Book - Vertebrate Embryology (1913) 5
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Jenkinson JW. Vertebrate Embryology. (1913) Oxford University Press, London.
- Vertebrate Embryology 1913: 1 Introduction | 2 Growth | 3 The Germ-Cells, their Origin and Structure | 4 The Germ- Cells, their Maturation and Fertilization | 5 Segmentation | 6 The Germinal Layers | 7 The Early Stages in the Development of the Embryo | 8 The Foetal Membranes of the Mammalia | 9 The Placenta | Figures
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Chapter V Segmentation
Apart from the exudation of the circum- vitelline fluid, and, in some cases at least, the assumption .of a definite bilateral symmetry, the first sign that the fertilized ovum gives of its activity is cleavage or segmentation. In this process the material of the egg-cell, which, as we have seen, has a certain structure, is cut up by successive nuclear and cell-divisions into an increasingly greater number of increasingly smaller elements. The division of the nuclei is always by karyokinesis.
These cleavages pass through the egg substances in a perfectly definite way, which may be readily described by reference to the structure and symmetry of the egg, its axis, poles, and equator.
As a type we may consider the cleavage of such an egg as that of the common frog {Ham, temporaria). The egg is of the smallyolked or microlecithal type, and its cleavage is total or holoblastic, that is to say, the whole substance of the germ is divided (Fig. 47).
The first cleavage is a meridional one, that is, is in a plane which includes the axis of the egg. The cleavage begins at the ammal pole, and is seen externally as a fairly wide f m-row. The division is extended inwards, and at the surface of the eg<r gradually round to the vegetative pole. At the very beginning" therefore, it may be seen that the protoplasm is divided more readily than the yolk. Prior to the division the surface of the egg at the animal pole is markedly flattened. At the sides of the furrow are a number of smaU wrinklings in the superficial skm or membrane of the cytoplasm (not the vitelline membrane but the surface layer of the egg itself). These wrinklings which are at right angles to the furrow at the animal pole and directed away from the animal pole at either end, are quite transitory effects of the internal forces to which cleavage is due. As soon as it is completed the furrow becomes narrowed.
By a meridional furrow the egg is necessarily divided into equal parts or blastomeres. The cleavage may, however, be parallel to a meridian and therefore unequal. This is not, however, in any way prejudicial to a perfectly normal development.
The two blastomeres soon prepare for the simultaneous divisions of the second phase. As before, the division is preceded by a flattening of the egg at the animal pole, and the same transverse wrinklings are seen. In each blastomere the furrow begins at the animal pole, proceeds internally and round to the vegetative pole.
Usually the two divisions of this phase are again meridional and intersect the first fuiTow at right angles. In that case there are four surfaces of contact between blastomeres intersecting in one line, the egg-axis. This is cleavage of the pure radial type. But it may happen, owing to slight inequality of division in either one or both of the two blastomeres, that the second furrows fail to meet, either at the animal or at the vegetative pole, or at both. There is then intercepted between them a small portion of the first furrow, known as the cross- or polar-furrow. By shifting of the blastomeres the polar-furrow soon comes to make an angle with the remaining portions, that is the ends, of the first furrow, these two ends being parallel to one another. There are now five surfaces of contact between blastomeres, surfaces which intersect approximately at angles of 120°, and cleavage is no longer radial.
There are now four blastomeres, each of which has a similar portion of the animal, pigmented, and vegetative, unpigmented, regions of the egg. The simultaneous divisions of the third phase separate these regions, for the cleavage is latitudinal, or parallel to the equator and nearer the animal than the vegetative pole. The divisions intersect the first two cleavages at right angles. The result is four small animal, four large vegetative blastomeres, the former wholly, the latter partly pigmented. During the progress of the division the transitory superficial wrinklings may again be seen.
In the fourth phase of cleavage division begins rather earlier in the animal than in the vegetative cells. In direction it is on the whole meridional, and at angles of 45° to the first two cleavages, and therefore of the radial type ; but there are manydepartures from this rule. Thus, instead of intersecting at the animal pole, the furrows may run into the first or second, and in a variety of ways ; in one variety the resulting arrangement is isobilateral (with two planes of symmetry), in another it is bilateral (with only one plane of symmetry), and the latter is stated to be the normal method of the fourth cleavage in the edible frog (Sana esculenta). But though liable to much variation the divisions are always parallel to a meridional plane, or only slightly oblique ; they are never parallel to the equator. As has been already pointed out, these irregularities of cleavage do not involve any abnormality in development.
