OF THE lORE-UIUiy VESICLE. 311
 
the foranifQ of Munro. When, further, one considers the
relatiou uf the fimliria to the apparent oi>ening into the ventricle, through which the pia mater i« invaginated (the transverse fissure), it is explained why the edge of the fornix
appears as a narrcjw white band, not only as viewed from
within tile ventricular cavity, Imt tiho in a nit-Mal section of
the brain (Fig. 156, C).
 
 
 
 
t^o. liiK.—Fctal tiriiln at thr \ieg\xm\ng of Ihe tiiihlh moiitli <MlliaIkovlm> :
A.iiiperloi, B. Inderal, C, rnvslnl aiirfaru: K. tliaiiK of KoUndo: prf, ■■reecnlral
fi«Buw; S|f, HylirlanHwure: inip, lntBrp»rletiil llHiiin.'; jhw, parfet(ww«lpil«l(i»aurB!
pU, psratlel tiiuurv: eailra, calloiomirglnnl Buure: u'T, unoui : cale. culCBTine
 
Another important region of fusion of the opposed mesial
surfaces of the hemispheres is that corresponding lo the
future corpus calloBum Throughout this area the liemispheres
closely unite with each otiier The line of fusion begins
at the Imses of the vesicles, some little distance in front
of the anterior parts of the fimbriie (Fig. 155, f-c). and after
passing upward and luruird, curves horizontally backward
 
 
 
kward I
 
 
 
312
 
 
 
TEXT-BOOK OF EMBUYOLOGY.
 
 
 
in close relation with the fused portions of the fimbria?, now
the body of the fornix. The atUiesiou begins at the anterior
part in the third month, and atfects the regit)n of the body
and gplenlum of the future corpus callosum in the fifth and
sixth mouths. Fibers penetrate from one hemisphere to the ,
other throughout this zone of contact, intimately uniting the
cerebral hemispheres. The corpus callosum is therefore &
great commissure lietween the two halves of the cerebrum,
and is necessarily composed of fibers having a transverse 1
direction.
 
While the back part of the corpus callosum lies over the
body of the fornix and is in close contact with it, the front
part of the body of the corpus collusum, as also its genu or |
curve and its rostrum or ascending part are at some distance (
from tlie front parts of the fimbrite. In other words, while the j
great longitudinal fissure extends at first to the bases of the
cerebral vesicles, this fissure is made relatively loss deep by
the adhesions which occur between the mesial walls and which
result in the development of the corpus callosum ; and the
space below the anterior part of the corpus callosum, between
it and the anterior parts of the fimhriie (Fig. 1.56, C), is an
isobtied part of the great lonrjitudinal fissure. This space is
bounded on either side by tliat jtart nf the wall of the corres- ,
ponding cerebral vesicle or hemisphere which is limited above |
and in front liy the corpus collusum, and behind by the anterior part of the fimbria or anterior limb of the fornix. The '
space is the so-cniled fifth ventricle of the adult brain. The |
circumscribed parts of tlie mesial walls of the hemisphei
which form the lateral walls of the space, together constitute '
the Beptum lucidum. The jtarts of the hemisphere walls that '
become the septum lucidum do not participate iu the procesa '
of fusion mentioned above. Their surfaces are iu contact) [
however, and do not develop the typical gray cortical matter, I
such OS appears elsewhere ujKin the surface of the cerebrum. J
Cortical gray matter is produced here, but only iu radi- 1
mentary form.
 
From what has been said, it will be seen that the t
layers of the septum lucidum are circumscritKid and opposed.
 
 
 
METAMORPHOSIS OF THE FOEE-BRAtS VESICLE. 313
 
parts of the mesial walla of the hemispheres ; that the fifth
ventricle is not a tnie ventricle but an isolated part of the
longitudinal tiggure having no connection whatever witli the
system of ventricular cavities ; and that this .so-called ventricle is not, like the true ventricles of the brain, lined with
ependyma, but with atrophic gray cortical matter.
 
The limbic lobe has been referred to as that part of the
mesial aurf'ace of the hemisphere which is circumscribed by
the calloso- marginal fissure, the post-limbic sulcus, and the
collateral fissure. It is limite<l centrally by the fissure of the
corpus callosum and the hippocampal fissure, which are
represented in the fetal brain by the single uninterrupted
arcuate fissure. Hence the limbic lobe would include the
gyrus fornicatus, the isthmus, and the gyrus uncinatua, which
constitute morphologically a single ring-like convolution.
Schwalbe, however, includes with this so-called limbic lobe
all the surface of the mesial wall of the hemisphere included
between the arcuate fissure and tlie fissure of the choroid
plexua (Fig. 154), designating it the folciform lobe (Fig. 157).
 
 
 
 
 
 
 
The falciform lobo therefore consists of two ring-like convolutions, one within the other, the two l)eing separated from
each other by the arcuate fissure (the adult callosal and dentaf« fissures) and being limited centrally by the fissure of the
choruiii plexus (the great transverse fitisurc of the adult
 
 
 
314 TEXT-BOOK OF EMBRYOLOGY.
 
brain). While the outer of these concentric convolutions —
the limbic lobe of Broca — develops into the fornicate or callosal, the isthmian, and the uncinate gyri, the inner ring
differentiates but slightly, its cortical matter remaining
atrophic. The atrophic condition of the cortex here is associated with those adhesions between the mesial walls of the
hemispheres that result in the formation of the corpus callosum and the septum lucidum. By these adhesions the
continuity of the inner concentric convolution is broken, and
it is therefore represented, after the development of the corpus
callosum, by the atrophic gray matter of the septum lucidum,
by the gyrus dentatus, and by the lateral longitudinal striae
on the free surface of the cor])us callosum, the latter being an
atrophic or rudimentary convolution. Since the transverse
fissure of the brain is the centric boundary of the ring, the
fornix is also a part of the falciform lobe. To sum up, the
falciform lobe includes the gyrus fornicatus, the isthmus, the
gyrus uncinatus, the lateral longitudinal striae or taenia tectae
of the corpus callosum, the gyrus dentatus, the lamina* of the
septum lucidum and the fornix.
 
The olfactory lobe or rhinencephalon is an outgrowth from
the vesicle of the cerebral hemisphere. Its development begins in the fifth week by the pouehing-out of the wall of the
vesicle near the anterior part of its fioor (Figs. l-t7 and 149).
This diverticulum, which contains a cavity contiinious with
that (»f the vesicle, grows forward and so<ni l)ecomes somewhat clul)-sha]>ed. In the selachian.'^ (.sharks and dog-fish)
the projection attains a great relative size, the olfactory lobes
in these animals being one of the most eonsj)icuous ]uirts of
the brain. In all mammals except man it is well developed,
and in the horse its cavity persists throughout life. In man
the cavitv soon becomes obliterated and the lobe itself in
part aborts. The ])rotru(le(l })ortion, becoming more distinctly club-shaped, differentiates into the olfactory bulb and
the olfactory tract, the ))osition of the original cavity being
indicated by a more or less central mass of neuroglia conspicuous in cross-sections of those structures. The proximal
portion (»f the olfactory lobe is represented in the adult
 
 
 
METAMORPHOSIS OF THE FORE-BRAIN VESICLE. 315
 
human Urain by the gray matter of tlie anterior perforated
lamina {or space), ami by tlie trigonuin ol facto riiiiii and the
area of Broca, as well aa by the inner and outer roots of the
olfactory tract (note olfactorj- lobe of dog's brain, t"ig. 156).
 
 
 
 
. l;iS.— Buc of dogabialn: al., o\lar\otj lube: a.ji,»., rcxiin corrcipandiiiK M
r perforaled spa<-c. which la Incluiled in the ulllictorv l<ibc ; /.S.. IliiBurt nT
i; a.h., hlppooampal ((>tus. deTelopvd to a Rn-iitor rtpRree than In human
1., leellonal siirCice of oltactoty lobe: m.. olltictury sulcus.
 
 
 
Because of the relation of the place of evagination of tlie
olfactory lobe to the fossa of Sylvius, it happens that a |>art
of this lobe, the anterior perforated lamina, is i^ititatcd at the
commeneeincnt of the fissure of Sylvius and that it is in eoiithmity with Iroth extremities of the ring lobe; hence, the
olfactory lobe Is connected with both extremities of the falciform lobe. To express it in the language of human anatomy,
the outer or lateral root of the olfactory tract is connected
with the gyiuB uncinatUB, while the inner or mesial root'may
be traced to the fore part of the gyrus fomicatns.
 
After what has been said, the reader need scarcely be reminded that the olfactory bulb and tract, often erroneously
referred to as the olfactory nerAe, are parts of a lobe of the
 
 
 
:il<i
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
iirain, a lobe which in man is rudimentary but which in all
o(h(M* mammals is well developed.
 
Tahulaied Rijnim^ of ihe Derivatives of the Brain-Besides,
 
 
 
Hkain
VKtiK'l.KM.
AfliT
tiralu
 
Ililidbrnln
vuhIcIo.
 
 
 
Mill tiruin
VVblcie.
 
 
 
IlltlT
 
hritiii
 
 
 
Floor.
 
 
 
Medulla
oblongata.
 
 
 
PonH Varolii.
 
 
 
Roof.
 
 
 
Tela choroidea
inferior.
 
 
 
Lateral walls.
 
 
 
Inferior peduncles of cerebellum.
 
 
 
Hiiroiidary
fi in*
bruin
Vfnirlt*.
 
 
 
Peduncles of cerebrum. Posterior perforated
space.
 
(N)rpora albicantlii. Tuber cintTcum, infundlbulum, and
imrt of hypojihvHis. Uptic
ehiiiHm.
 
AntiTlor |K'rfonittMl lamina.
CorpUM 8triiitum. Island of
\W\\. Olfai'tory
lolic.
 
 
 
Po8terit)r medullary velum.
Cerebellum.
Anteri(»r medullary velum.
 
Posterior part of
toKnientum.
 
Corpora quadriuemina. I^amina quadrigemina.
 
Pineal l>ody. Posterior commissure. Epithelium of velum
interpositum.
 
 
 
Middle and superior peduncles
of cerebellum.
 
 
 
Brachia. Internal geniculate
bodies.
 
 
 
Optic thalami.
 
 
 
Convolutions of cerebral hemisplieres. Corpus rallosum. Fornix. iSeptum lucidum.
 
 
 
Cavitt.
 
 
 
Fourth
ventricle.
 
 
 
Aqueduct
of Sylvius.
 
 
 
Third
ventricle.
 
 
 
Lateral
ventricles.
 
 
 
TIII2 DHVELOPMENT OF THE PERIPHERAL NERVOUS
 
SYSTEM.
 
'I'lif (l<»v<*l<>|)m<'nt of the pcri])heral nervous system is still
iiiV(»lvr<l ill some ilejxroe of obscurity. In general terms it
nmy Iw .slal4*(l that tin* ])eri])lieral nervous apparatus is derive<l as an e\t<>nsion of the central cerehro-spinal axis.
 
In the ease of the spinal nerves, e^'ich nerve-trunk is com*
poseil of hoth motor aiul sen.<orv fihers, tlie former being in
continuity with the spinal cord tlirough the medium of the
anterior (»r motor roots, and the latter through the posterior
or sensory roots, ea<'h sensory root ))o.^.sessing a ganglion.
The cranial nerves exhibit a less n^giilar composition. While
the trigeminal nerve, for e.xam|>le, arises by two roots, after
the manner of a spinal nerve, some others correspcmd in relative {N)sition and in mod(» of d<»velopinent to the ventral or
motor r(M>ts of th(» spinal nerves, and still others are equivalent to the Hi»nsory spinal roots.
 
 
 
ORIOIS OF THE GASGLIA. 317
 
The deTelopment of the sensoir nerve-Sbers ib d^jH-udent
upon and is preceiicd liy tliiit of the ganglia of the posterior
roots of the tspinal nervi'.=, and nf several ganglia of the head
rt'giiin which are related tu the development of certain of the
cranial nerves. Hence the consideration of the genesis of
 
 
 
Firi. IM.
 
 
 
oritr Rnbl). The
 
 
 
primitive ergments vn atlLt conneL'Icd wilh Ihe reiaiiluluK porllnn
genn-ltiTer. At the regiuu of tmnnltlon lliere ie to be ii
 
muHclc-plato ut the primiLlvc x
gemi-Uyer ; pmi. i»ri«Ul, imi6, vlwx-ral mldillB Ujrpr. 8, crotMCClion through a
Ilunl emhryo (nflvr Sagvuii'hl) : m, spinal cori] : tpa. lower thickened part of (he
nuurul ridge: ipp'. Its Dpptr allcnuated part, which Ii conllnuoiu with the Toorof
the Deural tube : ui. primitive aegmunt,
 
the ganglia niiist precede the account of the growth of the
sensory nerve-fibers.
 
The oriKin of the ganglia is connected with the early
history of the evoliilion nf the neural tuhe. Just after the
 
 
 
318 TEXT-BOOK OF EMBRYOJ.OGY.
 
Sides of the medullary plate {vide p. 279) have united with
each other to form the neural tube, there appear two ridges
of cells between the tube and the epidermis, one on each side
of the raphe or line of union of the sides of the tube. Thene
ridges are the neural crests (Fig. 159). They first appear in
the region of the hind-brain and advance from this point
both headward and tailward. The ganglia develop from
these neural crests. The cells of the neural crest are usually
described as growing out from the neural tube, though according to His it is probable that they originate singly from
the ectoderm.
 
Tlic mass of cells composing the neural crest grows out\vard and then ventrad along the Avail of the neural tube, and
very soon undergoes segmentation into a number of cellmasses which are the rudimentary ganglia. In the spinal
region the number of segments corresponds to the number of future spinal nerves. In the head region there are
four segments. These latter, the cephalic ganglia, will be
referred to sul)se(iuently.
 
The segmentation of the neural crest corresponds in the
main witli the sogmcntation of the ])araxial plate of the
inc.-odcrm, whereby the myotomes are ))ro(luced, and ench
segment lies upon the inner side of :i myotome. The connection of the segments with the neural tnl)e beccmies reduced in each case to a slender strand, the point of continuity
of which with the tube is shifted farther awav from the
median line, as (level(>])nient proi^resses, to eorresptmd with
tl)e dorsolateral position of tiie sensory nerve-roots in the
mature condition.
 
Tlieeells of the tranirlia ae<|nir(» eaeli an axis-evlinder process and a (h-ndrite or |)n>toj)la>iiiie process, beeoniing thus
bijKjIar e<'lls. Tli«' axons or axis-eylinder |>i'oeesses make
their way into the spinal e<)nl — in the ease of* the spinal
gauiilia — eunstitnting ilui> the dorsal nerve-roots, and ])nrsne
their eour>e within tiie eord as the e(>lnnins of (loll and Bnr<la('h. The (hndrites, eon>titntini:' llie distal ]>ortions of the
dorsal roots, join tlir ventral r(K)i< (»ii the distal si<le of the
iranulia and lM'<'onie liie sensory nerve-fibers of* the spinal
nerves. Allhonirji these two proe«.*sses grow out from oppo
 
 
ENVELOPES OF THE NERVE-FIBER. 319
 
site sides of the cell, the further growth of the cell is such
that both processes are connected with it by a common stalk,
thus producing the cell with T-sha|)ed process so characteristic of the spinal ganglia. Thus the ganglia are made up
of cells which are interpolated in the course of the sensory
nerve-fibers, and these cells may be regarded as having mignited from the developing cerebrospinal axis, or, if the
view of His be acc^ptwl, from the region of the e^ctoderm
from which the tube originates, their connection with the
axis l)eing maintained by the gradually lengthening out
axis-cylinder process.
 
The development of the motor nerve-fibers differs from
that of the sensory. These fibers, or at least, the axis cylinders of the fibers, are the elongated neurits of nerve-cells of
the spinal cord and brain. The neuroblasts of the thickened
neural tube, as they become fully differentiated nerve-cells,
migrate from their central position into the mantel layer, or
superficial stratum (Fig. 140). On the distal side of the nucleus of the cell, the protoplasm first becomes massed and
then lengthens out to form an axis-cylinder processor neurit,
which in all vertebrate animals grows out from the cerebrospinal axis to form the axis-cylinder of a motor nerve-fiber.
 
Although, in the wise of the spinal nerves, the motor and
sensory fibers are separated from each other at their origin
from the cord, they soon intermingle to constitute a spinal
nerve-trunk. In certain lower types, as cyclostomes and
amphioxus, the motor and the sensory fibers permanently
pursue separate routes to their ]>eriphenil distribution.
 
The envelopes of the nerve-fiber are acquired at a relatively late period. The appearance of the neurilemma precedes that of the white substance of Schwann. The
neurilemma is derived from the mesmlerm. The cells of
the latter apply themselves to the nerve and, penetrating
between the fibers, become arranged as an enveloping layer
upon each axis cylinder, ultimately forming a complete
sheath, the neurilemma. The persistent nuclei of these cells,
scantily surrounded with protoplasm, constitute the nervecorpuscles of the neurilemma. The medulla, or white substance of Schwann, is formed at a considerably later period
 
 
 
320
 
 
 
TEXT-BOOK OF EMIiRYOLOOY.
 
 
 
within the neurilemma. The ileixwit of the medullary sheath 1
varies as to time lor (Jifferent groups of SIhts — although the ]
time is constant for each groti|j — unJ proceeds always in a
direction away from the cell from which the fiber originates,
or, differently expressed, in the direction in which the fiber
eonveys impulses. Thus, in the spinal cord, groups of afferent
fibers may be distinguished from those that are efferent hf
observing the direction in wliich the medullary sheath develops — that is, whetiier tlio sheath appears first at the upper end |
of the fiber or at the lower end.
 
The cranial nerve-flbers in their development follow in tlie \
main the same general principles that govern the growth of
the iipina! nerves. That is to say, the motor fibers grow out 1
as extensions of the axis-cylinder processes of nerve-cells of i
the cephalic jmrt of the neural tul)e and the sensory fibers \
proceed from the cells of outlying ganglia, or in the case of
at least one nerve, the olfactory, from infolded and highly I
specialized ceils of the ectoderm.
 
The cephalic ganglia, four in number, have been referred I
to as resulting from tiie segmentation of the head-region of ]
the neural crest. As previously stated, the neural crest I
begins to grow first in the region of the hind-brain and j
extends from this point both forward and backward, occupying a iwsition upon the roof or dorsal wall of the hind-brain. \
The part of the neural crest belonging to the head-region i
then divides into the four masses or head-giinglia which are I
designated respectively the first or tiigemmaJ, the second or ]
acosticofadal, the third or glosBophanrngeal, and the fourth ;
or vagal, ganglia.
 
The trigeminal ganglion, which is very large, becomes di- i
vidwl into a smaller anterior (M>rtiou, the ophthalmic or ciliaiT
ganglion, and a. larger posterior segment, the tiigemlsal j
ganglion proper. These two become widely so|mrated durin^J
llic pnigres.'i of development, since they constitute respeo-J
lively the later ciliary and aasserian ganglia, the ciliary I
ganglion belonging to the ophthalmic division of the fifth I
nerve, while the trigeminal belongs to the sui^-rior maxillary J
division and the sensory part of the inferior maxillary divi- J
 
<n of the fifth. Their nerve-cells give rW. to the sensoryj
 
 
 
FOURTH CEPHALIC GANGLION. 321
 
fibers of these trunks in the same manner that the cells of the
spinal ganglia produce the sensory fibers of the spinal nerves.
 
The acusticofacial ganglion, afler its migration from its
original position on the dorsum of the hind-brain, lies just
in front of the otic vesicle. This ganglion subsequently
divides into the facial and the acoustic ganglia. The facial
ganglion, the geniculate ganglion or intumescentia ganglioform is of the facial nerve, situated in the facial canal of
the temporal bone, although described as a ganglion upon
a motor nerve, the facial, is, in reality, connected mainly
with the pars intermedia, a bundle of sensory fibers issuing
from the nucleus of origin of the glossopharyngeal nerve.
It is equivalent therefore to a spinal ganglion.
 
The acoustic portion of the acusticofacial ganglion divides
still further to become the ganglion on the vestibular part of
the auditory nerve, and the ganglion spirale of the cochlear
division of the auditory, which latter is situated in the spiral
canal of the modiolus. It is considered probable that the
lateral accessory auditory nucleus, which is connected with
the cochlear fibers of the auditory nerve and lies on the outer
side of the restiform body, is also a part of the acoustic
ganglion. From the cells of the vestibular ganglion, which
is situated in the internal meatus, centrifugal fibers develop
to form the vestibular nerve, while other centripetally growing
fibers become the ventral or mesial (vestibular) root of the
auditory nerve. The cochlear ganglion in the same way gives
rise to the cochlear branch of the nerve and to its dorsal or
lateral root. Thus the auditory nerve and its ganglia correspond respectively to the sensory root of a spinal nerve and
to a spinal ganglion.
 
The third cephalic ganglion becomes the ganglion of the
glossopharyngeal nerve, undergoing segmentation to form
the upper or jugular and the lower or petrous ganglia of this
nerve, while the axis-cylinder processes of its cells lengthen
out to become the sensory fibers.
 
The fourth cephalic ganglion similarly becomes the two
ganglia of the pneumogastric nerve and gives rise to its
sensory fibers.
 
21
 
 
 
VENTRAL MOTOR NERVE-ROOTS, 323
 
From what has been said, it will be apparent that the
cranial nerves develop in a far less regular manner than the
spinal nerves, and that consequently their trunks consist in
some cases of only sensory fibers, in other cases of only
motor fibers, and in still others, of both varieties. Typically,
each cranial nerve would have a dorsal sensory root with a
ganglion, and two motor roots, one lateral and the other ventral. But by the suppression of one or two of these typical
roots there will be produced a nerve, for example, representing only the ventral root, as the sixth and twelfth nerves, or
a trunk containing sensory and lateral motor fibers, as the
vagus, or a nerve consisting solely of sensory fibers, as the
auditory.
 
By way of recapitulation the cranial nerves may be briefly
considered seriatim :
 
First Pair. — The olfactory nerve-filaments grow centripetal ly from the olfactory epithelium of the nasal mucous
membrane.
 
Second Pair. — The optic nerve is not a true nerve (see
Chapter XVI.).
 
Third Pair. — The oculomotor nerve represents a persistent
lateral motor root of the first head-segment (the ophthalmic
division of the fifth nerve being the sensory root of the same
segment).
 
Fourth Pair. — The trochlear nerve represents a lateral
motor root and belongs to the second head-segment.
 
Fifth Pair. — The trifacial or trigeminal nerve, containing
sensory and motor fibers, represents a persistent lateral motor
root and a dorsal sensory root. The ophthalmic portion of
the sensory root belongs to the first head-segment, while all
the remaining fibers, with the fourth nerve, are assigned to
the second segment.
 
Sixth Pair. — The abducens develops as a ventral motor
root and belongs to the third and possibly to the fourth
segments.
 
Seventh and Eighth Pairs. — The acusticofacialis nerve, or
the facial and auditory nerves, develop as a single nerve with
^veral roots. The auditory nerve and the sensory fibers ^^
 
 
 
324 TEXT-BOOK OF EMBRYOLOGY.
 
the facial — that is, the pars intermedia — correspond to a dorsal sensory root, the division of the acusticofacial ganglion into
the several ganglia of the auditory nerve and the geniculate
ganglion of tlie facial accounting for the division of the root
into the auditory trunk and the ]xirs intermedia. (The sen*
sory tibors of the facial pass off through the chorda tympani
to go to the tongue as special-sense fibers.) The motor fibers
of the facial develop as a lateral motor root^ originating
from c-ells in the ventral zone. These two nerves, with the
sixth, belong to the third and possibly to the fourth head*
segments.
 
Xinth Pair. — The glossopharyngeal nerve, made up largely
of s(»ns()rv fibers?, re[)resents a dorsal sensory root and a lateral
motor root, the fib(»rs of which latter grow out from cells in
the dorsal ])art of the v(»ntral zone of His, the later micleus
ambigmis. It belongs to the fifth head-segment.
 
Tenth Pair. — The vagus develops in the same manner as
the glossoj)harvngeal.
 
Eleventh Pair. — The sj)inal acoessorv represents in part
motor spinal roots and in [)art probably the lateral motor and
dorsal scnsorv roots of the cranial nerves.
 
Twelfth l^iir. — The liy[)ogIossal develops as the ventral
motor roots of several segments, being identical in mode of
origin with the anterior roots of the sjnnal nerves. This
nerve and the vati^us b(*long to the head-sc»gments from the
sixth to the tenth inclusive.
 
THE DEVELOPMENT OF THE SYMPATHETIC SYSTEM.
 
There are two views as to the origin of the sympathetio
system. One theory, based uj)on the investigjitions of Paterson, is that the gangliated cord of the sym[)athetic is differentiated from mesodermie cells, the eell-eord thus formed
acquiring, secondarily, coinieetions with the spinal nerves,
and ])res<Miting still hiter the enlargements which C4mstitute
the ganglia.
 
The more genendly accept(»d vi(?w, based upon the researches of Rilfour and the later work of Onodi and His, is
that the Bsrmpathetie ganglia develop as offshoots from the
 
 
 
DEVELOPMENT OF THE SYMPATHETIC SYSTEM. 325
 
ventral extremities of the spinal ganglia. Each little mass,
which has budded off from a spinal ganglion, moves somewhat toward the ventral surface of the body, its bond of
union with the parent spinal ganglion being drawn out to a
slender cord, the representative of the future ramus communicans. Each primitive sympathetic ganglion sends out
two small processes, one growing tail ward from its lower extremity, and one in the opposite direction from its upper end,
the approaching processes from each two adjacent ganglia
meeting and uniting and thus secondarily establishing the
connection between the different ganglia of one side of the
body and forming the gangliated cord of the ssrmpathetic.
From these ganglia migrating cells probably pass out to
develop into the secondary ganglia of certain viscera, as His
has shown to be the mode of origin of the ganglia of the
heart.
 
THE CAROTID BODY, THE COCCYGEAL BODY, THE
ORGANS OF ZUCKERKANDL.
 
In connection with the sympathetic system may be mentioned the " carotid gland," or glomus caroticus, or intercarotid
ganglion, found at the bifurcation of the common carotid
artery ; the coccygeal body or " gland," Luschka's ganglion,
found at the lower extremity of the coccyx in relation with
branches of the middle sacral artery; and the organs of
Zuckerkandl, found in later fetal life and for a short time
after birth at the origin of the inferior mesenteric artery.
 
These structures present features in common with each
other in that they are made up of knots of blood-vessels
intermingled with collections of cells, among which are
numerous chroviaffine cells such as are found in the medulla
of the adrenal body and in the sympathetic ganglia ; and in
the furthor fact that they are penetrated by sympathetic
nerve-fibers.
 
That the cells of the carotid body and of the organs of
Zuckerkandl are derived from the adjacent sympathetic ganglia has been established, but whether these bodies are for that
reason to be classed as nervous structures is as yet uncertain.
 
 
 
CHAPTER XVI.
THE DEVELOPMENT OF THE SENSE ORGANS.
 
In the organs of the .senses we have to do with peripheral
nervous mechanisms of greater or less degrees of complexity,
the essential elements of which are elalK>ratelv modified or
specialized neiiro-epithelial cells. These neuro-epithelial
structures are specialized cells of the ectoderm, derive<l from
it either directly, hy the infolding of patches of ectodermic
epithelium, as in the case of the olfactory cells, or indirectly,
by growth outward from the central nervous system, as in
the case of the retina. The organs of the sense of touch, the
tactih' corpuscles of the skin and mucous membranes, are
distributed somewhat irregularly, while such highly Sjweialized struc^tures as the organs of the special senses of vision,
hearing, smell, and taste are provided with special protective
and accessory apparatuses.
 
THE DEVELOPMENT OF THE EYE.
 
It will perhaps larilitatc th<* eoinpreliension of the general
principles involved in the (Ifvclopincnt of the eye if its
function as tlie organ of vision is. krpt in mind, and if,
therefore, the retina and the o|)tic nerve are recognized as
the essential parts (»f the organ, and tlie other structures as
accessories. The retina an<l tlie optic nerve are an outgrowth from the brain, the rod- ai]<I eone-visual <'ells of the
former being epithelial cells so specialized as to serve as
j)ercipient eh-ments, while the (»j)tic nerve-fibers are the (-onducting medium. To alh)Wof tlu' penetration and refra<'ti<m
of the ravs of liirht, the (»verlving epich^rmis differentiates
into a transparent and refra<*tive medium, the crystalline
lens, and the necessary prote<»tion an<l means of nourishment
 
 
 
THE DEVELOPMENT OF THE EYE. 327
 
are provided by the other constituent of the eyeball. Further protection is furnished by two folds of modified skin
and subcutaneous tissue, the esrelida, and lastly for the
lubrication and still further protection of the exposed part
of the eyeball, there is formed still another set of accessory
organs, the l&crimal appuratiu.
 
The first step in the development of the eye is the growth
of a diverticulum from the side of the primary fore-brain
vesicle (Fig, 160). These optic evaginations are qnite large
 
 
 
 
II of two-d«j- chlck-embryn ; B. bmln of huni»n embrro of
Oiowii the development of tbe opllc leslclva and bnlD-resl<
h. inter-brain; ob, optic veulcles.
 
 
 
as com{)arGd with the h rain- vesicle. They begin to be evident even before the neural tube is completely closed. As
the attached part of the diverticulum expands less rapidly
than the distal ]>ortioii, the evagination soon assumes the
form of a sac or vesicle, the optic vesicle, connected by a hollow stalk with the primarj' fore-brain. When the secondary
fore-brain vesicles gn)w out anteriorly from the primary vesicle, the region of the latter that Itecomes in consequence the
inter-brain is the part to which the stalk of the optic vesiele
in attachetl. Hence the optic vesicle is an appendage of the
inter-brain or thalamoncephalon and its point of attachment
to the latter Is at the lateral iwrt of the base, in front of the
region of the infundihiihim (Fig. 147, A and C).
 
The optic vesicle expands laterally and dorsally until.it
lies immediately lieneath the epidermis, forming a prorai
 
 
328 TEXT-BOOK OF EMBBTOLOGT.
 
nenee on the side of the head (Fig. 62). The ectoderm
at the point of omtaet with the optic ver^iele becomes thick*
ened and depressed, the dilTerentiation of this lens-area being
the >tartin^ p«.»int of the erystaUine lem. The depressed
|iatoli of ectixlemi, sinking more <k»eply, is converted into a
sac, tlie lens-vesicle, the o >nnt*etion of which with the surfacecells is <mM\ l«»st. The ilistal wall of the optic vesicle, upon
coming into contact M'ith the lens-vesicle, undergoes invagination, this wall sinking in until the cavity of the vesicle is
ahno-t obliterated. Thus the vesicle is converted into the
druilile- walled optic cnp, the o|K'ning of which looks laterally
toward the surface of the head, and is occupied by the leusvesiele.
 
The invaginated wall of the vesicle — tliat is, the layer
nearer the c<*nter of the cuji — becomes tlie retina, except its
pigment-layer, the latter resulting fn)m the outer layer of
tlie cup. The stalk <»f the cup l)ecomes the optic nerre.
The surroimding mesodermic tissue grows into the openings
nrferred to above, and gives rise to the vitreous hnmor,
while the mesodermic cells that closely envelop the optic cup
produce the uveal tract and the sclera and cornea.
 
Having tra<*ed briefly the develripment of the organ, its
sevcnil parts may now bo considered in detail.
 
The Retina and the Optic Nerve. — These two structures, as stated abov<*, an; directly derive<l from the optic
vesieh; and its stalk.
 
To rep4*at, for the sake of continuity, some points already
mentioned, tin; optic vesicle grows forth as a diverticidum
from the side; of tin? primary fore-brain vesicle, its appearance beting foreshadowed by a lateral bulging of this vesicle
even before the neural canal is com[)letely eloswl. When
the primary fore-bniin vesicle divides into the secondary
fore-brain vesicles and the vesicle of the inter-brain, the
regir>n of origin of the optic vesicle falls to the latter, the
jM»int (»f attachment being at the outer edge of the base of
the vesicle in front of the infundibular evagination. The
optic nerve is to be regar<le<l th(Tefore as springing from the
inter-brain or thahunen<*e|)halon.
 
 
 
THE F E P T TEE 39
 
The out x m f d rt um xpand g m re
 
rapidly than ba f hni n a um h f n fa
 
vesicle wi h a na n k I" p h k nd n
 
 
 
 
bring
 
 
pre
 
 
n h
 
 
r rth
 
 
k
 
 
Th
 
 
 
 
"E
 
 
 
 
n he
 
 
outward d
 
 
n 3
 
 
f no
 
 
V »■
 
 
n n
 
 
n
 
 
ah
 
 
 
 
f he
 
 
head.
 
 
Tb
 
 
beng
 
 
bra n-ca at
 
 
h
 
 
m
 
 
h
 
 
 
 
mme
 
 
 
Fio. 162.— Three BUMtmlvc sMgca of dsTelopnient of the eye, showinE fOmt.tlon or aeenndary nptic cup and crrilaltlne leiu in human embryos or 4 mm. < J),
fl mm. (B), &nd H mm,(C]. (Tonnieui): a, a, primltlTe optic veilclei; b, extern *l
Uyer of neciuidary opllc cup (fliture pfttmenl-Uyor of rellna) ; c. Inner layer of cnp
(re linn proper) ; il, lens-pit llhlekened and depreued eatudena):
 
 
 
diattfly under the epidermis, separated from it by nnly a thin
layer of embryonal connective tisBtie. This lateral position
of the optic vesicles is characteristic of the early stages of
development. After tlie end of the first month the eyes
 
 
 
330
 
 
 
TEXT-BOOK OF EMBRYOLOQT.
 
 
 
gradually move forward and downward toward their permanent position, which is approximately attained probably
early in the third month.
 
Shortly after the fourth week the distal or lateral wall and
the under surface of the optic vesicle l>ecome invaginated.
The invagination begins when the vesicle comes into contact
with the lens-vesicle (Fig. 162). When the infolding is
complete, the vesicle has l>econie the secondsiy optic cap,
which latter consists therefore of two layers, an inner and
an outer. The month of the cup, which faces away ffom
the niMliaii plane of the head, is occupied by the lens-vesicle.
Since the under surface of the vesicle ]>artici))a(es in the invnginating pniee.'is (Fig. 163) there is also in this wall of the
enp an ajwrture, which is known as the choroidal flsmre.
The invagination likcwi.sc affects the under siirfatre of tlie
tubular .-^lalk of the vesicle sii that it is c<mvertc<l into an inverted (loultle-Iaycred trough. These invaginations bear an
important relation not only to the
further nK'taniorphosis of the optic
vcsieli" lunl its stalk into the retina
and thf.iptic- nerve, but also to the
(levc4o|)nnTit iif the vitreous body
ami of llic r<-iiti-al artery of tlic n.'lina. TliMs, the vitreous binly is
PrmIiiwiI in yurX at !ca>t by the
Kie^iilcrmic tissue that finds access
to the cui) tlin.Li<rh llic ehoniidai
fiTisure, and tlic arloria wntralis
n-i'mw is di'v.IojK'd in the vascular
■ tissue iliat invagliiates
-uriaee of the stalk of
 
 
 
 
leM-lcl
 
 
 
the I
the '
 
 
 
■si.-Ie.
 
 
 
"nrV^/van'Iiri.Kil-Ti'm'''" '^''*' '■'""■""''^'l fi'-^i""*' gnuhially
1-oiitraeis after ilic enti-!in<-e of the
niescRlerni, and in the liist motitli of fetal life it entirely closes.
Tiie mouth of ilie o]Hie cup einlinices tite lens, its rim being
always on the ilislal side of, or su]>crlii-ial to, that .-itructure.
This iijieiiiiig repiTM-ntf* the pupil of later stagi-s.
 
 
 
THE DEVELOPMENT OF THE EYE. 331
 
The further metamorphosis of the optic cup includes alterations peculiar to each of the two layers and also to the
different regions of the cup. The mouth of the cup contracts
somewhat by increased growth of the wall, and thus there is
a zone bordering this orifice which is anterior to the lens,
holding the same relation to the latter body that the future
iris holds. A second zone corresponds with the periphery
of the lens, while a third region, the fundus of the cup,
includes all the remaining part of its wall.
 
The flmdas of the cup undergoes much greater specialization
than the other regions. The outer layer of the cup remains
thin, consisting of a single layer of cells which assume the
cuboidal form and become infiltrated with pigment-granules.
This forms the pigmenlrlayer of the retina. The inner
lamina of the cup thickens, by the multiplication of its cells,
and soon consists of numerous spindle-shaped cells. The
thickened fundus is marked off from the zone that surrounds
the periphery of the lens by a slight groove which corresponds in position with the future ora serrata. These early
spindlc-cells give rise to two kinds of elements, the stroma
of the retina, or Miiller's fibers, and the various nerve-cells,
including the highly specialized rod- and cone-visnal cells.
 
