Book - A Text-book of Embryology 3
CHAPTER III. THE GERM-LAYERS AND THE PRIMITIVE STREAK. THE TWO-LAYRED STAGE OF THE BLASTODERMIC VESICLE
Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.
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 |
|---|
 |
In lii<lyiiij»; tint complicated miuI obscure iilienomciui of I lie ioriii.'itioii oi* the p*rni-layei*s in inamnials, eoin]Kiri.son willi what iK'riiis in c(?rtain lower forms is liel})fnl. In lln' rax' of tin* ampiiioxus^ the one-layered stajrc, the blastula in .-iir<'eed<-d liy the sac-like douhle-layered gastmla stage. TIh' ^a-lriila, in its typical form, consists of t\vt> layers of ctlls f-nrroijiidin^a rmtral cavity, \vhi<*h latter comniunicales w iih th<* exterior Iiy means of a small ajuTtuiv, the blastopore. 'VUr ravily is tiie archenteron or cu'lenteron or intestiiio-liodv cavitv. 'rh<* outer laver of <'ells is the ectoderm or epihla.-^l ; the inner layer is the entoderm or hy|K)l)liLst. riii-« t'onu of the pM'ni is hmmi in holohlastic invertehnite, as v^ell ar^ in Mtnie vertchrate ova, and is typically exemplified the developmenf of the amjihioxus. The hiastiila of this liiiijjai i«* a ^inlp>a<', iIm' wall of which is a single layer t)f ipir K-Iial eelU ^nrnMlnding the <"h'avajre-cavity ( Fi !Jv 1 :j'i;.>riiu^-in i*!' the vi'^<*lativr rells, the cleava«;:e-cavity s -.urn/aji.-iuxl ii|Hiu and fntaiiy i:^ cnmpletely nl)literate<l, l>eing i'iiiuv/-.'i vy the an*lientei'on iVi*^. -»». O. From this it is Hr JUi' r^^irulation occurs here hy a simple pnnM'ss of Lu.il I III, lu ova with a larjrc amonnt of f^Mwl-yolk, as , UixU, ami finhes. the process is m<Mlifi(»d uL tbti^ivudltion. Still iurther nnHlifications ill ihe development of mammal-, >ince AluetiiM» in thr amount t)ftoo«l-yolk. aI gaMni la lln-ory n(* llarrkrl, all ibraulnials a^ di-tinjrni-hnl fmni liTCiiUiMns — pa>^ ihrnujih a typiral \iivir ilevelopmrni. ihfii:{7«l \ffilloipl«' that thr higher animals during their development repeat, to a greater or less extent, the embryonic or the larval forms of the lower members of the group to which they belong. Huxley has pointed out the morphological identity of the adult form of the coelenterata with the two-layered gastrula.
Fig. 26.— Gastrulution of amphioxus (modifled from Hatschek). .1. Blastula: az, animal cells; vz, vegetative cells; fh, cleavage-cavity. J!. Beginning invagination of vegetative pole. C. Gastrula stage, the invagination of the vegetative cells being complete: ok, outer germ-layer; ik, inner germ-layer; ud, archenteron; a, bla8toiX)re.
