Talk:Book - A Laboratory Manual and Text-book of Embryology 2

From Embryology




The processes of cleavage, or segmentation, not having been observed in human ova, must be studied in other vertebrates. It is probable that the early development of all vertebrates is, in its essentials, the same. Cleavage may be modified, however, by the presence in the ovum of large quantities of nutritive yolk. In many vertebrate ova the yolk collects at one end, termed the vegetal pole, in contrast to the more purely protoplasmic animal pole. Such ova are said to be idolecithal. Examples are the ova of fishes, amphibians, reptiles, and birds. When very little yolk is present, the oviun is said to be isolecithal. Examples are the ova of Amphioxus, the higher mammals, and man. The typical processes of cleavage may be studied most easily in the fertilized ova of invertebrates (Echinoderms, Annelids, and Mollusks). Among Chordates, the early processes in development are primitive in a fish-like form Amphioxus. The yolk modifies the development of the amphibian and bird egg, while the early structure of the mammalian embryo can be explained only by assuming that the ova of the higher Mammalia at one time contained a considerable amount of yolk, like the ovum of the bird and of the lower mammals, and the influence of this condition persists.

Cleavage in Amphioxus

The ovum is essentially isolecithal since it contains but little yolk (Fig. IS). About one hour after fertilization it dixides vertically into two nearly equal daughter cells, or blastomeres. The process is known as cell cleavage y or segmentation, and takes place by mitosis. Within the next hour the daughter cells again cleave in the vertical plane, at right angles to the first division, thus forming four cells. Fifteen minutes later a third division takes place in a horizontal plane. As the yolk is somewhat more abundant at the vegetal pole of the four cells the mitotic spindles lie nearer the animal |X)le. Consequently in the eight-celled stage the upper tier of four cells is smaller than the lower four. By successive cleavages j first in the vertical, then in the horizontal plane a 16- and 32-celled embryo is formed. The upper two tiers are now smaller, and a cavity, the blaslocaU, is enclosed by the celb. The embryo at this stage is «»m(^lime« called a morula fmulbeiry). In subsequent cleavages, as development procccrlft, the size of the cells b diminished while the cavity enlarges (Fig. 15),

— CiMi-aitc of (h« tfn of Amphioxus (after Hatschek). X200. 1. Tbe egg before tiie m-rmrnt of ilcvi^lnpmenl; only one polar body, /'.A,, is pr«$«nt. Ih« other having btcD lost duiing oviiUtion, J, The n\-um in the act of dividing, by a vertical cleft, into two equal blastomefes. 3. Stage with four njual blaslomrtts. 4. Sta^r with i-ij^ht bla$tomere»: an upper tier of four slightly amaller ones and a Iiiutt tier of four slightly larj^r ones 5. t^tafie with sixteen bla^lomere? in tno tiers, each of eight. A. StaKC with thiily-twii Ida^iinKirs, in fiiur tiers, each of eishi: the embr\-o is represented bisected to «hii«' the c(e(t\-aj;e cavity or Nastivirle. fi. 7. l^terstaRe; the bla»ton>erc« have increased in number by fiinlier division. S. DUstula sta^e hjsecleil to show the lilastociele, B.

The enil*r\i> is now a N<istui<i. nearly spherical in form and about four hours old, The cleavage of the Amphioxus o\-um is thus holtM<istif. i. e.. compUte. and nearly

Cleavage in Amphibia

These ova contain so much yolk that the nucleus and most of the cjtoplasm lies at the upper or animal pole. The first cleavage spindle lies in this cytoplasm. The first two cleavage planes are vertical and at right angles, and the four resulting cells are nearly equal (^Fig, 16, 1). The spindles for the third cleavage arc located near the animal pole and the cleavage takes place in a horizontal plane. As a result, the upper four cells are much smaller than the lower four (2 and 3), The large yolk-laden cells divide more slowly than the upper small cells. At the blastula stage, the cavity is small, and the cells of the vegetal pole are each many times larger than those at the animal pole (4 and 5). The cleavage of the frog's ovum is thus complete but unequal.