The principal sustentacular elements, or Miiller^s fibers,
like the spongioblasts of the neural tube, are radially
arranged and extend throughout the entire tliickness of the
retina. Their inner expanded extremities, in close contact
with each other, form the inner limiting membrane, while
their outer ends, in the same way, constitute the outer limiting membrane, which latter is in contact with the pigmentlaver. The stroma of the retina receives a small contribution from the mesodermic tissue, which grows into it through
the choroidal fissure to furnish the vascular supply.
 
Of the nerve-cells, those near the pigment-layer undergo
great alteration in form and become the sensory epithelium
— that is, the rod- and cone-visual cells. At first these lie
entirely internal to the external limiting membrane, which
separates them from the pigment-layer. After a time, liowever, processes grow out — that is, away from the center of
 
 
 
332
 
 
 
TEXT-HOOK OF KMTIRYOLOGY.
 
 
 
the eyeljall — and ]>erforate the external limiting membrane
to i»eiietrale Iwtwecn the cells of the pigment-layer. These
 
jmxresscs are llu' rods and fonen, and colWtively constitute
the layer of rods and conea of (lit- iidult The bodies of the
 
 
 
 
KoMlsni pi. pi BtQ* 11 led I'plthcUiiTii iif Ihc eju I'luttr luniella uf the opUc enp,fl^
aceondary optic veiklcl ; r.rolinii(iniiprlain*llB of the optic cup); it, mKi^iUln
nf (he optic cup. which farma the pun clllarls el Iriills rellnie: g, Til
with blood-TCBKla : fv, tunlm tucuIou leiilla; U. blood-uurpuwlei : tA.ehon
tf, lens-flben: It, leiu-eplthcllum ; r.nnu of the leni-tlbcr nuclei; ft, ftindaB
at the iK>niea; he, exlernAl corneal epithelium.
 
 
 
rod- and cone-visual cells, situated on (he inner side of the i
membrana limitan.s externa, are elongated into narrow ele- j
ments, the position of tho nnctei being indicated by slight I
enlai^ments. They constitnte the outer nuclear layer of '
the matnre retina. The outer nuclear layer and the layer of
 
 
 
THE DEVELOPMENT OF THE EYE. 333
 
rods and cones are to be regarded, therefore, as one layer of
highly specialized neuro-epithelium, made up of the rodvisual cells and the cone-visual cells, the inner segments or
bodies of the cells being only apparently isolated from the
outer segments, the rods and cones respectively, by the fact
that the latter proj(?ct through minute apertures in the
external limiting membrane. The axis-cylinder pnxjcssesof
these cells pass toward the center of the eyeball.
 
The neuro-epithelium of the retina is the last of its elements
to develop. In man and in many mammals, it is present at
birth. In the cat and the rabbit, the rod- and cone-visual
cells develop after birth, and hence the new-born of these
species are blind. The macula lutea is developed after birth.
 
The cells of the inner part of the retina differentiate into
the remaining nervous elements, some becoming the bipolar
and other cells of the inner nuclear layer — the ganglion
retinae — while others form the large ganglion cells of the
ganglion-cell layer. The axis-cylinder processes of the
ganglion cells are directed inward to form the nerve-flber
layer, the fibers of this layer converging from all parts of
the inner surface of the retina toward the optic disk or
papilla. Here they perforate the retina, as well as the choroid and sclera, to pass, as optic nerve-fibers, to the brain.
 
This part of the optic cup, the ftmdus, produces then, in
the manner descrilKjd above, the functionating portion of the
retina, or the pars optica retinae, the anterior termination of
which is indicated by the orra serrata.
 
The lenticular zone of the optic cup, which is in relation
with the peripherj' of the lens, undergoes comparatively
slight specialization. Its outer lamella is pigmented, as in
the fundus of the cup. Its inner layer remains very thin
and consists of cells which at first are cuboidal, but which
later become cylindrical. At the end of the second month,
or the beginning of the third, the two layers of the lenticular
zone become plicated, owing to excessive growth in superficial extent. The folds are nearly parallel and are arranged
radially with reference to the lens, the margin of which they
surround. These folds are the first indication of the ciliary
 
 
 
334 TEXT-BOOK OF EMBRYOLOGY,
 
processes. The mcstKlermic tissue immediately external to
th(» <)j)ti(! cup (lifforentiates into the uveal tract, the part corresponding with the lenticular zone of the cup furnishing the
ciliary hody. The young growing connective tissue penetnitos between the folds of the lenticular zone of the cup,
acupiiring intimate union with the pigment-layer, and thus
provides tlie connective-tissue basis of the ciliary processes.
This lenticular zone of the two hiyers of the optic cup,
therci'on*, <'onstitutes the lining, or internal covering, of the
ciliary ImmIv, and must necessarily be reganled as the continuation of the retina, it is known as the pars ciliaris retiiUB
of tlie fully developed (?ye.
 
TIkj marginal zone of the optic cup, or the region bordering its orilice, is also related in its further growth with the
uveal traet. Although in the earlier stages of development
the lens lies in the mouth of the cuj), as time goes on the
relation is so altered that the a[K»rture and the zone which
borders it occupy a position in front of the lens. In this
marginal zone both lamelhe of the cuj) become pigmented
and aecjuire union with the layer oi' mes(Klermic tissue which
is dillerentiatiiiir into the iris, and tliev therefore contribute
to the formation of that structure, ecmstituting its pigmentlayer. Th(^ pigment-layer of the ]>osterier surface of the
iris is, therefore, an extended but rudimentary part of the
retina. It is called the pars iridica retinae.
 
FnMu what has been said, it will be ap])arent that the
retina f(>rms a (Mnnplete tunie with an anterior perforation,
the pupil, and that it consists of the funetionally active part,
or retina proper, th(» pars optica retinae; of the pars ciliaris
retinae, marked olV iVom the latter by the ora s(»rrata ; and
of the pars iridica retinae, which terminates at the margin
of the pupil.
 
The evolution of the optic cuj) or secondary oj)tic vesicle
mav b(» thus summarized :
 
I. Marginal or most anterior The thin atrof)liic pare iridica reregion of cup. tinir, or pi^rnicnt layer of the iris.
 
n. Ltnticular zone of cup. Pars ciliaris rt-tina*, covering inner
 
surface of ciliary body.
 
 
 
THE DEVELOPMENT OF THE EYK 335
 
III. Fundus of cup. Functionating part of retina, or pars
 
optica retins, including :
A. Outer layer. A. Pigment-layer of retina.
 
£. Inner layer. B. 1. Neuro-epithelial layer, made
 
up of layer of rods and cones (the processes of the rod- and cone-visual cells) ;
membrana limitans externa; outer
nuclear layer (the bodies of the rodand cone-cells).
 
2. Cerebral layer (representing an
interpolated ganglion with connecting
fibers), consisting of :
 
Outer reticular layer ;
Inner nuclear layer ;
Inner reticular layer;
Ganglion-cell layer;
Nerve-fiber laver.
 
 
 
The optic nerve is the metamorphosed stalk of the optic vesicle. When the distal and under surfaces of the vesicle suffer
invagination, the stalk participates in the process, its under
surface being marked by a groove which is a prolongation of
the choroidal fissure of the optic cup (Fig. 163). By this infolding, the cavity of the stalk is obliterated and the stalk is
converted into a double-walled tube enclosing mesodermic
tissue which follows the invaginating ventral wall. In this
mesodermic tissue is developed the arteria centralis retina.
In mammals the invagination affects only the distal part of
the stalk, the segment included between the eyeball and the
point corresjionding in the adult to the place of entrance into
the nerve of the central artery. It must be apparent that
the outer layer of the tube thus formed is directly continuous
with the outer layer of the optic cup, while the invaginated
lamina is the prolongation of the inner wall of the cup or
of the part that becomes the retina proper, since not only
the distal wall of the optic, vesicle is invaginated, but its
under or ventral wall as well.
 
Tlie primitive optic nerve at this stage consists of layers
of spindle-shaped cells, with a central core of vascular connective tissue.
 
The manner in which the nerve-fibers are developed is
 
 
 
336 TEXT-BOOK OF EMBRYOLOGY.
 
still a matter of controverjsy. According to His and K5lliker, the fibers gi*o\v out from the ganglion-cells of the optic
thahimi and the anterior corpora quadrigemina^ while Muller
and Froriep believe that they are the prolonged axis-cylinder
processes of the ganglion-cells of the retina. According to
Ramon y Cajal, growth occurs in both directions. In
eitlier ease, tlie cells of the optic stalk would furnish only the
sustentative tissue of the nerve. There is also a contribution of sustentative tissue or stroma fnmi the mesoderm, as in
the case of the central nervous svstem.
 
The Crystalline I^ens. — The lens, exclusive of its capsule, is, like the retina, of ectodermic origin. The first step
in its <leveIopm(»nt is the formation of a thickened and deI)ressed patch of the ectoderm on the lateral surface of the
hea<l, this area being situated at the place where the optic
vesicle is nearest the surface (Fig. U)2, By d). The depression is tlie lens-pit. It soon becomes converted into a
closed sac, the lens-vesicle, by the gradual a])proximation and
union of its edges. The pit receding from the surface as its
lips come together, the completed vesicle lies under the surface ectoderm, witli wliieh it is for a time connected by the
slender stalk of tho invagination. Upon the disapi)earanc(» of the strand of cells constituting the stalk, the lensvesicle is completely isolated from the outer germ-layer
(Kig. 1(12, (\i'l
 
The lens-voside in birds is a hollow epithelial sac several
lavers thick, but in nuunmals the central cavitv contains a
mass of (jells, which latter disappear in the later stages of
development.
 
I^pon the invagination of th<» optic vesicle to form the
se(M>ndary optic cu|>, the lens-vesich? is embracwl by the lips
of the cup and still later Cannes to lie within the cup, near its
orifice (Fig. 1^)4).
 
The further alterations in tlie vesicle are de|KMident primarily upon changes in its deep and sujH^rficial walls resjH^ctively, each of wliich consists of several layers of cylindrical cells. Tlie cells of the sui>erficial wall alter their
form, bcH?oming cul)oi<lal, while tlie posterior or decjwr cells
lengthen so as to become fibers. Thus the deeper wall of
 
 
 
THE CRYSTALLINE LENS, 337
 
the vesicle thickens at the expense of the central cavity — the
central mass of cells at the same time disappearing — while
the superficial layer remains thin. The two strata are continuous with each other at the equator of the lens, one form
gradually merging into the other at this region, which is a
zone of transition (Fig. 164).
 
The lens at this stage is composed, therefore, of a thin
superficial or anterior stratum of cuboidal epithelial cells and
a much thicker posterior or deep layer of so-called fibers, tlie
latter being simply the greatly elongated cells of the posterior
wall of the vesicle. Between the two laminse is a small
remnant of the cavity of the vesicle. The epithelial layer
persists throughout life as the epithelium of the lens, while
the fibrous layer is the basis of the lens-fibers of the mature
condition. The cavity sometimes persists as a small space
containing a few drops of fluid, the liquor of Morgagni.
 
The next important stage in the development of the lens
is the formation of additional lens-fibers. These result from
the proliferation of the cells of the epithelial or anterior
layer. The lens-fibers are formed in successive layers, as
may be made evident by the maceration of a lens. Each
fiber extends from the anterior to the posterior surface of the
lens. The ends of the fibers meet each other along regular
lines, producing thus the characteristic three-rayed figures or
stars of the lens, one of which belongs to each surface.
Hence, while the lens-fibers first formed are the elongated
cells of the posterior layer of the lens-vesicle, the fibers of
later gro^vth originate from the cells of the anterior wall.
The epithelial character of the lens-fibers is evinced by the
presence of a nucleus in each fiber of a young lens.
 
The lens-capsule results from the differentiation of the
mesodermic tissue which surrounds the lens. It is from this
enveloping vascular lamina, the tunica vasculosa lentis, that
the growing lens derives its nutrition. The capsule is well
marked in the second month. Its blood-vessels are derived
from those of the vitreous body. At the end of the seventh
month this well-developed, highly vascular membrane begins
to undergo retrograde alterations, the final result of which is
 
22
 
 
 
338 TEXT-BOOK OF EMBRYOLOGY.
 
its transformation into the thin, non-vascular^ transparent
capsii le of tlio mature lens.* The most active growth of the lens
itself occurs prior to the degeneration of the tunica vasculosa
lentis, so that even before the end of fetal life the lens has
nearly attained its full size. Thus the weight of the lens of
the new-born child is 123 milligrammes, while that of the
adult lens is but 11)0 milligrammes (Huschke).
 
Hence the crystalline lens has a double origin, the lens-substance or lens proper being derived from the ectoderm, while
the capsule oriirj nates from the mesoderm.
 
The Vitreous Body, — The vitreous body has been regarded usually as a roniparatively slightly differentiated form
of connective tissue, and as being derived from the middle
germ-layer. Recent investigtitions show, however, that it
originates in ])art at least from ectodermal tissue. According to these observations, processes grow forth from those
stromal elements of the optic cup which afterwanl Ix^come
Midler's libers, and these processes, advancing toward tlie
lens-vesiele, interlace to form a network, the primitive
vitreous (Kolliker, Froriep). This ])roeess continues for a
longer time at the marginal zone or month of the cup than
elsewhere, the j>roto|)lasniie fil>ers which grow from this
future ciliary and iridal j)ortion of the cup contributing to
the lorniaiion of the zonule of Zinn. In mammals the cells
of the lens-vesicle, another ectodermal structure, also send
torth processes which, according to Lenhossek, bear a prominent part in the d<velo|)ment of the vitreons body. The
mesodermic tissue, already in the sta<x<* <>f cnd)rvonal connective tissue, now gains access to the optic (Mip through the
choroidal fissure (Fig. 1 <>•>), its ingrowth, in fact, accomjKinying the invagination of the un<l(M' >urta<M' of the opti(^ vesicle,
and constitutes what K(")lliker designates the mesodermal
vitreous. The intermingling of these two constituent ele
' It sonu'titiu's li:i|>|»«.Mis that parts of the fetal lons-capsulc persist. The
most ootnmon exani]>le of 8\i<h persistence is the so-called meiiihrana pupillaris soinetinK*s present at hirth, pnuhicinK rmnjnn'tal otrtAia of the pupil.
This results from the persistence (»f that part of the fetal capsule which is
situated on the anterior surface of the lens, hehind the pupil.
 
 
 
THE MIDDLE AND OUTER TUNICS OF THE EYE. 339
 
ments produces finally the definitive vitreous. Since the
inferior surface of the stalk of the vesicle — the future optic
nerve — participates in the invagination of the optic cup, the
mass of mesodermic tissue which helps to form the vitreous
is continuous with that which invaginates the primitive optic
nerve to produce the central artery of the retina. As a consequence, the blood-vessels which soon develop so plentifully
in the vitreous b(xly are extensions from the central artery
of the retina, the latter itself being continued forward as the
hyaloid artery. The terminal branches of the hyaloid artery
pass on through the vitreous body to terminate in the vascular capsule of the growing lens, constituting the blood-supply
of that structure.
 
The intercellular substance of the young tissue undergoes
but little differentiation, while the cells become gradually
reduced to a few stellate elements which ultimately entirely
disappear. The peripheral part of the tissue develops into
the hyaloid membrane, which anteriorly acquires union with
the capsule of the lens.
 
The blood-vessels of the vitreous disappear during the last
two or three months of fetal life. The hyaloid artery persists, although in reduced form, for a longer time than the
smaller vessels. Upon its final degeneration it is replaced
by a canal, the hyaloid canal, or canal of Stilling, which is
present in adult life.
 
The Middle or Vascular and the Outer or Fibrous
Tunics of the Bye. — The outer fibrous coat of the eye, including the sclera and the cornea, and the middle tunic or
uveal tract, comprising the choroid, the ciliary body, and the
iris, are structures of mesodermic origin, being directly produced by the mesodermic tissue surrounding the optic cup.
The richly cellular mesoderm applies itself to the exterior of
the cup and differentiates into the two layers in question, the
changes involving on the one hand the metamorphosis of the
mesodermic cells chiefly into muscular and vascular elements,
and on the other hand the evolution of a tissue essentially
fibrous in structure. These two tunics are distinguishable
in the sixth week.
 
 
 
340 TEXTBOOK. OF EMBRYOLOGY.
 
Tlie cornea is formed from the thin laver of mesoderm that
penetrates l)et\veen the lens-vesicle and the surface ectoderm.
The lens- vesicle lies very near the surface, and the thin
stRitum of mesoderm that is interposed between the two is
the anterior layer of the lens-capsule (Fig. 164). This anterior layer thick(»ns by the immigration of other cells and subsequently splits into two lamime, a superficial one which produces the cornea (Fig. 104, A), and a deeiK?r, which is now the
proper anterior wall of the lens-c^apsule. Thus a space filled
with fluid a pjwars between the primitive cornea and the lens,
which (H)rresponds with the future anterior and posterior
chambers of the eye, the <livisicm of the sj>ace into these two
chambers being eflectcd subsequently by the development of
the iris. The further (leveloi)meiit of the cornea consists
simply in the differentiation of the mesodermic cells and the
int(M'eellular substance into the several characteristic elements
of the adult structure.
 
The uveal tract closely corresponds in extent with the two
layers of the oi)tic cup. The choroid is differentiated from
that portion of this primitive uveal tract which envelops the
pars optica of the retina. In this region the enveloping
layer of nicso<lcrniic cells develops into the several elements
of the choroid, the most C()ns])icuous of which are an inner
layer of capillary vessels, tlu^ choriocapillaris, and an outer
lay(»r of larg(?r vessels, th(» stroma-layer of the choroid. Tlie
development of the clioroi<l bears a certain relation to the
choroidal fissure of the optic cup. This tissure has been referred to as a gap in the iMi<lcr surface of the eup corresponding with the line of invagination through which the
mesodermic tissue, of which the developing choroid is a
part, grows into the cup to pnxluee the vitreous. Although
normally this fissure in the retina entirely disjippears, its site
be<*onies pigmented later than other regions of the pigment«laycr of the retina, and h<*nce there is, for a time, a clear
streak in this part of the retina which has the appeanince of
a fissure in that membrane. As the pigment-layer of the
r(»tina was formerly assigncnl to the choroid, this streak appeared to be a breach of continuity of the ch<»roid ; hence the
 
 
 
THE MIDDLE AND OUTER TUNICS OF THE EYE. 341
 
term choroidal fissure. In some cases, however, the choroidal
fissure fails to close, and as the development of the choroid
is largely dependent upon or is governed by that of the
retina there remains a corresponding gap in the choroid.
This defect enables the sclera to be seen from the interior in
a line extending forward from the optic nerve entrance. It
is known as coloboma of the choroid.
 
The ciliary body is developed immediately in advance of
the choroid and from the same layer of mesodermic tissue.
The deeper parts of the tissue in this region correspond with
the plications of the ciliary part of the retina, sending processes into and between the radial folds of this part of the
two layers of the optic cup, with which latter the highly
vascular mesodermic tissue acquires firm union. This results
in the formation of the ciliary processes. Some of the cells
of the more peripheral part of this zone are converted into
unstriated muscular tissue, thus producing the ciliary muscle.
All the characteristic or important elements of the ciliary
body are, therefore, derived from the mesoderm, while the
thin layer of tissue on its inner surface, representing an
undeveloped part of the optic cup, the pars ciliaris, is of ectodermic origin.
 
The iris, the most anterior zone of the uveal tunic, is produced from the same mesodermic tract that gives rise to the
choroid and to the ciliary body. As stated above, soon after
the lens-vesicle becomes constricted off from the surface ectoderm, it is enveloped by a mass of mesodermic cells which
constitute its primitive capsule, and the layer of these cells
lying between the lens-vesicle and the surface ectoderm splits
into an anterior layer, which becomes the cornea, and a posterior stratum which is the anterior wall of the lens-capsule.
This produces a space between the lens and the cornea. The
lens now recedes farther from the surface, and the margins
of the optic cup advance, so that the lens now lies within
the cup, the marginal zone of the cup being in front of the
lens, between it and the cornea, while its equator is in close
relation with the ciliary regions of the cup and of the uveal
tract. Thus the space between the lens and the cornea is
 
 
 
342
 
 
 
TEXT BOOK OF KMBIiYOLOGY.
 
 
 
V
 
H divided into an iiiitcrior cdnipartnicnl, tlic anterior chamber,
 
H ntid a jmsterior fl])ai-p, tin- posterior chamber, the orifice of
 
■ tlic cup being a mi-nns of comiTHinication l>plnceii the twd
 
H and representing ttie pupil of a iaier sla^, Tlie niar^ual
 
H zone of the cuj) furnishes the gniding line for the develo]*
^1 ment of the iris, The niesf«lcniiie tissue in rclutioD with
 
 
 
tm. Iffi.— BtgltEal iiectlon IhroUBh ttif pyp nf an embryo i
 
lUyi X 30 (Kailkerl: o,o|illc nttrc; j.. licmgimBl Hmni-nl-U
 
CllUry pun nf the rellnt ; p'. fi>rppiirl "f llie i>l.ll« PUp (rudiment of thB il
 
the srterli twnlralls retlnit unler II: f. trls; mj,, nienibr«n» pupllUriii «,«
with Piilib*tluro (,- pp.pn. iMiliwbne; I. lem; V. Icnt-cpllhcHum ; /. MlwottBjJ
 
 
 
the outer surface of the marginal stone of the euji difTerc
tiatca into the vascular, muscular, and couneclive-tiasuc elM
tnenta of the iris pn)iwr, while ils pfisterior pignient-laycr fa
constituted by the slightly specialized layers of the mot
anterior part of the optic cup, the part that in known a* th<
para iridica retinre. Recent investigations (Xushhauni, Her^
zog, etc.) indicate that the circular and the radial muscular'J
 
 
 
 
THE EYELIDS AND THE LACRIMAL APPARATUS. 343
 
fibers of the iris develop from the outer epithelial layer of
the optic cup or, iu other words, from the part of the optic
cup that becomes the pars iridica retina?. The circular fibers,
sphincter pupillee, are distinguishable in the fourth month,
the radial or dilator fibers, in the seventh mouth.
 
Since the anterior and posterior chambers of the eye are
spaces hollowed out of the mesoderm, they represent a
lymph-space and are, as such, lined with endothelial cells.
 
The cleft in the inferior wall of the optic cup referred to
above as the choroidal fissure necessarily affects the marginal zone of the cup as well as the region posterior to it.
If this part of the fissure persists, as it sometimes does, it
may be accompanied by a corresponding deficiency in the
tissues of the iris projjer. Such a congenital defect, appearing as a radial cleft in the lower half of the iris, is known as
coloboma of the iris.
 
The Eyelids and the I/acrimal Apparatus. — The
 
eyelids are developed from folds of the primitive epidermis that form over the superficial part of the developing
eyeball (Fig. 165, pp and pa). After the separation of the
lens- vesicle from the surface ectoderm, the latter pouches
out into two little transverse folds for the upper and lower
lids respectively. Each fold includes a certain quantity of
mesodermic tissue, from which are produced the connectivetissue elem.ent^ of the lids, as the tarsal plates, etc. After
the folds attain to a certain degree of development their
eilges approach each other and become adherent, thus enclosing a space between the primitive lids and the front of the
eyeball. The infolded ectodermic layers lining this space
acquire the characteristic features of mucous membrane and
constitute the epithelium of the coAJnnctiva, the part of this
membrane that covers the cornea adhering closely to that
structure as its anterior epithelial layer. The union of the
edges of the lids begins in the third month and lasts until
near the close of fetal life. A short time before birth the
permanent palpebral fissure begins to form by the breaking
down of the adhesions.
 
A part of the mesodermic tissue of the lids undergoes con
 
 
344 TEXT-BOOK OF EMBRYOLOGY.
 
version into fibmiis connective tissue, thus producing the
tarsal plates of the upper and lower lids^ with the iMJpebral
DasciflB and tarsal ligaments by which the plates are attached
to the margins of the orbit.
 
During tlie period when the edges of the lids are adherent,
the Meibomian glands and the eye-lashes are formed. The
ghmds develop from solid cords of epithelial cells that grow
from the deepest or Malpighian layer of the primitive epidermis into the tarsal plates. The conls become hollow
tubes by degeneration of their eentnd cells.
 
In addition to the two principal folds that produce the
lids, a third, vertical fold app(\ars at the inner, nasal side of
the conjunctival space, beneath the lids. This fold remains
quit(i small in man and forms the plica semilunaris, but in
most other vertebrates it attains much greater size as the
third eyelid or nictitating membrane. A small part of this
third fold develops sebacw)us glands and a few hair-follicles
and becomes the lacrimal caruncle.
 
The lacrimal gland is devel(>]KHl in the same manner as
tl)(i Meibomian glands, by the growth of solid epithelial
cords from the conjunctiva. The cords grow into the underlying inesodcnn at th(» outer part of the line of reflection of
the conjunctiva from the inner surface of the upj)er lid to the
front of the eyeball. The conls acquire lateral branches and
then become liollowcd out to form the secreting tubules and
efferent ducts of the gland, the connective-tissue stroma of
which is contribut<'d by the surrounding mesodermic tissue.
The orifices of the adult efferent ducts in the upj>er outer
j)art of the conjunctival sacj corr(»sjK)nd with the points from
which the primitive cell-cords first grow forth.
 
Th(> efferent lacrimal apparatus, consisting of the nasal
or lacrimal du<'t and the canaliculi, is related genetically
to the growth of the nose and the upjxT jaw. S(M)n after
the appearance of the nasofrontal ]>rocess, a lateral projection, the lateral nasal process, grows from its side near the
base and advances <lownward so as to form the outer boundary of the nasal j)it and consequently of the future nostril
(Fig. ()7, A, i>). This lateral nasal process is separated from
 
 
 
THE DEVELOPMENT OF THE ORGAN OF HEARING. 345
 
the maxillary process of the first visceral arch by an oblique
furrow, the naso-optic groove, which extends from the inner
angle of the orbit to the outer side of the nostril, or, before
the separation of the nasal pit from the primitive mouth, to
the upper boundary of the latter orifice. The naso-optic
groove indicates the situation of the lacrimal duct. By
some authorities — Coste and Kolliker — it is believed that
the duct results from the union of the edges of the groove.
Later investigations seem to indicate, however, that the
duct is formed by the hollowing out of a solid cord of epithelial cells that appears at the bottom of the furrow. In
either case the epithelial lining of the duct is an ectodermic
involution. When the nostrils are separated from the oral
aperture by the union of the nasofrontal, the lateral nasal,
and the maxillary processes (p. 133), the lower end of the
furrow is obliterated, and the partially formed duct is made
to terminate in the nasal cavity.
 
The canaliculi, representing the bifurcated upper extremity of the duct, result, according to one view, from the
division of the upper end of the epithelial cord into two
limbs, one for each lid, and their subsequent hollowing-out ;
according to another, from the continuation of the cell-cord
into the upper lid and the later addition of a limb for the
canaliculus of the lower lid. The lacrimal sac is merely an
expanded part of the duct.
 
THE DeVELOPMENT OF THE ORGAN OF HEARING.
 
As in the case of the other sense-organs, the auditory
apparatus consists of highly specialized nenro-epiiheliiim,
connected by nerve-fibers and interpolated ganglia with the
central nervous system, and of protective and auxiliary
structures. The neuro-epithelial structures, including the
organ of Corti and the cells of the cristsB and maculae
acusticae, result from the specialization of certain of the
epithelial cells which line the membranous labyrinth. The
perilymphatic space, which is a lymph-space, together with
its bony walls, the osseous labyrinth, serve for the protection of the delicate neural elements, while the middle ear
 
 
 
346
 
 
 
TKXT-llOOK OF EMBRYOLOGY.
 
nicclia for the oonductiou K^(
 
 
 
ami tlio nxleriial oar act ;
siinuroiis vibratiims.
 
Till' iiitcrniil ciir Iwiiig the css^scntial [lart of the organ of
hciiriiif^ ami hriiij; alsd tlie part first formed may i»ro|wrIy
n-ccive tirst eoiisiih-nit^m.
 
The Internal Car. — The membriuioiis labyrintli uf the
 
 
 
 
Uc vesicle of
> ilors pit: B.
Ihtf ol[c Malcle;
rftirc Lrloderm,
 
 
 
iiilerMal ear is tlic <il<h'.-l part df the "i-jran i>f licanng. Its
DiiM-iii is from a lliiekeiie.l .-in-iilar |»ateh i.|' eetiHlerm on the
liorsulateral Mirl'ace i.l" the hea.l-rrjrimi of tlie eiiihryo near
the liiirsil lerriiiitalioii i.f the fir.-l oilier viseerjil furrow. The
tiiieketi.'.! arra M1ll^^ l>i-lo\v the Mirfar*', forniiiig thus the
auditory pit, whic-li is iin>eiit in itie ihinl week (Kig, lOd, .1 ).
 
Tlie j.il 1 nrs .h-e|.rr, it^ e.lut- a|.|.rua.-h eaeh other and
 
tiiially iiieel ami niiitc lo form the otic vesicle or otocyst.
Tills "lilile e]>ithelial sae fira.lnally nve<les from the nirface
eeli«l.'i-m. At llii,- Ma.L'i- "f d-veloimieiit then- is no eraiiial
eapsule oiher than tlir imlilti'r<'nt itu-oileriaie tissue \v I lieli
surrounds ihe hi-ain-ve.-ieles ; heuee, the otic vcsiele, cmbedded ill tlii,- ti»m-, lies in elo^,- proxhnily to tlw aftor!>n»iti, and e.mies into r<>Iation with the neiisli«)(aeial ^Mtiglion (jt. ;t"21). Till' vcsiele, at lirst s|ilierii-al, soon heeoiiies
 
 
 
THE lyTEEXAL BAH.
 
 
 
347
 
 
 
pear-shaped owing to the protrusion of its tlorsal wall. Tliis
dursal projection, the recesaos vestibtiU or la.b3rrmthi (Pig.
16ii, C), lengtliens out into a slender tube, tlie ductus endolymphaticns (Fig. ^^^), llie slightly dilated end of wlneli,
the aaccua endolymphaticuB, is found in the adult occupying
the aqucdiictus veslilndi of the tempoml bone. ,
 
 
 
 
Fin. 187.— Development or the membranniu labjrrlnth of tbo human ear (W.
Hta, Jr.l : A. left labyrinlli of embryo of about four U'ecke, outer ildc; i>E, velUbutar and cochlear poitlons : rl. recessua Jabyrliithl. B, left labrrlnlh with part*
nftoclalanaaudllfiry nerre* of embryo of about four ani a half weeka: rl. reoealus labyrintlii ; hc. ptc, ok, saperlar. poaterior. and external nemlelrciilar canalg ;
I. Haccule: <. cochlea: vn./R. vdlbular and facial aervea; rg- rfi SB- veatlbular.
cochlear, and genlciilatcganElin. C, left lahyrlnlh of embryo of about (Ivcwoclu,
bom wllhnlit and below: labelllni; as In preredlngflKure.
 
The opiHJsite, anterior or ventral extremity of the otic
vesicle tilsti bulges out into a small cvagiuatioD, wliieh gradually elongates until it is a tapering tube, slightly curved
inward toward tlie median piano. This lengthens still more
and becomes spirally coiled, forming the cochlear duct or
scala media of the future cochlea (Fig. 168). The venicle
it'^e If becomes constricted in such manner by an inward projection of its wall as to indicate its <livision into an upper
larger and a lower smaller sac, the terms upper and lower
referring respectively to the head-eud and the tail-end of the
embryonic body. Before the con.strietion occurs, the wall
 
 
 
 
348 TEXT-BOOK OF EMBRYOLOGY.
 
of that part of the vesicle which is to become the future
iipjKT or utricular division presents two {)ouched-out areas
(Fig. I(j7, B). One of these gives rise to the extenud semicircular canal, while from the other are formed the saperior
and posterior canals. The pouch that produces the external
 
 
 
 
Flo. 168.— niafrram to illustrate the ultimate condition of the membimnotu lAbjMnth (after Wnldeyer): i/, utrieulus: if, sacculus; cr, oanaliH rcuniens; r, ductus
endolymphaticus: r. cochlea; k, blind sac of the cupola; r, vestibular blind cae
of the du(?tu8 coehleuris.
 
canal is scmieirciilar in form and flat, lying in the horizontal plane, its upper and lower walls bring in contact with
each other. The oppo.sed walls fuse, except at the periphery
of the pocket, and hence all that remains of its cavity is a
small marginal tube or channel, corresponding with its border and opening at each end into tlu* cuvity of the vesicle.
Throughout the region of fusion of the walls, the latter become thin and finally disaj)i)oar, being replaced by connective tissue. Thus a semicircular epithelial tube is formed^
which is the horizontal or external semicircular canal. One
end of the tube being dilated, the ampulla of the canal is
produced.
 
Tlie superior and posterior semicircular canals are formed
in a somewhat similar inanucr by the other evaginatcd
ponch or ])ockct, which is irregularly globular. To pro<bi<*e this result, the walls of tlie pocket contract adhesions throughout two regions, which (U)rresp<md with the
rcs])cctive sj)accs enclosed by <'acli of the* two future canals
in (jiiestion. The fusion of the walls takes place in such
manner as to leave two narrow channels or tubes, one of
which ahnost encircles the inner or mesial asjKHJt of the
pocket, while the other bears the same relation to its jH)stc
 
 
THE INTERNAL EAR. 349
 
rior wall, the inner limb of the latter semicircle coinciding
with the posterior limb of the former. The result of this
arrangement is that two vertical semicircular canals are
formed with their planes at right angles to each other, the
two communicating with the otic vesicle by three openings,
one of which is common to both canals. The other two
apertures, being dilated, are the ampullated individiial orifices of the posterior and superior canals.
 
The constriction in the otic vesicle referred to above increases until this sac is divided into two parts, a larger,
which includes the region from which the semicircular
canals have developed and which is now the utricle, and a
smaller vesicle, the sacculOi comprising the part from which
the cochlear duct was evaginated (Fig. 168). The line of
division coincides with the middle of the orifice of the ductus
endolymphaticus, the proximal end of which participates in
the division. Thus the ductus endolymphaticus becomes a
Y-shaped tube, and affords the only bond of connection between the saccule and the utricle (Fig. 168).
 
The beginning of the cochlear duct, failing to keep pace
in growth with the other parts, api)ears as a smaller tube relatively, and is known as the canalis reuniens (Fig. 168, cr).
 
The structures so far considered — the utricle, the saccule,
the semicircular canals, and the cochlear duct — being the product of the ectodermic otic vesicle, represent simply the adult
epithelial linings of those cavities. The fibrous layer of the
membranous labyrinth, in common with the walls of the bony
labyrinth, is a product of the enveloping mesodermic tissue.
While the cells of the otic vesicle thus for the most part constitute the walls of the several sacs and canals of the primitive internal ear, some of the cells specialize into neuro-epithelium. The most marked specialization of this sort occurs
in the cochlear duct, where most of the cells on that wall of
the duct which may be called its floor — the part corresponding to the future membrana basilaris — undergo such profound
modification in form as to produce the most highly specialized neuro-epitheliul cells anywhere to be found, the elements
that constitute the organ of Corti.
 
 
 
350 TEXT-BOOK OF EMBRYOLOGY.
 
In the utricle and the saccule, as well as in the ampulIsB
of tlio semicircular C4inals, there is a similar but less marked
sp(»cializati(m of epithelial cells to produce in the former case
the maculae acusticse, and in the latter, the crista acusticfle of
the ampnlhT. While, therefore, the cells of the otic vesicle
which are to s(Tve as the lining mucous membrane of the
membranous labyrinth become flattene<l polyhe<lral cells
arranpMl as a sintrle lay(T, those cells which are to functionat(» as the periphenil part of th(» acoustic mechanism l>ecoine
the specially modified C(>lumnar cells, many of them with
cilium-like appendages, of the maculie, the cristae, and of the
organ of C(>rti.
 
From the first the otic vesicle lies in close relation with
the aeustieofacial ganglion (Fig. 167, />). As pointed out
in a preceding chapter (p. 321), this ganglion subsequently
divides into two parts, corresponding with the two divisions
of the auditory nerve. This division (jf the ganglion and of
th(* nerve is correlated with the separation of the otic vesicle
into a coelilear part, the cochlear duct, and the two vestibular
vesicles, the saccule and the utricle. While the cochlear duct
IS still a short, slightly curved tube, the cochlear ])art of the
ganglion lies in close proximity to the tube, in the concavity
on its inner side. As the duct lengthens and becomes more
coiled, tiie ganglion likewise lengthens into a band which
follows the spiral course of the duct, lying parallel with the
latter and on the side toward the axis about which it is
coiled. Ai'ier the formation of the bony parts of the cochlea,
this ganglion octMipies the sjnral canal of the modiolus and
is known as the gangUon spirale. It helongs to the cochlear
division of the auditorv nerve, which is distributed to the
cochleii.
 