Fig. 47. - Segmentation of the egg of the frog [Rana iemporaria) except a {Raiia esculenta). (e, h, from Morgan, after Schulze ; g, after Roux.)
A. First furrow, from the side of the grey crescent. The furrow is in the plane of symmetry, that is, in the middle of the grey crescent. The furrow has not quite reached the vegetative pole.
B. First furrow from the animal pole. The division is not quite meridional in this case ; the two cells are therefore unequal.
c. From the vegetative pole. First furrow completed, and now closing up. Second division with a wide furrow coming round from the animal side. The first furrow has cut the grey crescent obliquely (the grey crescent is at the top of the figure).
D. Beginning of the second division, from the animal pole.
E. Second division in which the furrows do not intersect the first furrow at the same point ; the part of the first furrow intercepted between them is a polar furrow, and the first furrow is bent twice.
F. Typical division of the fourth phase, seen from the side opposite the grey crescent. The thu'd latitudinal division is completed ; the fourth is completed in the animal, but not yet in the vegetative cells.
G. The bilateral foiirth division in the egg of Rana esculenta, from the animal pole. The fii-st division runs up and down the page, the second from side to side. The third is in the plane of the page. The upper side of the figure is the side of the grey crescent (dorsal). The second division has been unequal, the two dorsal cells being smaller. On this side the furrows of the fourth phase run into the second furrows, while on the opposite ventral side these divisions run into the fii'st.
H. Fourth division. Abnormal case in which the furrows are paraUel to the first. (From the animal pole.)
I Fourth division. Usual appearance from the animal pole. The furrows do not meet exactly, but pass into the first or second near the animal pole.
J. Side view of the fifth (latitudinal) division.
K. Later stage, seen from the left side, the grey crescent being on the right of the figure.
L End of segmentation from the same point of view as the last. The grey crescent has become white and the original white area so enlarged.
In the fifth phase the furrows are once more latitudinal, and result in the production of four tiers of eight cells each. In the animal cells the division is approximately equal, in the vegetative imequal, four smaller pigmented blastomeres being separated from four larger, partly unpigmented. In this phase, again, division begins first in the animal cells. Departures from accurately latitudinal division are of irequent occurrence.
Up to this moment segmentation has been fairly regular, and synchronous in each phase, at least in cells belonging to the same region of the egg ; but from now onwards there is little regularity in direction, or simultaneity in time of division. The only rule that is rigidly adhered to is seen in the more rapid division of the small, pigmented, protoplasmic animal cells, the less rapid cleavage of the larger, yolky, unpigmented vegetative cells.
Further, while up to now all the furrows have been perpendicular to the surface, tangential divisions, separating the outer from the inner portion of a cell, now occur, and the segmentation cavity or blastocoel is formed (Pig. 48). The first sign of this may indeed be detected in the eight-cell stage, as a small space between the cells, in the axis of the egg, but nearer the animal than the vegetative pole. The cavity is soon enlarged, partly by the secretion of albuminous material, partly by the absorption of water from outside, and becomes eventually an extensive hemispherical cavity in the animal portion of the egg
As a result of all these processes - continued division, more rapid in the animal cells, and tangential as well as perpendicular to the surface, and enlargemerit of the segmentation cavity - the frog's egg, now known as a blastula, presents at the end of cleavage the following appearance (Fig. 48*) : the roof of the segmentation cavity is formed of about four layers of small animal cells. The cells of the outermost layer are deeply pigmented, and an-anged in a cubical, or shortly columnar epithelium ; in the next two layers the cells are rounded, or by mutual pressure polyhedral; in the innermost layer they are again in the form of a cubical epithelium. These animal cells contain only small yolk-granules. The floor of the segmentation cavity is occupied by about twenty layers (in the greatest thickness, that is, in the axis) of large cells heavily laden with large granules of yolk, while the intermediate region, round about the equator of the egg, is occupied by cells which in size, amount of yolk, and size of the yolk-granules, are intermediate between the above two kinds. The superficial pigment extends beyond the equator into the vegetative region, but on one side, that on which the grey crescent was formed and the unpigmented area Ton equen'y enlarged, it is less extensive than on th. other. The segmentation cavity is symmetrically placed about the eggaxis, but Hes wholly in the animal hemisphere.
Fig 48 - Meridional section through the egg of the frog in an early â– stage of segmentation, showing the segmentation cavity.