It must not be understood, however, that we find in mammals a gastrula stage such as that of amphioxus ; we do find, indeed, that the single-layered blastodermic vesicle as described above acquires two and, still later, three cellular layers, but to what extent the lavers of the diderniic blastodermic vesicle correspond to the ectoderm and entoderm of the gastrula of lower types is not quite clear ; nor are all the details of the growth of these layers as yet clear. The phenomena have been studied in the mole, the rabbit, the bat, the sheej), and the dog, as well as in some other mammals. Acconling to the investigiitions of Van Beneden upon the development of the rabbit and the bat, the inner cell-mass spreads out upon the inner surface of the enveloping layer and shows a differentiation into two groups of cells. One group, occupying the center of the mass, Van Beneden'sembryonic bud (Fig. 27) consit^ts of cells that are at first globular, but later cuboidal ; the second group, composed of flatter and darker cells, covers continuously that surface of the mass which looks toward the blasto<lerniic cavity and soon extends beyond the limits uf the muss to line tlic inner surface of the enveloping layer, thus constituting the entoderm (Fig. 27). Some of tiie cells of the lighter group now vacuolate, the several vacuoles later becoming confluent to form one cavity, which i.s the future amniotic cavity (Fig. 28). In the rabbit this vacuolation docs not occur, at least not at this stage. The more or less globular mass of cells remaining after the vacuolation is known as the embryonic bnd or
Fig. Ti.—n\am of bat : differentiation nf cmbryntii
embryonic button or embryonic di^k (Fig. 20). It will be obsorvtHl that tiie amniolii: cavity lies between the embryonic bud and the enveloping layer. It is important to note thai the embryonic buil is the anla^c of the bixly of the embryo, that it is from this group of cells alone that the embryonic body
The enveloping layer, which disappears in the rabbit at
alwnt the seventh d.iy, brit which |M'rsists in <)tiier mammals
so far as known, resolves ilsclf into two laminie in the region
overlying the embryonic bnd and the amniotic cavity, an
inner luycr, the cystobUst, an<l an outer, the plasmodoblast
or placentobUst, eomiK>sed of flattened cells. The cystoblast
now constitutes the inimediatc roof or vault of the nnmiotic
cavity, while a layer of cells ditfcrentiated from the amniotic
surface of the embryonic bu<l forms the floor of the cavity.
This latter lamina becomes the outer laver of tlic dtdermic embryo, that is, it represents the cmbrvonic ectoderm, while
the enveloping layer would correspond to the extra-embryonic ectoderm. Somowliiit later the eystoblast in the vault of
the amniotic cavity disappears and the himina of cells referred to above && forming the floor of the amniotic cavity becomes continuous at the with the enveloping layer.
ConijMirison of but and mole embryo,-; with the oviira of Peters (Fig. .10) shows that essentially the same jiheDomeDa tKCMT in the development of the human ovum. The Dviim of Peters was estimated by him to be Iwtween three and fonr davH ohl and wiis the youngest hnmun ovura as yet studied. One muv see here the embryonie bud or disk E. Sch., the
Embryonic L'^illiliut; £ltil.. vmbryuiilv liyimUiut: iti'ti,,Kiatity..uuMUvAl v<aMe; X<H..cliurli>n[c tpthlut; St.. foltl in cxoi|[cciiv(i)r liiH'il bf»»liiBl« lijcror flailMicl veil*, whlrh are
klngpunlrutwlib Ihi-liyi-ruf cylludrtcr editor the einbryunlc<-|ilb1iiBl.
L'mbrj-onic eetodenu formitodcrm Enl,, and likewise » somewhat more ad vanced ■ under cousiderutioii.
may see that the singlemnnils becornos eonvertcd
ainniotio cavity A. H., the exti ing the vault of this cavity, the the yolk-aac 1). S., sinee this ovni iu development tliun the stage ii From the foi-egoing aeeount i Inyered blastodermic vesicle of into the two-layered or diploblastic vcsiele, consistiDg of the entoderm and the ectoderm, not, as in the amphioxns, by a simple process of invagination, but largely by a rearrangement of the cells of the inner cell-mass ; and that, partly as a consequence of ibis rearrangement of cells, the amniotic cavity is proilnced and a different in tiun l)econies manifest between a group of cells, the embryonic hiid, which is to serve for the development of the embryonic body, and other cells which arc destinwl to produce the accessory organs or envelopes of the developing embryo.