The remaining part of the acoustic ganglion becomes rather
widely separated from the spiral ganglion, coining to occupy
a position in tluMuternal auditorv meatus, and the part of the
auditory nerve with which it is conne<*ted acquires relation
with the macular regions of the utricle and saccule as well as
with the crista' of the ampulhe of the semicircular canals.
These nerve-fibers constitute the vestibular division of the
 
 
 
THE INTERNAL EAR 351
 
auditory nerve, M'hile the ganglion is the vestibular ganglion
or intumescentia ganglioformis of Scar])a.
 
The development of the bony labsrrinth of the internal ear^
as well as of the connective-tissue parts of the membranous
labyrinth, is effected solely by the differentiation of the mesodermic tissue which surrounds the epithelial structures above
considered. As previously stated, at the time when the otic
vesicle is first formed there is no indication of a cranial capsule, the brain-vesicles being surrounded and separated from
the ectoderm by indifferent mesodermic cells. During the
progress of the alterations in the otic vesicle, this tissue
undergoes condensation and alteration to form the membranous primordial cranium, and shortly thereafter the
petrous portion of the temporal bone is outlined in cartilage
by the further specialization of a portion of this primitive
connective tissue. The formation of cartilage does not affect
all of the tissue which is afterward represented by the
petrosa, the region that borders the semicircular canals,
the cochlear duct, the saccule, and the utricle remaining soft
embryonal connective tissue. There is thus a cartilaginous
ear-capsule produced which is more than large enough to
contain the primitive epithelial labyrinth, and the walls of
which are separated from the latter by embryonal connective
tissue.
 
The bony semicircular canals are almost exact reproductions, on a larger scale, of the epithelial canals^ and they are
formed by the ossification of the cartilaginous petrosa. Even
before this ossification occurs the soft connective tissue
between the cartilage and the epithelial semicircular canals
differentiates into three layers. The inner layer, becoming
more condensed, is converted into fibrous tissue, and, adhering to the epithelial walls of the canals, furnishes the connective-tissue component of the completed membranous
canals. Its blood-vessels serve for the nutrition of the
canals. The outer layer also undergoes condensation and
forms a fibrovascular membrane, the perichondrium, which
later becomes the internal periosteum of the bony canals.
The middle layer, on the contrary, becomes softer — by the
 
 
 
352 TEXT-BOOK OF EMBRYOLOGY.
 
liquefaction nf tin* intercellular siul>stance ami the degeneration of the cells — so that gradually increasing, fluid-filled
cavities make their ap|H*a ranee, and these latter becoming
lar^(T and many of them coalescing, a ^pace is formed
around the niemhranous canals which is filled with fluid, the
perilymph. This perilymphatic space is bridged across at
intervals by con nectivt»-t issue processes that serve for the
convevanct* of l)hMMl-vess<*ls to the membranous canals.
 
The vestibule of the internal ear is formed in practically
the same manner as the Imny semicircular canals, the epithelial saccule and utricle at'quirinj;^ their cimnective-t issue
constituents in the same way. TheR* is the difference, however, that the bony v<»stibule dm^s not conform to the shape
of the vestibular |mrts of the membranous labyrinth, since
it is a sinjrle unilividinl cavitv enclosinjr the two little vesiclcs, the siiccule and the utricle.
 
The bony cochlea, while develo|K»d u|M)n the same general
plan as the other parts of the bony labyrinth, presents certain cons|)iouous uKNlitications. The epithelial cochlear duct,
as stati'd above, in its early sta^ is a short, tapering, and
sli«rhtly curved tube. While it is still in this condition,
chondritication of tlir petrous bone (K»curs, whereby the
duct ac(|uircs its cartilaginous capsule (Ki^. 109, hk). This
capsule is i)\\v\\ at the prnximal end of the duct and
thronjrh this o|K*iiinLr lh<' cochlear bninches of the audittuy ncrvi' gain access U) the capsule, beinj; connected with
the c(K'hlcar <livi>inn of the auditory «ran«rlion, which, owing
to its prcviou>ly having a>>iuncd a jK>sitiou beside the duct,
Ciuucs to be enclosed by th<* <'apsulc as the latter is formed
(Kig. 1^>9, m\ <j^it). It is only after the chondrification
that the ci>chlear duct lengthens out and becomes t>pirally
coiled. The coiling is in such n)ann(>r that the cochlear
nerve is surrounded bv the duct — tliat is, it lies in the axis
about which the duct is spindly wound. Within the cartilaginous capsidc, filling all tlie space not occupied by the
spirally coiled duct and the ccH^hlear nerve with its lengthened-out ganglion, is the end)ryoni<* conn<»ctive tissue of
which f(»nnerly the entin* cartilaginous {x^trosa ccmsisted.
 
 
 
THE ISTKBXAI. EAR.
 
 
 
353
 
 
 
Thu cochlea consists now of a spirally coiled epithelial tu!>e
Ijiug within an eloiigati'd cavity in the cartilaginous petrosa,
a cavity, the walls of which arc, therefore, cartilaginous.
The peripheral wuU of the coiled tube is in contact with the
inner surface of the wall of the iTnrtilaginous cnpfule (Fig.
169, x), a fact which has an imiHtrtaiit bearing upon ihc
further stages of growth.
 
 
 
 
I
 
 
 
Tio, Ue,— Pan of ■ Bectloii Itiniugh the cocblea or an embryo Ml. II cm. (3.l> in.)
long litter Boellchcrl: bt. cartl]*gln(nis capadte. In vhLch llie cocblmr duel
describe* aseending »|"lral turns i ilf. duetUB t'orhletrin ; c. iirgun of Cortl; It,
lamina vialibul«rt«: i, ouler wall of the membranous duclu«cochl«arl» nilh Ugamenlum aplrale; SV. acila vfitninll : ST*. S7'. acila tympanl; g. Kelallnoun tiaae,
which itni Hlls the tcala Tvatlbulf inO In Ita I'M tniat: a\ remnant of the gelatinous tliiue, which I> not yet llquvllcd : M. Arm connective tluua summnillne
the cochlear nerve lac); gip. ganglion splcnle; .v. nerrc which rum to Cortl'a
or>n>n in (he nilure lamina aplrallanuea: r, c<im|<Hct cnnnL-dlre-tiune UfCT.wblch
liepomo onined and shares In boundlBK ""o bone cochlear duct : P, pcrichon
 
 
The embryonal connective tissue within the capsule now
undergoes important modifications, which vary greatly in different regions. That portion of this tissue which immediately
envclo])ri the cochlear nerve becomes first dense connective
 
 
 
n.">4 TEXT-BOOK OF EMBRYOLOGY.
 
tissue, which is afterward <lire<*tly coiiverte^l into bone, constituting the modiolus, or axis, of the cochlea. The processes of eondensation and subsequent ossification extend
outward from the nuKliolus in a spiral line, which corresponds
with the intervals bi^tween the successive turns of the cochlear duet, until they meet the wall of the original capsule,
thus produein^ the bony cochlea. That is, by the development of this spiral plate and its connection internally with
tlu» uKHliolus and externally with the wall of the capsule, a
tub(* at first partly membranous and jwrtly cartilaginous,
and at a later sta^e osseous, is produced, which encloses the
mueli smaller cochlear duct, and like it is wound spirally
arc>und the modiohis. To repcnit, the original sample eavity
of the cartilaginous capsule is subdivided by the growth of
the modiolus and of th(» spiral shelf in such manner as to
become a t<jjinil/t/ coiUd tube.
 
The cochlear nerve, enclosed within the coil of the cochlear duct, semis branches (Fiji:. 1^^, -V) i»^ a continuous
spiral line to th<* duct, and the soft tissue surrounding and
supporting these branches condenses to form a connectiyetissue plate whicli extends outward from the modiolus tc
the cochlear duct and which, therefore, has a spiral course
about the modiobis, its entire inniM* edge being attached
to that central axis, while; its outer border is, throughout
its entire extent, in continuity with the inner wall of the
duct. At a later sta^e tliis s])iral plate undergoes direct ossification to form tlic two lainclhe of tlie bony lamina spiralis.
Thus it is that tlie ganglion s])ir:de and the successive terminal branches of the cochlear nerve come to be enclosed
within th(; s[)iral lamina. Recalling the condition of the
cochh»a before the growth of the spiral lamina, it will be seen
that the latter, in connection with the epithelial cochlear duct,
divides the tube into two parts (Fig. IfJO, aST, .ST). It
will be evident, to(), that the epithelial cochlear duct now
holds a relation to the* larger tube of the future bony cochlea
which is similar in principh* to the relation of the membranous semicircular canals to th(» bony canals, but with
the dilVcrcnce that the outer wall of the epithelial duct is in
 
 
 
THE MIDDLE AND THE EXTERNAL EAR. 355
 
close contact with the outer wall of the future bonv canal
at Xf and that the inner walls of the two are connected by a
spiral plate, the lamina spiralis.
 
The cochlear duct, then, is surrounded by undifferentiated
mesodermic tissue, except on the side farthest from the
modiolus, where its wall is in contact with and finally
adheres to the wall of the cartilaginous capsule. The lamina
spiralis divides this tissue into two parts which respectively
occupy the positions of the future scala vestibuli and scala
tympani. Tliis soft embryonal tissue, as in the case of the
corresponding tissue of the semicircular canals, develops differently in different regions. The innermost stratum, which is
in relation with the epithelial cochlear duct, becomes fibrous
connective tissue and constitutes the flbrons layer of the adult
cochlear duct ; that is, on the side of the duct toward the
scala tympani, it becomes the connective- tissue layer of the
membrana basilaris, while on the side toward the scala vestibuli it forms the fibrous stratum of the membrane of Beissner (Fig. 169). The peripheral zone of indifferent tissue,
that in contact with the now cartilaginous wall of the future
bony cochlea, as well as that which lies against the lamina
spiralis, also undergoes condensation and forms a fibrous,
or fibrovascular, membrane, the internal perichondrinm or
future periostenm. The tissue intervening between these
two layers retrogrades, the cells degenerating and the intercellular substance liquefying, until finally the spaces known as
the scala vestibuli and the scala tympani are hollowed out.
These channels are lymph-spaces and the fluid they contain
is the perilymph. This perilymphatic space is in communication with that of the vestibule. Therefore, while the cochlear duct or scala media encloses an epithelinm-lined space,
as do the saccule, the utricle, and the membranous semicircular canals, and in common with those structures contains the so-called endolymph, the scala vestibuli and the
scala tympani are in the same category with the perilymphatic spaces of the other parts of the internal ear.
 
The Middle and the External Ear.— The middle ear,
consisting of the tympanic cavity and the Eustachian tube,
 
 
 
356 TEXT'BOOK OF EMBRYOLOGY.
 
is devclojx}il from tlie back part or dorsal ond of the first
inner visceral fUrrow. The external ear, comprising the external auditory meatus and the auricle, comes from the dorsal extremity of the first outer furrow and the tissue about
its margins, the tympanic membrane representing in part the
closing membrane which sepanites the inner furrow from the
outer.
 
The first inner viscend furrow, in common with the
other inner furrows, is an evagination of the lateral wall
of the primitive pharvngcal ciivity, or head-end of the guttract. The ventral end of this groove suffers obliteration,
but the dorsal s(»gment, designate<l the tnbotympanic sulcus, becomes converted into a tube by the growing together
of its edges. The tube is composed therefore of entodermic
epithelial cells. It elongjites in the dorsal and outward
dire(?tion, and its dorsal extremity becomes enlarged to produce the cavity of the tympanum, the remaining part of the
canal becoming the epithelial lining of the Eustachian tabe.
The canal being formed before the development of the
cranium, and approximat(?ly its posterior half being surrounded bv the mesodermic embrvonal connective tissue
that al'terward becomes the petrosa of the temporal bone, the
tympanic, cavity and a part of th(» Eustachian tube come to
be enclosed within that bone, while the connective tissue
enchasing the anterior part of the tube differentiates into the
curved plate of ciirtilage that forms the cartilaginous part of
the Eustachian tube.
 
Since the posterior end of the primitive epithelial tube
insinuates itself between the otic vesicle and the surface,
the tympanum comes to o(*cupy its normal position on the
outer side of the internal ear. The tympanum, being derived from the back part of the first vis(»eral cleft, is in
close relation with the first and second visceral arches, and
the ossicles of the mid<ll(» ear ar(» derived from the dorsal
extremities of the cartilaginous bars (jf these arches in the
manner described in C-hapter XVIII. Necessarily the
primitive ossicles are exterior to the primitive epithelial
tympanic sac, as is also the chorda tympani nerve, which
 
 
 
THE MIDDLE A-VD THE KXTKRSAL EAR.
 
 
 
357
 
 
 
jKisses ulcDg its outer nide. After the ossification of the
temporal hone, these structures are emhedded within the
abundant soft connective tissue which is between the epithelial sac, now the mucous membrane, and the bony walls
of the tympanum. This mass of soft tissue undergoes verj'
considerable diminution, owing to which the mucous membrane comes into contact with the bony walls, and as a result
the ossicles and the chorda tympani are enclosed in folds of
the mucous membrane and seem to lie within the tympanic
cavity,' They are excluded, however, from the true cavity
of the tympjinum, since they are exterior to the epithelial or
mucous- membrane layer.
 
 
 
 
Fin. 170. -Showing tlie gracjua] cluvi;lii|iiiii.-iil uf tlio fiartiinf the external car
ftom promlncncss upon the mandibular and hyoldean Tistcral archus lHI»),varlnmly magnlHeil : 1. 2. prunilnenceB on mandibular arcb : 3. prominence between
the two archea, prolooged postertorly in leconil fixture tu Sr; t.b. and 0, pniminencea on hyoidcau nr lecond riaceral arph; ^.lowerjaw. Prominence I forms
the li^rua; 2, 3. ^, the helix; 4. Ihu anllbelli ; G, Ihc antitragug: S, the lubule.
 
The external auditory meatus is simply the persistent posterior part of the fir«t outer visceral furrow or hyomandibular cleft (sec pp. \\2, 1 Hil, this cleft doffing completely
everywhere but in this region. The closing plate of the firrt
cleft becomes the tympanic membrane. Hiiice the outer
layer of this membrane is of wt.xlennie orijjin, while the
inner layer is entodermic, being continuous with the epithelial tympanic lining, and the middle fibrous layer is
derived from the mesoderm. The relation of the malleus to
the membrane and of the latter to the bony tyinjKinie plate
 
 
 
358 TEXT'BOOK OF KMBUYOLOGY.
 
which forms part of the wall of the meatus is dealt with in
the chapter on the development of the skeleton.
 
The auricle is ilerived from the tissue around the margin
of the uucIoscmI hack part of the first outer cleft (Fig. 171, C).
Six little elevations make their appearance here, the projections
being mesodermic tissue covered with ectoderm. The mesodermic component of the elevations diiferentiates into the
<':irtilaginous and other connective-tissue jKirts of the auricle.
The nodules marked 2 and 3 in Fig. 170 bi^coming a continuous ridge, produce* the helix, while nodule 4 becomes the antihelix. The tragus and antitragus develop resijcctively from
the projections 1 and 5. At the end of the second month,
these parts are so far a<lvan(?ed as to be easily distinguishable, and the connective-tissue basis of the ridges and projections and the continuous plate-like mass to which they
all are attache<l be^in to undergo chcmdrification. From
the third month onward, this primitive auricle, by continued
growth and greater sepanition from the si<le of the head,
assumes more? and more the charactei>> of the fullv formed
member. The lobule, however, which results from the
growth of the little elevation marked G, lags behind the
other parts in development and is rather indistinct until the
fifth month, after which time it increases in size and gradually acquires its normal i)rop(>rtioiis.
 
 
 
THE DEVELOPMENT OF THE NOSE.
 
The nose is primarily a special sense-organ, although a
pjirt of its cavity serves, in air-br(»athing vertebrates, as an
adjunct to the respiratory system. The evolution of the
mature organ of smell may be epitomized by the statement
that the olfactory epithelium, the ess(>ntial part of this senseorgan, is a patch of dei)ressed or infolded ecjtoderm, the cells
of which are highly specialized and ar(» brought into relation
with the central nervous system by means of the outgrowth
from the latter of a part of its mass, the olfactory lobe.
 
Verv wirly in intra-uterine life — before the end of the
thinl week — the olfactory plates apjn^ar as loealizt^l thicken
 
 
rut: DEVELOPMEST OF THE yoSE.
 
 
 
359
 
 
 
iiigs of the ectoderm situated just in front of or above the
on»l fossH. Tbese nasal areas are the forerunners of the
future olfactory epithelium. It is worthy of note that the
olfiictory [ilatf.* un.- \n very close relation with thf iirimiiry
 
 
 
 
 
— Devetupment or (he flu» nr ihc bum&neiDbryn (TUB) - A, embryo of
it twenty-nluv dayi. The nuuCroalal plate dlOeren Hating into proccuui
gioDularea, toward whlph the mailllnry proccsies of llret vlucral arcb are eitendiag. B. embryo of about Ihlrty-fourdajra: Ibe global ar, lateral frontal, and nuiillnry prufetwea are In apposition ; the prlmlllTe openlnt; la now better deflned, C,
embryo of about the eighth week - Immediate Imiinilsrle* of moulh are more deflntte and the nasal oriHces are partly (brmed, external ear appearing. D. embryo
at end of oecond munth.
 
fore-brain vesicle, being, in reality, on the outer surfiiee of
the ectodermic covering of its ventral wall.
 
Owing to the rapid outgrowth of the surrounding tissue,
the olfactory plates l)ecome relatively depressed, constituting
 
 
 
;3()0 TEXT-BOOK OF EMBRYOLOGY.
 
now the nasal pits, which arc distinguishable at about the
twenty-eighth day. The pits are separated from each other by
a broad mass of tissue, the nasal or iia8ofh>ntal process (Fig.
 
171), which is, as it were, a localized thickening of the inesoderniic tissue on the ventral wall of the primary fore-brain vesicle ; and this process makes its appearance in the third week.
During the fifth week the nasofrontal process thickens greatly
along its lateral margins, the thick edges being known as the
globular processes (Fig. 171, A, B). At the same time the
lateral nasal processes bud out from the nasofrontal process,
one on each side, above the nasal pits, and, growing downward, form the external boundaries of the pits, each of which
depressions is bounded on its inner side by the corresponding
globular process. The nasal pits, therefore, have well-marked
walls on every side except below, where they are directly
contimious with the oral fossa.
 
In the latter end of the sixth week the nasofrontal process, which, it will be remembered, constitutes the upper
limit of the oral fossa, is joined on each side bv the united
maxillary, and lateral nasal, processes. This effects a division between the oral fossa and the nasal pits, and forms,
though as yet crudely, th(* external nose, and the upper lip
as well. The detinite formation of the external nose may be
said to be indicated about the cH/htli rack. The orifices of
the na>al pit.-^ are now the anterior nares, while the pits themselves have bectnne short canals, opening by their deep
orifices, the posterior nares, into the primitive mouth-cavity
ahove the palatal shelves. The nanvs are separated from
i'ach other by the still broad nasofrontal j)roeess. That
portion of the nasofrontal process that separates the nares
gradually becomes thinner and produces the septum of the
nos(\ while its external or superticial })art gives rise to the
bridge and tip of the organ.
 
The growth of the palate- shelves (Fig. 172) toward the
median line, resulting in their union with each other and
with the recently-fornn^d septum, definitely divides the nasal
chambers from the cavity of the mouth, th(» posterior nares
now opening into the pharynx. This separation is completed
toward the end of the third month.
 
 
 
THE DEVELOPilF.yr OF THE XOSE.
 
 
 
SGI
 
 
 
The complexity of the ndiilt nasal cavities js proclticetl W
the formation of ridges ami jKHielies on (he lateral walls of
the original nasal pits. Three inwardly projecting horizontal
folds of the eeio<lernii(i lining of the cavity, tho superior, middle, and inferior tnrbi&al folds, appear njion the outer wall nf
each nasal fossa (Fig. 173). Each fold contains a stratnni
of mesodermie tissue which develops into cartilage and nubseqnenily into bone, forming respectively the three turbinated
bones. The cartilaginons character of these folds becomes
apparent at the end of the second, or the early part of the
third, month. An cvagination on the lateral wall of each
 
 
 
 
Fin. ITS.— Roof or Ihe oral rnvlly ofa human emhirj-o with the niiidiunenta of (he
poUlal prouenes (alter HIa), < 10.
 
nasal fossa, between the middle and the inferior tnrbinnl processes, becomes the antnun of Higbmore ; this is fnrmcil in
the sixth month. Other cvafrinalions jirtxiiiec the ethmoidal,
the frontal, and the sphenoidal sinuses, the last two of which
are not completed, however, until after birth. Very early in
the development of the nose a small invagination appears on
the mesial wall of the nasal pit. In the fourth month of
gestation this invagination has become n canal in the iipptum
(Fig. 173, /), running from before backward and ending in
a blind extremity. It is the so-called organ of Jacobson,
which, in man, is merely a rudimentary strnctnre, but which,
in most other mammals, is more highly developed, Iwing
surrounded by a cartilaginons capsule and receiving a special
nerve-supply from the olfactory nerve.
 
 
 
J
 
 
 
362 TEXT-HOOK OF EMBRYOLOGY.
 
The olfactory plates lictKime sejiaratod from the fore-brain
vesicle and ajii.-cijiiontly from the later brain and its oulgrowtli, the olfactwry bulb, by the development of au intervening bony plate, the cribriform lamina of the etlimoid
bone. The ectoderraic cells of the olfactory plates differentiate into the highly specialized nenro-epithelial elements of
the olfactory mncoiis monibranc, the olfactory epithelliun, and I
their asiioeiuted supporting cells. The axon*; of the neuro- I
epithelial cells piws upwuiil (liroiifrli die tribrlform ]»late of
the ethmoid bone as the olfactory nerve-fibers, and, entering
the ventral surface of tlie olfac'lory Ijnlb, arliorize with the
proces-ses of the mitnd cells of ilie bull), whereby they acquire J
functional relationship with the olfactory centers in the brain, 1
 
 
 
 
Fm. 173.— Cr SB a i i tl t tl tl hmd f ai en I rj i
orown-rump meuureuient The iiuml cavlllw Bre (leen lot
wlih thcoralisTilyal IhepUieii dealsniileiJ by a* K cBrUlHseaf tbeoual Mp-fl
tuni;iH,turblnal»rlIl&ge J argan i f Jucnbson J the place when ItopaulBttfl
Ihe naiial dbtUs' ; gf, paUul proceis; of. mftxlUary proceM; il. dBntal rld|».|
|Hert»[g).
 
The esternal nose, as previously stated, first acquires defi- 1
nite form about the eighth week by the union of the distal J
ends of the lateral nasal processes with the nasofrontal proo* 1
C8.S, the former proilucing the ala and the latter thebiid|«|
and the tip of the nose. In the third month the organ is j
tmduly flat and broad, but from this time on it gradually j
a-ssume-s the familiar characteristic form. From the third i
month to the fifth each external naris is closed by a gelat- J
inous plug of epithelial cells.
 
 
 
CHAPTER XVII.
 
THE DEVELOPMENT OF THE MUSCULAR SYSTEM.
 
THE STRIATED OR VOLUNTARY MUSCLES.
 
The voluntary muscular system, genetically considered, is
divisible into (1) the muscles of the trunk and (2) those of the
extremities. The muscles of the trunk include two distinct
sets : (a) the muscles of the trunk proper, or the skeletal
muscles, and (b) the muscles of the visceral arches or the
branchial muscles.
 
To arrive at a proper comprehension of the evolution of
the muscular system it is necessary to revert to an important
fundamental emhryological process, the segmentation of the
body of the embryo, or, as it is sometimes expressed, the segmentation of the coelom, or body-cavity. As pointed out in
Chapter IV., this process of segmentation occurs in all vertebrate animals and in some invertebrates.
 
The Muscles of the Trunk Proper.— At a very early
stage of development the tracts of mesodermic tissue situated
one on each side of the median longitudinal axis of the future
embryonic body, the paraxial mesodermic tracts, undergo
division or segmentation, in lines transverse to the long
axis, into 'a series of pairs of irregularly cubical masses
of mesodermic cells. These masses are the mesoblastic
somites or primitive segments, often inappropriately called
the protovertebrse. The somite first formed corresponds
with the future occipital region, the second one lies immediately in front of the first, while two others, situated still
more anteriorly, that is, near the cephalic end of the embryonic area, and seven more, behind the first, are added almost
simultaneously. The formation of the primitive segments
 
363
 
 
 
364
 
 
 
TEXT-BOOK OF EMIIRYOLOQY.
 
 
 
tlicn pr(^>cociU tiiilwiinl until a considerable number have
l)e(.'ii luldi'd. Tliorie in front of tlic one first formed are
di'iiomiimted tlip head-segments, while the others are known
iis tlie tnmk-segments. Kacli mniito ii> at first triangular in
iToss-stt;fion, the haso of the triangle looking toward the
chunla dor.sidi:*. Siibsc<iiiently they assume a more ciiboidal
whiiiM'. In the lower vertebrates — amphibians and fishee —
the somite is hollow, its cavity being in these cases a constricted-iift" portion of the IxKly-cavity (hence the term " 8^
 
 
 
m 1
 
 
;if 1
 
 
Ibo prom
 
 
■P1.P.
 
 
« of a wlacllUn
 
 
 
 
 
 
 
 
 
 
,.r,«™.„r bring
 
 
 
 
 
 
 
 
•h.t\
 
 
ii>r<lii;<u, aorta:
 
 
.1 1
 
 
'H
 
 
■iivity, ..II
 
 
Slim
l..f .
 
 
iieiit ; w, nine "f
tc [fp-i : r«. trait
whirh an- dewl
 
li,i'
 
 
"in
 
 
.all .if tli.
iiLlle Illy
 
 
(.k.-l
 
 
el'iRciiuui tlMie
iiiiting tnirt rb!
roin wbo«e wall.
 
 
 
mentation of the eieloni" to exjin.-is this iirtHrsf). In the
higher vert eh rates, liowevcr. the eavitv is obliterated by the
eneroiielinieiit of the eells of the wall> of the soniit<-.
 
The eells of the somites soon iin(h'r<;o ditfereiitiation nnd
rearm iifreii lent. It is nsiiiilly stated that, preparatory to the
 
 
 
THE STRIATED OR VOLUNTARY MUSCLES, 365
 
segmentation of the paraxial mesodermic tract, this tract has
become separated from the remaining lateral plate of the
mesoderm. The separation is not complete, however, and
therefore, after the appearance of the primitive segments,
each segment is connected with the more laterally placed
lateral plate — by the separation of which latter into two
lamellae the coelom is formed — by a smaller mass of tissue,
the iieplirotome, also called the middle plate, or intermediate
cell-mass (Fig. 17-!, vb). As development progresses the distinction between the primitive segment proper and the nephrotome becomes more sharply expressed, and the former is
designated the myotome. The primitive segment on its mesial surface, near the point of union with the nephrotome,
sends forth cells which form a mass called the sclerotome
(Fig. 174, sk). The sclerotomes spread out and blend with
each other, forming a continuous mass of tissue which envelops the chorda and the neural canal, and which also extends
laterally between the myotomes, separating them from each
other and constituting the ligamenta intermuscnlaria {vide p.
375) ; this tissue, being concerned in the production of the
permanent vertebrse, has no further interest in this connection.
 
What remains of the primitive segment after the forma-*
tion of the nephrotome and of the sclerotome is the myotome
proper or the muscle-plate. Although, as previously stated,
the primitive segments of the higher vertebrates contain no
cavity, the myotome and the nephrotome each enclose a
space, that belonging to the former being known as the
myoccel. The myotomes or muscle-plates are so called because they give rise to the voluntary musculature of the
trunk. But not all of the cells of the muscle-plate undergo
transformation into muscular tissue. While the cells on the
mesial or chordal side of the myoccel are going through
certain alterations preparatory to their metamorphosis, the
cells nearer the body-wall become rearranged to form a
characteristic layer which is known as the cutis-plate from
the fact that it contributes to the formation of the corium of
the skin (Fig. 174, cp). The cutis-plate and the remaining
 
 
 
366
 
 
 
TEXTBOOK OF EMIiRYOLOCY.
 
 
 
j»art of the miisde-plate are conliniiniis around the myoctpl,
the trunsition from one to the other being more or less gradMal. To suminarize, the primitive segment is differentiated
into the nephrotome, the sclerotome, the myotome or mnaclaplate, and the cutis-plate.
 
The Metamorphosis of the Muscle-plate. — By the
terra inujKle-ftlate. ia meant here the thickened layer of cells
on the chordal or mesial side of the myotome proper, which
layer condtitiites what remains of the myotome after the
differentiation of the eutis-plate. These cells having pro- |
liferated and increased in size, and having encroached '
thereby upon the cavity of the myotome, next undergo
alteration in shape, becoming cylindrical, with their long axes
parallel with that of the body of the embryo. The length
of each cylindrical cell equals the thickness of the primitive segment, at least in the Amphibia and probably also |
in the chick. The next step in the transformation is the '
acquisition of the transrerse Btriation characteristic of ver- j
tebratc voluntary mupclc. Soon after this the protoplaf
of tlic cell uudergoc];) longitudinal division iVito minute]
fibrillie — which latter do not necessarily correspond, how- J
ever, with the primitive fibrillfe of mature muscle — and tlw I
cell-nucleus likewise divides. The metamorphosis of the f
now tibrillated protoplasm into muscular tissue is first com— 1
pleted at the periphery of the fiber, so that a young musole- 1
fiber contains a central core of undifferentiated material, |
including the daughter-nuclei resulting from the divis
of the original nucleus. Soon after the appearance of stria- 1
tion and the fibrillation of the fil>er, the fibers begin to sepa^l
rate from each other, and developing connective tissue witJtT
young blood-vessels penetrates between them, the fibers now 1
showing aggregation into bundles. For some time longer i
the fibers are naked, since the earcolemma is not acquired
until considerably later. The differentiation into muscular .
tissue gradually extends from the periphery of the fiber to 'J
its core, the process being complete in the human embryo at J
about the end of the fifth month for the muscles of the upper.jj
extremities and in the seventh month for tho.-ie of the lower.
 
 
 
THE STRIATED OR VOLUNTARY MUSCLES. 367
 
The embryonic muscle-fibers are smaller than the mature
elements and increase in size until the third month.
 
It is considered highly probable by most embryologists
that muscle-fibers undergo multiplicatioii during embryonic
life. There are several theories as to the method of this
multiplication. The most generally accepted view is that
put forth by Weismann, the essential feature of which is
that the fibers multiply by longitudinal division or fission.
Reference was made above to the repeated division of the
nucleus of the cell as one of the initiatory steps in the formation of the muscle-fiber. According to the fission theory,
there is one class of fibers in which the nuclei are arranged
in a single row, and the fibers of this class do not undergo
fission ; while there is another class, the fibers of which have
their nuclei arranged in several rows. Fibers of the latter
type divide longitudinally into as many daughter-fibers as
there are rows of nuclei.
 
Although many of the details of the development of the
muscular system are still involved in obscurity, it is a generally accepted fact that each fiber is derived from a single
cell, the protoplasm of which develops the function of contractility to the subordination of the remaining vital properties of protoplasm. With this specialization of function
there is nefcessarily a concomitant alteration of structure.
 
The muscular mass resulting from the transformation of
each myotome grows in the ventral direction between the ectoderm and the parietal leaf of the mesoderm, or in other words
into the somatopleure, to produce the muscular structures of
the ventrolateral body-wall. The off-shoots of the myotomes
which thus jKjnetrate the body-wall in the fourth week produce, in the fifth week, a muscle-mass which, for the most
part, is non-segmental, and which gives rise to a dorsal and
a ventrolateral division ; the dorsal division, derived from all
the spinal myotomes, l)eing destined for the musculature of
the back, while the ventrolateral division, springing from the
thoracic myotomes alone, gives rise during the fifth, sixth, and
seventh weeks to the muscles of the thoracic and abdominal
 
 
 
368 TEXT-BOOK OF EMBRYOLOGY.
 
walls (Banleen and Lewis *). The dorsal division extends
in the dorsal direction, covering and acquiring points of attachment to the vert<}bral column, which has meanwhile
Ix'cn forming. In addition to the ventnd and dorsal extension of the muscl(?-|)lates, each one grows both forward and
backward — cephahid and caudad — in such manner that overhipping and intermingling result. During the diflTerentiation of the various muscular mass(\s from the myotomes, ventnd and dorsal l)ranches of the corresi)onding spinal nerves
grow forth, their final distribution being to muscles developed from the particular myotcmies with which the respective
nerv(»s correspond. According to Bardeen and Lewis the
struc'turcs of the l)v)dv-wall are well differentiated by the end
of tlie sixth week, although their extension to the mid-line is
not completed until near the end of the third month.
 
What has been said above concerning the evolution of
the trunk-musculature from the primitive s(»gments refers to
those muscles that are develope<l from the segments of the
trunk. As to the evolution of the head-segments comparatively little is definitely known. It is generally accepted
tliat in ela^niobranchs — a group including sharks and rays —
there an? nine primitive segments in the region of the future
head. The number present in mammalian embryos has not
been clearly worked out. Three? oeeiptal and thirty-five
spinal myotomes have been seen in human embryos of the
fourth week, at which time the formation of myotomes is
said to cease. In th(» l(>wer vertebrates each segment contains
a eavitv lined with flattened e(»lls, the mesothelium, the metamorphosis of which into muscular tissue may be inferred to be
essentially as alreadv outlined ai)ove. The first head-segment,
which lies in contact with and partially envelops the optic
vesicle, gives rise to the su|)erior rectus, the inferior rectus,
antl the inferior t>bli(jue muscles of the eye-ball (innervated
by the thinl cranial nerve) : the second segment produces the
superior obli(jue (iiniervated by the fourth nerve); and the
third, the external rectus (iiniervated by the sixth nerve).
The fourth, fifth, and sixth segments al)ort and hence pnxluce
 
* Amerirnn Jtntrnal nj AniitomUj vol. L, No. 1.
 
 
 
THE STRIATED OR VOLUNTARY MUSCLES. 369
 
no adult structures ; while the seventh, the eighth, and the
ninth segments become metamorphosed into the muscles that
connect the skull with the shoulder-girdle.
 
From recent studies^ it would appear that individual muscles undergo peculiar and significant migrations during their
development, and that the origin of the nerve-supply of a
muscle indicates the location of the particular myotome or
myotomes from which it originated, since the segmental
nerves are connected with their respective myotomes and
supply the muscles derived from such myotomes. For example, the serratus magnus, being innervated by branches
of the cervical nerves, develops from myotomes in the neck
region, and subsequently moves down to become attached to
the scapula and the ribs.
 
The Branchial Muscles. — This term embraces the
muscles of mastication and the various muscles connected
with the hyoid bone, with the jaws, and with the ossicles of
the middle ear. They result from the metamorphosis of the
mesothelimn of the visceral arches and acquire connections
with structures that have arisen from the so-called mesenchymal cells of these arches or, in other words, from the
embryonal connective tissue which makes up the chief part
of their bulk. For an account of the growth of the visceral
arches the reader is referred to Chapter VII. From this
account and from that found in Chapter IV., it will be seen
that the formation of the visceral arches and clefts is in
reality the segmentation of the ventral mesoderm of the headregion of the embryo, or to express it in another way, it is
the segmentation of the ventral coelom of that region. It is
interesting to note that whereas in the trunk the segmentation of the mesoderm is restricted to the dorsal part of the
body, in the head-region the ventral mesoderm also participates in the process. Hence the visceral arches, as might be
exj)ected, consist of so many masses of mesodermic tissue,
each arch containing a small («ivity lined with mesothelium,
which cavity is a constricted-off part of the body-cavity or
 
* See '* Development of the Ventral Abdominal Walls in Man/* Franklin P. Mall, Johns Hopkins Papers, vol. iii., 1898.
24
 
 
 
370 TEXT'BOOK OF EMBRYOLOGY,
 
Od'loiii. It is those mesotholial ct4l8 that produce, by their
diift'rentiation, the niusoles under consideration. While so
nuieh ooneerninjj: the origin of tliis group of muscles is practically assured bv ol)servations upon the embryos of the
lower vertebrates, the details are still obscure. His assumes
the origin of the palatoglossus, the styloglossus, and the levator
palati from the second or hyoid arch ; of the stylopluayngeus,
perhaps the palatopharsrngeus, the hyoglossus and the superior
constrictor of the pharsrnx from the third arch ; and of the
middle and inferior pharyngeal constrictors from the fourth
arch. Further, it is held bv Rabl that the muscles of the
i\u\\ including those of the scalp and the platysma — the
muscles of expression — originate* from the mesothelium of
the hyoid anrh in the form of a thin superficial sheet, which,
gratlnally spreading out from the place of origin, breaks up
into the intlividual muscles.
 