Fig. 16. — Cleavage of iht !'u^'> ..sclickin MiirshallJ, ity; N, nucleus. ; 21), B, Blaslucirle or cleavage a

Cleavage in Reptiles and Birds

The ova of these vertebrates contain a large amount of yolk. There is very little pure cytoplasm except at the animal pole and here the nucleus is located (Fig. 3). When segmentation begins, the first cleavage plane is vertical but the inert yolk does not cleave. The segmentation is thus incomplete ox meroblastic. In the hen's ovum the cytoplasm is divided by successive vertical furrows into a mosaic of cells, which, as it increases in size, forms a cap-like structure upon the surface of the yolk. These cells are separated from the yolk beneath by horizontal cleavage furrows, and successive horizontal cleavages give rise to several layers of cells. The space between cells and yolk mass may be compared to the blastula cavity of Amphioxus and the frog (Fig. 18). The cellular disc or cap is termed the germinal disc or blastoderm. The yolk mass, which forms the floor of the blastula cavity and the greater part of the ovum, may be compared to the large yolk-laden cells at the vegetal pole of the frog's blastula. The yolk mass never divides but is gradually used up in supplying nutriment to the embryo which is developed from the cells of the germinal disc. At the periphery of the germinal disc new cells constantly form until they enclose the yolk.

Cleavage in Mammals

The ovum of all the higher mammals, like that of man, is isolecithal and nearly microscopic in size. Its cleavage has been studied in several mammals but the rabbit's ovum will serve as an example. The cleavage is complete and nearly equal (Fig. 17), a cluster of approximately uniform cells being formed within the zona pellucida. This corresponds to the morula stage of Amphioxus. Next an inner mass of cells is formed which is equivalent to the germinal disc, or blastoderm^ of the chick embryo (Fig. 17). The inner cell mass is overgrown by an outer layer which is termed the tropltectoderm, because, in mammals, it later supplies nutriment to the embryo from the uterine wall. Fluid next appears between the outer layer and the inner cell mass, thereby separating the two except at the animal pole. As the fluid increases in amount, a hollow vesicle results, its walls composed of the single-layered trophectoderm except where this is in contact with the inner cell mass. This stage is known as that of the blastodermic vesicle. It is usually spherical or ovoid in form, as in the rabbit, and probably this is the form of the human ovum at this stage. In the rabbit the vesicle is 4.5 mm. long before it becomes embedded in the wall of the uterus.

Fig. 17.— Dbgrann showinf* the cleavage of ihe mammalian (rabbits) o\Tim blastiHiermic vesicle (Allen Thomsitn, nflcr van Bencilcn).

Among Ungulates (hoofed animals) the vesicle is greatly elongated and attains a length of several centimeters, as in the pig.

If we compare the mammalian blastodermic vesicle with the blastula stages of Amphioxus, the frog, and the bird, it will be seen that it is to be homologized with the bird's blastula, not with that of Amphioxus (Fig. 18). In each case there is an inner cell mass of the germinal disc. The trophectoderm of the manunal represents a precocious development of cells, which, in the bird, later envelop the yolk. The cavity of the vesicle is to be compared, not with the vesicle of mammal.

Fig. 18. — Diagrams showing the blastulae: A, of Amphioxus; 5, of frog; C, of chick; D, blastodermic

Uastula cavity of Amphioxus and the frog, but with the yolk mass plus the rudimentary blastoccele of Ifie bird's ovum. The mammalian ovum, although almost devoid of yolk, thus develops much like the yolk-laden ova of reptiles and birds. This similarity has an evolutionary significance. Its cleavage, however, is complete and the early stages in its development are abbreviated.

In Primates, but one cleavage stage has been observed. This, a four-celled ovum of Macacus nemestrinus figured by Sclenka, shows the cells nearly equal and oval in form. This ovum was found in the uterine tube of the monkey and shows that, in Primates and probably in man, cleavage as in other mammals takes place normally in the oviducts.




1lir lilantula an^l early blastrjdermic vesicle show no differentiation into hiyiMH. Stidi difTcrrmtiation takes place later in all vertebrate embryos

, giving th\r fii'Ht to the eitoderm ami entoderm^ and finally to the mesoderm. From these ihiiT prinuiry germ layers all tissues and organs of the body are derived.

Till* pnuTHjM'S of ^astrulation, by which ectoderm and entoderm arise, and of nirsodcrin formation will l>e treaterl separately.