As previously stated, the process of gastrulation and the
form of the pastnila are modified in the t".ise of ova possessing a large proportion of deutoplasm. In the case of the frog,
for example, as well as in other amphibians, the blastula has
the form shown in Fig. 24. By an invagination of the bla»tula-wall at the place of transition from the animal cells to
the vegetative cells, all of the latter and a part of the former
arc carried into the interior of the blastula to form the lining
of the archenteron (Fig. 31). Compare this with the ampht
Fig. 3L— Bi^tUI ■ecllon Ilinmgli uii I'ltenfirlliiii (nncrthcendorfcutrulntlon): o*. onltr BOnn-laycr; ijt, ii ventral lipa of the ccdenteron; genu-layer (Hertwlg).
oxus gastruln as shown in Fig. 26. In the bird's e^, the form of wiiose gastnila is shown in Fig. 32, an infolding or invagination occurs, as in the frog's egg, at the place of transition from the animal cells to the vegetative cells, or, in other words, at the margin of the germ-«lisk. The gastrnla thus formed is represented in Fig. 32. Its archenteron is a narrow fissure, and its blasto]>orc, situated at the posterior margin of the gerni-ilisk, is exceedingly small.
The Embryonal Area. — tTpon the surface of the germ at the lieginning of gastrulation — that is, at about the fifth day of devclopmeut in the ease of the rabbit's germ — there is a roimd whitish spot, the embryonal area. Its position eorre6jK)uds to that formerly held by the inner cell-mass of the bla^to<lormic vesifle, as shown in Plate I., Fig. 2. It is only in this region that the wall of the vesicle is, at this particular etag*", composed of morn than a single layer of colls, the octotlvrm and the cnloilcrm not I'Xti.-ndinp; much, if at all, beyond its periphery.
Tin? embryonal area, soon becoming oval (Fig. 'i'i) and, later, pear-shaped, exhibits, at it^ posterior margin, a trans nsed with reference to the future body, the narrow end of the area embryonalis corresponding to the posterior pole or caudal extremity of the fetus.
In the chick's egg, the embryonic area (Fig. 34), or embryonic shield, appears while the egg is yet in the oviduct.
bryonk shield : pr, primlllru gruuvu.
Its embryonic crescent correH|>on»ls to the manialian terminal ridge. Segmentation being limited tu the gcnii-<li»k in the chick's egg, the resulting blastoderm, which is not a vesicle, but a flattened mass (Fig. 32) composed of several layers of cells, rest.-? by its margin upon the jKirtially liquefiwl yolk. the central region iif the blastoderm, which overlies the liquefied portion of the yolk, from its tnuisluccnee ia known as the area pellncida (Fig. 34), while the dark opaque rim, resting iipun the yolk is the area opaca. The inner rim of the area opacii is tiie area vasculosa. These regions are observed also in the mammalian egg.
It in In Ihe eiiihri/o)iri! firea tilotie that the bofly of the embryo is developed ; the other parts of the germ produce extra -embryonic .structures, such a?* the amnion, the yolksac, etc.
Partial longitndinal division of the embryonic area during development result.s in the pi-o<hiction of some form of doable monster ; its complete cleavage gives rise to homologous or homogeneons twins, wiiicli arc twins of the same sex and of almost absolutely identical structure. Oniiuary twins are developed frtim separate ova, which may or may not have come from the same ovary.
The Primitive Streak
The primitive streak is a linear median marking lying ju the long axis of tlic ombiyonal area and containing a median furrow, the primitive groove (Fig. 35j. A traoMverse section tiirough the primitive streak
Figs. 36 and 37) shows thatUna surface-marking is produced by a thickening of the ectoderm along the median liae, owing to a proliferation of wlls from its under aide. TV lenfth of the streak is alwut two-thirds of that of the ■oafafTOBal area. In the rabbit's ovum it is seen at about t vtrealh day ; in the hmiiun germ the time of its appear
h ItHi » pfobably about ihe third or fourth
» .^ tmtt a ^strula as that of the amphi »iit»«fl dte blastopore appniadi cuch
other and fuse in a line corresponding to the median longitudinal axis of the future embryonic area, the fusion or eoncrescence b^inning at the anterior extremity of this line
and proceeding toward its caudal end. The surface-marking produced by the apposition and partial union of the blastoporic lips was called the primitive streak, and its median
furrow was known as the primitive groove, long before their true significant^e was appreciated. Since the edge of the blastopore marks the place of transition from the entoderm to the ectoderm (Fig. 26), the two germ-layers after the union of the edges of this opening are in intimate association under the primitive streak, as shown in Fig. 37.