The Muscles of the Extremities. — The relation of
i\\o (It'vclopnirnt ot'tlic inn>cles of tiie limbs to the myotomes
i> >till a (li<j)uttMl point. Sdihc authorities hold that the
linil)-mu-cK> of inaiiinials orii^inate from the mvotomes, as
\\{\< >li()\vn l)v l)«)lirn U) l)c the ca>e with the fin-musculaturc of Selachian-. A tact adduced as a strong argument in
t'avnr «>f tht'ir niyotomic oriiiiii is that the ncrve-su|)[)ly of
each liiiil) ('(U'rc-jjonds with the nerves of the number of myotoniie x'unients in relr.tion with which the limb-bud <lcvelops
(rid, |). loi)). ( )n th(* other iiand, it is stated* that the myotome- do n(»t extend into th(» developini:- limb-buds, but that
the inn.-ch'< ol" the liinl).- are diil'ci-entiated from the mesenchvinal core ol' the liinh-bud, thi- procos following the entrance of the motor nerve-fihi'rs into the member. The
mn>chs of the npper lind) are so well advanced in their
devel<»|)ment by th(*' >ixth week a< to be individually distingni-hablc. th(»-^e (if the lower limb reaciiing a corresponding
stau'e in the >eventh week.
 
^ r»anU*oii Mini Lc\vi>, /-*(•. cit.
 
 
 
USSTRIATED MUSCULAR TISSUE. 371
 
THE INVOLUNTARY OR UNSTRIATED MUSCULAR TISSUE.
 
This variety of muscular tissue, like that considered
above, is of mesodermic origin. But while the voluntary
muscles arise from the flattened or mesothelial cells of the
primitive segments, involuntary muscle results from the
transformation of the embryonal connective-tissue elements,
the mesencliymal cells, of the mesoderm. It is for this
reason that some authors speak of the voluntary muscles as
the mesothelial muscles and designate the involuntary muscular tissue as mesenchymal muscle.
 
While it is a generally accepted fact that each of the fibercells which make up nnstriated muscle is a metamorphosed
mesenchymal or connective-tissue cell, the details of the
process have not been accurately worked out. One may
assume that necessarily the young connective-tissue cell
elongates and that its protoplasm must undergo such differentiation as will fit it for the exercise of its future function,
contractility.
 
THE CARDIAC MUSCLE.
 
The account of the development of the heart-muscle will
be found in Chapter X.
 
 
 
CHAPTER XVIII.
 
THE DEVELOPMENT OF THE SKELETON AND
 
OF THE LIMBS.
 
Although the skeleton is the framework of the body in
the aiiatoiuieal or meehanieal sense, it is not so embryologically, since its deveh)pnient is not l)egun, at least not to any
important extent, until nearly all the prineijial organs are
well differentiated, and its growth is largely subsidiary to
that of the structures which, in the mature state, it supports
and })r(>tects. M4»rph()logists s])eak of the exoskeleton and
tlie endoskeleton, the former having reference to the hard
structures found superficial to the soft parts, for whose protection they serve, such as the canipace of the lobster, and
the hartl scales of certain fishes ; while the latter term
si<rnifies the cartilairinous or bonv structures found within
the bodies of most vertebrate animals. Kven in the highest
vertebrates, certain bones, such as those (►f the vault of the
cranium, are usually cousi(l(»r(»d by morphologists as l>eing
thi' representatives <>f p;irt of the cx<>skeleton of lower ty])es.
 
The skeleton, using the W(>rd in its 4>rdinary sense, consists of the axial skeleton and the appendicular skeleton, or
skeleton of the limbs, '^fhe former, inclu<liug the head and
the trunk, is connnon to all vert(»brates ; the latter is not
found in the lowest members of tliis class and hence is to \ye
regarded as a later ac(|uisition in the evolution of the skeleton.
 
In studying the development of t]w skeleton, as in considering tliat of otht'r systems an<l organs, cleaner conceptions
of tlie y:rowth of tlie individual mav be obtained bv comparing it witli tlie evolution of the ty[)e. For example, the
simplest form of skeletal apparatus is that of the amphioxns.
 
In this animal the only representative of the skeleton is the
 
:i72
 
 
 
THE AXIAL SKELETON, 373
 
notochord, a cyliiulrical rod composed of cellular or gelatinous
tissue in which neither cliondrification nor ossification ever
takes place. Such an animal furnishes an example of the
notochordal stage of the skeleton. Tlie surrounding of the
chorda with a sheath of embryonal connective tissue, by
which it is strengthened and thereby better fitted to serve as
the body-axis, furnishes the membranous type of skeleton, a
stage a little farther advanced than the preceding. The next
higher type of skeleton is the cartilaginous form. In tliiscase
the eml)ryonal (ronnective tissue has undergone transformation
into cartilage, at which p%int development is arrested, the
stage of ossification never being attained. The cartilaginous
type of skeleton is illustrated by that of the selachian (sharks
and dog-fish).
 
The third and highest type of skeleton is the osseous. This
results from the replacement of the cartilaginous tissue l)y bone.
The process of ossification does not, however, affect every
part of the cartilaginous skeleton, there being some portions
of the latter which remain permanently unossified. As there
are, throughout the vertebrate series of animals, various gra(hitions in the degree of differentiation of the skeleton, so in
the course of development does the osseous system of every
higher vertebrate pass througli these stages from the simplest
condition, that of the notochordal skeleton, to the highest
form of the almost completely ossified skeletal apparatus.
 
THE AXIAL SKELETON.
 
The axial skeleton, as stated above, includes the bones of
the trunk and those of the head. Logically the development
of the former will first claim attention.
 
The Development of the Trunk.
The Stage of the Chorda.— The formation of the
(jhorda dorsalis or notochord is the earliest indication of the
axis of the embryonic body and it will be recalled that it is
also one of the earliest embryological processes. The mode
of development of the chorda from the entodermal epithelium
has been described at p. 73. The chorda serves the pur
 
 
374 TEXT-BOOK OF EMBRYOLOGY.
 
pose, as it were, of an axis about which the permanent vertebral column and a part of the skull are, at a much later
date, built up. The anterior or headward termination of the
chorda corresponds to the position of the later hypophyais, or
pituitary body, and thus the chorda is coextensive, not only
with the vertebral column, but also with a portion of the
cranium. The cells of the chorda enlarge and become distended with fluid, the protoplasm of each cell being reduced
to a thin layer. The peripheral cells, however, constituting
a distinct layer, the chordal epithelinm, remain small, and it
is by their proliferation that the •horda increases in size. In
the amphioxus the chorda is the only " skeleton *' that is ever
acquired, and in this animal it is a permanent structure. In
all other vertebrates it becomes surrounded by embryonal
connective tissue, mesenchyme, which latter undergoes chondrification, and in the higher types ossification also. While
in some of the lower vertebrates, as in certain classes of
fishes, the chonla persists as a structure of more or less importance, in the higher members of the series, birds and
mammals, it retrogrades as the processes of chondrification
and ossification go on, until finally it is represented only by
the pulpy centers of the intervertebral disks.
 
The Membranous Stage. — The notochordal stage of
 
the development of the vertebral column is succeeded by the
menihninous staire. The transformation is effected bv the
appearance of an ensheathing mass composed of embryonal
connective-tissue cells which surround not onlv the chorda
but also the neural canal or fundament of the nervous svstem
(Fig. 17-1, nh). The source of this embryonal connective tissue
or mesenchyme bears an imi)ortant relation to the primitive
segments. As the develo[)nient of the primitive segments
was described in the last chapter, and also in Chapter IV.,
it will suffice to remind the reader that each primitive segment undergo(»s differentiation into the myotome or muscleplate, the cutis-plate, the nephrotome, and the sclerotome (Fig.
174), the sclerotouK? occupying the mesial surface of the segment and lying in close proximity to the chorda.
 
AVhile the myotome originates from the flattened or meso
 
 
THE AXIAL SKELETON.
 
 
 
375
 
 
 
thelial cells of the primitive segment, the sclerotome is made
up of cells of the type characteristic of young-growing connective tissue — that is, of the mesenchymal part of the primitive segments as distinguished from their mesothelium. Owing
to the rapid multiplication of its cells, each sclerotome spreads
out headward and caudalward, and dorsad and ventrad, surrounding both the chorda and the neural canal, until both these
structures become enclosed in a common, continuous sheath of
embryonal connective tissue. That part of this tissue which
surrounds the chorda is often designated the skeletogenous
sheath of the chorda and also the membranous primordial vertebral column. The cells of the sclerotomes not only surround the chorda and the neural canal, but they also spread
out laterally into the intervals between the muscle-segments
to constitute the ligamenta intermuscnlaria or the bands
or strips of connective tissue which separate adjacent muscle
 
 
MuscU'Segments
 
 
 
Intersegmental
arteries
 
 
 
 
ist spinal nerve
 
Ligamentum
intermuscularium
 
2d spinal nerve
 
Ligamentum
intermuscularium
 
jd spinal nerve
 
 
 
Skeletogenous sheath of chorda^ \^horda
 
Fig. 175.— Frontal projection fVom a series of sections through a cow embryo of
 
8.8 mm.(0.35 in.). (From Bonnet, after Froriep.)
 
 
 
segments from each other (Fig. 1 75). It is worthy of note
that while this skeletogenous sheath of the chorda originates
from segmented structures, the somites or primitive segments,
and is to that extent related to the segmentation of the body,
it now presents no trace of segmentation.
 
 
 
376 TEXT-BOOK OF EMBRYOLOGY,
 
Very soon, however, this ensheathing membranous tissue
exhibits areas of condensation alternating regularly with less
dense areas. Eaeh such condensed area has the form of u
somewhat obli(|uely i)lace<l bow or half-arch. This halfareli of condensed mesenchymal tissue is called the primitive
vertebral bow by Froriep (^Figs. 175 and 176), whose investigations upon chick and cow embryos established most of
the facts known concerning the development of the vertebra?.^
The median portion of the lx>w is on the ventral side of the
cliorda and is known as the hypochordal brace. The lateral
extremities of the bow abut against the eorresj>onding myotomes, eadi extremity becoming bifurcated. The dorsal
limb of the bifurcation, the neural process, extends gradually
over the dorsal surface of the primitive spinal cord, forming
the neural arch; while the ventral limb advances ventrad,
foreshadowing the hemal arch or costal process of the vertebra, or, as regards the thoracic region of the body, the future
rib.' Both dorsal and ventral processes grow into the intervals between iidjaoeiit myotomes and hence are intersegmental, that i>, they, n> well as the vertebral \m)\\ from which
they >j>rintr, (•orre>pon(l to the intervals between the primitive segments of the body. Sub>e(|iieiitly these j)roeesses of
the bow uive rise to the variou«- processes of the ecmipleted
vertebra. The median part ol' tlie bow, the hypochordal
brace, subseqncMitly becomes cartilaginous and assists in
forniiii<r th(» bodv of the vertebra in l)inl>, but in mammals
it reuiaius unchondrified and becomes an inconspicuous and
transitory part of the intervertebral ligament — the future
intervertebral disk — exce]>t in the case of the first cervical
vertebra, the atlas, the ventral arch of which it furnishes.
The nieinbran(nis anlage of x\w cartilaginous body of the
vertebra is fcnind in a special condensation of the mesen
' M<»re rcH'ently the ]>r(>cess lias K'cii stiKlicd m tlu* human embryo by
IJiinh-eii. Afurrn'tin Jourunl nf Annfonv/. vol iv , No. 'J, \W\.
 
' Mnr]»h(>l()irir:illy, each vertehni is po^n^ssod of a ii'iirnl nrrh^ f(f>r tlio
protection of the spnial cord ; and a bnnni nrrh^ for the protection of the
ortnms of emulation, re<y)i I'll turn, and diirc^tion, the ril)> of man and the
higher Vi'rtebnites U'lny the ])er»^istent hemal arches in tfie rei^ion of the
tfiorax.
 
 
 
THE AXIAL SKELEl'Oy.
 
 
 
377
 
 
 
eliymatous slieath of tlio i-lionla on tin; caudal aide of the
liyi>ocliortlal brace. The intarrertebral ligament is develowd
fnmi tlie (erichonlal tissue on tlie dorsal side of tlie liyi>ocliordal bniee. Banleen's piimilivc rlluk iiiultides this aniiigc
nf the iiitorvtTtebnil ligament phis the hyiwK'lionial brace of
Froriep, which latter Bardceii regards as a transitory thicken of e n r 1 gi. Ilk
 
 
 
 
The Cartilaginous Stage.— This stage of the devcloi>niciit (if titc spine is bi-onght abont by tlie metamorphosis of
IKirts of the niembrannus vertebral column into the cartilaKinons vertebra. Other and alternating \arts of the same
strncttiro furnish the Intervertebral disks and the ligaments
that hind together the individual elements of the spine. 'J'he
hisliilogic:il changes neeessiry to effect the transformation of
the embryonal connective tissue into cartilage are, briefly,
the moving ajwrt of the cells and the modification of both
the cells and the intercellular substance, the latter acquiring
the characteristic qualities of the matrix of cartilage.
 
For each vertebral bodr there are two centers of chontlrifieation, one on each ^ide of the chorda within the mass of
tissue referred to above (Fig, 176), The formation of cartilage l>egins in the second month. The two centers are soon
connected with each other by a third, which lies on the ventral
side of the chorda, the three forming now a cartilaginons half
 
 
378
 
 
 
TEXT-BOOK Of- EMBRYOLOGY.
 
 
 
crliuder wliicli js later coniplete<l by the development of c
tilage on tLe dorsal side of the chorda (Fig. 177). Accord- '
ing to Bardeen, the tartilage of llie body grows at the exjwhse of the primitive disk anterior to (above) it. At the
time when the chorda is completely encased in cartilage the
Bi>inal cord is still ensheatlied by merely membranous tig
Before the end of the second month tlie neural uches of the J
vertebrte are indicated by small isolated niassi's of cartila)
which develop in the connective tissue BUrrounding the spinal I
cord, the lateral parts of the membranous vertebral bows. 1
 
 
 
Parachordal carlilagt
 
 
 
 
In the eighth week these fuse with the bodies and appear I
llieri as projections from them. By the end of the third I
month the processes, or neural arches, have grown snfficiently ]
to" meet with their fellows on the dorsal side of the spinal J
conl, and in the fourth month the corresponding arches of ,
the two sides become niii ted, thuscompletingthecartilariiuma I
sbeath of tbe cord.
 
The masses nf connective tissue occupying the intervalsj
 
 
 
 
THE AXIAL SKELETON, 379
 
between the vertebral bodies, originating, as stated above, in
condensations of the mesenchymal sheath of the chorda on
the dorsal aspect of the hypochordal braces, become the
intervertebral ligaments (Bardeen's primitive disks) upon
their fusion, in mammals, with the hypochordal braces.
Subsequently they become the intervertebral disks. The
tissue between the cartilaginous arches becomes differentiated into the ligamenta subflava.
 
While the unsegmented skeletogenous sheath of the
chorda is gradually differentiating into the separate elements
of the cartilaginous vertebral column, the chorda itself begins
to retrograde. Within the bodies of the vertebrae its development is completely arrested, while those portions of it contained within the intervertebral disks continue to grow. The
chorda at this stage consequently shows alternating enlargements and constrictions. In certain fishes it persists as a
structure of more or less importance. In vertebrates above
cartilaginous fishes, all traces of the parts of the chorda
within the vertebral bodies are lost as soon as ossification
occurs, while in the intervertebral disks parts of it jxjrsist
as the soft pulpy cores of the latter.
 
Thus the cartilaginous vertebral bodies or centra originate
in masses of mesenchyme situated between the primitive
vertebral bows and are, according to Froriep, segmental, that
is, they correspond in position with the muscle-segments,
each centrum being developed within the limits of a single
segment ; while the processes develop from the lateral parts
of the vertebral bow and later unite with the body. Bardeen, on the other hand, refers the origin of each vertebral
body to two segments, since, according to his observations, the
body grows at the expense of the next anterior primitive
disk.
 
The cartilaginous trunk is completed by the chondrifieation
of the ligamenta intermuscularia to form the cartilaginous
thorax.
 
The Osseous Stag^e. — The process of ossification begins
in certain parts of the trunk at the end of the second month.
 
 
 
:»S0 TEXTBOOK OF EMRRYOLOGY.
 
W\\\vv \\w work (»f olioiulrifiration is entirely completed. As
i)ii« liiisioloirical details of Ixuio-foriiintion are to be found in
I 111' h'\l-lMM)ks of lii>toloirv, it will not 1)C necessary to enter
into till' suhjcct lu»ix'. The place's in any individual cartilage
wlu-rr tissificMtion houiiis aix» called the cHintcrs of ossification.
I'lh' proiM'ss is oni' of siii)stitutk>n, the cartilage becoming
broki'ii down an<l absorlunl as the fonnati«)n of bone goes on.
riit' oBHification of each vertebra is l)ogun at three conliT-n, onr in the bodv and one in cjieh arch. The centers
l»»r the arches appear in the x'venth week. The centers
ti>r ilic bodies a)){M'ar a little later and are found first in the
d»n-;d MTtehiie, ap|)«'arinLr sueeessively later in the vei"t<»bi.i- I'iiiihrr lip and farther down. The ossified arches unite
wtili liu'ji other diiriiiii: the lirst vear of life, but their nnicin
Willi ihr ImmIv of the vertebra takes place betwtHjn the thinl
.Old rii.ditli vciirs. At a much later periinl five accessory canU-.ia ul' H-.-i Ural ion are added to each vertebra. Two of these
til I.. Oil to the body an<l jrive rise to two annular ]>lates of
i.i.iu . I In- epiphyses, one for tlu' upper or cephalic surface and
.•III lix ihr opposite t)r eautlal snrt:u*e. The remaining three
I . (Oi I ■ briMiin rr>p<M*tively t<) the spinous process and the two
ii.iii.»viii:ii» pi'orusses. The i*piphyses do not acrpi ire osseous
Mil. Ill wiih ilu' vrrtrbra projuT until al)out the twenty-fifth
 
1 1 1. ... 1 .1 1 hi I transverse process of a e<'rvieal vertebra, enI III. I i.'i.oniii, and <'onsistini: of an aiiteriorand a j>osterior
I III III liuh • iimrr than th«' tran>vcr>r process ])roper, since
I. ml iiii iM \ i-iiir:i I port i(>iM> t hi' rudiment of a cervical rib.
I '.II 1.1 ihi (Mitr (it' the fii>ion ot' this nidimentarv rib with
I J, I. Ill ^.1 . pitMi-s, the vertebral artery, which passes
i , . ,1 (lu III i' -iMTtimidctl by tlu» two processes, and thus
.1, . h.ii I \ ii .d luiii-vcrM* processes di tier from those of
,1, I t, III III. f III the possession of a foramen.'
I I. aUi- III. I ill*' rtxlH, bein^ strikintrly modified eervic^al
 
I I 111. 1 .1.1.1. li '.uiir iiiitlii>rili«'s. :iN Miiiot, that tht* >k)ium]()08
I ti.. ..«., I » . tiul iliiii thi' arti'iy j^rows through the* ossifying
 
 
 
I ii
 
 
 
I » lilt
 
 
 
THE AXIAL SKELETON. 381
 
vertebne, require special mention. The atlas contains less
and the axis more than an ordinary vertebra, since that which
corresponds to the body of the atlas never unites with it but
fuses witli the body of the axis to constitute its odontoid
process.
 
The atlas presents two centers of ossification for its neural
arches — the so-called posterior arch — just as other vertebra?
do. Unlike other vertebrae, these centers do not unite with
the body but become joined to each other on the ventral side
of the position of the cliorda by a piece of cartilage which
results from the chondrification of the hypochordal brace,
referred to on page 376. This forms the cartilaginous ventral or anterior arch of the atlas, which, in the first year of
life, develops a center of ossification. The arch acquires
bony union with the lateral parts between the fifth and
sixth years.
 
The axis or epistropheus develops from the usual centers of
ossification and from an additional one for its odontoid process. Bony union of the odontoid process with the proper
bodv of the axis occurs in the seventh year. The odontoid
process, in common with every other vertebral body, is
traversed in the cartilaginous stage by the notochord.
 
The transverse processes of the lumbar vertebrse, like those
in the cervical region, include not only the transverse process proi)er but also the rudiment of a rib.
 
The sacral vertebra each present the usual ossific centers.
Inasmuch as they become articulated firmly with the pelvic
bones and undergo fusion to form a single adult bone, the
sacrum, their form is much modified during the course of
development. The transverse processes of each side coalesce to
form the lateral mass of the sacrum. Each transverse process
consists, as in the cervical and the lumbar vertebrae, of the
transverse process proper and a rudimentary rib, the center of
ossification for the latter being quite distinct during early
stages of development. The intervertebral disks of the sacral
vertebrae begin to ossify in the eighteenth year, the process
being completed in the twenty-fifth year.
 
 
 
a82 TKXT-nOOK OF EMBRYOLOGY,
 
Tlio coccygeal vertebrse are quite rudimentary. Each one
is ossified from a single piece of cartilage, and usually from
hut a single (H'uter of ossification. Occasionally the first
piece of tli<.' coccyx <levelops from two ossific centers, the pnx?ess l)eginning at hiiili. Ossification begins in the second
vert<*l)ra lu'twcen the lifth and the tenth years ; in the third,
shortly l)elon' puberty ; in the fourth, scnm after puberty.
The lower three pieces fuse into one before middle life, and
this unites with the first, and the latter with the sacrum, at
variable periods thereafter.
 
The Development of the Ribs and Stemtun. —
 
Ivcference has been made in the preceding pages to the liganieiita iulcriiuiscularia as strips or l)ands of embryonal conn<'('live tissue lying between adjacent muscle segments, which
have ()rigiii;i(<'d, in conuunn with the sheath of the chorda,
iVoiii tlx' c(>lls of the sclerotomes. TIh^ ligamenta intermnscularia become invaded by the costal j)roeesses of the primitiv<' v<'rt<'bral bows, th(^ costal process, which is the ventral
division ol* the tip of tlw i)ow, growing ventnul and j)enetratiiig th(^ substance of the ligament to constitute a curved
rod of connective tissue, the forerunner of the future rib.
Thus (lu'rc are form<'d coiniective-tissne rej)resentatives of
the ril)s, ench of which is enibedded in the looser connective
tissue nl' th<' corresponding intei'nniscular ligiunent. It is
b\ the development of* cartilage within these curved rods of
conden-ed moenchvme, the membranous ribs, that the cartiliiginous ribs are jn'odnced. 'Vhv pi*ocess of chondrification
commences in the >econd month, but does not involve the
proximal <'n(U of the ribs, the tissue heie becoming ligamentous and servinir to bind to<reth<'r the ribs and the vertebi.e. Ivibs are formed throughout the entire extent of the
Vertebral <M>lumn, except in the coccygeal region, but while in
tlu' lower vertebrates the entire series goes on to mature de\.lt»|>meut, in mammals, including man, their growth is
arrc-ted in the cervical, hunbar, and sacral regions. In the
case ot' man and mammals only the thoracic ribs persist and
iM'conh' adult structures.
 
 
 
THE AXIAL SKELETON. 383
 
As the distal (ventral) extremities of the ribs advance
toward the ventral median line, the tips of the first five, six,
or seven each exhibit an enlargement. These broadened ends
soon coalesce, thus forming on either side of the median line
a continuous strip of cartilage, the anlages of the sternum.
The other ribs remain free at their ends. The sternum is
therefore produced from two lateral halves, a circumstance
that explains some of its anomalies, as for example, cleft
sternum, which is a condition due to arrested development
or deficiency of union.
 
The ossification of the ribs begins in the second month of
fetal life and from a single center for each. The process does
not involve the entire rib, a portion near the distal extremity
remaining cartilaginous and becoming the adult costal cartilage. Accessory centers of ossification for the head and
tubercle appear between the eighth and fourteenth years
of life.
 
The ossification of the sternum proceeds from numerous
centers. There is one for the manubrium and from six to
tw^elve for the gladiolus. The ensiform acquires a center of
ossification in the early years of life, but for the most part
remains cartilaginous.
 
Although, as stated above, the ribs of adult human anatomy are limited to the thoracic region, their rudimentary
representatives are found throughout the other regions of
the vertebral column. In the cervical, lumbar, and vsacral
regions each rudimentary rib becomes blended with the
transverse proceas of the corresponding vertebra to form the
transverse process of human anatomy. It is from the persistence of the seventh rudimentary cervical rib and its failure to fuse with the corresponding transverse process that
the anomaly of a free cervipal rib results ; while the presence
of a thirteenth or lumbar rib, as occasionally met with, is due
to the unusual development of the first lumbar rudimentary
rib.
 
 
 
384 TEXT-HOOK OF EMBRYOLOGY.
 
The Development of the Head Skeleton.
 
Just as the wkck'tdii of tlif trunk consists of a doraally
sihiatcd bony casir tor tlit- protection of tlie spinal con! aud a
wrics of vcntrul nr licnisil arclics (or tlie protection of tlie
orgiins of circiilutioii iiiiil respiration; so does the head
skeleton comprise n Iiony eiise for the accommodation of the
lirain with smaller ac(.^;ssory osseous coni[>artment3 for the
orgiuis of special sense, as the orbits and the nasal chamlwrs;
and also a ventrally situated ai>i>aratus which constitutex
both a receptacle for the oral anil the pbarii'ngeal jxirts of
the digestive system and a nicebanlsm for the mastication of
 
 
 
 
r.
 
<l..velop,.
 
rounding' th<> h<'adsimilar l.> that i.l" ■
till' ventral parts, -.v
stni<'tTircs,<-oiistiliit
from the nie.-ioilcnn
 
 
 
iirt. the cranial capsule, or brain-case, is
exli'iit IVnm tlie c-i.iinective tis-^iic sai-id of tlic cliorila. its .>riL'in thus being
r spiml chmm. On the other hand,
he jaws and the liyojd bone and related
^' till- <(>-ealle<l viBceral skeleton, develop
tissue of the visceial aicbes. As in the
of ih.' trnrik skeleton, the eraniimi is first outlined in
branous tis^n<' re>nliiii,n I'rom the dilliTcnliatioii of the
 
 
 
THE DEVELOPMENT OF THE HEAD SKELETON. 385
 
embryonal connective tissue which ensheaths the head-enJ
of the chorda, and also of the connective tissue of the visceral
arches, this differentiation producing the membranotis primordial cranium. The metamorphosis of the membranous cranium
into cartilage brings about the cartilaginous stage of the
cranium, while the replacement of the cartilage by bone is
the final step in the process.
 
Bones that develop from centers of ossification in previously formed masses of cartilage are styled primordial
bones, while those that are pixxiuced independently of cartilage,, either in the skin covering the membranous cranium,
or in the mucous membrane lining indentations in its walls,
arc known as coveiing or dermal bones. The development
of bone is therefore said to be either endochondral or membranous. For the most part, the bones of the base of the
skull are of endochondral formation, while those of the vault
are develoj)ed in membrane. The membranous or dermal
bones are similar in point of origin to the exoskeleton —
placoid and ganoid scales — of certain fishes.
 
The Membranous Cranium. — The membranous braincase is differentiated from the mesenchymal tissue which
ensheaths the anterior or head-end of the chorda. As previously stated, the anterior end of the chonla is at a point
ventrad to the mid-brain vesicle, in the angle formed by the
latter with the fore-brain, at a position corresponding with
that of the i)ituitary body (Fig. 178). The skeletogenous
sheath of the chorda, in this situation as elsewhere, results
from the multiplication of the cells of the sclerotomes, since
this region of the body undergoes segmentation in common
with the trunk. The number of bead-segments is uncertain.
According to recent investigations upon shark embryos, there
are at least nine primitive segments formed in the headregion.
 
The skeletogenous sheath of the chorda spreads out dorsad
to cover the brain- vesicles. From the terminal point of the
chorda, beneath the inter-brain, the sheath advances anteriorly to invest the fore-brain, which latter at this stage is
 
25
 
 
 
3S0 TKXT-nOOK OF EMBRYOLOGY.
 
bent over vcntrcul. From tlie part investing the fore-brain,
a protiibcnint mass, the nasofrontal process, extends toward
the j)riinitivo inoutli-cavity, constituting the anterior or upper
h()iin<lary of the hitter. Meanwhile the mesencbsrmatic tissue of the visceral arches — that is, that part of the mesodcrnii(^ tissue of these structures wliich does not form
muscular tissue — is un<lergoing similar transformation into
menibninous tissue. The first visceral arch divides into an
anterior or upp(T part, the maxillary process, and a posterior
or hnvvv mass, the mandibular arch, these being the membninous jaw arches. The four jaw arches, with the nasofrontal process, form the boundaries of the primitive mouthcavity, the mandibular arches of the two sides having united
in the median line to form its lower border, and the maxillary
arches having fused with the lateral nasal and the nasofrontal
j)n>cesses to c( institute its upper boundar}'.
 
"^rhe membranous primordial cranium, then, consists of a
cnniplete connective-tissue investment for the brain-vesicles,
of tlui nieiiibi-anous jaw arches, and of the hyoid and the
branchial a relics, and presents in its walls the indications of
the cavities for speeial-sens<» organs in the shape of the surface iuvauinations which constitute rtv-^pectivcly the otic vesicle, the Icns-vesieh', and the nasid pits.
 
The Cartilag^inous Cranium. — By the further differentiation of tlie memi)ranous cranium the cartilaginous stage
is attainecl. The development of cartilage begins in the
second month. \Vhih» thc^ membmnous cranium furnishes a
coni]>let(* <*a])snle for tlie brain, tJK* cartilaginous brain-case is
deficient, sinc(^ the process of ehondrification <loes not affect
the i-egions of tlie future parietal and frontal bones. This is
true at lea>t <>f man and the high<*r vertebrates. In those
case^ where the >i<elet«)n rcMuains })ermanently cartilaginous,
as in selachians (sliarks, <log-tish, ct<\), the entire brain-case
partici])ates in the chondritying ])rocess. As the skull ext<'n(ls verv nnich farther forward than the end of the chorda
— wliich latter terminat<*s at the j)osition of the future sella
turcica — the regions of the |)rimitive skidl are <lesignated
 
 
 
THE DEVELOPMENT OF THE HEAD SKELETON. 387
 
respectively chordai and prechordal (Kolliker), or vertebral
and everid>ral (Gegenbauer), according as they fall behind or
in front of the end of the chorda.
 
The formation of cartilage begins in the region corresponding to the ))ase of the future skull. On each side of the end
of tlie chorda a mass or bar of cartilage is formed, extending
forwani and backward, this pair of parallel bars being designated the parachordal GartUagflB(Fig. 179,1). Farther forward,
 
 
 
 
 
Pio. 1T9.— Kirtt tundameat of the cartilaginous primordial c
Wlcderebelm) ; 1. J^ntStage! Cchordai jpE, parachordftl cartilage TV. Batbke'a
trabecule cranll ; PR, passage for the bypophysla S, A, O naeal pit apcic veiiiclc, otncysl. 2. Second Stage: C, chorda; B, basilar plate TV trabeculie cranll,
which have become united In front to eonrtilute the nagal Beptum (S) and the ethmoid plat«; a.AF, proccsBusnf the ethmoid plate enclosing the naial otsan ; (H,
foramina olfactorla for (he passage of the olfactory ni^nca FF poilorhltal process; SK. nasal pit; A, 0. optic and lahyrtnihlne veskle*
 
in the prechordal r^Ion, another pair of cartilaginous masnea
is producetl, known as the trabetmla cituiU. The latter are
not straight bars, but have somewhat the form of a pair of
calipers. In a short time the cranial tni)>eculfe unite with
each other, but not throughout their entire extent, an aperture
being left at the position of the pituitary body. It is through
this aperture that the oropharyngeal diverticuluni, which
forms tlie anterior lobe of the pituitary body, projects to
come into rolatioa %vith the diverticulum from the inter-brain,
which pnxliices the posterior lol)e. At a later period ossifi
 
 
388 TEXTBOOK OF EMBRYOLOGY.
 
ration fM-curs lien*, as elsewhere in tlie Ikiso of the skull, thus
eoniph'tely isolating the pituitary IkkIv from the wall of the
j)harynx. The jKiraehonhil eartilages also fuse with each
other an<l with the eraniti! tral)ecula?, the four pieces now
foniiinji: <»»H^ mass. The j»n)oess of ehomlrification extends to
iither parts of the memhranous enuiiuni so as to produce a
eartilagiiKnis hrain-ease, just as, in the case of the vertebral
column, the «lorsal extrusion of cartilage-formation gives rise
to a case (»r canal for the sj»inal cord. As before stated, however, the chondrifyiug j)nx'ess does not affect the entire
niembnuious cranium in the higher vertebnites, chondrificatiou oceurring around the ]»osition of the foramen magnum
and in the lateral walls of the cranial capside, while parts of
tli(? vault remain membranous. The anterior extremities of
\\n\ unite(l cranial trahecuhe become so modified in form as to
constitute the plate of the ethmoid and the nasal capsule for
tlie lo<lg('ment of th<» olfactory epithelium. In each lateral
n'<rion the cartilaginous ear capsule is differentiated.
 
Meanwhile the cartilaginous visceral skeleton is developing
from the memhranous .-truetures of the visceral arches. As
in the ease of the hniin-eapsule, the ehondrifying process does
not involve all \yav{< of the membranous visceral skeleton,
]>arts of the latter heing replaced later by dermal or c<^vering
l)one — that i<, bones that develop in membrane without
having been ])revion>ly mapped out in cartilage.
 
In tlu; first visceral arch, the formation of cartilage occurs
only in the mandil)nlar jMU'tion, the maxillary pnx'ess contiimintr memhranou*^. The eartilairc <>f the mandibular arch
a])p<'ars in the form of a eurved bar running ventrodorsally.
'^rhi> bar divides into a smaller j)roximal or dorstd piece, the
palato(juadratum of comparative luiatomy, and a longer distal
or ventral s(^gment, Meckel's cartilage, "^rhe ])alato-quadratum
sul)s<'(|nently divides into two ])arts, the cartilaginous anlages
resj)ectiv<'ly ol* tin- ])aIato-j)teryg<)id plate and the incus.
MeckeFs cartilage like\vi>e undergoes division, there being
se])arate<l from the chief mass a small ju'oximal st»gment
called the arti<*ulare, which is the forerunner of the future
malleus. Thus th(» cartilaginous bar of the mandibular arch
 
 
 
THE DEVELOPMENT OF THE HEAD SKELETON. 389
 
has to do with the formation of certain of the ossicles of the
middle ear as well as, to a limited extent, with the development of the mandible.
 
In the second visceral or anterior hyoid arch, chondrification also occurs, but not throughout its entire extent. A bar
of cartilage, the hyoid bar or Beichert's cartilage, is produced
in this arch and undergoes division into three segments, of
which the proximal or dorsal is the forerunner of the future
stapes of the middle ear, while the other two pieces represent
respectively the styloid process and the lesser horn of the
hyoid bone. The tissue intervening between the position of
the styloid process and the lesser hyoid cornu does not chondrify in man but remains membranous and becomes the stylohyoid ligament (see Fig. 185).
 
In the third visceral arch, or the posterior hyoid arch, a rod
of cartilage develops which represents the greater cornu of
the future hyoid bone. Ventral to this, there is formed a
median unpaired piece of cartilage, the copula, belonging to
the arches of the two sides, which later develops into the
body of the os hyoides.
 
To summarize, the head skeleton in the cartilaginous stage
of development presents an imperfect cartilaginous brain-case,
capsules for the organs of smell, sight, and hearing, and a
cartilaginous visceral skeleton, the several parts of which map
out the lower jaw, the hyoid bone, the styloid process, and
the ossicles of the middle ear.
 
The Osseous Stage. — The bony condition of the head
skeleton is brought about in part by the development of bone
from centers of ossification in the cartilages described above, and
in part by the growth of covering or dermal bones in the integument covering those areas which are deficient in cartilage ; in
other words, by both endochondral and membranous ossification. It may be stated in general terms that the bones of the
baseand of the sidesof the skull, including the auditory ossicles,
the ethmoid, and the inferior turbinated bone, are produced by
ossification in cartilage and are hence called primoi'diaJ bones;
and that the bones of the vault of the cranium, and for the
most part of the face, result from the membranous method of
 
 
 
 
whi.li h,-t\
 
 
 
3!»0 TEXT-ROOK OF EMBRYOLOQY.
 
nssi Rent ion, unci :irp tlicrcforo stylod (Icrmal or covering bones.
Smio of tii(! iiulivkliial bones, however, are partly of carlila}rinouij and partly of nicnibraiioiia origin, the several {Mrtioiis reniuining iwrnmncntly distinct in certain lower vertebrates,
but in niEin nuiting so iutiniately
witli (iich otlitT a» to present no
trai-e tif their previously separate
comlition.
 
