Book - A Text-book of Embryology 17: Difference between revisions

From Embryology
mNo edit summary
mNo edit summary
Line 274: Line 274:


+++++++++++++++++++++++++
+++++++++++++++++++++++++
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
the 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 thatof otht'r systems an<l organs, cleaner conceptions
of the y:rowth of the individual mav be obtained bv comparing it witli the 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 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. the 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 purpose, 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 mesothelial 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.
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' the 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.
eliymatous slieath of tlio i-lionla on tin; caudal aide of the
liyi>ocliortlal brace. The intarrertebral ligament is develowd
fnmi the (erichonlal tissue on the dorsal side of the 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 the 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
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 ensheathed by merely membranous tig
Before the end of the second month the 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
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.
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
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.
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.
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.
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 the spinal con! aud a
wrics of vcntrul nr licnisil arclics (or the protection of the
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 the 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 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 bent over vcntrcul. From the 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 the 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 the brain, tJK* cartilaginous brain-case is
deficient, sinc(^ the process of ehondrification <loes not affect
the i-egions of the 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 designated 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 base of the future skull. On each side of the end
of the chorda a mass or bar of cartilage is formed, extending forwand and backward, this pair of parallel bars being designated the parachordal GartUagflB(Fig. 179,1). Farther forward,
Fig. 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 the 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 ossification fM-curs lien*, as elsewhere in the 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
the 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 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
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 the occipital bone proper,
rijiin. The OBsiflcation of the latter
(Hie on each side of the 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 the 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.
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
Fig. 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
the 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 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
.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 the 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 the 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, 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
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 the 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 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.
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.
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 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  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, the
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, the 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 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 the 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 the 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).
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, 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 the 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, other 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 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
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 the 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,
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
i.\'/*!ol' the third ('crvical >cgm«'nt,
^. N.-^v* 10 its adult position ; and that 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 the future dorsal surface of
eacrh limb looks toward the back of the fetal Ixxly (dorsad),
the future flexor surface toward its anterior aspect (ventrad), 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.
•*?SS!i.«"'
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).
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.
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
Tnav»jr«i of t>thmoid. Internal pt<'ryiroi<l plate Aisea
witlj •'Xternal. Intermaxillarifs fuse with maxilla.
I^'us longer than arms.
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.
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




----
----

Revision as of 15:42, 31 July 2014

CHAPTER XVII. THE DEVELOPMENT OF THE MUSCULAR SYSTEM

Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

   Text-book of Embryology 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs


Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.

Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)


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


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 the 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^




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 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 part 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 now tibrillated protoplasm into muscular tissue is first completed at the periphery of the fiber, so that a young muscle 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 striation 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 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 its core, the process being complete in the human embryo at about the end of the fifth month for the muscles of the upper extremities and in the seventh month for tho.-ie of the lower.


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 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 the 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 thatin 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.


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.

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.


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.


+++++++++++++++++++++++++


Retrieved from ‘https://embryology.med.unsw.edu.au/embryology/index.php?title=Book_-_A_Text-book_of_Embryology_17&oldid=139835’
Powered by MediaWiki