Book - Vertebrate Embryology (1913) 6

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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 VI The Germinal Layers

By the germ-layers we understand certain groups of cells which contain in themselves the materials for certain definite groups of organs and tissues. These groups of cells are definitely separated from one another at an early period of development, and the process of their separation is spoken of as the formation of the germinal layers.

The germinal layers in a Vertebrate are three in number, the ectoderm, the endoderm, and the mesoderm. The ectoderm is that group of cells which contains within itself the material for the formation of the epidermis and epidermal derivatives like hair, feather, skin-glands, the enamel of the teeth, the nervous system both central and peripheral, and the sense organs, and further the stomodaeum and proctodaeum, or entrances to the mouth and anus ; the endoderm contains the material for the lining epithelium of the alimentary canal and its outgrowths, such as gill-slits, thyroid, thymus, lungs, liver, pancreas, bladder ; while from the mesoderm- with which we include the notochord - skeleton and connective tissues, muscles, blood and vascular system, coelom and urogenital organs will be derived.

The germ-layers are thus definable by thek fate in development. They may also be defined with reference to their position in the embryonic body when they have been definitely segregated from one another, for then the ectoderm is the outside layer, the endoderm the inside layer, while the mesoderm with the notochord is in between. Prior to that moment, however, it is difficult if not impossible, to give generaUy valid definitions of these sets of cells by their position, since the method of theur origin from the different cells into which the substance of the ovum is divided by cleavage varies in the several groups.

In a Vertebrate the germinal layers are segregated durmg a process which is known as the formation and closure of the blastopore, or in an older terminology ' gastrulation the ' gastrula ' being the name bestowed on this stage in which a new cavity, the ' archenteron ' or primitive gut, is formed and is in communication with the exterior by an aperture, the blastopore.

This opening, and -ndth it the germinal layers, is from the first bilaterally symmetrical. This is true of all Vertebrates, but in the method of its origin the phylum must be divided into two great groups, those in which the blastopore arises at the edge of the blastoderm - ^the Anamnia - and those in which it appears inside the blastoderm - the Amniota. By the help of the Gymnophiona, however, the gap between the two may be bridged.

Anamnia

We shall begin with the Anamnia, in which the conditions are much simpler.

As a type we shall take the common English frog {Eana iemporaria) .

The first sign of the formation of the germ-layers is given as soon as segmentation is at an end by the appearance of the structure known as the dorsal lip of the blastopore (Fig. 61). This is a short, deeply-pigmented rim bounding a groove, placed parallel to the equator, and a little below it (about 25°) at that point in the boundary between the pigmented and unpigmented regions of the egg where the latter area is most extensive. This is the side on which the grey crescent was formed and the original unpigmented area so increased. The plane which includes the egg-axis and the dorsal lip will shortly become the median longitudinal or sagittal plane of the embryo ; it coincides evidently with the plane of symmetry of the unsegmented ovum.

The egg is still in the position into which it turned at the time of insemination with its axis vertical, and the heavy white pole below. The changes that now take place as seen from this vegetative pole are as follows. The rim of the groove begins to travel downwards over the surface of the egg towards the vegetative pole, the area over which it passes becoming covered by cells which are as deeply pigmented as those of the animal portion of the egg. At the same time the rim elongates, becoming crescentic ; in other words, the processes of rim formation and overgrowth are extended to the right and left along the margin of the pigmented area, and the lateral lips of the blastopore come into being. As the dorsal lip (the middle region of the rim) continues on its course towards the vegetative pole, and as continually fresh parts are drawn into the process at the sides, the blastoporic lip becomes first semicircular, and then three parts of a circle, until finally, when that part which is diametrically opposite to the dorsal lip, namely the ventral lip, also begins to grow down, it attains the form of a circle enclosing the still uncovered portion of the vegetative hemisphere, the yolk-plug. The dorsal lip has now moved down to or a little beyond the vegetative pole.

Fig. 61. - Diagrams of the closure of the blastopore in the egg of the common frog {R. temporaria). In a-d the egg is viewed from the vegetative pole, in E, F from below. The dorsal lip is at the top of the figures. In D the ventral lip has just been formed and the blastopore is circular. In E the rotation of the whole egg has begun, and in f is complete.