The occipital bone consists of
 
two genetically distinct jKirts,
 
the )iUiK>rior or intetpatietal -por
tion, which \n a dermal bone,
 
and tlie occipital bone proper,
 
rijiin. The OBsiflcation of tlie latter
 
(Hie on each side of tlie fommen
 
mitions, one in frtmt of the foramen
 
ihi' liii^ilar process, and one (losterior to that fli)Ort«re for
 
III' lalmliir imnioii nfilic !»iiie ni>t belonging to the iuter
>ial Minium. ( Wiliciiiinn lnjiiii^ in these centers early
 
K' ihird li'ial ni«n[h and pr«<-<rds at ^yv]\ rate that at tlic
 
■ ol' bii'ih ihi- lioin' n>n-^i-.ts of fmir bony jwns which are
 
tliii) hiyers of cartilage.
 
tiiaiii separate tlntingboiit
 
ijrir-ls, iTsjteetively, the ex
supra-occipital (Fig. 181).
 
ihennion with it of tlie in
riial bone that ossifies from
 
!■ wifli the .-iUpra-oiK^iintal
 
iilli ot'fi'tal life. Consisting at
 
■panilcil liv caililagc, the oceip
1 be<-onii-s a ,-ii,._de l...nc by the end i.f the thin! or fourth
 
ir by thi- buiiy uiiiuii c.fllie sciKinitt' se-rmeiits.'
 
J'lic temporal bone is made up of thi-ee genetically distinct
 
' In s.iiiir- m^,-' \}\i- union of llir iii(i'rii:irifl;il willi llii' sii]irH-<>n-ipitnl is
iiniili'iiv Ihi' ailiill Ihiiu' tluii iiiTwiitin;! Iwii iniusvcrau linsures which
s, iiui: rriilii LMi'li laliTjl :iii^lv, tiiwunl tlio lULiliun line.
 
 
 
.-d fn
 
 
 
occipitals, ill
Tl„.s„|,r.-,.
l,T|.;iiiHal |«
 
IWl (its.
 
mar Ih I
 
l.irlli <.f I'uiii
 
 
 
iiilvlai
 
 
 
■h liv
 
ii„id.ll,..«. |.art'-.n
 
l.'>is.Kii...I In- Til.,!
 
basi-occipital, iiml llif
■i|,il„li,.„tn„,.„l,..ll,y
'• .vTi,iB.i.-.l,.,
 
.1.1 timi l.,.ri.i« ... r..»
 
 
 
I |).ir(s ■■
 
 
 
THE DEVELOPMENT OF THE BEAD SKELETON. 391
 
parts, the smuunoBal or BqaamosTgomatic, the petrosal or petrom&stoid or periotic, and the tympanic. At the time of birth
these three elements of the bone are still separate from eacli
other, the tympanic being an incomplete ring, and the petro
 
 
 
mastoid being still without a mastoid process. The petromastoid is the only part of the temporal bone that is outlined
in cartilage, the squamozygomatic and the tympanic being
represented in the eartilaginous stage of the cranium by
mem bran OU3 tissue.
 
The. sqnunosygomatic (Fig. 182) is ossified in previously
 
 
 
392
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
 
V\i\. l^J.— S«iuamozy«<>mjitic 1^7) and tyin|»aiiic ('),
of t«'iu|M>ral IxtUL' at birth.
 
 
 
formed niemhrano from a single center of ossification, which
ai>j)oar.s in the lower part of this segment at about the seventh
\v<»('k. The proeess of bone-formation extends in all directions from this center, but especially
upward into the squamosa and outward and forwaril into the zygoma.
The periotic or petromastoid results
from the ossification of the cartilaginous ear-oaj>sule, which latter constitutes a part of the cartilaginous portion of the early cranium. It should
be remembered that the essential part
of the orgiui of hearing, the internal
ear, is differentiated from a small
pouch of epithelium, the otic vesicle,
wliieh is produced by an infolding or
iiivairination of the surface* ectoderm, and that it is the cartilat^inniis tissue cuclosiuir the otic vesicle and its outgrowths,
the semicircular canals and the cochlea, that constitutes the
cartilai^iuous car-capsulc.
 
The (Ksitication of (he ])criotic is usually descrilx?d as pro(•('(Mlinir iVom three ceuter<. The first of these, the opisthotic,
inake< its apix-araiiec iu the hitter part of the fifth month on
tlie outer wall ot' th<' ea]>sule, at a poiut corresj)onding to the
])o>i{iou of the |U"ouiout<»ry, \\ heuee the formati<m of bone
-preads iu such uiauuer a> to ]>ro(lu(M' that part of the petros:i
which is below the iuterual auditory eanal. A secoml center,
the pro-otic, aj>]>eais a little later over the superior seniiein'ular eaual aud <::ive< rise to that ])art of the ])etrosa above
the iutiMMial auditory uieatus, aud also to the inner and up|)er
part of the uia>toidea. The third nucleus, the epiotic, arises
iu the ueiiihborhood «»f the ])o-iterior semicircular eanal.
( )>sifieatiou proeee(N rapidly, the three parts speedily uniting
to t'orui oue boue, the [xriotic or petroiuastoi<l. The |)etrous
portion of the ju'riotic is the ruon' important and the more
constant. I'he luastoid is of variable size in different animals, and in the human sp<'cies, at birth, it is fiat and devoid
of the ma>toi<l pnx'ess w hich is so conspicuous in the mature
 
 
 
THE DEVELOPMENT OF THE HEAD SKELETON, 393
 
condition of the skull. The mastoid process develops during
the first two years of life, but its air-cells do not appear until
near the age of puberty.
 
The pars tyznpamctis, or the tsrmpanic (Fig. 182), whicli constitutes the bony part of the wall of the external auditory meatus, is ossified in membrane from a single center of ossification.
This center appears in the third fetal month in the lower part
of the membranous wall of the external canal, from which
point the pnxiess of bone-formation extends upward on either
side so as to form an incomplete bony ring, open above.
This tympanic ring is situated external to both the ear capsule and the ossicles of the middle ear and gives attachment
to the periphery of the tympanic membrane. The further
growth of the tympanic ring being in the outwanl direction,
it becomes a curved plate or imperfect cylinder of bone
which constitutes the bony wall of the external auditory
canal. At birth, the pars tympanicus still has the form of
the incomplete ring, its further development taking place
during the first few years of life. The extremities of the
ring unite with the squamozygomatic before birth. The
tympanic unites also with the petrosa except in a region
adjacent to the proximal end of Meckel's cartilage, where
an aperture is left which is the petrotympanic or Glaserian
fissure. Since upon the part of Meckel's cartilage which is
thus enclosed bv the union of the two bones is formed the
long process of the malleus, the presence of this process in
the Glaserian fissure is accounted for.
 
The stybid process of the temporal bone belongs to the
visceral-arch skeleton. It ossifies in two parts in small
masses of cartilage that belong to the anterior hyoid arch.
One, the tympanohyal, gives rise to the base of the process
(Fig. 186); it begins to ossify before birth and soon unites
with the temporal. The other segment, the stylohyal, undergoes ossification later and joins with the tympanohyal only
after adult age is reached. Sometimes it remains separate
throughout life.
 
The sphenoid bone is for the most part ossified in cartilage.
The body of the bone is represented in the fetus by two
sej>arate parts, the posterior body, or basisphenoid, or post
 
 
;5<)4
 
 
 
TKXT-BOOK OF EMBRYOLOGY,
 
 
 
.spli<'noi<l (Fijr. 1S.3, hs\, wliicli incliulos all that part of the
IhmIv ni* th<» niatiiro bono whicli is jx)stcrior to the olivary
rinlnrnco and to whirh belong the greater wings (alisphenoitls): and an anterior body or presphenoid (;>^), situattnl in
front of the olivary ominonoe, to which belong th|? lesser
wings (orbitos|)h('noids). The ossification of the basisphenoid {)roeeeds from two centers placed side by side, which
 
 
 
 
%^
 
 
 
/^
 
 
 
 
 
Ki«.. l.s:;. - Splu'noiil Imhu\ fiflh or sixth A tnl month: seen from above: p«. pre>I))h-iiui«1 itr iiiiti-riur Ixuiy, \\ itti h'sscr wiii}:-^; (i{(, greater wings ; 6<, baffisphenoid
 
or |»i»'ti'i lur I'luly.
 
 
 
;i|»jMar in the eii::htli w(M'k. 1\vo months later two secniid.-irv (M-nh rs M])pear for tlie lateral parts of the b(Kly.
'rih- presphenoid likewise develops fnjni two centers, which
nre appMniit in ilie ninth week. The union of the
|)ir-plu'n<)id with tlir basisplieiioid occurs in the seventh or
eiL'lith nionili. Maeli greater wing develops from a sinprle
eenlci* of o»ili<:ition, uliieli is ]>resent in the eighth week.
The pmrrssof o>sifieaiion >j)read> from this center to produce
imi (nilv the LTi'eatrr wini: but also the external pterygoid
j)lal('. i'lie ureatrr win^s remain separate from the body
until <nnie tiinedurinsr tli(» lir>t vear after birth. P^aeh lesser
wing <»s>ities from a center that appears about the ninth
wrcL. 11ie les<er >vinij:s unite with the j)rcsj)henoid in the
>i\ih fetal montii.
 
The internal pterygoid plate dilfers from the other parts of
the .vpheiioi<I in that it does not os>ify in cartilage but in
membrane. It is stated. howev(»r, that its hamular process
tir-^t hreomes eartilaiLrinons bet'ore it ossities. It is, therefore,
a run rin(/ hum'. \\< center or <'enters of os>itication aj>pear
in tlie fourth month in the connective tissue in the lateral
walls of the oro])harvnireal t-avitv. In manv animals this
plat<' ac(|uires n«» connecti(»n with the external pterygoid plate,
 
 
 
THE DEVELOPMEST OF THE HEAD HKELETOS. 39j
 
but remains throughout life a distinct l>one, the ptervgoiil.
In man it fuses witli the external plate in the fifth month.
 
The presphenoid with its attaclied leaser wings, and the
basisphenoid, to which are united the greater wings and the
pterygoid plates, remain permanently separate bones in some
animals. In man, as noted above, the two parts of the body
of the bone unite shortly before birth, although the greater
wings remain separate until some ranutha after that event.
 
The etlunoid bone and the Inferior turbinate are formed in
cartilage, resulting from the ossification of the jKiHterior portion of the cartilaginous nasal capsule (Fig. 184, m). The
 
 
 
 
vtlli Ih^ nntl ruTltj at (hcplMea dmtEnntri by n 't K. »rt1lnge»t ibe iiikwI Hptum ; n, (urbliml cartilage ; J. omnn nf Jacobsnn : J', the iilaoe wbeie II apclW into
tlia nasal raviij- ; gf, palatal proeeia; of, maiillarj pruccu; iJ. dental rtdgc
 
IHuTlWlB).
 
latter represents the anterior extension of the cartilaginous
trabecule cranii so mmlified as to constitute a rc<»ptacle for
the olfactory epithelium. The anterior part of this capsule
remains cartilaginous throughout life as the septal and lateral
cartilages of the nose. By the ossification of the posterior
part of the nasal capsule the ethmoid and the inferior turbinate bones are produced. Ossification, beginning in the
fifth month, involves the lower and the middle turbinals and
a psirt of the lateral masses. The ixwificatlon of the superior
turbinal, of the vertical plate, of the crista galli, and of the
 
 
 
.n
 
 
 
396 TEXTBOOK OF EMBRYOLOGY.
 
rcintiiiiinjL^ jxirts <»f tho lateral masse? is efiected after birth.
Tlic Imiiiv iiiiinii <»f till' lateral masses with the median plate
i> <'oinplct(*<l U'twii'ii tlie fifth ami seventh years.
 
Tlif frontal bone is a ot evening or dermal l)one, being ossitird ill iiK'inhran*.' tVom two centers of ossificatioUy one for
earh iatt-nil half. These centers are situateil above the
<)rl>ital arrhrs and are tir>t a]»|mrent in the seventh week. At
hirtli, tin* two halves of the bone are still se|)aratey their
niiiuii not nr<Mirriiig until during the first year of life. Sonieiinic> till* union fails to take platv, the ciimlitiou of the per>i>t<Mit frontal or metopic suture being known lus metopism.
M<toj»i-in is ronsid(*ral)ly more common in European skulls
than in tlm.-t' «»f lower tyjH'.
 
TIk- parietal bone i< also o.-sificnl in membrane. It develops
from tuo mhlci wliirli soon c<»alesce. Their position eonx?-jMnid- to that of tile future parietal eminence.
 
TIh.' bones of the face are for the most jxirt dermal l>ones.
< )f tin-**, tin* nji|K'r and the lower maxilhe and the palate
Imiiu- IxJoiiLT to the vi-ccral-arch skeleton. The others devrlnp ill tin- iiHinhranous wall <>f the cranial eapside.
 
Tlw nasal jithI lacrimal bones ossify each from a single
(•(•lit*!-, uhirli a|»|M':ir- in the ci^dith week.
 
The malar i- o--ifn d in nicniKrane from three nuclei, the
ppKM-- iMMiniiiiiir in tlic eighth week.
 
riic palate bone is iorinr<l in ninrous membrane fn>m a
-iiw^lc cciitf r whirli i- .-itnal<_'(l at the jniiction of the vertical
and tlic horizontal pl.itc^.
 
riic vomer d<'V('l(»|>s from two center- of ossification which
a|>|H':ir at the hack |)art of the cartilaLrinotis nasid septum.
Ivi'h <'eiiirr Liive- rise to a laniina ol' hone, the two laminae
;:rMdually iinitiiiLr with each other from behind forwanl, and
eiiil>r:uinL'" Ix'tween them anteriorly the septal cartilage.
 
The vomer and the palate bone are examples of the formation of JM)iie in iniieoiH in<'inl>rane. The centers of ossification lir-t aj)peMr in the eighth week in each ca-e.
 
'i'he skeleton of the visceral arches includes the upi>er and
lower niaxilhe, the liyoi<l Ixnu* with a ])art of the styloid
|)rocess, tlu? ear ossicles, and the j)alate bones. The ]>alate
hone- have heeii referred to above. These hones of the
 
 
 
THE DEVELOPMENT OF THE HEAD SKELETON. 397
 
visceral-arch skeleton are partly primordial and partly membranous.
 
The snperior maxilla comprises two parts, the superior
maxilla proper and the intermaxillary bone. While these
intimately unite in man, in some animals, as the dog, they
are permanently distinct, the intermaxillary lK)ne constituting
the important and conspicuous premaxilla of the dog. The
superior maxilla ossifies in membrane — within the membranous maxillary process of the first visceral arch — from
an uncertain number of centers. It seems probable that
there are five nuclei of origin, one for the palate process,
one for the malar or external part of the bone, one for the
portion internal to the infra-orbital foramen and a part of
the nasal wall (orbitonasal center), one for the part of the
bone between the frontal process and the canine tooth, and
one for the premaxilla. The formation of the antmm begins
in the fourth month by the development of a recess or fossa
on the inner or nasal wall of the bone.
 
The palate process is formed by the growth, on the inner
aspect of the bone, of a shelf-like projection which advances
toward the median line until it meets and unites with its
fellow of the opposite side (Fig. 172j. The horizontal plate
of the palate bone develops similarly and very shortly after,
and thus is produced the hard palate, which separates the
nasal chambers from the mouth. The two halves of the
hard palate unite first in fnmt, their union being completed
by the twelfth week. If union is incomplete, the anomaly
of deft-palate results. The intermaxillary segment begins
its development in the seventh or eighth week upon that
part of the nasofrontal process which lies between the nasal
apertures. In the fifth month the intermaxillaries fuse with
the maxillse, the line of union being indicated by a suture
which is apparent upon the oral surface of the palate processes. The intermaxillaries contain the germs of the four
incisor teeth. As previously mentioned, deficiency of union
between the maxilla and the intermaxillarv results in the
deformity of hare-lip. Obviously, the hiatus in hare-lip
will be found to be not me<lian, but lateral, corresponding to
the position of the line of normal union.
 
 
 
;i!IM TEXT-JWOK OF KMBRYOLOOY.
 
Tliti lower jaw or mandible is intimatcl}' associated ia its
(K^vclopriiuiit witli that of the malleus and incus of the middle
(•(ir. Inasmuch as thoric thrt-e Inmes are dilfcrentiatcd from
ihi! it:irti[:i;riiioiis anil inumliniiioiis visceral skeleton of the
first vi.-wrrtil arch it is di'sir.ibk< to consider their develop
IIHlll toj^tiuT.
 
As dfscrilied aliove, the inenihmiiotis jaw-arches form the
hiliral and Idwit Iiniiiidtirics of the month-cavity, the first
vis<i'ral arch dividing into the nuixillary process and the
iiuiiidihiihir :m-li. Thenr apjH'iirs in the mandibular arch a
bar of cartilage which abuts liy its jmiximal extremity upon
th Iter wall of the au<litory labyrinth. This cartilaginous
 
 
 
 
" r'ii;)il<'t'ii n-i-Lka nld with the
1til. 1'lii- liiniT Jaw lomewhU
'li i.'iiti'iiil:> III till' niHlkiii. The
lyuitaiiicu!! iKvikible: Aa.nwlfiirlilnip-. .Vt: uk, biiiijr lower
 
 
 
>. iia*
 
 
,,:,h,l,..iu.dm.u,
 
 
 
orliim, Meckel's cartilage (Fifr.
 
•.iNii.ial pi.ve. whifh is called,
 
- |>al:ito<|ii:iiIi-:itLnti. From the
 
Hie {i:<[iili>|ilery[ruiil process.
 
 
 
THE DEVELOPMENT OF THE HEAD SKELETON, 399
 
grows toward the roof of the mouth-cavity and becomes a
separate segment. The piece of cartilage remaining, which
represents the proximal end of the original bar, undergoes
ossification, becoming the incus (Fig. 185, am). The posterior or proximal extremity of Meckel's cartilage, becoming
a partly sej)arated cartilage, the articulare, ossifies to produce the mallens (Fig. 185, ha). Though the form of the
malleus is recognizable, it is still in direct continuity with
Meckel's cartilage. In the opposite direction it is articulated
with the incus. As the tympanic ring develops, and the interval below, between tliis ring and the petrosa, is gradually
narrowed to the petrotympanic or Glaserian fissure, the malleus comes to He within the tympanic cavity, being continuous,
through the fissure, with Meckel's cartilage. Upon the separation of the malleus from the cartilage of Meckel, the long
process of the malleus represents the former bond of union
and therefore occupies, in the mature state, the Glaserian
fiasure. The joint between the malleus and the incus represents the primitive vertebrate jaw articulation. In the shark,
for example, the mandibular joint is between the two pieces
into which the cartilaginous bar of the first visceral arch
divides — that is, between the palatoquadratum and the representative of Meckel's cartilage, the mandibulare. In mammals, however, the malleus, as we have seen, loses its connection with the mandible, the joint between the latter and
the skull, the temporomaxillary articulation, being secondarily acquired in a manner to be pointed out hereafter.
While the malleus develops for the most part by ossification
in cartilage, its long process develops in membrane as a small
covering or dermal bone, the angulare.
 
The membranous lower jaw with its enclosed bar of cartilage becomes osseous, not by the ossification of the cartilage, but by the development of a casing of bone within the
surrounding membrane. In other words, the lower jaw
develops chiefly by the intramembranous method of boneformation. Several centers of ossification appear, and from
these the process of bone production extends rapidly, forming, by the fourth month, a covering or dermal bone, the
 
 
 
400 TEXT-BOOK OF EMBRYOLOGY.
 
dentale (Fig. 185, uh)^ which is situated mainly on the outer
side of Meckel's cartilage. A smaller plate appears on the
inner side. Thus the cartilage comes to be surrounded by an
irregular cylinder of bone. The cartilage of Meckel plays
a comparatively unimportant part in the ossification of the
lower jaw-bone and begins to degenerate in the sixth fetal
month. Its distal extremity, however, undergoes ossification, thus aiding in the formation of a small part of the
mandible near the symphysis; while a posterior segment,
with the fibrous tissue encasing it, which extends from the
temjx>ral bone to the inferior dental foramen, persists as the
internal lateral ligament of the lower jaw. With these
exceptions, Meckel's cartilage entirely disappears. The
angle of the mandible and a small part of the ramus are
also ossified in cartilage, which latter is developed independently of Meckel's cartilage. From the posterior part of the
dentale the condyloid process develops and becomes articulated with the glenoid fossa of the temporal bone, thus establishing the temporomaxillary articulation. This joint, as previously stated, is a secondary one and replaces in mammals
the primitive articulation between the mandibulare and the
palatoquadratum of the lower vertebrates.
 
At birth, the two lateral halves of the inferior maxilla
are united at the symphysis by fibrous tissue ; bony union
occurs during the first or second year after birth.
 
To summarize, the inferior maxilla develops as a part of
the visceral -arch skeleton and is chiefly a covering bone, since,
with the exception of the angle, a portion of the ramus, and
a small part near the symphysis, which are of cartilaginous
origin, it is formed by the membnmous method of ossification. The two other products of the mandibular arch, the
malleus and the incus, are ossified from cartilage, with the
excej)tion of the processus gracilis of the malleus, which is
of membranous origin.
 
The development of the hyoid bone, of the styloid process of
the temporal bone, and of the stapes was referred to in considering the c4irtilaginous visceral-arch skeleton, but for the
sake of clearness and completeness it may not be amiss to
 
 
 
THE DEVELOPMENT OF THE HEAD SKELETON. 401
 
repeat, in this connection, Bome points previously mentione<!.
 
The membranons ulterior hyoid or second Tisceral arch, at a
certain stage of development, presents, in ita interior, tlie
dorsoventral cartilaginoiiii bar known as Keichert's cartilage. This is parallel with Meckel's cartilage, and, like it, is
in contact by its dorsal or cranial end with the outer wall of
the auditory labyrinth. A shorter bar of cari:ilage appears
in the third visceral arch, which latter is known also as the
posterior hsroid arch. Together, these two cartilaginous elements furnish the stapes of the middle ear and the hyoidean
apparatBS, tlie latter consisting of tlic hyoid bone, the stylohyoid ligaments, and the styloid processes. In man the
 
 
 
 
llfOldeaii apparatuB and Inryni at dog.
 
 
 
hyoidean apparatus is somewhat nidlmontary, but in the dog
and many other mammals it is present in its typical form
(FIr. 18G). In such animals the stylohyoid ligament of human anatomy is roprescntrd by a b()ne, the epihyal, the hyoid
bone being, therefore, connected with the skull by a series
of small bones artieulatod with earh other. AH the elements
of the hyoidean apparatus, ^ave the IwmIv and the greater
cornna of the livotd liono, are produced bv Reichort's cartilage ; the hyoid Iwdy, known in comiKirutive anatomy as the
basiliyal, .tihI the greater eornua, or the thyrohyals, ossify
 
 
 
402 TEXT-BOOK OF EMBRYOLOGY.
 
from the cartilage of the third arch, the cartilage for the
body being a median unpaired segment known as the copula.
Beicliert's cartilage undergoes division into five s^ments.
The segment at the cranial end, upon ossification, becomes
the stapes/ This ossicle, by the closing of the walls of the
tympanic cavity, is isolated from the other segments. The
second piece, the tsrmpanohyal, ossifies to form the base of the
styloid process and ankyloses firmly with the temporal bone
at the point of junction of the periotic portion of that bone
with its tympanic plate. The third portion, the stylohyal,
forms the lower part of the styloid process. It undergoes
ossification later than the tympanohyal and does not acquire
osseous union with it until the time of adult age. It sometimes remains separate throughout life. The fourth segment, the epihyal, does not even become cartilaginous in
man, but remains fibrous, constituting the stylohyoid ligament. In most mammals it ossifies, to form a distinct bone,
the epihyal. The ventral extremity of the cartilage of
Roichert, the ceratohyal, produces the lesser cornu of the
hvoid bone.
 
THE DEVELOPMENT OF THE APPENDICULAR SKELETON.
 
The upper and lower limbs articulate with tlie trunk
through the modium respootivcly of the pectoral and pelvic
jj^irdlcs, tho fornicr being constituted bv the scapula and the
elaviele, and the latter by the ossa iiinoniiiiata. As in the
ease of the axial skeleton, the hones of the limbs in their
develo]nu(jnt ])ass sueeessively through a nienibnmous and a
cartilaginous stage.
 
The general (h^velopnient of the upper and lower extremities is deseribed in a later section. As stated in that account,
each linii)-bud is to be regarded as an outgrowth from, or as
eorresj>ou(ling in posili(ni to, several primitive segments, the
tissue composing tlie little bu(l-lik(» ])rocess subsequently differentiating into the muscular, cartilaginous, and connectivetissue elements (►f the member. The origin of each limb
from more than one primitive segment has been established
 
^ Si'u ft)ot-n()te, i>:igt.' J 15).
 
 
 
DEVELOPMENT OF THE APPENDICULAR SKELETON. 403
 
chiefly by eml)ryological investigations upon the lower vertebrates, and is borne out by the fact that each extremity
receives- its nerve-supply from a series of spinal nerves instead of from the nerve-trunk of any one segment.
 
The Development of the Pectoral and the Pelvic
Girdles. — The pectoral or shoulder girdle consists in its
earliest stage of a pair of curved bars of cartilage, each of
which is made up of a dorsal limb occupying approximately
the position of the future spine of the scapula and approaching but not touching the spinal column, and a ventral segment lying near the ventral surface of the trunk. At the
angle of union of the dorsal and ventral parts is a shallow
depression, an articular surface, which represents the point
of articulation with the future humerus.
 
The scapula is developed, except its coracoid process,
from the dorsal part of the primitive shoulder-girdle. This
soon acquires a form resembling that of the adult scapula
with the infraspinous portion of the bone very much shortened. Ossification begins at the neck of the scapula about
the eighth week, and in the third month extends into the
spine. The ventral part of the cartilaginous shoulder-girdle
extends almost to the median line of the chest-wall. It
divides into two diverging bars, the lower one of which
undergcx^s ossification in birds and in some other vertebrates
to form the conspicuous coracoid bone. In mammals, however, it aborts and gives rise to a smaller element, the
coracoid process of the scapula. At birth the human scapula
is but partially ossified, the coracoid process, the acromion,
the edges of the spine, the base, the inferior angle and
margins of the glenoid cavity being cartilaginous. The
coracoid process ossifies from a single center and acquires
osseous union with the body c>f the bone at about the age of
puberty. The acromion ossifi(s from two or three nuclei
and joins the sj)ine between the twenty-second and twentyfifth years. Still other centers of ossification ai)pear from
time; to time. Thus there is an accessory center for the base
of the cf)racoid and the* adjacent part of the glenoid cavity,
 
 
 
404 TEXT-BOOK OF EMBRYOLOGY,
 
and one at the inferior angle of the hone, from which latter
ossification extends along the vertebral border.
 
The clavicle does not develop from the primitive shouldergirdle, but is formed in membrane, for the most part, as a
dermal bone. 1\^ ossification begins in the sixth or seventh
week, before that of any other bone in the body. Subsequently, cartilaginous epiphyses are added, one at each end.
It is by means of the epiphyses that the bone grows in
length.
 
The cartilaginous pelvic girdle consists of a pair of cartilages, which are united with each other by their ventral
extremities, and each of which, by its dorsal end, is articulated with the sacral region of the cartilaginous spinal
column. At about the middle of each cartilage, on its outer
surface, is a depression representing the future acetabular
fossa. Anterior to the depression is a large ajjerture, the
thyroid foramen, the upper and lower boundaries of which
are respectively the pubic and ischiatic rwls or bars, which
make up the ventral portion of the cartilage, while posterior
to the fossa is the iliac segment, which has a somewhat
irregular jilatc-likc form. Ossification Ix'^ins in the third
month, ]>roccc(Iiii<2; from three centers, one for each of the
tlire(? divisions of tlie innominate l)one. At the time of
birth a large pro|)oriion of the orijxinal cartilage is still
present, the os pnl>is, the ischium, and the ilium being separated from each other np to the age of puberty by strips of
cartilag(». The ischium and the pubes unite first, and later
acquire osseous union with the ilium. In addition to the
three ])rimarv centers of ossification, otlier and secondary
nuclei apj)ear at a later date in the crest of the ilium, the
tuberosity of the ischium, and in i\w various spines and
tubercles.
 
The skeleton of the free portions of each extremity, consisting at first of a continuous mass or rod of partially metamorphosed mesenchymal tissue, undergo(»s division into segments
which represent the skeleton of the arm or of the thigh, of
the forearm or of the leg, and of the hand or of the foot.
This segmentation corresponds with that of the entire mass
 
 
 
DEVELOPMENT OF THE APPENDICULAR SKELETON. 405
 
of the limb, both as to extent and order of appearance (see
page 406). Nuclei of chondrification now appear, one in
the center of each skeleton-piece, from which cartilage formation extends toward either end. The several cartilaginous
elements thus pnKluced present approximately the respective
forms of the future bones. The larger cartilages are present
in the upj)er extremity in a six weeks' embryo, but not until
somewhat later in the lower limb. All the bones of the
extremities are of endochondral origin.
 
The long bones develop in a fairly uniform manner. The
shaft or diaphysis ossifies from a single center, while the two
epiphyses each present several centers. The centers for the
diaphyses appear at about the eighth week, ossification proceeding at such rate that at birth only the ends of the long
bones are cartilaginous. The centers for the epiphyses appear
at various times after birth. Osseous union between the
diaphysis and the epiphyses does not occur until the growth
in length of the bone is completed. As the details conceniing the time of appearance and the number of these centers
are to be found in the text-books of anatomy, they are omitted
here.
 
Each bone of the carpus and of the tarsus ossifies from a
single center, except the os calcis, which has two ossific
nuclei. The bones of the carpus are entirely cartilaginous at
birth, their ossification beginning in the first year with the
appearance of a center in the scaphoid. The pisiform bone
is the last of the series to ossify, its ossification beginning in
the twelfth year.
 
The bones of the tarsus begin to ossify earlier than those of
the carpus. The os calcis and the astragalus present osseous
nuclei in the sixth or seventh ft^tal month, and the cuboid
shortly before birth. With thos(^ excej^tions the tarsal bones
undergo ossification between the first and the fourth years.
 
The metacarpal and the metatarsal bones and the phalanges
present each a center of ossific^ation for the shaft and one
epiphyseal center. In the case of the phalanges and of the
metacarj)al bone of the thumb and of the great toe, the epiphyseal center is at the proximal extremity, while in the
 
 
 
4<)»] TEXTBOOK OF EMBRYOLOGY.
 
n'inaininj: nu'tatarsiil an<I inetacarjKil l)onos it is at the distal
vuiV The ossification of tlio >haft begins in the eighth or
ninth wwk of fetal life; <»f the epiphyses, n(»t until scvenil
y«irs after hirtli. The development of the ungual or distal
phalanges — of the hand, at least — is jH-euliar in that the
ossification l)egins at the distal extnMuity, instead uf in the
middle uf the shaft.
 
 
 
THE DEVELOPMENT OF THE LIMBS.
 
The linihs of vertehnites develop fi-om little bud-like
i)r(KH'sses (Fig. iVl) liiat spring from two lateral longitudinal
ridges, situattnl one on eaeli side of the body. Thes(» ridges
are not exactly i^anillel with the nuMhau ]>hine of the body,
but converge somewhat toward that jilane as they api)roaeh
the «uidal end of tlie embryo. It results from this circumhiancc that the jMistcrior limbs are jilaeed chaser together
than the anterior. In man, the limb-buds ai)pear soon after
the thin! wivk. Kju'Ii bud contains a basis (»f j)rimitive eonnivtivc tissue oontributt»<l by several somites, as >vell as muscubr <tnietnrt\ whioh is th(? ofi«hoot from the muscle-plates
0-' J 1.^ numlKT of primitive si^gments.
* Thr assumption of the origiu (►f each limb-bud from more
i «r^mitivc socmont is borne out by the nerve-sup])lv
Th' fu]lv-l>"^*^' l"»l^' ^*"*''* extremity being innervated by
' ' ' ^ ■ •' <nir.-il nerves (compare page ;}GS). The eon.>a<m of the limM)ud i)ro(iuces the bony structures
while the i»utgn)Wths from the mustrle-plates
\r.y mnsriilntun*. Previous reference has been
-.. t.^ ihe work of Banleeu and Lewis on
v^ ihr limbs. Aci'ordiug to their findings,
 
•tail «■ m ...
 
 
 
TT^ni o1
 
 
 
^,,.^:,...t inr*i
 
 
 
T^y fVTcnd into the limb-buds, i)ut the limb
.^ f«rtni the mestMichymal core of the bud.
 
""""'* -,,*; thf bnJ for the arm is at first ojiposite
 
' "• "^^ ^ ^^ tho first thonicic s«'guuMits, subse
 
 
—.•- ••«,
 
 
 
i.\'/*!ol' the third ('crvical >cgm«'nt,
^. N.-^v* 10 its adult position ; and that
 
 
 
THE DEVELOPMENT OF TEE LIMBS. 407
 
the hud for the leg, at first attached at the region of the lower
four lumhar and first sacral myotomes, extends to include the
first lumbar and the second and third sacral segments,
assuming later a more caudal position.
 
In the fifth week each limb-hud becomes divided, by a
transverse groove, into two segments (Fig. 59, 12, 13), of
which the distal part becomes the hand or foot, while the
proximal j>ortion very soon afterward divides into the
forearm and arm or leg and thigh. Even as early as the
thirty-second day, the digitation of the limb-buds — in the
case of the up]>er extremities — is indicated by four longitudinal parallel lines or grooves on the distal extremity
of each (Fig. 59, 14). By the conversion of these grooves
into clefls, the fingers appear, in the sixth week, as separate
outgrowths. The development of the upper extremities precedes that of the lower by twelve or fourteen days, so that,
when the fingers are present as distinct projections, the toes are
just being marked off in the manner noted above for the
fingers. The toes begin to separate, by the deepening of the
intervening clefts, from the fiftieth to the fifty-third day. By
the end of the eighth week, the fingers are perfectly formed,
with the exception of the nails. The nails have their beginning
in the seventh or eighth week, in little claw-like masses of epidermal cells, which are attached to the tips of the digits
instead of to the dorsal surfaces. Subsequent transformations
result in bringing the nail into its normal position on the
dorsal surface of the distal phalanx. The nails are well
formed by the fifth month, at which time the covering of
modified e])idermal cells begins to disapi>ear. The extremity
of the nail, however, does not break through so as to project
beyond the finger-tip until the seventh month. A more
complete account of the development of the nails will be
found in connection with the origin of the skin (page 270).
 
The Position of the Wmbs.— The paddle-like limbbuds at first project laterally almost at right angles with the
axis of the trunk. At this time tlie future dorsal surface of
eacrh limb looks toward the back of the fetal Ixxly (dorsad),
the future flexor surface toward its anterior aspect (ventrad),
 
 
 
408 TEXT-BOOK OF EMBRYOLOGY.
 
while the first digits — the future thumb and great toe — and
consequently the radius and tibia, occupy the side of the
member that is directed headvvard or cephalad, the future
little finger and fifth toe with the ulna and fibula looking
caudad. As the limbs enlarge and differentiate into their
respective segments, they apply themselves to the ventral
surface of the body, this change in position being facilitated
by the occurrence of the future elbow- and knee-flexions,
which cause the flexor surfaces of the forearm and leg, respectively, to approach the corresponding surfaces of the
upper arm and thigh. At abf)ut the same time, the distal
segments, the hand and foot, become bent in the opposite
direction, producing the condition of the limbs that is permanent in the Amphibia — that is, the condition in which the
dorsal surface of the proximal segment of the limb faces in
the same direction as the dorsal surface of the trunk, while
the middle segment is flexed and the distal is extended. To
establish the permanent condition of the human limbs, there
occur an outward rotation of the arms and an inward rotation
of the lower extremities, on their long axes. The thumb and
nulius, therefore, instead of looking cephalad, are now directeil dorsad — with the forearm in the supine jxjsition and
the arm outstretched — or laterad, away from the median
plane of the body, if the arm hangs by the side in the anatomical j)osition. By the inward rotation of the lower limb,
the great toe and the tibia come to lie toward the median
plane of the body, cjiusing the extensor surface to look ventrad, the flexor surface, dorsad.
 
 
 
TABULATED CHRONOUXIY OF DEVELOPMENT.
 
 
 
PertillMtioS!"*
 
 
STAGE OF THE OVH«.
FiiCT Weik. Second Week.
 
 
Obanctwi.
 
 
SegmenUtlon of fartlllied
while pauine along ovl
Great Increase In tl«.
Cella of loner cell-maw rearranged lo form cnto
cell* of Rauber.
 
eytlum (.--3 dar).
 
 
Ovnm Id ntema, embedded
 
C°ortSn'and il> Tilll (FlgJ
49). YapeularitalloQ of
Fliorlau and ila villi.
 
Yalk-uc partly formed.
 
 
ViLtenUr
 
 
 
 
rf"yolk-wc. '''""
 
 
"Vy'SS,.
 
 
 
 
Oral pit (12thMl«h day).
«m-lmrt partly «.i-.n.ted
from yHlk-BBc.
 
 
■XiS?
 
 
 
 
 
 
-XSJS^
 
 
 
 
 
 
■Us.
 
 
 
 
 
 
VWTOUM
 
8r*l«m.
 
 
 
 
Mednllary plate (Hth day).
 
 
■lM<d>l Seme
Oigini.
 