Amphloxut and Amphibia. In these animals the larger cells at the vegetal pnir nf ihr blasttila n'thcr fold inward, i. e., invaginate (Amphioxus, Fig. 19), or WW lor the most part overgrown by the more rapidly dividing cells of the animal


Vwi 1^ (U^dulrtlionor nmphio\u7« UtiU>Hhck in UcislcrV X220. .4, Blastula: a, animal cells; fi N vH>*t«*M\v \vlU» » t » \ Un»n rtR\' \ rt\ ity. H, Ut^inning in\*n):ination of vcgetath-e pole. C, GastruU, the uu<^4(M{^U\M\ wl tho wni^lrtliNxMvIN Mi\K \vm|4rtr: n7.. ivtvxlerm; rfi/.. entoderm; arcA., aichenteron;

|H^lo v<n^^philM{0. KATntunUy the invaginatinj; colls obliterate the blastula cavity M\\\ \\\\\w \\\ \\\\\\iW\ with tho oulor layer of ivUs vFig. l^V The new caWty thus Km uh\J iv \ he i^inul ivv ^iut \>r *m Ai^^itihi and its nanxnveil mouth is the blastopore. \\w \^\\W\ la\ev v\t vvH> is the t\:%sirrm, iho innor, ncwlv formed laver is the enio^\- %y Vh\^ ouusKMiual vyH> a«x^ honvvforih \\xiux^mi\l in the nutrition and metab\Nl\>*n\ nM Ov KsU rho on^Mw^ iv lu^w iornu\l a Gasirula . little stomach).

K^|M\li^* Ami ttinU. rho j^^vuuual x?isv\ or Wasuviom*,. in these animals lies »0vo .^ X A5^ >v,^ ^iv v\u",axv \M iux^t; w^lk xFxj: .^\ Sinoc the oniMrmous amount of wvK ev^V> V <,wi;*,v.x>^;u^^^\ .^v v.\ Vv,^5^^,^^\us ar*xi ar^'.jxhil^^ur.s in:poiS!5iMe, the process

Origin of the Mesoderm, Notochord and Neural Tube

There appears caudally on the blastoderm of reptiles a pit-like depression. From this slight invagination a proliferation of cells forms a layer which spreads beneath the ectoderm (cf. Fig. 21 A). The inner layer originating in this manner is the enloderm, and the region of the pit where ectoderm and entoderm are continuous is the blastopore.

In birds the caudal portion of the blastoderm b rolled or tucked under, the inner layer formed in this way constituting the entoderm. The marginal region where ectoderm and entoderm meet bounds the blastopore, while the space between entoderm and yolk is the arckenteron.


As in cleavage, so also in gastrulation the mammalian ovum exhibits a modified behavior indicative of an ancestral yolk-rich condition. The entoderm apparently arises by a splitting off, or delamination, of cells from the under side of the inner cell mass (Figs. 16, 74 A and 75) . In the blastoderm of the rabbit, opossum, and mole, however, a minute pore has been observed at which the ectoderm and entoderm are continuous. This opening is believed by some to represent a true blastopore where the ingrowth of entodermal cells has occurred.

Amphioxus and Amphibia

The dorsal plate of entoderm, which forms the roof of the archenteron in Ampkioxus, gives rise to paired lateral diverticula or ccdomic pouches (Fig. 20) . These separate both from the plate of cells in the mid

FiG. 20.— Origin of the mesodenn in .^mphioxus (after HaUchek). X about 425. n.j., Neural (groove: nx,, neural canal; ch., aniage of notochord; mrs. torn., mesodeimal segment; M.. ectoderm; ml., entoderm: at., cavity of gut; ca., ca\om or body cavity.

dorsal line (which forms the nolochord), and from the entoderm of the gut, and become the primary mesoderm. The mesodermal pouches grow ventral and their cavities form the ccelom or body cavity. Their outer walls, with the ectoderm, form the body wall or somalopleure ; their inner walls, with the gut entoderm, fonn the intestinal wall or splanchnopleure. In the meantime, a dorsal plate of cells, cut off from the ectoderm, has formed the neural tube (anl^e of the nervous system), and the notochordal plate has become a cord or cylinder of cells (axial skeleton) extending the length of the embryo. In this simple fashion the ground plan of the chordate body is developed.

In Amphibia the mesodermal diverticula grow out from the dorsal entoderm as solid plates between the ectoderm and entoderm. Later, these plates split into two layers and the cavity so formed gives rise to the coelom.

Fig. 21. — l.ongitui]inal sections oF the snake's blastoderro at various stages to show the origin of the notochordal plate (adapted aFter Hertwig).