At this moment the whole egg begins to rotate about a horizontal axis in the opposite direction to that in which the dorsal lip moved ; and this rotation continues - the circle of the blastopore becoming smaller all the time - until the dorsal lip has returned, rather beyond the point from which it started, to the new equator, or horizontal plane through the centre of the egg. The end now occupied by the blastopore is posterior. The angle subtended by the arc traversed by the dorsal lip - both before and during the rotation - is 75°, and the angle through which the whole egg rotates is 100°. It follows that the vertical line now drawn through the centre of the egg, which will be the dorso-ventral line of the embryo, makes the same angle of 100° with the original egg-axis ; that the animal pole is situated below what will be the anterior end of the embryo (Fig. 62, f), since the blastopore is posterior ; and that the antero -ventral haK of the embryo is developed over the animal, the postero -dorsal half over the vegetative hemisphere of the egg. The dorsal and ventral lips are now actually dorsal and ventral.

It is clear that the lip of the blastopore which is thus formed and closed arises along the whole of the boundary between small pigmented and large yolk-cells, and that the process is bilateral, taking place, as it does, first and most rapidly at the dorsal lip, last and least rapidly at the ventral lip, and at an intermediate rate at the lateral lips in between.

The examination of sections (Fig. 62) will now show us that the closure involves (1) a movement of the yolk-cells into the segmentation cavity together with (2) an overgrowth and ingrowi^h of cells at the blastoporic lip, resulting in the formation of a new cavity, the ' archenteron ' ; and that during the process the material for the germinal layers is brought into position and laid down.

A sagittal section of the egg passing through the dorsal lip at its first appearance shows the groove placed about 25° below the equator in the zone of intermediate cells. The radial disposition of the cells immediately about the groove marks the beginning of a process of overgrowth and ingrowth which becomes more obvious a little later, when it is seen that a fold of small cells has grown over a certain area of yolk-cells. This fold consists naturally of two sheets, an outer and an inner. The cells of the outer sheet resemble closely the small pigmented cells of the animal hemisphere into which they are uninterruptedly continued ; like the latter, they are arranged in about four layers, the outermost of which is epithelial. At the lip of the blastopore the outer passes into the inner sheet, the cells in the outermost layer of the former being gradually turned over into the innermost layer of the latter. This inner sheet also consists of several layers of cells, the innermost of which is pigmented and epitheUal, the remainder being more irregularly disposed. The inner sheet forms the outer, or, as it will be when the egg has rotated, the upper wall of the slit-like cavity between itself and the yolksurface now covered up. This cavity is the archenteron and the inner sheet of the fold is its roof ; the original vegetative surface of the egg forms its floor.

This overgrowth and ingrowth of cells, with consequent formation of an archenteric cavity, takes place in an exactly similar fashion at the lateral (Fig. 63, a) and ventral lips. By the time the latter has appeared the archenteric cavity is much enlarged, first by its being extended in an anterior direction into the yolkcells that have meanwhile been pushed up into the segmentation cavity on the dorsal side, and secondly in a lateral and finaUy a ventral direction by a movement of the mass of yolk-cells towards these regions of the egg also. The segmentation cavity is thus first reduced to a small space upon the ventral side and then obliterated altogether. In a small percentage of cases however, the segmentation cavity communicates with the front end of the archenteron, is surrounded by yolk-cells, and mcorTiorated in the front end of the gut.

' t the shifting of the heavy yolk-cells to the ventral s.d tZ alters the eentre of gravity and so ca,.es the rotafou of the egg until equilibrium is regained.

arch

mes

mes. Y.

Fig, After rotation ^'*^°P°'^'^- before rotation ; e, During rotation ;

archenteron; y p! yolk fri^f/-'^ segmentation cavity; arcL,

formed below th;;^! ^ ^'^"'^ = mesoderm

pushed into the segment; Sty."""^"'" ""^'^ '^'^"^ "^'^ ^-^k-cella

With the exception of the yolk-plug the outer surface of the egg is now covered by a sheet of small cells, disposed in about four layers, the outermost of which is epithelial and pigmented. This sheet is the ectoderm. In part it comes from the original animal cells which formed the roof of the segmentation cavi. r â–  but part of it is derived from the outer sheet of the blastoporic fold.

Fig 63 - Transverse sections of the frog's egg. A, During the closure of The blastopJie • B, After, mes. 2, mesoderm differentiated from the yoM Tus^^^^^^^ segmentation cavity (in B these are seen to be

Central); U., lateral lip of the blastopore; n.c/^., notoehord.