 
 
 
Naaal areaa.
 
 
Byatem.
 
 
 
 
 
 
•*?SS!i.«"'
 
 
 
 
 
 
 
410 TEXT-BOOK OF EMBRYOLOOY.
 
Tabdi^ted CmtoNOLoaY or Dkvelopmeht {Ootuimud).
 
 
 
 
 
STAGE OF THE EMBKYO.
TuiRD WEEK, Fotiavn W««.
 
 
Ouiarml
Cbuutan.
 
 
Body of embryo indicated.
 
VUcural arche. and clefia
 
NMo^fciltkr"™™
AllBiitoiCBtnlklFlgisTt.
lHjllnctton betweuii chorion
Jevc and rhorion fWn
 
Marked flexion of body (21irt
to lOd day): sradual uncoiling after aSd day.
 
Vlacerararchea and jolk-aac
 
to flatten.
CephBllo Bcxurea.
 
 
"nsas..
 
 
Heart wllb einitle caTlty
present, aooo dlvldltig Into
 
VlaCBral-areh -veueli beglu
to appear.
 
 
IMviilon of atrium begini.
 
 
DlceiUTa
TlyaMm.
 
 
(5ut-lracl a strBlght lube connetted wlih yolk-«ae by a
 
Anal Plate.
 
 
Alimentary canal ptcocnla
pharvni. euipbapu, atomach. and intwliue.
 
Fancrcaa bvguii.
 
l.lver-dlTPrileulum divides,
 
Blle-diicta acquire himlna.
 
brealiB down,
 
 
-C£'
 
 
tral wflU of esophagus,
arterward becoming a
 
 
Pulmonary anlage blftircall's, the two poaches
being eonne-ted by « pedicle,^he prlraltlTC irwhea.
with the pharynx.
 
 
"XS^""
 
 
Wolffian bodies reeoBnlsuble.
 
 
 
 
BUn.
 
 
SeKmenlation of paraxial
mesoikTm,
 
HeurBl canal : Its cells sho*
 
El obi oats and Kerm-cellH
Fourth ventricle Indlcaled.
Fore-bralQ mtd-brBln, and
hind-brain -reniples. s-kiu
illvidlns Into five vealelcs.
 
Auditory pll followed by ollc
 
nlfaclory plalM.
Optic vL-hlclcflbi-frtn,
 
 
I'utL-plite.
 
 
Vettmu
87item.
 
 
thicken.
Yenlral roots of aplual
 
Anterior' lobe of hypopbrila
 
begins.
 
 
Smel&l Bsnae
 
OTgMM.
 
 
Ollc vealcle with recenu
 
NHMlplu dl'itlnd.
lJl4lc veaicic atalked and
tmuBformetl IntooptlccDp,
 
 
MOBCular
ByaCem.
 
Bkflleton Mid
 
Limbi.
 
 
iiicsodcrm.
ft-Km.nlallon of paraxial
 
 
Somites or primitive acf
 
Hyiilomea.
 
S.iniltcs or prlmlllre aeg
Sk"ci(.'t.ien.ina ahcath of
 
phorrla.
Llmb-hiidii Biijarent (aboot
 
2lst day).
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
411
 
 
 
Tabuulted Chronoloot of Development (Oontinued),
 
 
 
STAGE OF THE FETUS.
 
 
 
Fifth Week.
 
 
 
Sixth Week.
 
 
 
Body shows dorsal coDcavity in neck- NasofVontal, lateral nasal, and maxil
 
 
 
region
 
Globular and lateral nasal processes.
Lacrimal groove.
Third and fourth gill-clefts disappear in
 
sinus prscervicalis.
Umbilical cord longer and more spiral.
Umbilical vesicle begins to shrink.
Length of fetus 1 cm. ({ inch).
Larynx indicated.
 
 
 
lary procesnes unite.
I'mbilical vesicle shrunken.
Amnion Larger.
 
 
 
Primitive aorta divides into aorta and
I)ulmonary artery.
 
The only corpuscular elements of the
blood during the first month are the
primitive nucleated red blood-cells.
 
 
 
Vitelline circulation atrophic and replaced by allantoic circulation.
 
 
 
Intestine shows flexures, notably the
U-loop, inaugurating the distinction
between large and small bowel.
 
Anal pit.
 
 
 
Right and left bronchi divide into three
and two tubes respectively (5th to 7th
week).
 
 
 
First indication of teeth in the form of
the dental shelf.
 
Submaxillary gland indicated by epithelial outgrowth.
 
Duodenum well formed; caecum; rectum (end of week).
 
 
 
Larynx indicated as dilatation of proximal end of trachea.
 
Arytenoid oartiliiges indicated (though
not cartilagiiiouM).
 
Thyroid and thymus bodies begun.
 
 
 
Genital ridges appear on wall of b<»dycavity and soon t>ecome the indifferent genital srlands.
 
Ducts of Mtiller apfiear.
 
 
 
(ienital tubercle, genital folds, and gen- '
ital ridge (external genitals). '
 
 
 
Epidermis present as two layers of
cells.
 
 
 
CoIIh of oiiti>)-t>late proliferate and gradually hpreau out beneath epidermis.
 
 
 
Olfactory lobe begins.
 
Arcuate and choroidal Assures on mesial surfaces of fore-brain vesicles.
: Cells of central canal of cord ciliated.
i Ridge-like thickening of roof of mid! brain.
 
 
 
Membranes of brain and cord indicated.
I*inenl body U;gins.
Dorsal roots of spinal ner\'es.
Home tracts of spinal cr>nl indicated,
and its lumen alters (Fig. 139).
 
 
 
Semicircular canals indicatcnl. Semicircular canals.
 
Eyes begin to move forward from side Concha of external ear.
of head. ' Outer flbrous and middle vascular tu
1 nicR of eye.
Eyelids
 
 
 
I Mandibles unite (a')th day).
 
, Meckel's cartilage.
 
I Limb-buds segment.
 
I Digitation indicated (32d day, for hand.
 
 
 
I^wer jaw begins to ossify.
 
Clavicle iHJgins to ossify.
 
Rilw l>egin to chondrify.
 
Bodies of vertebra: are cartilaginous.
 
Fingers as sefMirate outgrowths.
 
 
 
412
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
Tabulated Chronology of Development {Contiuued).
 
 
 
•
 
 
STAGE OF THE FETUS.
Seventh Week. Eighth Week.
 
 
General
Characten.
 
 
Fetal body and limbs well
defined (Fig. 64).
 
Head less flexed.
 
No longer any trace of syncytium on dccidua vera.
 
 
Head more elevated (Fig. 65).
Free tail begins to disappear.
Subcutaneous lymph-vessels
 
present.
Oils lining the coelom are
 
true endothelium.
 
 
VaBcular
Bystem.
 
 
Interventricular septum of
heart completed, tne heart
now having four chambers.
 
Other corpuscular elements
added to blood during second month.
 
 
 
 
Dlgeetlve
System.
 
 
Transverse colon and descending colon indicated.
 
 
Parotid gland begins.
 
True endothelium lines the
body-cavity.
 
Gall-bladder present (2d
month).
 
Anlage of spleen recognisable (2d month).
 
 
Respiratory
System.
 
 
Median and lateral lobes of
thyroid unite.
 
 
lArynx begins to ehondrify.
Formation of follicles of
thymus.
 
 
Oenlto-nrlnary
System.
 
 
Maximum development of
Wolffian body.
 
 
Mailerian ducts unite with
each other. Genital groove.
 
Bladder present as spindleshaped dilatation or allantois.
 
Suprarenal bodies recognizable.
 
 
SUn.
 
 
Nails indicated by claw-lIkc
masses of epithelium on
dorsal surfaces of digits.
 
 
Corium indicated as a layer
of spindle-cells beneath
epidermis. Development
of mammary glands began.
 
 
Nervous
System.
 
 
Fore-brain vesicles increase
in size disproportionately.
Cerebellum indicated.
 
 
Sympathetic nerves discernible.
 
 
Special Sense
Organs.
 
 
 
 
External nose definitely
formed (Fig. 171).
 
Lens-capsule.
 
Palpebral conjunctiya separates from cornea.
 
 
Muscular
System.
 
 
Muscles begin to be recognizable, though not having
as yet the characters of
muscular tissue.
 
Ossific centers for vertebral
arches and for vertebral
l»odies; ossiflc renters for
frontal bone and for sciuamosa.
 
Membranous primordial cranium begins to ehondrify.
 
Claw-like anlages of nails.
 
 
 
 
Skeleton and
Limbs.
 
 
Ribs begin to ehondrify. Centers of ossification of bestsphenoid, of greater wings.
of nasal and lacrimal
bones, of malar, vomer, palate, neck of scapula, diaphysos of long bones and of
metacar)>al bones. Fingers
perfectly formed. Toes begin to seiMirate (53d day).
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
413
 
 
 
Tabulated Chronology of Development (Continued),
 
 
 
STAGE OP THE FETl'8.
Ninth Week. Third Month.
 
 
 
Weight, 15 to 20 frrams ; length, 25 to 30 Weight (end of month), 4 ounces: length,
 
mm. (1 to Ij inchefi). 2) Inches.
 
Hard palate completed. - At first chorion Icve and chorion fron
• Free tail has disapfteared. dosiim prt'oent : later, formation of
 
Differentiation or lymph-nodes begins placenta (see second frontispiece).
(O. Schultze). (Uoaea divided. I
 
 
 
Pericardium indicated.
 
 
 
Placental system of vessels.
Blood-vessels fienetrate spleen.
 
 
 
Anal canal formed by division of cloaca. Mouth-cavity divided fh)m nose (end of
 
 
 
(Anus opens at end of '2d month, ac-,
cordiuK to Tourncux.)
 
 
 
month). Soft (Mlate completed dlth
week). Papillie of tongue. Evagination for tonsil. Intestine begins to rect»de within abdomen (10th week). Rotation of stomach. Vermiform api>endix as a slender tube. Omental bursa,
(tastric glands and glands and villi of
intestine fairly well formed (lOth
week I. Liver verj* large. Peritoneum
has its adult histological characters.
 
 
 
Epiglottis.
 
 
 
External genitals begin to show distinctions of sex.
 
Ovary and testis distinguishable fVom
each other.
 
Kidney has its characteristic features.
 
Urogenital sinus ac<iuires its own aperture by division of cloaca.
 
 
 
Union of ti-stis with canals of Wolftian
 
lM)dy conn>lete.
Testes in false jx-lvis.
Ovaries descenM.
Prostate K*gun d'ith week).
 
 
 
r«>riuin projK»r present as distinct layer. |
Nails not (juite iH»rfeetly fi>rmed.
Hepinning Jif <ievelopiiient of hair as '
solid ingrowths of epithelium.
 
 
 
Corpus striatum in«licated.
j Corpora quadrigemina represented by
two elevations on mid-brain roof.
 
 
 
Cerebrum covers inter-brain. Fornix '
and corpus callosum iK'gun. Fissure '
of Sylvius. Calcarine fissure. Crura
cerebri. Kestiftirm bodies. Pon.s.
 
 
 
! External ear indicated (Fig.
Ciliary processes intlicated.
 
 
 
170).
 
 
 
Eves nearly in normal iK>8ition.
Eyelids begin to adhere to each other.
 
 
 
\
 
 
Centers of ossillcation of presphenoid. Ik'jrinningossiticationof occipital bone. "
of les.«ier wings of sphenoid, and t)f of tympanic, of spine of scapula, of
shafts of metatarsal bones. ossu innominata. '
 
Cnrtilacinovis arches of vertebrse close.
Limbs have definite shape ; nails almost
|)erfeetly formed.
 
 
 
414
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
Tabulated Chronology op Development (Coniinwd).
 
 
 
General
CliaracterB.
 
 
 
I
 
 
 
Vasciilax
System.
 
 
 
Digestive
System.
 
 
 
Respiratory
System.
 
 
 
Oenito-urlnary
System.
 
 
 
SUn.
 
 
 
Nervous
System.
 
 
 
Special SenBo
Organs.
 
 
 
STAGE OF THE FETUS.
Fourth Month. Fifth Month.
 
 
 
Weight, 7j ounces: length, 5
 
inches.
Head constitutes about oneI quarter of entire body.
 
 
 
Enamel and dentine of milkteeth. Germs of permanent
teeth tl7th wk) : (for 1st molar, 16th wk). Muscularis
(longitudinal and cirt>ular)
of stomach and esophagus.
Intestine entirely within
abilomen. Acid cells of
peptic glands. Malpighian
I bodies of spleen. Anal
' membrane disappears.
 
! Cells of tracheal and bron' chial mucous membrane
ciliated.
 
 
 
Weight. 1 lb. : length, 8 in.
Active fetal movements begin. Two layers of decidua
(Mialesce, obliterating the
space between vera and reflexa. Lymphatic glands
begin to appear.
 
Heart very large.
 
 
 
Salivary glands acquire lamina.
 
Villi of large intestine begin
 
to disai>piear.
i Liver very large.
 
Meconium shows traces of
bile (sometimes early in
fourth month).
 
 
 
Sexual distinctions of external organs well marked.
Closure of genital farrow.
Scrotum. Prepuce. I*rostate well formed.
 
 
 
Pftpilln? of corium. Subcutaneous fat first appears. I^iiiugo or embryonal down
on scalp and some other
parts.
 
rnrieto-orcipital fissure.
 
 
 
Distinction between uterus
 
and vagina.
Hymen begins.
 
 
 
j CorfHtra alhicantia.
Tmnsvei
 
 
 
»rse fibers of p<m«.
 
Middle i>eduncle8 and chief
fissures of cerebellum.
 
Spinal cord ends at end of
riM'cyx.
 
Deposit of myelin on fihrrs
of |Mist«Tior* roots, extending to liurdaeh and (ioll.
 
 
 
I
 
 
 
Panniculus adiposus.
lanugo more abundant.
Sel>aceous and sweat-glands
iH'gin.
 
 
 
Fissure of Rolando. Body of
fornix and corp. caliosum.
I>on>:itudinal fibers in crura cerebri. Superior peduncles. A nterior pyramids of
medulla. Chief transverse
fissures of lateral lobes of
cerelwllum. Deposit of myelin completed for tract of
(roll and later of Burdach.
and for short commissural
fibers (Tourneux).
 
 
 
Kyolids and iiostriN closed.
("urtiljiKcof Kustnchiun tnlK?.
 
 
 
Orj^m of Corti indicated.
 
 
 
Muscular
 
 
 
 
Difiirentiation of muscular
 
 
System.
 
 
 
 
tissue of arms.
 
 
Skeleton and
 
 
Os»i<'(ms crntcr for inteninl
 
 
(>s*!ifl<«tion of stapes and petriisji. Opisthotic and prootic apjH-ar. Ossificati<m
 
 
Limbs.
 
 
ptrrvjroid |»ljite.
 
 
 
 
Antnnn of lliirhmore b^'cins.
 
 
 
 
<Ksin«'Mtioii of malleus and
 
 
iM'irins in middle and infe
 
 
 
incus.
 
 
rior turt)inals and lateral
 
 
1
 
 
'^ymp""i*' f^'nK
 
Tnav»jr«i of t>thmoid. Internal pt<'ryiroi<l plate Aisea
witlj •'Xternal. Intermaxillarifs fuse with maxilla.
I^'us longer than arms.
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
415
 
 
 
Tabulated Chronology of Development (OonHwued),
 
 
 
STAGE OF THE FETUS.
Sixth Month. Seventh Month.
 
 
 
Weight, 2 poands ; length, 12 inches.
Vemix ca«eo«a begins to appear.
Amnion reaches maximum size ; amniotic fluid of maximum quantity.
 
 
 
Weight, 3 pounds ; length, 14 inches.
Sur»ce less wrinkled owing to increase
of fiit.
 
 
 
Peyer*s patches. I Meconium in large intestine.
 
Trypsin in pancreatic secretion (fifth ■ Ascending colon partly formed,
or sixth month). Csecum below right kidney.
 
 
 
Air-vesicles of lungs begin to appear.
 
 
 
Walls of uterus thicken.
 
 
 
Vernix caseosa begins to appear.
Eyebrows and eyelashes begin.
 
 
 
Testes at internal rings or in inguinal
canals.
 
 
 
Epithelial buds for sebaceous glands acquire lumina. Branching of cords of
milk-glands. Eponychium of nails
lost; nails said to break through,
lanugo over entire body.
 
 
 
Collateral and calloso-marginal Assures.
 
Body of fornix and corpus callosum
complete.
 
Hemispheres of cerebrum cover midbrain.
 
 
 
Cerebral convolutions more apparent.
 
Cori>ora nuadrigeraina.
 
Myelination of fibers of direct cerebellar
 
tracts. (Crossed pyramidal tracts not
 
until after birth.)
 
 
 
Lobule of ear more characteristic.
 
 
 
Lens-capsule begins to acquire trans- ,
mrcnry. Kyelids permanently open. ,
rupilliiry membrane atrophies.
 
DiflTorontintion of muscular tissue of ,
lower extremities.
 
 
 
i I^esser winps unite with presphcnoid. i Basisphenoid and presphenoid unite
' Mockel'H cartilHffo h<»cin« to rctrogrn<lo. I (7th or blh month).
' Ossific nuclei of os calcis and astragalus.
 
 
 
416
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
Tabulated Ciironoloot of Development (Conduded),
 
 
 
 
 
STAGE OF THE FETUS.
Eighth Month. Ninth Month.
 
 
General
Charftcters.
 
 
Weight, 4 to 5 pounds ; length,
 
16 Inches.
Body more plump.
 
 
Weight, 6 to 7 pounds ; length,
 
20inches.
Umbilicus almost exactly in
 
middle of body.
 
 
Vascular
System.
 
 
 
 
9
 
 
Digestive
System.
 
 
Ascending colon longer.
Caicum below crest of Ilium.
 
 
Meconium dark greenish.
 
 
Respiratory
System.
 
 
 
 
 
 
Oenito-orinary
System.
 
 
Testes In inguinal canals.
 
 
Testes in scrotum.
T^bia m^Jora in contact.
 
 
SUn.
 
 
Vemix caseosa covers entire
 
body.
Skin briehter color.
Lunug(j begins to disappear.
Nails project bi'yond hnj^er
tii>a.
Increase of subcutaneous
 
fat.
 
 
Lanugo almost entirely absent.
 
Galaetopherous ducts of
milk-glands acquire lumina.
 
 
Nervous
System.
 
 
 
 
Spinal cord ends at last lumbar vertebra.
 
 
Special Sense
Organs.
 
 
 
 
Ossification of bony lamina
spiralis and of modiolus.
 
Neuro-epithelial layer of retina completed; macula
still absent.
 
Choroidal fissure closes.
 
 
Muscular
System.
 
 
 
 
 
 
Skeleton and
Limbs.
 
 
 
 
Ossification in lower epiphysis of femur, sometimes
also in ui>i>er ei>iphy8e8 of
tibia an(i humerus.
 
Tyinpanohyal begins to ossify.
 
Ossitlc nuclei for body and
great horn of hyoid bone.
1
 
 
 
INDEX.
 
 


Abdominal cavitv, development of,  
Abdominal cavitv, development of,  
Line 18,156: Line 68:


.skehjtal apparatus of, 372  
.skehjtal apparatus of, 372  
*>7


Ampull(e of semicircular canals, development of, 34rt  
Ampull(e of semicircular canals, development of, 34rt  
Line 18,187: Line 95:
pyramidal tmcts of medulla, development of, 2JK>  
pyramidal tmcts of medulla, development of, 2JK>  
Antitragus, formation <»f, 35*<  
Antitragus, formation <»f, 35*<  
Antrum of liighmore. di'velopment
Antrum of liighmore. development


of, 301  
of, 301  
Line 19,537: Line 1,445:


position of, 407  
position of, 407  
Limiting membrane, intier, formatioD
Limiting membrane, intier, formation
of, 3:ji  
of, 3:ji  
outer, formation of, 331  
outer, formation of, 331  
Line 20,675: Line 2,583:


l'oss;» <)f. 1504  
l'oss;» <)f. 1504  
Syujpatlietie nervous sy.stem, 324  
Syujpathetie nervous sy.stem, 324  
Syncytium. 93, 97  
Syncytium. 93, 97  
Synovial sacs>. dev<'lopment of, 126  
Synovial sacs>. dev<'lopment of, 126  
Line 21,410: Line 3,318:
London Lancet ( Typhoid volutti,)  
London Lancet ( Typhoid volutti,)  


" Wc weloonic; tlie translation into English of this cxct'llrnt practice of medicine. The  
" Wc weloonic; the translation into English of this cxct'llrnt practice of medicine. The  
first volumi* contains a \ast amount of useful information, and the forthcoming volumes are  
first volumi* contains a \ast amount of useful information, and the forthcoming volumes are  
awaited with interest. '  
awaited with interest. '  
Line 21,503: Line 3,411:
the aim of the authors to elucidate the practical aspects of the subject, and to  
the aim of the authors to elucidate the practical aspects of the subject, and to  
this end the text has been beautifully illustrated with clinical pictures, showing  
this end the text has been beautifully illustrated with clinical pictures, showing  
the condition before tlie use of the X-rays, ac various stages of their application,  
the condition before the use of the X-rays, ac various stages of their application,  
and the final thera|ieutic result obtained. Details are also given regarding the use  
and the final thera|ieutic result obtained. Details are also given regarding the use  
and management of the apparatus necessary for X-ray work, illuslratinE the  
and management of the apparatus necessary for X-ray work, illuslratinE the  

Latest revision as of 22:03, 29 October 2012

INDEX

Abdominal cavitv, development of,

215 Accessory sii])nirenul oi*};ans, 242

thyroid. 227 Acetabiilur fossa, 404 Achoria, 1)4 Achroiuatin, 27 Acid colls, formation of, 200 Acoustic gaii)j:lioii, ;J21 Acusticofacial jraiijilioii, 321 Adaniantoblasts. I'.VJ Adenoid tissue. develo|>ineut of, 129 Adipose tissue, formation of, 12G After-birth, 104 After-brain, 2S7 Age of fetus, estimation of, 122 Air-chambtT of hen's e;;i;, 29 Air-sjK's, development of. 2"2o Ala* of n<)se. <h'volopment of, 3<)2 Aiiir lamina. 290 Alecithal ova, 2()

Alimentary c^inal, develoi)ment of, Ls5 * diflcrentiatiou into separate region^. 11>7 histolojiical alteratitms in. 20.")

tract, alteration in position of parts, 202 in«.'rease in length of. 201 Alisphenoids. .'{91 Allantoic arteries. 90, 104

circulation. 90 formation of, 103

stalk. .N'>

veins, im, 104 Allantois, ^9, 190. 2.")

function of. JM)

resi)iratory function of. 200 Alveoli, ]>ulmonarv, development of,

225 Ameloblasts. 1.39 Amnion, HI, t<'2

false, si

of man, >^^ Amnion-fold, 80. J^l. R3 A mil iota. H3 Amniotic cavity, 54, 84. 5i5

lluid. 85. SO functi<ui of, J^O

suture, S3 Amphibians, blastula of. 51 Amt)hioxus. blastula of. 50

.skehjtal apparatus of, 372

Ampull(e of semicircular canals, development of, 34rt

seminal, 240 Anal canal. 257

membrane, 195

l»late, 195 Anamnia, Si Angioblast. 147 Animal ]Mjle. 27 Animalculi^ts. 18 Anhige, 175

median, of thyroid body. 228 Annular sinus, 179 Annulus ovalis, 157 Anomalous arrangements of aortic

aivh, lOs Anterior chamber of eye, 342

nares. development of. 1 10, 3«U»

pyramidal tmcts of medulla, development of, 2JK> Antitragus, formation <»f, 35*< Antrum of liighmore. development

of, 301 Anus, development (»f. 195

imi)erforate, 1!»7 Aorta, caudal, KJO

develo]iment of. 159

l)rimitive, 151. 1(J5 Aortic arch, anomalous arrangements t.f, HW

arciies. 105

septum. 159 Api>enda«j:es of skin. 27n Appendicul.-ir skeleton. 372

deveIo])ment of. 4<>2 Aque«lu«t of Sylvius, development of,

29(> Arch, hyoid. 115

mandibular. 115

maxillary. 115

of aorta. d«*velo]»ment of, 107 Arched eolle(>ting tubule of kidney,

240 An'heiitenm. 52 Ar;lie>. aortic. 105

branehial. 114

mandibular. 1.35

visceral, 112 Arehibla«<t. Of! Arcuate fissure. 300. .307 Area, embryonal, 5S

glamlular, 275

opaca, 59

417


418


INDEX.


Area pellucida, 59

vasculosa, 59, 88, 150 Areas, iiasail, 145, 359 Areola, development of, 276 Areolar tissue, develo))tueiit of, 125 Arrectores piloruiii, 2b9 Arteria centralis retinae, development

of, ;«5 Arterial system, fetal, 165 Arteries, allantoic, 90, 164

umbilical, 103, 165

vitelline, 151 Artery, carotid, common, development of, 166 external, development of, 166 internal, development of, 166

innominate, development of, 167

middle siicral, development of, 166

pulmonary, development of, 168

subclavian, left, development of, 168 right, development of, 167

superior vesical, 182 Arytcno-cpiglottidean folds, 226 Arytcn»>id cartilages, development of, 22()

ridges, 22(> Ascending colon, formation of, 203

me.socolon, formation of, 203

root of lifth nerve, 224

root of vagus, 290 Aster. 45

Atlas, formation of, 381 Atresia of pupil, 3:tt. Atrial crescent, 157 Atrioventricular canal, 156

valves, ]56 Atrophic tubules of Wolffian bmlv,

23() Attract ion -sphere, 45 Auditory apparatus, development of. 3*15

meatus, external, formation of. 357

nerve, formation of. 321

nucleus, lateral accessorv. 321

]>it. 3I(» Auricle, (levelopuKMit of, 35R Auricles, division into right and left,

157 Auricular ap])eudages. 159

canal. 15<)

<;cptuni. 157 Auriculovcntricular a]»ertures. 161

valves. \i]'2 Axial tiher of spern«arozo<m. 20, 22

skeleton. 372

development of. 37'> Axis. (leveio]>ment of. 3>0 Axis-cylinder process. 2>1

IUui>kkn's primitive disk. 377, 379 Hartholiji. glands uf. '2til Basil ganuHia. .".(K]. ,3Hl

lamina. 2!>o Hasi-ocripital bojie. .390 Rasisphcnoid, 394


Belly-stalk, 85 Bifid uterus, 253

Bile-capillaries, formation of, 209 Bile-ducts, formation of, 209 Bladder, development of, 255 Blastema, Wolffian, 236 Blastodermic vesicle, mammalian, 50 stage of, 49

two- layered stage of, 52 Blastopore, 52 Blastula stage, 49 Blood, development of, 126, 147 Blood-islands. 148 Blood-lacunee, 97 Blood-platelets, 150 Blood-vessels, 150 "Blue baby," 158 Bodie^i, polar. 3.3, 34 Body of vertebra, formation of, 377 Body-cavity, 63, 6(>, 214 Body-wall, development of muscles of, 367

formation of, 79 Bony cochlea, development of, 352

labyrinth, development of, 351

semicircular canals, 351 Bowman, capsule of, 238, 240 Brain, development of, 286 Brain-case, 384 Brain-membranes, development ot,

302 Brain-vesicles, 287

derivatives of, 316 Bninchial arches, 114

development of, 369 Bran chiome res, 78 Bridge of nose, development of, 362 Broad ligament of uterus, 255 Brunner. glands of, 206 Bud, embryonic, 54 Bulbus arteriosus. 156

vestibuli. 259 Burdax'h. tract of. myelination of, 414 Bur>;i. omental. 20l,'21S

pharyngeal. 136 Bni-sal sacs, development of, 126

('ADrcors niembrjm<»s, 195 C'tecum, develo]mient of, 202, 203 Calcaravis. 308 Calcarine fissure, 303, 308 Callosoniarginal fissure, 309 ("'anal, anal, 257

atrioventricular, 156

auricular. 15()

hyaloid. 339

medullary. 70

neural. 70. 279. 281

jieurenteric. 74. 281

of anus. 197

«.f His. 145. 227

of Xuck. 255

of Stilling, 339 Canaliculi. hurrimal, development of, 345


INDEX.


419


Canalis reuniens. 349 Capsule of Bowman, 238, 240

of kidney, 241 Cardinal veins, 164 anterior, 189 posterior, 169 Carotid artery, common, development of, 166 external, development of, 166 internal, development of, 166

body, 325 Carpus, development of bones of, 405 Cartilage, formation of, 126

Meckel's, 115, 398

Reicbert's. 115 Cartilage-cells, 126 Cartilaginous capsule of cochlea, 352

cranium, 386

ear-capsule, 351

ribs, 382

sheath of spinal cord, 378

stiige of skeleton, 373 of trunk skeleton, 377

vertebral bodies, origin of, 379 processes, origin of, 379 Caudal aorta, 166 Cavity, amniotic, 54, 84

cleavage-, 50

pleuroperitoneal, ^

segmentation-, 50 Cell-cords, 150 Cell-mass, inner, 50

intermediate, 77, 232

outer, 50 Cells, sexual, 31

mesenchynjal, 66 Cementum of tooth, 137

development of, 141 Central canal of cord, formation of, 286

lobe, formation of, 305 Centrolecithal ova, 27 Centrosome, 45 Cephalic flexure, 112,288

ganglia, development of, 320 Ceratohyal, 402

Cerebellum, development of, 292 Cerebral fissures, development of, 302

vesicles, 287, 288 Ce numinous glands. 273 Cervical fistula, 116

flexure. 112

rib. 380. 383 Chalazsp, 29 Chambers of eye. 342 Chin ridge. 13.'> Chorda dorsjilis. 73 formation of. 373 stage of. 373 Chordte tendiiu'jp. 162 Chordul ei)ithelium, 374

plate, 74

region of primitive skull, 387 Choriata, 94 CIiorioc:;pillaris, 340


Chorion, 92

frondosum, 93

leve, 93

primitive, 92

true, 92 Choroid, coloboma of, 341

development of, 340

fissure, 306, 307

plexus, 308

plexuses of fourth ventricle, 291 Choroidal fissure, 330, 341 ChromafiSne cells, 325 Chromatin, 27

Chromosomes, reduction of, 23 Cicatricula, 28 Ciliary body, development of, 341

ganglion, 320

muscle, development of, 341

processes, development of. 333, 341 Circulation, allantoic, 90, 163

placental, 147

portal, 177

vitelline, formation of, 147 Claustrum, 303 Clavicle, development of, 404 Cleavage, kinds of, 47

of ovum, 45

partial discoidal, 48 peripheral, 48

total equal, 47 unequal. 47 Cleavage-cavity, 50 Cleavage-nucleus. 43 Cleavage-planes, 46 Cleft palate, formation of, 137

stem urn, 383 cause of, 82

uvula, fonnation of, 137 Clefts, visceral, 112 Climacteric, 38 Clitoris, development of, 259 Cloaca, 190, 196, 256 Cloacal depression. 197, 256 Closing membrane, 113, 117, 106 Coccygeal body, 325

curve, 112

vertebra?, ossification of, 382 Cochlea, bony, development of, .352 Cochlear duct, formation of, 347

ganglion. 321

nerve, .'154 Coplenteron, 52 Ccplom, 63. (>6, 214" Collateral fissure, 30,3. .308 Collecting tubules of kidney, 237 Coloboma of choroid, 341

of iris, 343 Colon, ascending, formation of, 203

descending, formaticm of, 201, 203

transverse, formation of, 203 Coluinnre carnete, 154 Commissures of brain, development of, 303

of cord, white, 285 Conarium, 298


420


INDEX.


Conariuiii. modificatious of, 298 Coue-visuul cells, 331 Congcuital iitresia of pupil. 33d <lia])brHKniatic heruia, 177 fecal fistula, 207 hernia, 249 umhilic4il hernia, 205 Colli vasoulosi, formation of, 246 Connective tissues, development of,

124 Constructive stage of menstrual cycle,

39 (.'oi>iila of hyoid hone, 3S9 Conicoid hone, 403

process of scapula, 40,3 Cord, spinal, <levelopment of, 2sl

umhiiical, 102 (^)^ds of cells. 147 Corium, development of. 2()8 Cornea, development of. 340 Cornicular tuberch'S. 2i<) Corona radiata. 2."). 31 Coronarv lipvment. 210 of liver. 221 sinus of heart, 172 valve. Kil Corpora alhicantia. 296 hijreniina. 29.1

cavernosa, formation of, 262 (juadrijjemina. '^9.") Corpu«; callosum, formation of, 309, 311 hemorrhaKicum. 37 luteum of prej^nancv. 37, 38 false, :W

of menstruation, 3S true. .38 spongiosum, formation of, 262 striatum, 303 Corpus.;le of Hassal. 230 Corti. or;ran of, 319 Costal process of vertebra, formation

of. 37«). 3'-2 Ci»tyl»Mlous of placejita. !»9 Coveriiii; l)ones, .3*»r> Cowper. ulaiuls <»f. 2<J3 Cranial capsulf, IJ*^!

nerve-fibers. dev«'lopmcnt of, 320 Cranium, cartilairiuous. 3s(> membranous, .385 ossL'ous. 3*^9 Crescent, atrial. l'>7 <'ricoid cartilai^e. 22»» ( 'rista' Mcusticu'. 350 (rossrd pvramidal tract. mvelinati<ui

of, 115 ('rum cerebri, develoi>ment of, 295 Crusta |)etrosa, 1 11 Cryptorcbism. 219

CrystjiUine lens. cl('veloi)ment of. .336 Cuueitorm tubi'rcle>. 22<» Cu-.]iioiis, endocardial. 15«) Cutis-plat'*. 77. 26S. :ij;.-> Cuvi«'r. duct of. Ml. 17(K 176 Cv-^tic dtirt. develo])ment of, 209 cVt.»bIast. 54


Daughter-cells, 22 Daughter-wreaths, 45 Decidua nienstrualis, 39, 95

of pregnancy, 96

reliexa, 9rocesses, 141

ridge, 1,37

shelf. 137 Den tale, 400 Dentate tissure, 30.3, 307 Dentinal fibers, 141

tubules, 141 IX'ntine, 137 Dermal bones, 385

navel, 82 Descending colon, formation of, 203 Descent of testicles. 218 Destructive stage of menstrual cycle,

39 Deutoplasm of hen's egg, 28

of ovum. 26 Develo])inent during eighth mouth,

during eighth week, 119

during fifth month. 121

during fifth week, 118

during ninth month, 122

during secoiul moiAh, 118

during seventh unrnth, 121

during sixth month, 121

during third month, 120

•luring tiiird week, 117

length of time necessary for, 18

tabulated chnmology of, 409

theories of. 17 Diaphragm. develo]>ment of, 177 Diaphragmatic hernia, congenital, 177

ligament. 248 Dieiicepbalon. 287 Digestive svstem. development of,

1.S5. 411-416 Diiritation of limb-buds, 407 Dipbyodont. 137 Dire<t cerebellar tra<*t. nivelination

of. 411 I)isc«>idal cleavage, partial, 48 Discus proiigerus, 31. 251 Disk, germinative. 28 Distal convoluted tubule of kidney,

210 Divrrticubi of primarv renal pelvis,

237 l)ors;il curve, 112

mesrnter.v. 1{H»

nerve-roots of s])inal ganglia, 318

pjincreas. 211 Double monster, origin of, .58

uterus, 2.53


Duct of Cnvier, 164, ITO, ITS luuouephric. £U of UutuKr. 251

ur Miuicr. 24a. air, 253, se&

i>f Batbkc. 246 <if SaotoriDi, 812 of Wirauug. as proDcphrlc, 233

8i'KllieuUl. 233

tlivrtiKlossal. 145. 227

tlivruld, B2T

vitelline, HO, fC, 1S»

Wuiffiau, 234 Ductus Arunlii. 180

arteriosus, 16H

commuDia chuledochus, formation of, 20!)

vndol Till p till lie us, 347

veiuhiUs. lli:i, l-u Duodi'Dum. furtuution of, 217

Ear. eitemiLl, development of, 355. 358 iiiliTiiiil. Ji'velopnient of, 316 iiiiildl, ilcvelopmonC of. 35G

l;.riin-iiU\ cartiUginomi, 351

Ectudemi, 'fi

Egg. iihiinate origin of, 31 E^Wuiuns. 31. iHQ E«K-^'>v«lapeB, 85 Egg-plasm. 26 EKg-lubi», priniary, 31 EigbCh mouth, ilovelopnicat during. 123.416

pair cranial nervea, developmeat of, 323

week 'development during, llil, 412 Ellaculiitarjr iluct. furmntlan of, 347 Elaatii: tissue, formatian of. ]^ Elevontb paircrnnial ii.TTes. 324 EmbpddiiigofDX'iini. M Embryo, diScreiitiuIion of. ffi>

of elffht and B half weclu, 121

of lin«ctitlj riBT, Its

c.frtli weeks, I'lP

of hirtwnthdnr. lOS

of three weekx. 112

of tirenlv-eight days, 118

sejtnii-iiUlion nf body of, 78

slSKi-of. in. 107 Eiubryolijay de fined, 17 Enibryouiil area, .>8

Embryonic bud. I>4 cn-KCcnl. -")!)