It is evident that by the process of cjeavage the unlike material of the egg has been cut. up into a number of cells, the characters of which are derived directly from the characters of that region of the egg-substance from which they come. There is thus, during segmentation, no differentiation- beyond the formation of the blastocoel- no new structure formed, and the significance of the act is probably to be sought in the reduction of the c3^oplasm relatively to the nucleus. " Initially, the cytoplasm is too large, by cleavage it is reduced, and when a definite numerical nucleo-plasma ratio has been reached segmentation as such comes to an end and new events - of differentiation - begin.
But while cleavage is thus not a process of differentiation, it is yet true that the particular pattern adopted in cleavage - ^in our own case the radial pattern - is very definitely related to the initial structure of the ovum. That structure, as we have seen, is a polar one, with a radial symmetry about the axis, and the first three furrows are very definitely related to this axis, being successively meridional, meridional, and latitudinal. The direction taken by these and subsequent divisions may very probably be particular cases of the rules known by the names of Balfour and Hertwig.
According to Balfour's rule yolk impedes nuclear and celldivision, and that, as we have seen abeady, and shall see again when we deal with large-yolked eggs, is certainly the case in the Vertebrates. According to Hertwig's first rule the nucleus places Itself m the centre of its sphere of activity, and this in a telolecithalegg is in the axis but excentrically, and nearer the animal than the vegetative pole. According to the second rule of Hertwig, the dividing nucleus - or mitotic spindle - elongates in the direction of greatest protoplasmic mass (Fig. 49), or as Pfliiger phrased it, the direction of least resistance, the resistance being offered by the yolk, and by the surface of the egg. Hence the first spindle has its equator in the axis in the animal portion of the egg, and elongates in a plane perpendicular to the axis, as the disposition of the yolk about the axis makes this a direction of least resistance ; the resulting division is therefore meridional. Similarly in each of the first two blastomeres the greatest protoplasmic mass or least resistance is again in a plane perpendicular to the axis, and, in this plane, in a direction parallel to the first furrow. In this direction the spindle elongates, and once more the division is meridional, and at right angles to the first.
Fig. 48*. Sagittal section through the frog's egg at the beginning of the formation of the blastopore. d.l., dorsal lip; i.z., intermediate zone.
In the third phase, however, the greatest protoplasmic mass in each cell is in a direction parallel to the axis ; at the same time, in each cell the nucleus lies in the protoplasmic portion, near'the animal end : hence the latitudinal division. The direction of division in subsequent stages, as far as it can be followed, may be similarly explained.
Attention has already been called to the effect of the yolk on the rate of division and on the size of the cells.
We have still, however, to inquire into the cause which determines the particular meridian occupied by the first furrow out of the infinite number possible.
As we have seen, prior to segmentation the egg assumes a bilateral symmetry, owing to the formation of the grey crescent on the side opposite the entrance of the spermatozoon. The plane of the grey crescent or plane of symmetry of the egg is of course a meridional plane, and it might be inmgmed that it is this which determines the meridian to be occupied by the first fui-row But this is not so. There is no definite or necessary relation between the two. The first furrow may pass through the centre of the grey crescent (Fig. 47, a), or be oblique (Fig^ 47 c) or at right angles to the plane m which it lies. The reVon for this divergence of the plane of the first furrow from he plane of symmetry lies in the fact that tha latter is determined by the first, the fo.mer by the second part of the path taken by the spermatozoon in the egg, and that the two parts do not necessarily lie in the same meridional plane (Fig. 46).
The sperm enters the egg and immediately passes inwards ; this is the first part of its path, and opposite this the grey crescent is formed. The sperm then turns to meet the female pronucleus,
Fig 50.- Diagrams illustrating the various relations between thp fir
and this is the second part of its path. The centrosome divides at right angles to the meridional plane which includes the line ot union of the pronuclei, the spindle is formed between the two centrosomes, and the division falls in the equator of the spindle that IS, in the second part of the path (Fig. 50). The two parts ot the path may, but need not, lie in the same meridional plane hence the first furrow may, but need not, lie in the plane of the grey crescent.
While, therefore, as far as it is meridional, the direction of the first furrow is definitely related to the original polar eggstructure, the particular meridian it occupies is not so related to the bilateral structure imposed on the egg at the time of fertilization. That structure, on the other hand, as we are soon to see, is the actual forerunner of the bilateral symmetry of the embryo.
It is clear, then, that the factors which determine differentiation are distmct from those to which the pattern of segmentation is to be attributed. The former, for reasons which cannot be more fully set forth now, are to be looked for in certain cytoplasmic organ-forming substances, the latter in the relation between the nuclei, with their centrosomes, and the cytoplasm.