Morphologically the primitive streak of the higher vertebrates is regarded as the fnsod and extended hlastoiwre of lower types. The terminal ridge of the mammalian embryonic area, as well as the crescent of the ombrvonic shield of avian and reptilian e^js, represents, as 6tate<] above, the anterior lip of the blastopore. Since the embn.-onal area is
increasing in circnmferpnee while the li^js of the blastomere are nndei^iing union or concrescence, the transversely directed terminal ridge, wliich lies at the ]>osterior edge of the
embryonal area, and which remains a fixed point, becomes a longitudinal marking, and tiiis marking or primitive streak
<*oinc.s to lie, therefore, behind the site of the . blastopore.
Hc'ference to DuvaFs diagram (Fig. 38) will make this clear.
Fig. — I)lHKriiiii fliicidiitiiiK the formation of the primitive groove (aAer
lUi\tit . Thi' liicrt'ii.tiiiK Hi/j' of the f;erm-disk in the course of the <Ieveh)pment is
iiiiht'HU'tl l»y iloltr«l rirtMihir lines. The heavy lines n-present the ereseentic
<niii\i- mill the primitive groove which arises from it by the fusion of the edges
After ihe development of the primitive streak, there is
'*4Tii, ill I Ik* nirdian line of the enibrvonal area, anterior to
\\w MifaU, another marking, the head-process of the ])rimitive
^irrak. This is ahnost i(h'ntieal with the primitive axis of
\liiioi, wich tlial investigat(>r (les<*ril)es as a median band of
«i'il.^ «*«iiiiiivtol with the ent<Mlerm and extending forward
'niiii ih«' Ma.slo|M»re.
HouMu's Utfdi^ In an neennnilation of cells on the nnder
-iiii'h-f .'t" I ill* t'ohHh'rni at the anterior end of the ])rimitive
-.ir«iv. M »^ iiinH»ruint lieean.s<» of its relation to the nenren r-* . mal. wiiiili will be ilt'seribcd later.
. iMOttmi iu' primiiive streak and l)last«>pore j)l:iy no part
•I -KM ^u»c«'^ "' deNrlopnuMit, it is worthy (jf note that
„, 1.^ ti :iu' line of the longitudinal axis of the
— .^.K. imI iuii ilie |Hi?»ilion of th(? bht-^topore niarks .iiiuai end of the embryo. l>evctO|Mtt«iiC of the Mesoderm. — Hie mesoderm - ,; -.iMiiitiv vi'miH^stHJ of sevenil layers of eells r . fKurui :uul the ento<K'rm. It is earliest
,N i' 'lie fn>nt end of the primitive
iu'ld by the i)lastoj)ore. From .i t.:si.> ukI Tvr^eriorlv and, later, ant<'rier, until (»ther imj)ortant 1 -Atciuls eompletely around
The terms gastral memderm and peristomal mesoderm are used to designate respectively that portion developing from the region of the head-process of the primitive streak and that portion growing from the region of the blastomere.
Concerning the origin of the mesoderm much difference of opinion prevails. The simpler and more primitive method is seen in the amphioxus, in which it develojis as two evaginations from the dorsal wall of the arclienteron, one on each side of the mid-line. These entodermic folds, containing each a. cavity, the enterocoel, grow out laterally between the
inner and the outer germ-layers. By transverse constriction,
each fold divides into a series of segments, the somites, which
lie on either side of the median line from the head-end to the
tail-end of the embryo. Each somite divides into a dorsal
part, the " protovertebra," and a ventral jmrt, the lateral
plate. By the fusion of the lateral plates of each side their
several cavities become one, the body-cavity or coelom.