Eutl-kuob i.f lipunUBtozooii, 21. 22 EriducardiaU'nslii..iis lat* Eudoutrdiiim. 1>4 Endochoudral Uiuus, 3»5 Eudolyniph. aoA Cndoskeloton, 3T^ KiLili'ih'liuiii. I'liniintluu of. 66, 126 Eiid-piPd- i,r swrmaf


] I ' . i'.ii.. ; \.:\:r of loammaliau

l>l:if ttnlrtmjo vesicle, QO Ependyma. 310 Epcndymal celU, 392. £83


E|.ihyttl, 402

Epiotic center uf usdtlcation, 392 Epitbeliul lHHlie«. ^9 Epithelium, terminal. '29, 31, 244 Epitriebium, 2U0

Efn^'phoroiuVVl Erythruhlustii. 14^ Erythrocytes, 119 Elhniold bune. iwifiuition of, 305

cribriform plate of. 388 Ethmoidal sinuB. dDvelopmcnt of, 3^1 Euatnchian tube, development of, fJW fominli'iuof, 19-1

Vhtve


Exoccipitals. 300

Eioskclelon, 372

ExBtmphy iif lihiddor, cause of, fi3

Eitcrniil uaditory meatus, formation




of, 31.-),


jimiitiils, ^mu!.^ 250, 266

.uiiic. an. iW

Eyp. devoli.pment of. I'lH. 326 Evclashoi. duvclopment of, 344 Eyvlid, third. 344 Eyelids, development of. 343 primitive, 134

Fack, derclopmcnt of, 117, 130 Fai'ial KHKRllon, 321 Falciform ligament of liver, formation of. 210 lobe, 309. 313 Fnllopian tube«, development of, 253


1. SI I Fall cerebri, 303


422


INDEX.


Fecal fistula, congenital, 207 Female external genitals, 259, 266

internal genital organs, 249

pronucleus, 34

sexual system, 266 Fertilization, 41

artificial, 44

external, 42

internal, 42 Fetal arterial system, 165

membranes at birth, 104

vascular system, final stage of, 181

venous system, 169 Fetus, length of, at term, 122

stage of, 20, 118

weight of, at term, 122 Fiber-tracts of cord, development of, 285 myelination of. 414, 415 Fibrillee of muscle, formation of, 366 Fibrous tunic of eye, development of,

339 Fifth brain-vesicle, metamorphosis of, 289

month, development during, 121, 414

pair cranial nerves, development of, 323

ventricle. 312

week, development during, 411 Fimbria, 309

Fingers, development of, 407 First pair cmnial nerves, development of, 323

week, development during, 409 Fissure, arcuate, 306, 307

calcarine, 303, 30H

calloso-niarginal, 309

choroid. :W), 307

choroidal, 330

collateral, 303, 308

dentutc. 303, 307

great transverse. 304, 308

hippocampal. 307

of choroid ]>loxus, 3^)7

of Kohmdo. 30,S

of Sylvius, 303, 301

purieto-occii»ital. 308 Fissures, cerebral, development of, ,^02. 303

median, of cord. 285 Fistula, congenital fecal, 207

M Ml hi Heal urinary, 25<j Flexure, cephalic. 112. 288

nuchal, 289

pontal, 2.^9 Floor-i)late, 2.'-l. 282 Fold, ])h'uropericardial, 176 Folds, medullary. 72 Folliflo. (Jraafian. 2J»

of tooth, 1 11 ForauHMi cR'cuni. 115, 227

eomnunu' anterius. 30()

of Monro. 301. 3<M)

of V/inslow, 221


Foramen ovale, 157

thyroideum, 226 Fore-brain, 286, 302

secondary, 287

vesicle, 73

metamorphosis of, 302 Foregut, 81 Formative yolk, 26 . Fornix, formation of, 309, 310 Fossa of Svlvius, 304

oral, 192

ovalis, 158 Fourth month, development during, 120, 414

pair cranial nerves, development of, 323

ventricle, 291 development of, 290, 294

w^eek, development during, 410 Fretum Halleri. 156 Frontal bone, ossification of, 396

lobe, 306

sinuses, development of, 361 Funiculus solitarius, 290 Furcula, 225

Gall-bladder, development of, 209 Ganglia, cephalic, 320

spinal, 317 Gangliated cord of the sympathetic,

325 Ganglion, acoustic. 321 acusticofacial, .'^21 cephalic, fourth, 321

third, 321 ciliary. 320 cochlear. 321 facial. 321 Gasserian, 320 intercarotid, 325 Luschka's. 325 ophthalmic, 320 s]>irale, 350 trigeminal, 321 vestibular, 351 Ganglion -cell laver, development of,

333 Gartner, duct of, 254 Gasserian ganglion, 320 Gastrjil mesoderm, 63 Gastrohepatie omentum, 209, 220

formation of, 205 Gastrosplenic omentum, 214 (rastrula, 52

stage, 52 Generative organs, external, development of. 258 internal, development of, 243 Genital cord. 243 eminence. 259 in male, 261 folds. 259

in female. 259 in male. 262 gland, indifierent, 265


INDEX.


423


Genital groove, 258

ridge, 243, 258 in female, 259

ridges, 31 Genito-urinary system, development

of, 232, 409-416 Germ-cells, 224 Grcrm-disk, 27 Germ-layers, 52

derivatives of, 67 Germinal epithelium, 29, 31, 244

sj)ot, 25, 27

vesicle, 25, 27 Grerraiuative disk, 28 Giral<les, organ of. 247 Glands of alimentary tract, formation of, 206

of Bartholin, 261

of Brunner, development of, 206

of Cowper, development of, 263

of intestine, development of, 206

of Lieberkiibn, development of, 206

of Moll, 273

of stomach, development of, 206 Glandular area, 275

hy]K>spudias, 262 Glaus clitoridis. formation of. 259

penis, formation of, 259, 262 Glasorian fissure, 393, 399 Globular processes, 118, 132, 360 Glomerulus of kidney, 233, 238, 240 Glomus caroticus, 325 Goll, tract of, myelinatiou of, 414 Graafian follicle, 29 development of, 251 formation of new, 252 Gray matter of bniin, formation of, 303 of medulla, development of, 224 Great omentum, formation of, 204,

220 Groove, dental. 140

lacrimal, 119, 132

medullary, 71

naso-optic, 345

primitive, 60

pulmonary, 22.S

transverse, crescentic, 398 Gubernaculum testis, 248 Gum, development of, 136 Gut, postanal, 196 Gut-tract, 80. HI. 186, 188 Gyrus fornicatus, 315

uncinatus, 315

Hair, development of, 271 Hair-bulb. 271

development of, 272 Hair-follicle, 271.

development of. 272, 273 Hair-germs. 272

Hard palate, development of, 397 Hare-lip, 134, 397 Hassal, corpuscles of, 230


Head, muscles of, development of, 367

of epididymis, 246

of spermatozoon, 20, 22 Head-fold, 80

of amnion, 80, 83 Head -gut, 188 Head-kidney, 232 Head-process of primitive streak. 62,

70 Head-segments, 364 Head-skeleton, development of, 384 Heart, development of, 152

lymph-, 128 posterior, 128

metamorphosis of single into double, 156

valves, development of, 161 Helix, formation of, 358 Hemal arch, formation of, 376 Henle's loop, 240 Hen's egg, description of, 27 Hensen's node, 62 Hepatic cylinders, 209

vein, development of, 181 Hermaphroditism, 263, 2()7 Hernia, congenital, 249

umbilical, 205 Highmore, antrum of, development

of, 361 Hilum folliculi, 31 Hind-brain. 28(J. 292

secondary, 287

vesicle, 73, 292 Hindgut. 81, 1H8 HipiKicanipal fissure, 307 Hippocampus migor, 307

minor. 308 His, canal of. 145, 227 Holoblastic ova, 47 Homogeneous twins, origin of, 59 Homologies of the sexual system, 263 Hyaloid arterj*, formation of, 339

canal, 339

membrane, formation of, 339 Hydatid of Morgagni, 247

8i»s8ile, 247

stalked, 247

unstalked, 247 Hydramnios, 86 Hymen, formation of, 261 Hyoglossus. origin of. 370 Hyoid arch, anterior, 389 l)osterior, 3'^9

arches. 115

bar. 3H9

bono, development of. 389, 401 Hyoidean apparatus, 401 Hyonmndibular cleft, 115 Hypobhist. 5'^ Hypochordal brace, 376 Hypophysis, ,'^(X>

formation of. I'i5 Hypospailias, 262

glandular, 262


424


lyDEX.


Iliac segment of pelvic girdle, 404 vein, left cuiumun, development of,

I m perforata anus, 197 luipresisious, maternal, 120 Ini'us, development of, 388, 399 Indifferent genital gland, 265

si'xual gland, 244 Inferior medullary velum, 292

peduncles of brain, 290 Infundibula of lungs, development of,

225 Infundibulum of brain. 2i>6, 300 Inguinal ligament, 248

in female, 254 Inner cell-mass, 50 Innominate artery, development of,

liu Inter-brain, 287, 296

vesicle, metamorphosis of, 296 Intercarotid ganglion, 325 Intermaxillary bones, formation of,

l.iei, 397 Intermedial cell-mass, 77, 232, 365 Internal ear, development of, 346

fertilization, 42

lateral ligament of lower jaw, 400

limiting membrane of spinal cord, 3v'^3 Interpallial fissure, 302 Interrenal organ, 242 Intervertebral disks, 377, 379

ligament, development of, 377, 379 Intervillous spaces. 97, 102 Inte.stinal canal, formation of, 79

glands, development of, 20*>

mesentery, 216

mucosa, formation of. 189

villi, formation of, 206

]>ortals, 81. 186 Intestine, small, development of, 202,

205 Intestino-bodv cavitv, 52 Intumescentia ganglioformis. 351 Involuntarv muscle, development of,

371 Iris, coloboma of. 343

development of, 341 Ischiatic rod. 404 Island of Keil, :J05

.Tacobson's organ, development of.

3()1 Jaw. upper, development of, 134 Jaw-arch. 115 .lellv of Wharton, 103 Joint-cavities, development of, 128 Jugular vein, primitive. 1({4, 169 transverse. 172

Kidney, development of, 232

Labia majora, 260

minora, formation of, 259 Labyrinth, bony, development of, 351


Labyrinth, membranous, development of, 346 Lacrimal bones, ossification of, 396

canal iculi, 345

caruncle, 344

duct, development of, 344

gland, development of, 344

groove, 119, l.'i2

sac, development of, 345 Lamina cinerea, 296, 299

quadrigemina. 295

spinilis, bony, development of, 354

terminal is, 309 Langhans' laver, 97 , lanugo, 121, 273 I I^iryux, development of, 225 I Uitebra, 29 Lateral cartilage of nose, 395

folds of amnion, 80

frontal processes, 118, 132, 134 in formation of nose, 146

ligaments of liver, 210

nasal process, 344, 360

plate of mesoderm, 65

plate of somite, 63

ventricle, development of, 303 length of fetus at term, 122 Lens, crystalline, development of,

336 Lens-area, 328

Lens-capsule, development of, 337 Lens-pit, 336

Lens- vesicle, 110, 134, 328, 336 Lenticular zone of optic cup, 333 Lesser omentum. 220 formation of, 205 Leukocytes, 149 Levator palati, origin of, 370 Lids, union of edges of, 343 LieberkiJhn, glands of, 206 Ligament of ovary, 255 Ligamenta intermuscularia, 365, 375

subflava, 379 Ligaments of liver, formation of, 209 Ligamcntum venosum Arantii, 184 Liguhr. 2\yZ Limb-buds, 119. 406 Limbic lobe, 309, 313 Limb-muscles, development of. 370 Limbs, bones of. development of, 405

development of, 406, 409-416

position of, 407 Limiting membrane, intier, formation of, 3:ji outer, formation of, 331 Lin in, 27 Lip ridge. 135

upper, development of, 136 Liquor amnii, 85. 86 function of. 86

n.lliculi, 31. 251

of Morgagni. 337 Liver, development of, 207

first rudiment of, 198

ligaments of, formation of, 209


INDEX.


425


Liver-ridge, 175, 208 Iiobes of liver, 208 Lobule of ear, development of, 358 Longitudiual liber-tracts of medulla, 2«0

fissure of brain, 302 Loop of Henle, 240 Lower jaw, ossification of, 398 Lumbar rib, .'i83

vertebne, ossification of, 381 Lungs, development of, 223 Luschka's ganglion, 325 Lymph, formation of, 126 Lymph-clefts, development of, 128 Lymph-hearts, 128

posterior, 128 Lymph-sacs, development of, 127 Lymph-spaces, development of, 127 Lymphatic system, development of, 127

vessels, development of, 128 Lymphoid follicles of tonsil, 195

tissue, development of, 1'^

Macula lutea, formation of, 333 Maculse acustictc. development of, 350 Malar bone, ossification of, 396 Male external genitals, 261, 267

internal genital organs, 245

pronucleus, 42

sexual system, 245, 266 Malleus, development of, 388, 399 Malpighian corpuscle, development of, 213, 238 primitive, 236 Mammalia deciduata, 99

indeciduata, 99 Mammals, blastula of, 49 Mammary gland, development of, 274 Mandible, ossification of, 398 Mandibular arch, 115, 135, 386 Mantle layer, 284 Marginal sinus, 102

velum of spinal cord, 283, 284

zone of optic cup, 334 Marshall, vestigial fold of, 172 Maternal impressions, 120 Maturation of ovum. 32 Maxilla, superior, ossification of, 397 Maxillary arch, 115

process, 135, 386 Meatus, external auditory, 357

urinarius, male, 262 Meckel's cartilage, 115, 388, 398

diverticulum, formation of, 207 Meconium, 122 Median fissures of cord, 285

lobe of cerebellum, 292 Medulla oblongata, development of,

289 Medullarv canal, 70

cords, 246. 252

folds, 72. 279

farrow, 71

groove, 71


Medullary plate, 70, 279 tube, 279

velum, anterior, 294 inferior, 292, 294 Meibomian glands, development of,

344 Membrana adamantiua, 139 basilaris of cochlea, formation of,

eboris, 141

granulosa, 31 formation of, 251

prseformativa, 141 Membrane, anal, 193

closing, 113, 117, 194

nuclear, 27

of Xasmyth, 140

of Reissner, 355

pharyngeal, 117, 131, 188, 192

vitelline, 25, 26

tympanic, 194, 357 Membranes, caducous, 95

deciduous, 95 Membranous bones, 385

cranium, 385

labyrinth, development of, 346

ribs, 382

stage of skeleton, 373 of trunk, 374 Menopause, 38 Menstrual cycle, 39 Menstruation, 38

relation of, to ovulation and conception, 40 Meroblastif ova, 48 Mesencephalon, 286, 294 Mesenchymal cells, 66

muscle, 371 Mesenchyme, 66 Mesenteric artery, superior, 152

vein, superior, 181 Mesenteries, 190 Mesentery, intestinal, 216

ventnil, 204 development of, 220 Mesoblast, 62

Mesoblastic somites, 65, 75 Mesooardium anterius, 153, 174

posterius, 153, 174 Mesocolon, ascending, production of, 203

formation of, 203 Mesonephrogenic tissue, 239 Metanephrogenic tissue, 239 Mesoderm, 62

derivatives of, 68

gastral, 63

paraxial, 65

peristomal. 63

somatic, 66

splanchnic, 66

structures developed from, 12Aet8eq. Mesodermal vitreous, 338 Mesogastrium, 204, 216 Mesonephric duct, 235


426


JSDEX.


Mesonephros, 234, 264 Mesorchiuu, 24b, 255 Mcsothelium, (Hi, 126 McHovarium, 24>::^

Metacar|>al l>oneg, development, 405 MetamorphoHiH of single into double

heart, l.VJ MctanephroH, 237, 265 Metatarsal boneif, development, 405 Meteiiceplialon, 287, 292 Me topic suture, 396 Metopism, 396 Micropyle. 25, 42 Mid-brain, 2»6, 294

prominence of, 288

vesicle, 73. 294 Mid-gut, las Middle ear, development of, 194, 355

piece of Kpermatozoon, 20, 22

plate. 77, 232, 3<I5

sacral artery, development of, 166

tunic of eye, development of, 339 Milk-lines. 275 Mi Ik -ridges, 275

Mo<liolus of cochlea, development, 354 Moll, glands of. 273 Monorchism, 249 Monro, foramen of, .'$01, 306 Mons v<?neris, formation of, 259 Morgagni, hvdatid of, 247

liquor of, 337 Morula, 45 Mother-cells, 22

Motor nerve-fibers, development, 319 Mouth, development of, 134, 192 Mucous tissue, formation of, 125 Mulberry -mass, 45 Miiller, duct of, 243, 247, 253, 265 Miillcr's lib«;rs, :W1 Muscle, involuntary, development, 371

Vdluntary, development of, 363 Muscle-plate, 78, .365

metamorphosis of, .366 Muscles, bnmchial, development of, .3()9

of extremities, development of, 370

of trunk, development of, .363 Muscular coat of inti'stines, formation of, 205

system, development of, 363, 409416 Musculi papillares. 163

pectinati. 154 Myeleiucphalon. 287. 289 Myelin. <leposit of. 319 Mv<K-(el, .365 Myotonu", 77, 365

N'ml-rki). 271 Nail-phite. 270 Nails, development of, 270

of toes, 271 Nail-welt. 271 Nares, anterior, formation of, 146

develo])ment of, 3<)0


Nasal areas, 145, 359 bones, ossification of, 396 capsule, 388

cavities, development of, 361 pits, 118, 132, 145,360 process, 132. 360 lateral, 344, 360 Nasmych, membrane of, 140 Nasofrontal process, 115, 118, 132, 134, 360, 386 in development of nose, 145 Naso-optic furrow, 132, 134 in formation of nose, 146 groove, 345 Nephridial funnels, 233 Nephrogenic tissue, 236 Nephrostomata, 2«33 Nephrotome. 77, 234, 264, 365 Nerve-cells, formation of, 2c»2

of cord, formation of, 284 Nerve-corpuscles of neurilemma, 319 Nerve-fiber, envelopes of, formation of, 319 layer, development of, 333 Nerve-fil>ers, cranial, development of, 320 motor, development of, 319 sensory, development of, 317 Nerve-trunk, spinal, development of,

319 Nervous system, development of, 278, 409-416 peripheral, development of, 316 sympathetic, development of, 324 Neural canal, 70, 279, 281 crest, si'gmentation of, 318 crests, 318 process of vertebra, formation of,

376 tube, 279 Neurenteric canal, 74, 281 Neurilemma, formation of, 319 Neurit, 278, 284 Neuroblasts, 282, 284 Neuro-epithelium of retina, development of. .3.33 Neuroglia. 2S2.283

layer, 284 Neurons. 278

Nictitating membrane. 344 Ninth month, development during, 122. 416 pair cranial nerves, development of,

.324 week, development during, 413 Nipple, development of, 276 Node. Hensen's. (>2 Normoblasts. 149 Nose, development of. 145, 358 Nott>chord. 73

Notochordal stage of skeleton, 373 Nuchal flexure, 2S9 Nuck. canal of. 255 Nuclear.juice. 27 layer of retina, outer, 332


INDEX.


427


Nuclear membrane, 27

ttpiiidle, 45 Nucleus amygdalae, 303

cleavage-, 43

of uvuiii, 27

scgmeutation-, 43 Nutritive yolk, 26 Nymphffi, formation of, 259

Obex, 292

Occipital bone, ossification of, 390

lobe. 30<) Odontoblasts, 141 Odontoid process, development of,

3m Olfactory bulb, 314

epithelium, 359, 362

lobe, 314

nerve-fibers, 362

plates, 132, 145, 358

tract, 314 Omental bursa, 204, 218 Omentum, gastrohepatic, 209, 220 formation of, 205

gastrospleuic. 214

great, formation of, 204, 220

lesser, 220 formation of, 205

phrenicosplenic, 214 Omphalomesenteric veins, 151 Ontogeny, 17 Oocytes, 32 Oogenesis, 29 Oogouia. 32

Ophthalmic ganglion, 320 Opisthotic center of ossification, 392 Optic cup, 328

secondary, 330

lobes, formation of, 295

nerve, development of, 335

thalami, 29fj

vesicle, 287. 327 Ora serrata. 'Xil Oral cavity, development of, 192

f ( JSSft 1 ' f"^

pit. lOH. 117, 131. 135, 192

plate, l.SO, 134, 192 Orbitonasal center, 397 Orbitosphenoids, 394 Organ of Corti. 349

of Giraldts, 247

of .Tacobson, development of, 361

of Kosenniuller, 254 Ors^ans of Ziirkerkandl. 325 OsstMMis cnmiuni, 3H9

stag«» of trunk skeleton. 379

tissue, formation of, 126 r)ssirlcs of ear. d(*velopment of, 356 Ossification of ribs, 383

of skull. :}H9

of st<Tnuni. '.V<\

of vcrtcbne.'JrsO Ostium int(?rv«'ntrieulare, 158 Otir v<'sirlc. 109, :i46 Otocyst, 34()


Outer cell-mass, 50 Ova, alecithal, 26

centrolecithal, 27

classification of, 26

formation of, 29

holoblastic, 47

meroblastic, 48

primitive, 31, 245, 250

telolecithal, 26 Ovaries, change of position of, 254 Ovary, development of, 249 Oviducts, development of, 253 Ovists, 18 Ovulation, 36

relation of, to menstruation, 40 Ovum, 24, 251

embedding of, 95, 96

maturation of, 32

rii)ening of, 32

segmentation of, 45

stage of. 19, 106

Palate bone, ossification of, 396

formation of. 136

process, development of, 397 Palate-shelves, 3<)0 Palatoglossus, origin of, 370 Palatopharyngeus. origiu of, 370 Palpebral fasciae, 344

fissure, 343 Pancreas, development of, 211

dorsal, 211

first rudiment of, 199

ventral, 211 Pancreatic duct, development of, 211 Pander's nucleus, 28 Panniculus adiposus, 269 Papilla? of tongue, formation of, 145 Parablast, (Hi

Paraehoi-dal cartilages. 387 Paratlidymis, 247 Parathyroid bcnlies. 229 Paraxial nies<Mlerm. 65 Parietal bones, ossification of, 396

elevation, 288

eye, 299

fomnien, 2JW

layer of pleura. 177

lobe, 3(KJ

zone. 76 Parieto-oocipkal fissure, 308 Pan»oj)horon. 254 Parovariunj. 254 Pars ciliaris retime. 334

i n te rm ed i a 1 i s, 259

iridiea retina?. 334

menibranacea septi, 159

optica retinje. 333 Parthenogenetic <'ggs. 34 Patuh)us foramen ovale, 157 Pectoral girdle, development of, 403 Pelvic girdle. 404 Pelvis of kidney, primary, 237 Penis, development of, 259 Perforated lamina, anterior, 315


428


ISDEX.


Perforated space, posterior, 295 Pericar<lial aivity, 175 Pericardium, development of, 174 Perilymph, 352, 355 Perilymphatic space, 352 Perineal bwly, 197 Perineum, formation of, 197 Perionyx, 271 Periotic bone, 392 Perijiheral cleavage, 48

nervous system, 316 Peristomal mesoderm, 63 Peritoneal cavity, 215 Peritoneum, development of, 214

visceral layer of, 189 Perivitelline space, 25 Permanent kidney, 237

teeth, development of, 141 eruption of, 143 Petromastoid bone, 392 Petrotympanic fissure, 399 Pfliiger's egg-tubes, 250 Phteochrome bodies, 242

cells, 242 Phalanges, development of, 405 Pharyngeal bursa, 136

constrictors, origin of, 370

membrane, 117, 131, 188, 192 in formation of mouth, 135

pouches, 113, 188, 193 Pharynx, 193

Phrenicosplenic omentum, 214 Phylogeny, 17 Pial processes, 283 Piiimcnt-lavcr of retina, 331 Pillars of Uskow, 177 Pineal body. 296

or gland, 2f)7, 298

eye, 299 Pit, auditory, 316

oral, 1 OS 117 Pits, nasi I, 360 Pituitary body, 300

formation of, 135 Placenta, 98

at term, 101

discoidoa, 99

pnevia. 102

zonaria, 99 Placental sinuses, 100

spaces. 102

system of blood-vessels, 164 PlaVentoblast. 51 Plaues of cleavage, 46 Plantar born, 270 Plasiuodobljist, 54 Plate, chordal. 71

medullary. 70

vertebral, 65 Pleura, parietal layer of, 177

visceral layer of, 177 Pleune. develoi)ment of, 174, 175. 226 Pleural sjics, formation of, 174, 175 P]euroi)eri('ardial fold, 176 Pleuroperitoneal cavity, 66, 215


Plica semilunaris, 344 Pocket of Rathke, 301 Polar bodies, 33, 34

striation, 45 Polarity of egg, 27 Pole-corpuscles, 3ii Polyphyo<lont, 137 Polyspermia, 42 Pontal flexure, 289 Pons, formation of, 292 Portal circulation, 170, 177

vein, development of, 181

venous system, 170, 177 Postanal gut, 196 Postbranchial bodies, 229 Posterior chamber of eye, 342, 343

nare«, development of, 360 Post-limbic sulcus, 309 Postsphenoid, 393 Preformation theory, 18 Prehepaticus, 175, 208 Prehyoid gland, 227 Premaxilla, 397 Prepuce, formation of, 262 Prespbenoid, 394

Primary collecting tubules of kidney^ 239

egg- tubes. 31

renal pelvis, 237 Primitive aorta, 151, 165

chorion, 92

disk, 377, 399

enamel -germ, 138

eyelids, 134

groove, 60

heart- valves. 161

jugular veins, 161, 169

Malpighian corpuscle, 236

nails, 270

ova. 31, 2-15, 250

segment i)late, 65

segments. 65. 75

sexual e«dls, 245

streak. 59

vertebral bow, 376

vitreous. 33S Primordial bones, ,*^5 Proamnion. i\\ Process, latenil frontal, 118, 132, 134

nasal. 132, 3()0

nasofrontal. 115, 118, 132, 134, 360 in formation (jf nose, 145 Processes, dental, 141

globular. 118, 132, 360 nasal, 360

maxillary, 135

of vertebra, development of, 376 Processus vnginalis, 249 Proeborion, 50, 92 Proctodeum. 196 Pronei)bric duet, 233 Pronephros, 232. 264 Pronucleus, female, 34

male, 42 Pro-otic center of ossification, 392^


INDEX.


429


Prosoncephalon, 28(5 Prostate kI&"^* formation of, 257 Prostatic urethra, formatiou of, 257 Protoplasmic processes, 264 Prottivertebru, 6.3

Proximal convoluted tubule of kidney, 240 Pterygoid plate, internal, development of, 394 Pubic rod, 404

Pulmonary alveoli, development, 225 artery, development of, 159, \G6 diverticulum, 223 grtK>ve, 223 Pulp of spleen, development of, 213

of teeth, 137 Pupil, :VM) congenital atresia of, 333 development of. ;W2 Pvnimiilul process of thyroid gland, 22H tracts, anterior develojmient of, 290 crossed, of cord, mvelination of, 415

Ramus communicans. 325 Riithke's piK-ket, 13<>, 193, 301 Kiiuber's layer, 50 Ki'ceptaciilu chyli. 12S Keceptive i)rominence, 42 Kecessus labyrinthi, kWI

vestibuli. .'M7 Kectuui, 197

Ri'current laryngeal nerves, 1(38 Ke<luctiou of chromosomes, 23 Reduction-division, 23 Reichert's cartilage, 115, 389, 401 Reil, island of, 305 Reissner, membrane of, 355 Renal vein, left, 173

vesicles, 210 RepHMliiction, theories of, 17 Re>pinitorv svstem. development of,

222,'40i>-41() Restiform bodies, develoimieut of, 290 Rete mucosum. 270

testis, formation of. 21() Retina, development of, \^Z6 Rhinencephalon, 314 Rhombencephalon, 2H() Rhomboidal fossji, 2i)l i:ib, cervical, :W0. 3^3

lumbar, 38.3

thirteenth, 383 Rib-j, development of, 382 Ri«lne, genital, 213

terminal, 58 Ring lobf, formation of, 304 Ri|M'Mlug of ovum. 32 Roil-and-cone laver, formation of, 332 Rod-visual cells,\'{;n R(»lando, fissure of. 30^ R(M)f-plate, 2'^1, 2>^2 Rotation of stomsu'h, 203, 217 Round ligament of liver, Irtl


Round ligament of liver, formation of, 210 of uterus, 248, 255

Saccule, development of, 349 Saccus endoh'mphaticus, 347 Sacral vertebrse, ossitication of, 381 Sacrum, formation of, 381 Salivary glands, development of, 143 Santorini, duct of. 212 Sauropsida, blastula of. 51 Scala media of cochlea, development of, 347

tympani, development of, 355

vestibuli. development of, 355 Scapula, development of, 403 Schwann, white substance of, 319

deposit of, upon libers of tract of conl, 411, 415 Si'lerotome, 77, 365, 375 Scrotum, development <tf, 263 Selniceous glands, development of, 274 Second month, development in. lis

IMiir cranial nerves, development of, 323

week, development during, 40i) Secondary hair, 273

optic cup, ;J30 Secreting tubules of kidney, 237, 239 Segmental duct, 233 Segmentation of body of embryo. 78

of ovum, 45 S(;gmen tat ion -cavity, 50 Segmentation-nucleus, 43 Semicircular canals, bony, 351

<levelopment of. ,'i4.S SiMuilunar valves, development of. KC Sc>minal ampulhe, 21<)

vesicle, format i(m of. 247 Seminiferous tubules, formation, 246 Sense (ngans, development of, 32t),

409-416 Sensory epithelium of retina, 331

nerve-libers, development of, .317. 31s Septa placenta*. 102 Si'ptal cartilage of nose. 395 Septum, aortic, 159

auricular. 157

intermedium. 1.56

luciduni. fornuition of. 312

primum. 157

secundum, 157

spurlum, 1.59

transvcrsum. 164, 175 SeroMi. si

Si'rous membranes, development, 126 Sertoli's columns. 21. 246 Sessile hydatl«l. 217 Seventh month, development during. 121, 415

pair cranial nerves, development of, 32,3

week, development during, 412 Si^xual cells. 31


430


INDEX.


Sexual cells, primitive, 245

cords, 31, 245 female, 250

gland, inditl'erent, 244

system, female. 24f», 2t>6 homologies of, 2t).'i inditferent type, 243 male, 245, 2m SJiell of lien's egg. 29 She 11 -membrane, 21) Shoulder girdle, development of, 403 Sinus, annular, 179

pcK'ularis, 247, 257

pneeervic^ lis, 110

reuuiens, 159

terminal is, 150

urogenital, UK), 256

venosus, 159, 169 Sixth month, development during, 121, 415

pair cranial nerves, develo])mentof, 32.3

week, development during, 119. 411 Skelelogenous sheath of chorda dorsal is, 375

tissues, 77 Skeleton, appendicular, 373

development of, 402, 409-416

axial. 373

development of, .372

of head, development of. 384

of trunk, cartilaginous stage, 377 chonlal stage of. 373 development <»f. 373 membranous stage of, 374

visceral. 3>4 Skin, ajipeiulujjes of, 270

development of. 2(»S. 409-116 Small intestine. develo]unent of, 205 Snu'gnia enibryonum. 270 Somatic mesmlcrni. 6(5 Somatopleure, 6»l, 1>6 Sornitrs. 6.3, 75

mesoblastic, 65. 75 Space, peri vitelline. 25 Spaces, intervillous. 97. 102 Spermatie cord, 249

veins. 173 Spermatids. 22 Spermatoblasts. 2*? Sperniatogenesi>. 'Jl Spermatoyenic eell>. 21 SpiTUiatocytes. primary, 22

S4'0niid:irv. 22 SjM'rmato<;(»Tiia. 22 Spernialozoon. tjo

power of loeoiMotion of. 21

vitJility of. -Jl Splu'uoid b«t!ie. os>iri(at ion <»f. 39."» S]>heTioi«lal sinus, development of. ."',61 Spinal eonl. <lev<]«ipmeiil of. •^•'1 Spinous p^oee«^> of vertebra, develop nn'Tit of. .'Jsn Splaiiebuie nn>.od<'rni, 66 Splanebnopleure. i\i\, INJ


Spleen, development of, 212 Spongioblasts, 282, 283 Spot, germinal, 27 Sprouts, vessel, 150 Squamozygomatic bone, 391 Stage of embryo, 19, 107

of fetus, 20, 118

of ovum, 19, 106

of quiescence of menstrual cycle, 40

of rei>air of menstrual cycle. 40 Stalked hydatid, 247 Stapes, development of, 389 Stem-zone, 75 Sternum, cleft, 383

development of, 383 Stigma. 31

Stilling, canal of, 3:i9 Stomach, development of, 203

first rudiment of, 198

glands of, development of, 206

rotation of, 203, 217 StomodsBum, 131, 192 Straight collecting tubules of kidney,

239 Stratum Malpighii, 270 Streak, j)rimitive, 59 Striated muscles, development of, 303 Stroma-laver of choroid, development

of, 340 Styloglossus, origin of, 370 Stylohyal, 402

cartilage, 393 Stylohyoid ligament, 389 Styloid process of hyoid. 389

temponil, development of, 393 Stylopharyngeus, origin of, 370 Subclavian arterv, left, development of, 1(38 right, development of, 167 Submucosaof intestines, formation of,

205 Substance-islands, 147 Subzonal layer of mammalian blasto <lerniic vesicle, 50 Sulcus interventricularis, 1.58

of corpus callosum. ;i07

terniinalis, 160 Suju'rior maxilla, ossification of, 397 Suprahyoid gland, 227 Supra-<M'(*ipital bone, 390 Su]M"aperi('ardial bmlies, 228 Suprarenal bo<lies, development of,

241. 265 Suspensorv ligament of liver, fonna tioil of. 210 Sustentacular cells of seminiferous

tubule. 21 Suture, amniotie, KJ Sweat-glands, development of, 273 Sylvius. a<iueduet of, 2fW>

' ti-sure of. ;}n:;, :ioj

l'oss;» <)f. 1504 Syujpathetie nervous sy.stem, 324 Syncytium. 93, 97 Synovial sacs>. dev<'lopment of, 126


/


INDEX.