We may now turn to the cleavage of other Vertebrate eggs.
Petromyzon, Ceratodus, and the Urodelous Amphibia have small-yolked holoblastic ova. The course of segmentation is very similar to that in the frog, and, as a result, a similar blastula stage is reached with segmentation cavity in the animal hemisphere. In Petromyzon, however, the roof of the segmentation cavity consists of but one layer of cells. In Ceratodus the third furrows are stated to be meridional, the fourth and fifth latitudinal.
In the so-called Ganoid fishes Lepidosteus, Amia, Acipensei , as well as in Lepidosiren and in the Gymnophiona, the egg is intermediate between the small-yolked and large-yolked types. The additional amount of yolk exerts an influence upon the cleavage, and the vegetative portion of the egg is divided but slowly. Indeed, nuclear division here outruns cell-division, and at the end of segmentation there is produced a larger or smaller cap of small animal cells resting on an incompletely cUvided but multinucleate yolk-mass. The segmentation cavity is small are again meridional or parallel to the first, in Lepidosteus the fuiTOws of the third phase are parallel to the first, those of the fourth phase parallel to the second (as in Teleostei). In Amia the third are meridional, the fourth latitudinal and quite close to the animal pole, the fifth tangentia I in the animal, meridional in the vegetative blastomeres (Pig. 51, a).
(Fig. 51, B). , The direction of the furrows of the first few phases differs m the different cases, although the first two are always meridional and at right angles to one another. In Acipenser the third furrows
53. - Sections tlu-ough the segmenting blastodisc
Elasmobranchs. (After Riickert.) A, B, c, Successive stages ;
A and B, Torpedo ; c, Pristiurus. In c the left side is the anterior end. /. fine, c. coarse, yolk ; m., merocj^^es.
Fig. 52. - Surface views of segmentation of the blastodisc in Elasmobranchs. ( After Riickert. )
A, 7-8-ceIled stage {Torpedo).
B, 8-16-celled stage [Torpedo). c. Blastoderm (Sci/llium).
In all the figures the upper is I the anterior end. Note the larger cells at this end in c.
In the Myxinoids, Elasmobranchs, Teleostei, Reptiles, Birds, and Monotrematous Mammals the egg is very large-yolked and segmentation is meroblastic or partial - that is to say, is confined to the blastodisc or cap of cytoplasm which lies at the animal pole upon the voluminous yolk. As a result of segmentation the blastodisc is cut up into cells and becomes the blastoderm, resting upon an unsegmented yolk. The yolk may, however, contain nuclei, and these may continue to divide for a considerable time.
In all these cases the first two divisions are meridional and at right angles to one another. In Myxinoids the furrows of the third phase are again meridional. Cleavage then becomes irregular. When it is ended the blastoderm consists of a columnar upper layer, some lower layer rounded cells, and a third layer closely appUed to the yolk. In the yolk are nuclei without cell-divisions. In Elasmobranchs (Figs. 52, 53) the third division is in some cases meridional, in others latitudinal, but so near the animal pole as to have the appearance of a circular furrow. Divisions perpendicular to the surface contmue, and tangential divisions soon begin to occur.
At one end- the future anterior- of the blastoderm the marginal cells are distinctly larger than at the opposite end. The first furrow, however (Fig. 52, a), bears no definite relation to the antero -posterior axis.
The tangential divisions separate cells lying at the surface from cells which are still continuous with the yolk below. More tangential divisions increase the number of layers of cells, but the lowermost layer is always continuous with the yolk. Iii the same way at the margin of the blastoderm divisions perpendicular to the surface separate cells at the edge which are continuous with the yolk from cells inside. The marginal cells and the cells of the lowest layer feed upon the yolk, grow, divide again, and so the whole blastoderm increases in diameter and in thickness.
Segmentation therefore leads to the formation of a manylayered blastoderm. The cells of the uppermost layer are arranged in a columnar epithelium. Those below are rounded elements, aggregated especially at the (future) anterior end ; they are known as the lower layer cells, and between them and the yolk is a segmentation cavity, more spacious at the posterior end. In the yolk are numerous nuclei, some of which are derived from the fertilization nucleus, while others, as we have seen, are due to accessory spermatozoa (merocytes).