The origin of the middle germ-layer in higher vertebrates is far less clearly made out. Some investigators hold that it
arises in essentially the same manner as does that of amphioxus — that is, by evagination or outfolding of the entoderm
bounding the coelenteron ; the investigations, however, of
Bonnet and of Duval respectively upon sheep and chick
embryos, point to a different conclusion. Bonnet's observations show that the mesodermic tissue, starting from Hensen's
node, grows out laterally between the ectoderm and the entoderm, and that at some distance from the median line of the
embryonic area there is a delamination or splitting-off of
cells from the entoderm ; and, further, that these two primitive areas grow toward each other and unite to form one
continuous sheet of mesoderm. It may be said, therefore,
that the mesoderm originates from a double source, chiefly
from the entoderm, hut also from the ectoderm, since the cells
giving rise to the part that grows from the region of Hensen's node are ectoderm ic. A section of the germ transverse
to the long axis of the embryonic area (Figs. 36 and 37)
shows the mesoderm to be a distinct and independent layer,
sharply defined from the other germ-layers everywhere except \a the region of the mid-line, in which position the three
layers are so closely related as to constitute one structure
The mesoderm does not extend completely around the germ
at this stage, being deficient on the side opposite the embryonic area.
The mesoderm, after its formation, grows by the proliferation of its own cells, independently of the eotiKlerm and the
entoderm.
If the expansion of the mestxlerra, as indicated by the
surface appearance of the germ (Fig. 391, be noted, it will be seen that at first it is present throughout a pear-shaped
area whose narrow end is directwl forward. Somewhat later,
two wing-like expansions grow forward fivim the front end
of this area (Fig. 40); these winga, meeting at their tips,
enclose a sjKice, the proamnion, which is devoid of mesoderm.
Referring again to the transverse section (Fig, 37), it is
evident that the middle germ-layer in the vicinity of the
median line is composed of a somewhat irregular mass of
cells, while farther away it constitutes u lamina on each side.
Fig, 9.— DlagmniniMlc »urfiies vli-w i.r r«bl)i('» uvum of SOS hours (alter Tnnrneuil. The dtrkly ahadHl area indicate! Ihe pilGtit of Ihe meandcmi. n. Pvriphenl llrnll nf area n|>ar> ; A. of area pellucldai e, nf parietal tune: d, cif itvin-ioDe ; /. Henien'a nodi: ; g. proamnion.
As development advaix^x, these two portions become more
differentiated from eacli oilier, althungli thej- are not entirely
separated niilM mmli lutt-r. The thick ma^^ adjiiwnt to the
median line is tin- Tertebral plate, ()r primitive Hegment plate,
or paraxiaJ mesoderm; the niorf Haitcnod hitor.il [wrlion is
the lateral plate. The mesodtjrm at this stage, therefore, consists of four fwrts — the two pamxial musses, lying one on each side of the median line, and extending from the head end to the tail-end of the embryonal area, and the two lateral plates, situated upon the outer sides of the paraxial colnmns.
Fig. --DU((r«mm«tlp surluui: iku- uf n nenxl. The darklf sliuded nrua IndlcBlei tb enl limll vf ■roopoiMi; 2. of area pellnc[da: S. of pariiiti 6, Hemen's node; 7, proamnion.
Each primitive segment plate nndor^ocs transverse division into a inimhrr of irrofiiihirjy rnlncal masses, thu mesoblastic somites, or primitive segments, often improperly called the protovertehrffi. The presence and position of the primitive segments are indicated by transverse jiarallel lines on the surface of the germ, which constitute a series on either side of the primitive streak and its head-process (Fig.*. 40 and 46). The formation of the somites begins at the cephalic end of the emliryo and progresses tailward.
The lateral plate of the mesoderm splits into two lamellae, of which the outer or parietal layer ig the Bomatic mesodenn.
and the inner or visceral layer is ihe splanchnic mesoderm.
The somatic mesoderm unites with the cctiHlerin, forming the
Bomatoplenre ; the splanchnic mesoderm unites with the entoderm, forming the BpIasctmopleuTe. The tissiire-like cavity
between the soniatoplciire and the splanchnopleure is the
cffilom, or body-cavit:r, or p leu rojieritj meal cavity (Fig, 45).