431


Tail of spermatozoon, 20, 22

Tail-fold, «0

Tarsal ligameuts, 344

plates, 344 Tarsus, development of bones of, 405 Teeth, development of, 137

permanent, development of, 141 eruption of, 143

temporary, development of, 137 eruption of, 142 Tela choroidea, 297 Telencephalon. 287, 302 Telolecithal ova, 26 Temporal bone, ossification of, 390

lobe, formation of, 304 Temporary teeth, development of, 137

eruption of, 142 Temporomaxillary articulation, 400 Tendon, development of, 125 Tendon-sheaths, development of, 128 Tenth pair cranial nerves, development of, 324 Terminal filament of spermatozoon, 20,21

ridge, 58 Testicle, development of, 245

descent of, 248 Thalamencephalon, 287, 296 Thebesius, valve of, 161 Theca foUiculi, 29 Thecal sacs, development of, 126 Theory of evolution, 17

of unfolding, 17 Third eyelid, 344

month, development in, 120, 413

pair cranial nerves, development of, 32:i

ventricle, formation of, 296

week, development during, 410 Thirteenth rib, 383 Thoracic prolongations of abdominal

cavity, 175 Throat-pockets, 113, 188, 193 Thymus body, 194, 230 Thyroglossal duct, 145, 227 Thyroid body, accessory, 227 development of, 194, 226

cartilage, 226

duct. 227

foramen, 404 Thyroids, lateral. 226. 228 Tissue fungus, 97 Toes, development of, 407 Tongue, development of, 143, 194 Tonsil, development of, 194 Tonsillar pit, 195 Trabecula; cranii, ;J87 Trachea, develoiuuent of, 225 Tragus, formation of, 358 Transverse colon, formation of, 203

crescentic groove, 80

fissure of brain. 2})H

processes of vertebne, 380 Trigeminal ganglion, 320 Trophoblast, 92


True chorion, 92

Truncus arteriosus, 113, 151, 154, 165 Trunk, skeleton of, development of^ 373 osseous stage of, 379 Trunk-muscles, development of, 363 Trunk -segments, 364 Tuber cinereum, 296, 300 Tubercles, corn icular, 226

cuneiform, 226 Tuberculum impar, 144, 194 Tubotympanic sulcus, 356 Tunica albuginea of ovary, 250 of testicle, 246 fibrosa, 30 propria, 30 vaginalis testis, 249 vasculosa, 29 lentis, 31^7 Turbinal folds. 361 Turbinate bone, inferior, ossification

of, 395 Turbinated bones, development of,

361 Twelfth pair cranial nerves, development of, 324 Twins, origin of, 59 Tympanic cavity, formation of, 194 membrane, 194

development of. 357 portion of temporal bone, development of, 393 Tympanohyal, 402

cartilage, 393 Tympanum, development of, 356

Umbilical aperture, 87, 186

arteries, 103. 165

cord, 102

hernia, congenital. 206

urinary fistula, 256

vein, 103.165, 169

vesicle, 80, 87, 186 function of, 89 human, 89

vessels, 103 Uncinate gyrus, 315 Unstriated muscle, development, 371 Urachus. 91, 256 Ureter. 237

development of, 232 Urethra, female. 257

male, formation of. 262

prostatic, formation of. 257 Urinary fistula, umbilical, 256 Urogenital aperture, 257

sinus, 190,196,256 Uskow, pillars of, 177 ,

Uterus bicomis, 2.53

development of. 253

double. 25.3

masculinus, 247. 257 Utricle, development of, 349 Uveal tract, development of, 340 Uvula, formation of, 137


432


INDEX,


Vagina, development of, 253

median septum iu, :i^ri Valve, coronary, IGl

Eustachian, 160

ofThebesius, 161

of V'ieussens, 294 Valves, atrioventricular, 156

auriculoventricular, 162

of heart, development of, 161

semilunar, development of, 163 Van Beneden's embryonic bud, 54 Vas aberrans, 247

deferens, formation of, 246 Vasa eflerentia, 246

recta, formation of, 246 Vascular area, 88

system, development of, 147, 409-416 fetal, final stage of, 182

tunic of eye, development of, 339 Vegetative pole, 27 Vein, <'ardinal, 164

hepatic, 181

iliac, leftc(mimon. development, 172

portal, development of, 181

renal, left, 173

superior mesenteric, 181

umbili(tal, 103 Veins, allantoic, 90, 164

cardinal, 164 anterior, 169 pdsterior, 1(>9

omphalomesenteric, 151

primitive ju^iular, 169

spermatic. 173

umbilical, 165, 169

vitelline, 151, 169 Velum interposi turn, 296. 297 Vena azygos major, 173 minor. 174

cava, inferior, 171. 174 superior, 170 Veiue hepaticie advehentes, 179

reveheiites. 179 Venous segmeut of heart, 156

system of fetus, 1()9 ])ort:il, 170 V'^iitral nu'Sfiitery. 190. 204 developuient of, 220

pancreas, 211 Ventricles, separation of, 158 Vermiform appendix. dcvelopment,203

process of cerebellum. 292 Virnix caseosa. 87, 121, 270 Vertfbni'. ossification of, 3^0 Vertebral bow, primitive, 376

column, <levelopment of. 373-3S2 meuibranous primordial. 375

])lati'. 65

region of primitive skull, 3'^7 Vesicle, blastodcrmii'. stage of, 49 tWi)-layered stage of, 52

germinal. 25, 27

lens-, 110

otic, 1(K», 34(5

umbilical, 80, 87, 18<)


Vesicles, cerebral, 287, 288 Vessel sprouts, 150 Vestibular ganglion, 351

nerve. 321 Vestibule of ear, development of, 352

of vagina, 259

of vulva, 257 Vestigial fold of Marshall, 172 Vieussens, valve of, 294 Villi of chorion. 93

of intestine, formation of, 206 Visceral arch, first, function of, 115, 131

arches, 112

metamorphosis of, 115 moqdiological significance of, 113

clefts, 112

layer of peritoneum, 189 of i)leum, 177

skeleton. 384 Vis«'eral-arch vessels, 113, 151. 165 Vitelline arteries, 151

artery, right, 152

circulation, formation of, 147

duct, 80, 87, 186

membrane, 25, 26

veins, 151, 169 Vitellus, 25, 26 Vitreous body, development of, 338

mesodermal, 338

primitive, 3,'i8 Voluntary' muscles, development, 363 Vomer, ossification of, 396

Weight of fetus at different stages, 412-416 at term. 122 Wharton, jelly of, 103 White commissures of cord, 285 tibrous tissue, formation of, 125 matter of brain, formation of. 30.3

of ct»rd. development of, 285 of hen's egg. 29

substance of Schwann, development of, 31 J». .|(»9. 41 Winslow. fonimen ol. 221 Wirsung, duct of. 212 Witches' milk. 276 WoltUan blastema, 236 body, 2:M duct. 235

in female. 253 ridge. 232. 234 Wolff's doctrine of epigenesis, 18 Wreath, 45

VoLK of ovum. 25 Yt»lk-sac, M), v87, 186

ZiNX, zcmule of, 3:58 Zona pellucida, 25. 31

r.idiata, 31 Zone, parirtal. 7(>

stem-, 76 Zonule of Zinn. .338 Zuckerkandl. organs of, 325


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is conceded by scholars to be without question the best Practice


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of Medicine in existence. So necessary is this book in the study of Internal Medicine that it comes largely to this country in the I w^«w«w«^^w original German. In view of these facts, Messrs. W. B. Saunders

  • Company have arranged with the publishers of the German edition

to issue at once an authorized American edition of this great Practice of Medicine.

For the present a set of 12 volumes, selected with especial thought of the needs of the

practising physician, will be published. These volumes will con


rOR THE PRACTITIONER


tain the real essence of the entire work, and the purchaser will therefore obtain, at less than half the cost, the cream of the original, loiter the special and more strictly scientific volumes will be offered from time to time. The work will be translated by men possessing' thorough knowledge of both English and German, and each volume will be edited by a prominent .specialist. It will thus be brought thoroughly up to date, and tlic American edition will be more than a mere translation ; for,

in addition to tht; matter contained in the original, it will represent


PROMINENT SPECIALISTS


the very latest views of the leading American and English specialists in the various departments of Internal Medicine. Moreover, as each volume will be revised to the date of its publication by the eminent editor, the objection that has heretofore existed to treatises published in a number of volumes will be obviatetl, since the subscriber will receive the completed work while the earlier volumes are still fresh. The American publication of the entire work is under the editorial supervision of Dr. Al.KREi) Stenckl. who has selected the subjects for the American Edition, and has chosen the editors of the tHfferent volumes.

The usual method of publishers when issuing a publication of this kind has been to require {physicians to take the entire work. This seems to us in many cases to be undesirable, 'I"herefore, in purchasing this Practice physicians will be given the opj^ortunity of subscribing for it in entirety; but any single volunie or any number of volumes, each complete in itself, may be obtained by those who do not desire the complete series. This latter method offers to the purchaser many advantages which will be appreciated by those who do not care to subscribe for the entire work. Subscription.

SEE NEXT TWO PAGES FOR LIST


VOLUMES SOLD SEPARATELY


PRACTICE OF MEDICINE.


AMERICAN EDITION


NOTHNAGEL'S PRACTICE


VOLUMES NOW READY


Per volume x Cloth, f 5*00 net Half Morocco, ^.00 net


Typhoid and Typhus Fevers

By Dk. H. Curschmann, of Leipsic. The entire volume edited, with additions, by William Osler, M. D., F. R. C. P., Regius Professor of Medicine, Oxford University, Oxford, England. Octavo volume of 646 pages, fully illustrated.

Smallpox (including Vaccination), Varicellat Cholera Asiatica, Cholera Nostras, Erysipelas* Erysipeloid, Pertussis, and Hay Fever

By Dr. H. Immermaxn, of Basle ; Dr. Th. von JUrgensen, of Tubingen ; Dr. C. Liebermeister, of Tubingen ; Dr. H. Lenhartz, of Hamburg ; and Dr. (1. Sticker, of Giessen. The entire volume edited, with additions, by Sir J. W. Moore. M.D., F. R. C. P. I., Professor of Practice, Royal College of Surgeons, Ireland. Octavo, 682 pages, illustrated.

Diphtheria, Measles, Sccurlet Fever, and R5theln

By William P. Northrup, M. D., of New York, and Dr. Th. von Jurgensen. of Tubingen. The entire volume edited, with additions, by William P. Northrup, M. D., Professor of Pediatrics. University and Bellevue Hospital Medical College, New York. Octavo, 672 pages, illustrated, including 24 full -page plates, 3 in colors.

Diseases of the Bronchi, Diseases of the Pleura, and Inflammations of the Lun^s

By Dr. F. A. Hoffmann, of Leipsic ; Dr. O. Rosenhach, of Berlin ; and Dr. F. Ai'FRECiiT, of Magdeburg. The entire volume edited, with additions, by John H. Musser, M. D., Professor of Clinical Medicine, University of Pennsylvania. Octavo, 1029 pages, illustrated, including 7 full-page colored lithographic plates.

Diseases of the Pancreas, Suprarenals, and Liver

By Dr. L. Oser, of Vienna ; Dr. E. Nei'sser. of Vienna, and Drs. H. Quincke and Cf. Hoppe-Seyler, of Kiel. The entire volume edited, with additions, by Reginald H. Fritz. A. M., M. D., Hersey Professor of the Theory and Practice of Physic, Harvard University ; and Frederick A. Packard, M. D., I-ate Physician to the Pennsylvania and Children's Hospitals. Octavo of 918 pages, illustrated.


of the Stomach

By Dr. F. Riegel, of (liessen. Edited, with additions, by Charles G. Stockton. M. D., Professor of Medicine, University of Buffalo. Octavo of 835 pages, with 29 text-cuts and 6 full-page plates.


of the Intestines and Peritoneum

By Dr. Hermann NotHnagel. of Vienna. The entire volume edited, with additions, by H. D. Rollf^ston, M. D., F. R. C. P., Physician to St. George'u Hospital, London. Octavo of 1050 pages, finely illustrated.


.SAUNDERS" BOOKS ON


AMERICAN EDITION


NOTHNAGEL'S PRACTICE


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Per volume : Cloth, $5.00 net Half Morocco, ^.00 net


Tuberculosis and Acute General Miliary Tuberculosis

By Dr. G. Cornet, of Berlin. Edited, with additions, by Walter B. James, M. D., Professor of the Practice of Medicine, Columbia University, New York. Octavo of 806 pages.

Diseases of the Blood { Anemia, chlorosis. Leukemia, and Pseudoleukemia)

By Dr. P. Ehrlich, of Frankfort-on-the-Main ; Dr. A. Lazarus, of Charlottenburg ; Dr. K. von Noorden, of Frankfort-on-the-Main ; and Dr. Felix Pinkus, of Berlin. The entire volume edited, with additions, by Alfred Stengel, M. D., Professor of Clinical Medicine, University of Pennsylvania. Octavo of 714 pages, with text-cuts and 13 full-page plates, 5 in colors.

Malarial Diseases, Influenza, and Dengue

By Dr. J. Mannaberg, of V^ienna, and Dr. O. Leichtenstern, of Cologne. The entire volume edited, with additions, by Ronald Ross, F. R. C. S. (Eng.), F. R. S., Professor of Tropical Medicine, University of Liverpool ; J. W. W. Stephens, M. D., D. P. H., Walter Myers Lecturer on Tropical Medicine. University of Liverpool ; and Alhkkt S. Griinijaum. F. R. C. P., Professor of Experimental Medicine, University of Liverpool. Octavo of 769 pages, illustrated.

Diseases of Kidneys and Spleen, and Hemorrhagic Diatheses

By Dr. H. Senator, of Berlin, and Dr. M. Litten. of Berlin. The entire volume edited, with additions, by James B. Herkick. M. I)., Professor of the Practice of Medicine, Rush Medical Colle^i^e. Octi*vo of 815 pages, illust.

of the Heart


By Prof. Dr. L. von Sch rotter, of Vienna ; Prof. Dr. Th. von JurGENSKN, of Tubingen ; Prof. Dr. L. Krkhl. of Greifswald ; and Prof. Dr. H. Vihrordt, of Tubingen. The entire volume edited, with additions, by Geor(;k Dock, NL D.. Professor of Theory and Practice of Medicine and Clinical Medicine. University of Michigan, Ann Arbor. Octavo of about 1000 pages, with 72 text illustrations and 6 colored plates.


SOME PRESS OPINIONS


London Lancet ( Typhoid volutti,)

" Wc weloonic; the translation into English of this cxct'llrnt practice of medicine. The first volumi* contains a \ast amount of useful information, and the forthcoming volumes are awaited with interest. '

Journal American Medical Association ( Tuberculosis volume)

" We know of no single tr<'atise covering the subject so thoroughly in nil its Mspect> as this great German work. ... It is one of the most exhaustive, practical, and satJMfactorv works on the subject of tuberculo'^is."

Medical News, New York ( A/zr/- volume)

" Leaves nothing to be desired in the way of completen^ss of information, orderlv arrangement of the text, thoroughgoing up-to-dateness, handiness for reference, and exhaustive discussion of the subjects treated."

EACH VOLUME IS COMPLETE IN ITSELF AND IS «ni o SEPARATELY


THE PRACTICE OF MEDICINE


Anders' Practice of Medicine

Recently Issued— New (7th) Edition


A Text-Book of the Practice of Medicine. By James M. Anders, M. D., Ph. D., LL. D., Professor of the Practice of Medicine and of Clinical Medicine, Medico- Chirurgical College, Philadelphia. Handsome octavo, 1297 pages, fully illustrated. Cloth, II5.50 net; Sheep or Half Morocco, J6.50 net.

OVER 27,000 COPIES SOLD

The success of this work as a text-book and as a practical guide for physicians has been truly phenomenal, it now having reached its seventh edition. This success is no doubt due to the extensive consideration given to Diagnosis and Treatment, DifTerendal Diagnosis being dealt with under separate headings, and the points of distinction of simulating diseases presented in tabular form. Among the new subjects added are Rocky Mountain Spotted Fever, Splanchnoptosis, Cammidge' s Test for Glycerose, Myasthenia Gravis, Pseudotuberculosis, Benign Cirrhosis of the Stomach, Intestinal Lithiasia, Intestinal Calculi, Red Light in Variola, Emulsion-albuminuria, and Adams-Stokes' Syndrome. Important additions have also been made to diseases which prevail principally in tropical countries.


PERSONAL OPINIONS


James C. \^niK>ii, M. D.,

Professor of the Practice of Medicine and of Clinical Medicitu, Jejferson Medical ColUge Philadelphia.

" It is an excellent book^-concise, comprehensive, thorough, and up-to-date. It is a credit to you ; but, more than that, it is a credit to the profession of Philadelphia — to us."

Wm. E. Quine. M. D.

Professor of Medicine and Clinical Medicine, College of Physicians and Surgeons, Chicago. " I consider Anders' Practice one of the best single-volume works before the profession at this time, and one of the best text-books for medical students."

Bulletin of the Johns Hopkins Hospital

'• The success of this work js well deserved. . . . The sections on treatment are excellent and add greatly to the value of this work. Dr. Anders is to be congratulated on the continued success of his text-book."


Pusey and Caldwell on X-Rays

in Therapeutics and Diagnosis


The Practical Application of the Rontgen Rays in Therapeutics and Diagnosis. By William Allen Pusev, A. M., M. U., Professor of Dermatology in the University of Illinois; and Eugene W. Caldwell, B. S.. UifL-ctor of the Kdward N. Gibbs X-Ray Memoiial Laboj ratory of the Univtrsity and Believue Hospital Medical College. New York. Handsnine octavo of 625 pages, with 200 illustrations, nearly all clinical. Cloth, SS-OO net; Sheep or Half Morocco, s6.oo net

RECENTLY ISSUED NEW i2il) EDITION. REVISED AND ENLARGED TWO LARGE EDITIONS IN ONE YEAR

Two large editions of lliis work wuhin a year testify to its practical value to both the specialist and general practitioner. Throughout the work it has beeo the aim of the authors to elucidate the practical aspects of the subject, and to this end the text has been beautifully illustrated with clinical pictures, showing the condition before the use of the X-rays, ac various stages of their application, and the final thera|ieutic result obtained. Details are also given regarding the use and management of the apparatus necessary for X-ray work, illuslratinE the descriptions with instnictive photographs and drawings. In making the reviri<m

listories of the cases cited have been brought down to the present time.


OPINIONS or THE MEDICAL PRESS

f BrilMi Joucnal of DennMology

' The most compleli; rind up-lo-dale conlrjbulion on llip subjecl of llie Iherapeu of Ihe Ranti;pn riv? wliicli h.i, been publiiheti iii English. '■ k.Boitoa Msdical and Sur^Ckl Journal

' It 15 indi;ptiis,i))lc to Ihose wlio use Ihc X-rays B&n ihi-rapeullc agFnl ; and lis illi ) numerniis . , . ttiBi ii bedoniM viiluable to every one."

Haw Yoi^ Medical Jounial

<We have noibini; but praise for lh< a aa one n better filled by Ironing oi


PRACTICE OF MEDICINE


Sahli's Diagnostic Methods

Editors: Francis P.Kinnicutt^ M.D.^ and Nath'I Bowditch Potter, M.D.


A Treatise on Dias^nostic Methods of Examination. By Prof. Dr. H. Sahli, of Bern. Edited, with additions, by Francis P. KinniCUTT, M. D., Professor of Clinical Medicine, Columbia University, N. Y. ; and Nath'l Bowditch Potter, M. D., Visiting Physician to the City and French Hospitals, N. Y. Octavo of 1008 pages, profusely illustrated. Cloth, j»6.50 net ; Half Morocco, J87.50 net.

JUST READY

Dr. Sahli's great work, upon its publication in German, was immediately recognized as the most important work in its field. Not only are all methods of examination for the purpose of diagnosis exhaustively considered, but the explanation of clinical phenomena is given and discussed from physiologic as well as pathologic points of view. In the chemical examination methods are described so exactiy that it is possible for the clinician to work according to these directions.

Lewellys F. Barker, M. D.

Professor of the Principles and Practice of Medic ine^ Johns Hopkins University " I am delighted with it, and it will be a pleasure to recommend it to our students in the Johns Hopkins Medical School."

Friedenwald and Ruhrah

on Diet

Diet in Health and Disease. By Julius Friedenwald, M. D., Clinical Professor of Diseases of the Stomach, and John Ruhrah, M. D., Clinical Professor of Diseases of Children, College of Physicians and Surgeons, Baltimore. Octavo of 728 pages. Cloth, II4.00 net.

JUST ISSUED— NEW(2nd)EDITION

This work contains a complete account of food-stuflfs, their uses, and chemical composition. Dietetic management in all diseases in which diet plays a part in treatment is carefully considered. The feeding of infants and children, of patients before and after anesthesia and surgical operations, and the latest methods of feeding after gastro-intestinal operations are all taken up in detail.

George Dock, M. D.

Professor of Theory and Practice and of Clinical Medicine, University of Mickigtm

" It seems to me that you have ost valuable work of the kind now

I am especially glad to see the Ion ^ different kinds of foods."


SAUNDF.RS' nOOfCS ON


I


Rolleston on the Liver


Diseases of the Liver. Gall-bladder, and Bile-ducts. By H.

D. Rolleston, M. D. (Cantab), F. R. C. P., Physician to St. George's Hospital, London, England. Octavu volume uf 794 pages, fully illustrated, including a number in colors. Cloth, g6.00 net.

ENTIRELY NEW-RECENTLY ISSUED

This work covers the enlirc field iif diseases of the liver, and i.'i die most voluminnus work on this subject in Enj;hsh, Dr. Rulleston has for many years past devoted his tiinc exclusively to diseases of the di^;estive organs, and anything from his pen, therefore, is authoritative a.nd practical. Special 3 given to pathology and treatment, the former being profusely illustrated.

Medical Record, New York


Boston's Clinical Diag>nosis

Clinical Diagnosis. By L. Napoleon Boston. M.D., Associate in

Medicine and Director of the Clinical Laboratories. Medico-Chirurgical College, Philadelphia. Octavo of 563 page.'^. with 330 illustrations, many in colors. CInth, £4,00 net.


Dr. Boston here presents a practical manual of ihe clinical and laboratory examinations which furnish a guide to correct diagnosis, giving only such methods. however, which can be carried out by Ihe busy practitioner in his office as well as by the student in Ihe laboratory. In this new second edition the entire work has been carefully and thoroughly revised, Incorporating all the newest advances. Borton Modic&l Mid Sur^nl Journal

" Up h:is produccil a book winch muy he regarded emineolly as a praelical luid servioable guide. . . . The iliuslralions are both namerous and good."


■w^ T»« 


MATERIA MEDICA.


GET A • THE NEW

THE BEST /\m6riC2kn standard


Illustrated Dictionary

Just Issued— New (4th) Edition


The American Illustrated Medical Dictionary. A new and complete dictionary of the terms used in Medicine, Surgery, Dentistry^ Pharmacy, Chemistry, and kindred branches; with over lOO new and elaborate tables and many handsome illustrations. By W. A. Newman Borland, M. D., Editor of " The American Pocket Medical Dictionary," Large octavo, over 800 pages, bound in full flexible leather. Price, $^^0 net ; with thumb index, J5.00 net.

WITH aooo NEW TERMS

The immediate success of this work is due to the special features that distinguish it from other books of its kind. It gives a maximum of matter in a minimum space and at the lowest possible cost. Though it is practically unabridged^ yet by the use of thin bible paper and flexible morocco binding it is only i % inches thick. In this new edition the book has been thoroughly revised, and upward of two thousand new terms have been added.

Howard A. Kelly, M« D.» Professor of Gynecology, Johns Hopkins University, Baltimore.

"Dr. Dorland's dictionary is admirable. It is so well gotten up and of such convenient size. No errors have been found in my use of it."


Goepp's State Board Questions

state Board Questions and Answers. By R. Max Goepp, M. D.,

Professor of Clinical Medicine, Philadelphia Polyclinic. Octavo of

800 pages.

READY SOON

Every graduate who desires to practice medicine must pass a State Hoard Examination, and to aid him in successfully passing such an examination this work will be of inestimable value. Dr. Goepp has taken great pains to collect the many questions asked in the past by Boards of the various States, and has arranged and classified them under subjects in such a manner that the prospective applicant can acquire the knowledge on any branch with the least difficulty.


THE PRACTICE OF MEDICINE, ii


Hatcher and SoUmann's Materia Medica

A Text-Book of Materia Medica : including Laboratory Exercises in the Histologic and Chemic Examination of Drugs. By Robert A. Hatcher, Ph. G., M. D., of Cornell University Medical School, New York City ; and Torald Sollmann, M.D., of the Western Reserve University, Cleveland, Ohio. i2mo of 41 1 pages. Flex, leather, $2.00 net.

RECENTLY ISSUED— A NEW WORK

This work is a practical text-book, treating the subject by actual experimental demonstrations.


Journal of the American Medical

"The book is well written, the classifications are good, and the book is to be recommended as a practical guide in the laboratory study of materia medica."


Eichhorst's Practice

A Text-Book of the Practice of Medicine. By Dr. Hermann EiCHHORST, University of Zurich. Translated and edited by Augustus A. EsHNER, M. D., Professor of Clinical Medicine, Philadelphia Polyclinic. Two octavos of 600 pages each, with over 150 illustrations. Per set : Cloth, g6.cx) net ; Sheep or Half Morocco, $7.50 net

Bulletin of Johnt Hopkint Hospital

    • This book is an excellent one of its kind. Its completeness, yet brevity, the clinical

methods, the excellent paragraphs on treatment and watering-places, will make it very desirable."

Bridge on Tuberculosis

Tuberculosis. By Norman Bridge, A. M., M. D., Emeritus Professor of Medicine in Rush Medical College, in aflfiliation with the University of Chicago. i2mo of 302 pages, illustrated. Cloth.

Ji.50 net.

Medical News, New York

" Thoroughly representative of our practical methods of diagnosis and treatment of the disease."


12 SAUNDERS' BOOKS ON


Thornton's Dose-Book

Dose-Book and Manual of Prescription-Writinsr. By E. Q. Thornton, M. D., Assistant Professor of Materia Medica, Jefferson Medical College, Phila. Post-octavo, 392 pages, illustrated. Flexible Leather, $2.QO net.

Recently Issued— New (3d) Edition

Dr. Thornton, in making this revision, has brought his book in accord with the new (1905) Pharmacopeia. Throughout the entire work numerous references have been introduced to the newer curative sera, organic extracts, synthetic compounds, and vegetable drugs. To the Appendix, chapters upon Synonyms and Poisons and their antidotes have been added, thus increasing its value as a book of reference.

C. H. Kfiller. M. D.,

Professor of Pharmacology, Northwestern University Medical School, Chicago,

" I will be able to make considerable use of that part of its contents relating to the correct terminology as used in prescription-writing, and it will afford me much pleasure to recommend the book to my classes, who often fail to find this information in their other text-books."


Lusk on Nutrition

Elements of the Science of Nutrition. By Graham Llsk, Ph.D., Professor of Physiology in the University and Bellevuc Hospital Medical College. Octavo of 325 pages. Cloth, $2.50 net.

JUST READY

This practical work deals with the subject of nutrition from a scientific standpoint, and will be useful to the dietitian as well as the clinical physician. There are special chapters on the metabolism of diabetes and fever, and on purin metabolism.

Lewellyt F. Barlcer. M.D..

Professor of the Principles and Practice of Medicine^ Johns Hopkins University. " I shall recommend it highly. It is a comfort to have such ;i discussion of the subject."


Mathews' How to Succeed in Practice

How to Succeed in the Practice of Medicine. By Joskpii M. Mathews, M.D., LL.D., President Amerir^*^ ^edic»il Association, 1898-99. i2mo of 215 pages, illustrated. "CO net.


THE PRACTICE OF MEDICINE, 13


Gould and Pyle's Curiosities of Medicine


Anomalies and Curiosities of Medicine. By George M. Gould, M. D., and Walter L. Pyle, M. D. An encyclopedic collection of rare and extraordinary cases and of the most striking instances of abnormality in all branches of Medicine and Surgery, derived from an exhaustive research of medical literature from its origin to the present day, abstracted, classified, annotated, and indexed. Handsome octavo volume of 968 pages, 295 engra^rfngs, and 12 full-page plates.


Popular Editioa : Cloth, I3.OO net ; Sheep or Half Morocco, I4*00 net

As a complete and authoritative Book of Reference this work will be of value not only to members of the medical profession, but to all persons interested in general scientific, sociologic, and medicolegal topics ; in fact, the absence of any complete work upon the subject makes this volume one of the most important literary innovations of the day. •

The Lancet, London

"The book is a monument of untiring energy, keen discrimination, and erudition. . . . We heartily recommend it to the profession."

Saunders' Pocket Formulary

Recently Issued— New («th) Edition— Adapted to the New (ljK)5) Phannacopeia

Saunders' Pocket Medical Formulary. By William M. Powell, M. D., author of "Essentials of Diseases of Children"; Member of Philadelphia Pathological Society. Containing 1 83 1 formulas from the best-known authorities. With an Appendix containing Posological Table, Formulas and Doses for Hypodermic Medication, Poisons and their Antidotes, Diameters of the Female Pelvis and Fetal Head, Obstetrical Table, Diet-list, Materials and Drugs used in Antiseptic Surgery, Treatment of Asphyxia from Drowning, Surgical Remembrancer, Tables of Incompatibles, Eruptive Fevers, etc., etc. In flexible morocco, with side index, wallet, and flap. $i-7S riet

Johns Hopkins Hospital Bulletin

" Arran.£jed in such a way as to make consultation of it as easy as possible. It is remarkable how much information the author has succeeded in gettmg into so small a book."


TfD'SMS' Hd'Ol^S ON


SoUmann's Pharmacolo^

Including Therapeutics, Materia MedicB: Pharmacy, Prescription -writing. Toxicology, etc.


A Text-Book of Pharmacology. By Torald Sollmann, M.D.

Proretisor of Pharmacol. icry and Materia Medica, Medical Deiiartment of Western Reserve University, Clevjiand, Ohio, Handsome octavo vohjme of 1070 pages, fully illustrated. Cloth. S4.00 net.

RECENTLY ISSUCD—NEW [2d| EDITION

Because of the radical alterations which have been made in the new (1905) Pharmacupeia, it was found necessary to reset this book cnlirelv. The aulbi bases Ihe study of therapeutics on a systematic knowledge of Ihc nature and properties of drugs, and thus brings out forcibly the intimate relation betivee pharmacology and practical medicine.

J. r. Foflwrintflum, M. D.

/Vfl/. n/ Tkir.ifti,tici ,iaJ Tkrary airJ Praaic^o/ Prtrcriiiig Trinilt Med. CnUige. Tanid. " TJie work teriaiiiiy r>couplES groimd not covered in so conciu. luctul. and scientific mannor hv :iny oihi^i I<:>1 [ tiuvc re.id on Ihe ^ubjccla embm^ed."

Butler's Materia Medica

Therapeutics, and PharinacoIo{^


A Text-Book of Materia Medica, Therapeutics, and Pharmacologyk

By Georgf. F. Buti.er, Ph. G., M. D., Associate Profes.sor of Therapeutics. College of Physicians and Surgeons, Chicago. Revised bjr Smith Elv Jelliffe, M. D., Profes-ior of Pharmacognosy, Columb^ University. Octavo of 694 pages, iilustratcd. Cloth, S4.00 net; Half Morocco, 1S5.00 net.

RECENTLY ISSUED~NEW 15th EDITION Adapted to the New ( 1905) Pharmacopaa For this fifth edition Ur. Butlers textbook has been entirely remodeled, written, and re.sel. All obsolete matter has been eliminated, ami special at. lion has been given to the toxicologic and therapeutic effects of the newer c«  pniinds, A classification has been adopted which groups tOKelher those di the predominant action of which is on one system of organs.

M«dicnl Record. New York

■ Niillimg hiis littn omuled by the author whicli, J" hi. ■ijdgmtnl. «i pletenesa of lb« lexl, anrf Ihp itmlenl or general rej Ibe l,en.

bearini; upon Ibe value of ilruRs and remedi« lonr



PRACTICE, MATERIA MEDIC A, Etc. 15

The American Pocket Medical Dictionary. 4ihCd. Recently issued

The American Pocket Medical Dictionary. Edited by W. A. Newman Dorland, M. D., Assistant Obstetrician to the Hospital of the University of Pennsylvania. Containing the pronunciation and definition of the principal words used in medicine and kindred sciences, with 64 extensive tables. Flexible leather, with gold edges, ^l.oo net ; with thumb index, j$i.25 net.

"I can recommend it to our students without reserve."— J. H. Holland. M. D., of tkt Jefferson Medical College, PhilaeUlphia.

Vierordt's Medical Diagnosis. Fourth Editioii, Revised

Mkdical Diagnosis. By Dr Oswald Vierordt, Professor of Medicine, University of Heidelberg. Translated from the fifth enlarged German edition by Francis H. Stuart, A. M., M. D. Octavo, 603 pages, 104 wood cuts. Cloth, $4.00 net; Sheep or Half Morocco, %^.oo net.

    • Has been recoKnized as a practical work of the highest value. It may be considered indispensable

both to students and practitioners."— F. Minot, M. D., late Professor o/ Theory and Practice in Harvard University.

Cohen and Eshner's Diagnosis. Second Revised Editioii

EssENTiAi^ of Diagnosis. By S. Solis-Cohen, M. D., Senior Assistant Professor in Clinical M^icine, Jefferson Medical College, Phila. ; and A. A. Eshner, M. D., Professor of Clinical Medicine, Philadelphia Polyclinic. Post-octavo, 382 pages; 55 illustrations. Cloth, $1.00 net. /// Saund^rs^ Question -ComJ^^nd Series.

"Concise in the treatment of subject, terse in expression of fact." — American Journal of the Medical Sciences.

Recently Issued

Morris' Materia Medica and Therapeutics. New (7th) Edition

ESSENTIAI^S OF MATERIA MeDICA, ThERAPEITICS, AND pRKSCKinioN-WRn ING.

By Henry Morris, M. D., late Demonstrator of Therapeutics, Jefferson Medical College, Phila. Revised by AV. A. Bastedo, M. D., Instructor in Materia Medica and Pharmacology iit Columbia Univer.sity. 1 2mo, 300 pages. Cloth, ^ 1. 00 net. In Saunders* Question- Compcnd Series.

    • Cannot fail to impress the mind and instinct in a lasting manner." — Buffalo Medical Journal.

Williams' Practice of Medicine Recently issued

• Essentials of tiik Practick ok MiniriNi. By W. R. Williams, M.D.. formerly Instructor in Medicine and Lecturer on Hygiene, Cornell Universiiv ; and Tutor in Therapeutics, ('<.lumbia University, X. V. l2mo of 450 pnge«-, illustrated. In Satmdrrs* QnestioH-ConiptnJ Series. Double nuniler, $1.75 net.

Stoney's Materia Medica for Nurses N^S^ i£n

Materia Medica ior Nirsivs. By Emily M. A. Stoney. Supvrintmdent of the Training School for Nurses ;it the Cnrney Hr>spital. South Boston, Mass. Handsome i2mo vohime of 300 pai^es. Cloth. 51.50 net.

"It contains about everything that a nurse ouKht to know in reg^ard to Atvl%^."— ^Journal of the American Medical Association.

Grafstrom's Mechano-therapy Seco^d*E&n!^ia,««l

A Text- Book of Mf( hano-therapy (Massai^e and .Medical GymnasticsL Bv Axel V. Grafstrom. B. Sc, M. D.. .\ttendini,' Physician to Augustus Adolphus Orphanage, Jamestown. N. Y. i2mo, 200 piijes, illustrattMl, 51-25 net.

"Certainly fulfills its mission in rcnderinR^ comprehensible the subjects of massage and medical gymnastics." — Xew Vork Medical Journal.


i6 SAUNDERS' BOOKS ON PRACTICE, Etc.

Jakob and Eshner's Internal Medicine and Diagnosis

Atlas and Epitome op Internal Medicine and Clinical Diagnosis. By Dr. Chr. Jakub, of Erlangen. Edited, with additions, by A. A. Eshner, M. D., Professor of Qinical Medicine, Philadelphia Polyclinic. With 182 colored figures on 68 plates, 64 text- illustrations, 259 pages of text. Cloth, j^j.oo net. In Sounder^ Hand- Atlas Series.

    • Can be recommended unhesitatingly to the practicing physician no less than to the student." —

Bulletin 0/ J^knx Hopkins Hospital.

Lockwood's Practice of Medicine. Re^^ia^ed

A Manual of the Practice of Medicine. By Geo. Roe Lockwood, M. D., Attending Physician to the Bellevue Hospital, New York City. Octavo, 847 pages, with 79 illustrations in the text and 22 full-page plates. Cloth, ^4.00 net.

A work of positive merit, and one which we gladly welcome."— AV«v York Medicml Joumai.

Keating's Life Insurance

How TO Examine for Life Insurance. By the late John M. Keating, M. D., Ex-President of the Association of Life Insurance Medical Directors. Royal octaro, 211 pages. With numerous illustrations. Cloth, j$2.oo net. *

" This is by far the most useful book which has yet appeared on insurance examiaation." — Medicml New*.

Corvrin's Physical Diagnosis. Thiid Cditioii. Revised

Essentials of Physical Diagnosis of the Thorax. By A. M. Corwin, A. M., M. D., Professor of Physical Diagnosis, College of Physicians and Surgeons, Chicago. 220 pages, illustrated. Cloth, flexible covers, j$i.25 net.

" A most excellent little work. It arranges orderly and in sequence the various objective pheaomena to logical solution of a careful diagnosis."— y<7vr»a/ 0/ Nervous and Mental Diseases.

Barton and Wells* Medical Thesatnrus

A Thi<:saurus of Medical Words and Phrases. Hy W. M. Barton, M. D., and W. A. Weli-S, M. D., of Georgetown University, Washinjjion, D. C. l2mo of 535 pages. Flexible leather, $2.50 net ; thumb indexed, $3.00 net.

Jelliffe's Pharmacognosy Recently issued

An Introduction to Pharmacognosy. By Smith Ely Jelliffe, Ph. D,, M. D., of Columbia University. Octavo, illustrated. Clolh, I2.50 net.

Stevens' Practice of Medicine. New (7th) Edition— Recently issued

A Manual of thk Practice or Medicinf. By A. A. Stevens. A. M., M. D.,

Professor of Pathology, Woman's Medical College, Phila. Specially intended for

students preparinp for j^raduaiion and hospital examinations. Post-octavo, 556 pages; illustrated. Flexible leather, $2.50 net.

"An excellent condensation of the essentials of medical practice f«ir the .student, and may be found also an excellent reminder for the bu»y physician." — Buffalo Mt'dical Journal.

Paul's Materia Medica for Nurses just Ready

Materia Mkhica for Nursf:s. By Georc.e P. Paui, M.D., Assistant Visiting Physician and Adjunct Radiographer to the Samaritan Hospital, Troy, N. Y. i2mo of 240 pages. Cloth, ^1.50 net.

In Dr. Paul's new work the physiologic actions of the drui^s an: arranged according to the action of the drug and not the organ acted upon. Another important section is that on pretoxic signs, giving the warnings of the full action or the bejjinninij toxic effects of the drug. If these signs be known many cases of drug poisoning may be prevented.

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