The nuclei in the yolk divide for some time by mitosis, but eventually abandon this method of multiphcation. They enlarge, become highly and coarsely chromatic, irregular in shape, probably amoeboid, and divide amitotically. Their function is now to break up the yolk, possibly by some fermentative action, and render it suitable for absorption by the embryo. Eventually, when this duty has been performed, they disintegrate and disappear without participating in the development of any embryonic tissue.
In the Teleostei (Fig. 54) the second division is parallel to the first, the fourth parallel to the second. In the nest phase the division is radial (with regard to the centre of the blastoderm) in the corner cells, parallel to the edge in the remaining marginal cells, and tangential in the four central cells. Further divisions result in the completed blastoderm, a compact discoidal mass of cells resting upon, but not contmuous with, the yolk. The upper layer of the blastoderm is epithelial : in the lower layers the cells are polyhedral. The Teleostean segmentation is often described as ' discoidal '.
The detachment of the blastoderm from the yolk is effected at a fairly early stage, but not before a very important process has taken place, the separation of the yolk-nuclei. At a certain stage the marginal cells with their nuclei sink back into the periblast- or hyaline layer surrounding the yolk. cell-divisions disappear and there is left a ring of nuclei round the margm of the rest of the blastoderm. The latter becomes now completely separated from the periblast around it and below it.
.eitation of the egg of the plaice, a, 2 cells ; b, 4 cells the fir, are so placed t at
Se f^urt?rr. P^g^)' ^oniPleted, the furrows o
the lourth phase are appearing ; d. Fifth division.
eggs).' segmentation of the hen's egg (from oviducal
in pro'J-Sr""""""^ l^^^-^ = the fourth (latitudinal) is
posterior end ^' small-cellod area is towards the
The yolk-nuclei then migrate into the layer of periblast underneath the blastoderm. There they cease to divide mitotically, swell up, become vacuolated, irregular, and very chromatic. As in the Elasmobranchs they are concerned with the liquefaction and elaboration of the yolk, and eventually disintegrate and disappear (Fig. 54*).
In some cases (Salmonidae and others) the periblast or yolknuclei are formed not only at the margin but also at the under surface of the blastoderm.
In the Reptiles and Birds (Fig. 55) segmentation quickly becomes irregular after the first two or three meridional divisions. Tangential divisions soon occur, and a blastoderm is formed composed of two layers only, an upper and a lower. In the hen's egg the cells at what will be the anterior end of the blastoderm are larger than at the posterior end. At the margin the two layers are united with one another and continuous with the yolk, as in other cases (Fig. 56). In Reptiles, though not so easUy in Birds, there may be distinguished in the upper layer two regions, an oval are-i in the centre composed of columnar cells (this is the embryonic shield), and a surrounding extra-embryonic area of flattened cells. The lower layer consists of scattered rounded cells, many of which are, in the Reptiles, still continuous with the subjacent yolk. The lower layer cells umte together finally to form a flat epithelium, the lower layer or paraderm (Fig. 57) (endoderm of most authors, but the term should be avoided at this stage).
Fig. 54.- Sagittal section through the blastoderm of Serramis during the formation of the germinal layers (after Wilson) ; showing beginning of overgrowth at dorsal Hp {d.l). par. parablast (periblast).
In the Reptiles the upper and lower layers remain continuous at one pomt up to the time when the germ layers begin to be tormed. This point is at what will be the hinder margin of the embryonic shield, and is known as the primitive plate. It is here that the blastopore will be formed (see Fig. 83).
In the Monotremata amongst the Mammals segmentation is also meroblastic, and it is known that the first two
Fig. 56. - a, Longitudinal section through the segmented blastoderm of the hen's egg ; s.g.c, subgerminal cavity. The anterior end with the larger cells is on the right.
B, Section through the blastoderm of the unincubated hen's egg. In the centre of the blastoderm the lower layer cells lie scattered in the subgerminal cavity ; at the edges the lower layer cells are closely packed and rest on the yolk.
Fig. 57 - Part of a section of the blastoderm of the chick after six " hours' incubation, u.l., upper layer ; l.l, lower layer.
furrows are meridional, the third parallel to the first and at right angles to the second. At the end of segmentation there are two layers.
In all these Amniota the blastoderm originates by cleavage in precisely the same way as in the large-yolked eggs amongst the Anamnia. In the next stage of development, however, their behaviour is different, for in the latter group the blastopore is formed at the edge of the blastoderm, while in the others it always arises inside the blastoderm at the edge of the embryonic shield, so that it is the latter structure which in its future conduct is comparable to the Anamnian blastoderm.