The great serous i-avities of the adult body — pleural, pericardial, and peritoneal — are later subdivisions of the ooe
The mesodermio cells bounding the body-cavity become
flattened and endothelioid lu character, and constitute the
mfluthelinm ; from them are descended the various endothelial cells lining the serous cavities of the mature organism.
According to some authorities, among whom Hertwig may
be especially mentioned, there develop from the mesothelium
at an early stage certain (x.d]s whose jtarticular function is
the formation of the differt-nt kinds of connective tissue,
8uch as bone, i-arlilitge, fibrous ti.ssue, etc ; these elements
are often distinguished as mesencliyinal cella, or collectively,
as mesencliyine. According to this classification, the importance of wliich is insisted ujwn by Minot, the mesenchyme
includes all the mesodennic tissue except the flattened cells,
the meBothelium, lining the body-cavity.' "Mesenchyma
consists of widely separated cells which form a continuous
network of protoplasm, the meshes of which arc originally
filled by a honn^neous intercellular substance or matrix."
—Minot.
His claims a double origin for the mesoderm. He maintains that the mesothelium and the smooth musculature of
the h«iy are of intra-enibryimic origin, and these structures
he terms the archibUst; while all other parts of the mesoderm, which he designates the parablast, have, in his opinion,
an extra-embryonic source, being derived [»ossibly from the
granulosa cells of the ovary. These views are not shared,
however, by the majority of cmbryologists.
Minot holds Willi Cio«tt« Hint the uiesondiyin:!! fcll^ are tlio product or till! mfwillii'liiim. IIprtwiK mninlninH (lint the mesenchyma artsei from all the other germU.Teni by the emigration of i«ol*tcd cellA.
The Derivatives of the Germ-layers
From the three primary germ-layers are developed the various tissues and organs of the body by metamorphoses which may be referred to the two fundamental processes of specialization or the adaptation of structure to function, and of unequal growth which latter results in the formation of folds, ridges, and constrictions.
From the ectoderm are produced : —
The epidermis and its appendages including the nails, the epithelium of the sebaceous and sweat-glands and their involuntary muscles, the hair, and the epithelium of the mammary glands.
The infoldings of the epidermis including the epithelium of the mouth, with the enamel of the teeth, the epithelium of the salivary glands, and the anterior lobe of the pituitary
body :
The epithelium of the nasal tract with its glands and communicating cavities :
The epithelial lining of the external auditory canal, including the outer stratum of the membrana tympani :
The lining of the anus and of the anterior part of the urethra :
The epithelium of the conjunctiva and of the anterior part of the cornea, the crystalline lens.
The spinal cord, the brain with its outgrowths, including the optic nerve, the retina, and the posterior lobe of the pituitary body.
The epithelium of the internal ear.
From the entoderm are produced : —
The epithelium of the respiratory tract.
The epithelium of the digestive trad, from the back part of the pharynx to the anus, including its associated glands, the liver, and the pancreas.
The epithelial parts of the middle ear and of the Eustachian tube.
The epithelium of the thymus and thyroid bodies.
The epithelium of the bladder, and of the first part of the male urethra and of the entire female urethra.
From the mesoderm are developed : —
Connective tissue in all its modified, formsy such as bone, dentine, cartilage, lymph, blood, fibrous and areolar tissue.
Muscular tissue.
All endothelial cells, as of joint-cavities, bursal sacs, lymph-sacs, blood-vessels, pericardium and endocardium, pleura, and peritoneum.
The spleen.
The kidney and the ureter.
The testicle and its system of excretory ducts.
The ovary, the Fallopian tube, the utei*us, and the vagina.
From the foregoing tabulation it may be seen that, generally speaking, all epithelial structures originate from either the ectoderm or the entoderm, the notable exception to this rule being that the epithelium of the sexual glands and their ducts, and also that of the kidney and of the ureter, proceed from the mesoderm.