The notochord and the dorsal mesoderm are differentiated out of the roof of the archenteron (Figs. 63 B, 64). The latter sheet of cells becomes split into (1) a thin layer next the cavity (this will be the roof of the ahmentary canal) and (2) a layer next the outside. This outer layer is divided into (a) a median strip or rod, which is the notochord, and (6) two lateral shee s, the dorsal mesoderm. The notochord is not separated miti after the sheets of mesoderm have been detached. The separation o both notochord and mesoderm begins at the anterior end and proceeds backwards. At the lip of the blastopore there is thus for a time an undiiferentiated mass of tissue in which ectoderm notochord, mesoderm, and roof of the alimentary ^canal are all continuous (Fig. 62, e). It will be remembered that the front end of the archenteron arises by an extension of that cavity into the yolk-cells ; here, therefore, yolk-cells form the roof, and it is from them that the anterior portions of notochord and dorsal mesoderm are formed. The posterior portions, however, arise in that part of the archenteric roof which comes into position as the inner sheet of the blastoporic fold.

The ventral mesoderm (Fig. 63, b) has a similar double origin. In front the floor of the archenteron is formed of the yolk-cells pushed into the segmentation cavity ; the cells next the ecto

Fig. 64.- Three stages in the differentiation of the roof of the archenteron in the irog. arch, archenteron; n.ch., notochord ; mes., dorsal mesoderm.

derm subdivide and become mesoderm. Behind mesoderm arises from the inner sheet of the fold at the ventral lip. At the sides of the embryo dorsal and ventral mesoderm pass continually into one another. The middle layer, therefore, taken as a whole, arises anteriorly and ventrally from the yolk-cells, posteriorly and dorsally from the blastoporic overgrowth ; the former is in the onginal animal, the latter in the original vegetative hemisphere.

Smce mesoderm is formed also at the lateral lips, the two sheets of this tissue which flank the notochord aje necessarily continuous, around the blastopore (Fig. 64*), with the mesoderm at the ventral lip (Fig. 62, e) ; only at the dorsal lip, where the notochord is formed, is there an interruption in the middle layer. The endoderm or lining of the gut cavity is what is left of the roof and floor of the archenteron, the roof of the gut being the thin layer left when the notochord and mesoderm have been detached, the floor the bulky mass of yolk-cells after the separation of the ventral mesoderm.

It must be remembered that though the differentiation of these germ-layers is only completed when the blastopore has closed, it has in reality been in progress during the earlier stages.

Fro 64* - Horizontal section of an older stage showing the sheets of mesoderm passing back into the lateral Ups of the blastopore (b.j).).

It still remains for us to discuss very briefly the origm of the cells from which the blastoporic fold is derived, that is, the origin of parts of each of the three germ-layers. The inner layer of the fold is certainly derived neither wholly from the smaU cells of the animal hemisphere, nor whoUy from the large cells of the vegetative hemisphere, but from the region about the egg-equator, in which the cells are of a character intermediate between these two (Fig. 62, i, z.). The outer layer of the fold comes from the same source, and from an extension of the roof of the segmentation cavity. These intermediate cells divide rapidly and give rise to the fold, which, as we have seen, contains ectodermal, endodermal, and mesodermal elements.

To sum up, the ectoderm of the frog comes partly from the cells of the animal hemisphere, partly from the intermediate cells ; the endoderm in part from the latter, in part from the yolk-cells, while the mesoderm and notochord have a similar double origin ; and the materials for these layers are brought into their definitive positions during the bilateral closure of the blastopore, which arises all along the line separating animal from vegetative cells.

We shall see that a similar statement may be made for the remaining Anamnia.

Fig. 65. - ^Formation of the germ-layers in Petrornyzon. (After Scott.) A, Sagittal section ; b, c. Transverse sections of two stages ; arch., archenteron ; d.l., dorsal lip of the blastopore ; n.ch., notochord ; d.m., dorsal mesoderm ; v.m., ventral mesoderm ; m.t., medxillary tube (here a solid wedge of cells).

Cyclostomata

In Petrornyzon (Fig. 65) the formation and closure of the blastopore, the origin and extension of the archenteron, resemble the same processes in the frog, with the exception that a ventral lip is never developed. The ventral mesoderm is diflferentiated from the yolk-cells pushed into the segmentation cavity, as in the frog, and these latter cells form the floor of the gut. They give rise, however, to much more than that, since the roof of the archenteron is converted wholly into the notochord and the gut is then completed by the upgrowt.h of yolk-cells from the sides and underneath the notochord. The dorsal mesoderm arisea m connexion with the overgrowth at the lip of the blastopore

In the Myxinoids (Fig. 66) segmentation produces a blastoderm at one end of the elHpsoid egg. At one point in the edge of this blastoderm a dorsal blastoporic lip appears, and the material for the germ-layers of the embryo is laid down during the bilateral overgrowth and ingrowth of cells in this region. The yolk is not wholly covered by this process, but as soon as

Fig, 66- Bdellostoma. Overgrowth of the posterior edge or dorsal lip of the blastoderm over the yolk, d.l, dorsal lip (posterior edge) ; v.l, ventral lip (anterior edge) ; op.r., operculum of the shell. (After Bashford Dean.)

the body of the embryo is formed all parts of the edge of the blastoderm grow down and the blastopore eventually closes at the vegetative pole.