The same pocket-like depression in the caudal portion of the hlasttxlerm, which gave rise to the cells of the entodermal layer, now invaginates more extensively and forms a jxiuch which pushes in between ectoderm and entoderm (Fig. 21 A and B). The size of the in\'ngination ca\-ity varies in different species; in some it is elongated and narrow, being confined to the middle line of the lilast(xlerni. The tl«*ir of this jxiuch siK>n fuses with the underlying entoderm and the two thin, niplure, and disapix'ar. thus putting the ca%ity of the pouch in communication with the space (archenteron) beneath the entoderm (Fig. 21 C). The cells of the roof persist as the notochordal plate which later gives rise to the notochord. The neural folds arise before the mouth of the pouch (blastopore) closes up, and, fusing to form the neural tube, incorporate the blastopore in its floor. This temporary communication between the neural tube and the primitive enteric cavity is the neurenteric canal (Fig. 21 C); it is found in al the vertebrate groups (cf. Fig. 78). A transverse section through the in vagina ted pouch, at the time of rupture of its floor, and the underlying entoderm will make clear the relatively slight lateral extent of these changes (Fig. 22).

From about the blastopore, and from the walls of the pouch, mesodermal plates arise and extend like wings between the ectoderm and entoderm (Fig. 22). As in amphibia they later separate into outer (somatic) and inner (splanchnic) layers enclosing the coelom (cf. Fig. 29 B). The relation between notochordal plate, mesoderm, and entoderm shown in Fig. 22 resembles strikingly the conditions in Amphioxus (Fig. 20 ^4).

Fig. 22. — Transverse section of a snake's blastoderm at a level corresponding to the middle of Fig. 21 C (adapted after Hertwig).


Due to the modified gastrulation in reptiles, birds, and mammals through the influence of yolk, a structure known as the primitive streak becomes important. An account of its formation and significance based on conditions found in the bird may be introduced conveniently at this place.

Shortly after the formation of entoderm there appears in the median line at the more caudal portion of the blastoderm an elongated opaque band (Fig. 23). Along this primitive streak there forms a shallow primitive groove^ bounded laterally by primitive folds. Cranially the groove ends in a depression, the primitive pit. In front of this pit the streak ends in a knob, the primitive knot (of Hensen).

The primitive streak becomes highly significant when interpreted in the light of the theory of concrescence, a theory of general application in vertebrate development. It will be remembered that the entoderm of birds arises by a rolling under of the outer layer along the caudal margin of the blastoderm. As the blastoderm expands it is believed that a middle point on this margin remains longitudinal slit must also be an elongated blastopore whose direction has merely been changed. The lips of the slit fuse, forming the primitive streak. The primitive groove may be interpreted as a further futile attempt at invagination in the region of the blastopore. The teachings of comparative embryology support these con. elusions, for the neurenteric canal arises at the cranial end of the primitive streak, the anus at its caudal end, while the primary germ layers fuse in its substance. All these relations exist at the blastopore of the lower animals.

Area opaca

ti.xed while the edges of the margin on each side are carried caudad and brought

together. Thus a crescentic margin is transformed into a longitudinal slit as in

Fig. 24. Since this marginal lip originally bounded the blastopore (p. 29) the

From the thickened ectoderm of the primiti\'c streak a proliferation of cells lake's plaw and then.- gniws out literally and caudally between the ectoderm and

ontixKTnt a solid plate of mostxlorm (Fig. 31 B and C). From the primitive knot

« niesixlcrinal shwl alsti extends cephalad forming along the midline a thicker

layer, the sti-oalU-d /icut f> or nolofhordal f4aic. which fuses intimately with

the entrtierni O'lRS. -S. -'0 and ,tl .41.

Siiuv the primitive .streak and Knxn-e

represent a iwixlitieit blast ojH>t\\ it is

evident thai this erauial extension, the

h«Md pi\Ht>ss, ^^*rr^•s^^H^ds to the (xweli++++liWe inxaRinalion i\*naTn<xl in the forn»a++++lioHof mesoderm and noto.hoitl i» rt-p++++tiU-s In bii>is \\\c lUMon of the head

pi\>,x'-.x wi(h ihe ent.xteim. the Mation

oi in.M>.leimaI ^hivts u\ ii ImeiaUv . ihe

ivMnianoH i^t the ii.>l.Hh.v>xl h\M» its ti^

vne ,t»d llv ,s. ,,i^i,mi(l ti;i,vv in it ,*( .i *■

Fig. 2,*.— niMtodcrm of a chick cmbij-o I <hc stage of the pnmiiivc siroak and ^w\T \\t> hours). X ».