Fig. 58. - Segmentation of the ovtim of the Marsupial Dasyurus. (After Hill.) 1, 2 cells ; 2, 4 cells ; 3, 6 cells ; 4, 16 cells, sh., shell ; a., albumen ; z., zona pellucida ; y.b., yolk-body.
Fig. 59.- Formation of the blastocyst in Dasunruti. (After Hill.)
1, Blastocyst in which all the cells are alike.
2, Older blastocyst, one half (e.) of which is the embryonic, the other {ir.) the trophoblastic area.
In the Amniota the groAvth of the blastoderm over the yolk goes on independently of the development of the blastopore and of the embryo ; only at a comparatively late period is the yolk finally enclosed at the vegetative pole. In the Monotremata, however, this enclosure is effected with great rapidity, and as the edges meet there is produced a spot where yolk, upper layer, and lower layer are all continuous with one another. This (accepting Assheton's interpretation) is the proper explanation of what Wilson and Hill have identified erroneously with the primitive plate of Reptiles (see Fig. 92).
In the remaining Mammalia the egg, as we have seen, is small by loss of yolk, and its cleavage is holoblastic.
In the Marsupial Dasyurus (Fig. 68) a curious phenomenon occurs before cleavage, the extrusion of the yolk-body at the vegetative pole. The first two divisions are meridional and at right angles to one another, the third again meridional. The eight cells lying in a ring round the centre of the zona pellucida now divide unequally, the smaller cells lying on that side on which the yolk-body is situate. Further divisions, perpendicular to the surface of the zona pellucida, lead to the formation of a hollow sphere, the blastocyst, in which two regions become later distinguishable (Fig. 59, 1, 2). In one hemisphere the cells are small, in the other large. The former is the embryonic and derived (according to Hill) from the small cells of the sixteencelled stage, and therefore from the vegetative hemisphere, since it is on this side that the yolk-body was extruded. The latter hemisphere is the trophoblastic, and derived from the larger cells (of the animal region of the egg) in the sixteen-celled stage. In the embryonic hemisphere individual cells become amoeboid, migrate below the surface, and there unite to form a lower layer or ^ endoderm ' (Fig. 69, 3-6), which presently grows round the mside of the blastocyst and forms a completely closed sac, the yolk-sac or umbilical vesicle. The remaining cells of the embryonic hemisphere become cubical or columnar, and are thus sharply marked off from the flat elements of the trophoblast.
In the Placental Mammals the first division is meridional (Fig. 60). After this segmentation is irregular, and results in a spherical mass of cells, in which an outer layer of cubical soon becomes differentiated from an inner mass of rounded elements. By the absorption of fluid a cavity is then formed between these two groups of cells except at one point, the future embryonic pole, where the inner mass remains adherent to the outer layer. Thus the blastocyst stage is reached. The cells of the outer layer or trophoblast (which will give rise to the ectoderm of the false amnion) now become flattened, while the inner mass is differentiated into a round embryonic knob of closely packed cells (this contains the material for the embryo and the ectoderm of the true amnion) and a lower layer of flattened cells (Fig. 60*). The latter quickly grow round the inside of the trophoblast and form the closed yolk-sac. Rarely (in the guineapig) the distal, lower or anti-embryonic wall of the yolk-sac is never developed. The lower layer gives rise to the gut, the yolk-sac epitheUum, and the allantois.
At the stage we have reached, therefore, the material for the embryo, true amnion, yolk-sac, and allantois is shut up in a completely closed sac, the trophoblast or false amnion. All Placental Mammals pass through such a stage, however the amnion may ultimately be formed, and in this respect differ strikingly from the Marsupials, where the embryonic area is at the surface of the blastocyst, and the trophoblast confined to one hemisphere of the latter.
It will be noticed that in the Placental Mammals it is not possible to state what relation exists, if any, between the axis of the blastocyst - ^the line drawn from embryonic to antiembryonic pole - and the original axis of the ovum. In the Marsupials, if Hill's interpretation is correct, these axes coincide, but the vegetative becomes the embryonic pole.
Fig. 60 . - Blastocyst of the mouse. The inner mass has been differentiatedinto embryonic knob (e.k.) and lower layer {l.l.). tr., trophoblast.
Literature
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R. Assheton. The segmentation of the ovum of the sheep. Quart. Journ. Micr. Sci. x\i, 1898.
R. Assheton. The development of the pig during the first ten days. Quart. Journ. Micr. Sci. xli, 1898.
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W. Heape. The development of the mole. Quart. Journ. Micr. Sci. xxvi, 1886.
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