Elasm obe anohh Germ-layer formation begins with the appearance at one point in the edge of the blastoderm of a fold or overturning of cells of the superficial layer. This point is, as will appear, in the middle line and at the posterior end. The fold, the rim of which is the dorsal lip of the blastopore, is slightly raised and covers over a space- the beginning of the archenteron- between itself and the yolk (Figs. 67, 68). By the continued backward growth of the fold and by the ingrowth of its under layer the archenteron attains a considerable length. The floor of the archenteron is formed of yolk, into which yolk-nuclei subsequently make their way ; its roof consists of a columnar epithelium derived in part from the overturning of cells at the lip of the blastopore, in part possibly from the posterior marginal cells of the lower layer.

Fig. 67. - Overgrowth of the lip of the blastopore and formation of the embryo in Elasmobranchs. (a-c after Riickert, d-f after Ziegler.) c.s., caudal swelling ; l.L, lateral lip. In r the formation of a lip has extended almost to the anterior edge. In d, e, the medullary folds are still open, in r they are closed.

But while this process is taking place at the dorsal lip, that is, at the median posterior margin of the blastoderm's edge, it is also being extended, though in a far less degree, to the neighbouring regions, the lateral lips, on the right and on the left. The archenteron thus comes to assume a crescentic shape, with a median anterior prolongation ; the latter underlies the embryonic portion of the blastoderm, while the crescentic part is wholly extra-embryonic, and remains very shallow, though it is subsequently prolonged to the right and left round the edges of the blastoderm until a slight overgrowth is formed even at the anterior margin.

With the overgrowth at the lips of the blastopore the material for the germinal layers is laid down (Fig. 69). The superficial layer is now the ectoderm. The mesoderm consists of two parts : (1) two sheets of cells lying one on each side of the middle line over the embryonic portion of the archenteron ; posteriorly these sheets pass into the caudal swellings- two thickenings at the edge of the blastoderm, one on each side of the middle line - where they are continuous with the roof of the archenteron, out of which they have been differentiated ; (2) the formation of mesoderm is, however, not limited to the parts immediately adjacent to the dorsal lip, but is carried on at the lateral lips, and, as these extend forwards round the whole edge of the blastoderm, at the anterior edge as well. This extraembryonic mesoderm is naturally continuous in the caudal swellings with the embryonic mesoderm first described ; it takes part only in the formation of the area vasculosa.

The notochord is formed from a median strip of cells which is cut out of the roof of the archenteron ; the process, like the differentiation of the mesoderm, takes place from before backwards. With the separation of the notochord and mesoderm the remainder of the archenteric roof is endoderm, and gives rise to the alimentary canal, the front end and sides bending dovm

VJ

THE GERMINAL LAYERS

123

Fig. 69.- Five successive transverse sections through the hinder (embryonic) portion of the blastoderm of the dog-fish during tlie formation ot the germinal layers a is posterior, cutting the two caudal sweUings ; E, Anterior through the head of the embryo, arch., archenteron; mes mesoderm; e.m., embryonic mesoderm; ex.m., extra-embryonic mesoderm ; w.c^., notochord ; lateral lip of the blastopore

124

THE GERMINAL LAYERS

VI

and meeting to form the ventral wall. The yolk in the floor of the archenteron plays no part in this process (Fig. 70).

Up to the present it is the posterior edge or dorsal lip which has been principally active, but now the anterior and lateral margins of the blastoderm become exceedingly vigorous and begin to grow over the yolk, the overgrowth being accompanied, as stated above, by a slight marginal invagination ; and eventu

y.n.

Fig. 70.- Two stages in the formation of the gut of the dog-fish by the bending down and fusion of the edges of the roof of the archenteron. y.n., yolk-nuclei.

ally the anterior edge makes the whole ckcuit of the yolk, passmg round the vegetative pole and reappearmg behind the embryo as the ventral lip of the small ' yolk-blastopore ' (Fig. 71). At the dorsal hp backgrowth of the caudal sweUings is responsible for the posterior elongation of the body of the embryo alone, the body being raised above the surface of the yolk. Where the body passes into the hinder edge of the blastoderm growth of the latter ceases, but the lateral edges immediately adjacent to this point swing backwards untU they bound a narrow median strip of yolk by which alone the aperture at the dorsal lip now communicates with the rest of the blastopore.

as

B

a.e = i/.l.