Fig. M.— llucnm ductdating the fonna(itm tif the primuHv simk (Duval m Heisler). rW in>-rc>sin$ siif tif Htc fxna disc in the v>i«r>* .if iSc ikAvlopment is indicated by dotted! .m-Mlat lincsi. The htav^- lines represent tth- .prs.vnii.- jnw\ic and (he priDiiti\-e streak n hk-h »ri«s iriim it by tlw fusion of the edges

.ill the ivn«lili.>ii

... ^^r^y\■

\\\\ tW

ous with the primitive pit rilxxi lor the less modi&ed

.Ml- a priniiii\t streak and knot essentially as in birds (Figs. 26 A and 28). Similarly from the keel-like ectodermal thickening of the primitive streak, mesoderm grows out laterally and caudally, and from the primitive knot it is continued cranially as the head process. All three primary germ layers fuse in the primitive streak and knot, this condition being known in man. The head process of many mammalian embryos contains a cavity {notockordal canal), which in some cases is of considerable size, opening at the primitive pit. As in reptiles, the floor of this cavity fuses with the entoderm and the two rupture and disappear. A still persistent portion of the floor is shown in Fig. 27. Thus a neurenteric

Fig. 25. Bead process -Median longitudinal section a chick embryo i process. X 10

Enlodertn Mrsodrrm

t the stage o( the primitive streak and head

canal, later enclosed by the neural folds, puts the dorsal surface of the blastoderm into communication with the enteric cavity beneath the entoderm (Figs. 77 and 78). The roof of the head process or notochordal canal is for a time continuous with the mesoderm and entoderm (compare these relations in reptiles. Fig. 22), but it eventually becomes the notochord.

The extent of mesoderm in rabbit embryos is shown in Fig. 28. Cranial to the primitive node the notochord is differentiated in the midline, the mesoderm being divided into two wings. The mesoderm rapidly grows around the wall of the blastodermic vesicle until it finally surrounds it and the two wings fuse ventrally (Fig, 29). The single sheet of mesoderm soon splits into two layers, the cavity between being the calom or body cavity. The outer mesodermal layer (somatic), with the ectoderm, forms the somatopleure or body wall, the inner splanchnic layer, with the entcxlenn, forms the intestinal wall or splancknopleure. The neural tube having in the meantime arisen from the neural folds of the ectoderm, there is present the ground plan of the vertebrate body, the same in man as in Amphioxus.

Fig. 26. The primitive streak of pig embryos (Keibel). X 20. A, Kmbryo with primitive streak and prim'e knot; B, a later embryo in which the neural grtM%'e is also present, cephalad in position.

No stages of gastrulation or mesoderm formation have yet been observed in the human embryo, but the primitive streak may be recognized in later stages (Fig. 77), and there is evidence also of an opening, the neurenteric canal, leading from the exterior into the cavity of the primitive gut (archenteron). In Tarsius, an animal classed by Hubrecht with the primates, the mesoderm has two sources: (t) From the splitting of ectoderm at the caudal edge of the blastoderm; this forms the extra-embryonic mesoderm and takes no part in forming the body of the embryo. (2) The inlra-embrycmic mesoderm, which gives rise to body tissues, takes its origin from the primitive streak and knot as in the chick and lower mammals. The origin of mesoderm in the human embryo is probably much the same as in

Fig. 27. — Median longitudinal section through the blastoderm of a bat (Vcspertilio murinus) (after Van Beneden).

Fig. 2S. — Diagrams showing the extent of the mesoderm in rabbit embryos (Ktilliker). In A the mesoderm is represented by the pear-shaped area about the primitive streab at the caudal end of the embryonic disc; in B, by the circular area which surrounds the embryonic disc.

The Notochord or Chorda Dorsalis

Unlike in Amphioxus ana amphibia, the head (notochordal) process and mesoderm of higher vertebrates are not clearly of eritodermal origin, but are derived from the ectoderm, any union with the entoderm being secondary. As the primitive streak recedes caudalward during development the head process is progressively lengthened at the expense of the former. Ultimately the primitive streak becomes restricted to the tail region, whereas the entire remainder of the body is built up around the head process as an axis. In later stages, the rod-like notochord extends in the midline beneath the neural tube from the tail to a dorsal out-pocketing of the oral entoderm, known as Seessel's pocket (p. 81). It becomes enclosed in the centra of the vertebrae and in the base of the craniimfi, and eventually degenerates. In Amphioxus it forms the only axial skeleton and it is persistent in the axial skeleton of fishes and amphibians. In man, traces of it are found as pulpy masses {nuclei pulposi) in the intervertebral discs.

Fig. 29. — Diagrams showing the origin of the germ layers of mammals as seen in transverse section (modified from Bryce).