C. D.

f M^*^" Extension of the blastoderm over the yolk after formation and loldmg ofiE of the embryo in an Elasmobranch. a, The lateral lips liave swung back parallel to one another behind the dorsal lip, so enclosino- a narrow strip of yolk. B, Side view of the same, c, The anterior ed-o («.e.) has passed beyond the vegetative pole, and in d it appears behind the embryo as the ventral lip (yi.) ; y.b., yolk- blastopore.

P. 124

VI

THE GERMINAL LAYERS

125

Teleostei

The processes are essentially the same as in the Elasmobranchs. Blastopore formation begins at the posterior edge, where the

Fig. 72.- Growth of the blastoderm over the yolk after the formation of the material for the embryo in the Teleostean fish Serramis. (After Wilson ) d.L, dorsal hp of the blastopore (posterior edge of the blastoderm) â–  a e anterior edge of the blastoderm or ventral lip {v.l.) of the blastopore • s.c, segmentation cavity ; o.g., oil-globulc. '

backward growth of the dorsal lip with concomitant development of an archenteric cavity gives rise to the body of the embryo, but the process is extended to the lateral and anterior edges, where there is a slight invagination. By the growth of

Fig. 73.- Sagittal sections through the blastoderm of Serranm during the formation of the germinal layers. (After Wilson.)

A, Beginning of overgrowth at dorsal lip {d.l.).

B, Overgrowth at anterior edge.

c, Later stage of posterior edge. t u i

D, The anterior edge has become theventraUip {v.l.); n.cA., notochord ;

end., endodorm ; m.p., medullary plate ; par., parablast (periblast) ; y.f.,

yolk-plug ; K.v., Kuppfer's vesicle.

Fig. li.- Serra^ms. Transverse sections showing differentiation of the roof of the archenteron into notochord (n.ch.), mesoderm (mes.), and cndodprm {end.); j^ar., parablast (periblast). (After Wilson.)

aic. 3

Fig. 75. - Serranus. Formation of the gut (a/.c.) by the bending down of tlie sides of the roof of the archenteron. s.n.ch., sub-notocliordal rod; aid., cndoderm. (After Wilson.)

VI

THE GERMINAL LAYERS

127

these extra-embryonic edges the yolk is finally enclosed and the anterior margin is then the ventral lip (Fig. 72). Notochord and mesoderm are differentiated in the roof of the embryonic part of the archenteron, the rest of this layer giving rise to the alimentary canal, as in Elasmobranchs. Extra-embryonic mesoderm arises at the remaining edges of the blastoderm (Figs. 73-75).

v.l.

Fig. 76.- Formation of the germ-layers in Ganoid fishes, a, b, in the Sturgeon {Acipenser) (after Bashford Dean) ; c, d, in Amia (after Sobotta) ; arch.^ archenteron ; d.l, dorsal lip ; v.l., ventral lip ; n.ch., notochord ; mes., mesoderm.

Ganoidei

Our knowledge of the differentiation of the germinal layers is very slight, but it is known that the closure of the blastopore is bilateral, and that mesoderm is formed at its lips, the notochord in the middle dorsal line (Fig. 76).

128 THE GERMINAL LAYERS VI

DrPNOi

The holoblastic egg of Ceralodus resembles that of the frog very closely in the development of its archenteron. The roof

Fig. 77. - Formation of the germ-layers in Dipnoi. A, B, in Ceralodus (after Semon) ; c, D, in Lepidosiren (after Graham Kerr), arch., archenteron-; d.l, dorsal lip ; n.ch., notochord ; ines., dorsal mesoderm.

of this cavity, however, takes no part in the formation of the gut, but is differentiated simply into median notochord and lateral plates of mesoderm. The yolk-cells then grow up to complete the dorsal wall of the aUmeutarx, canal (Fig. 77, A, u).

VI

THE GERMINAL LAYERS

129

Lepidosiren resembles the frog in all respects, except that the yolk is more vokimmous and that a ventral lip is never developed (Fig. 77, c, D).

Urobelotjs Amphibia

The method of germ-layer separation is here practically identical with that which is observed in the frog, except in one important respect. In the bilateral closure of the blastopore, the presence of a ventral as well as of a dorsal lip (Fig. 78, A) and the formation of the mesoderm from a double source, the two groups closely resemble one another ; but while in the frog the under layer of the roof of the archenteron persists as the dorsal lining of the alimentary tract, in the Urodeles the roof of the archenteron becomes wholly converted into the notochord, as in Petromyzon, and the gut must be completed dorsally by an ingrowth of yolk-cells from the sides (Fig. 78, b, c).

The Anurous Amphibia, such as the toad, generally resemble the frog in this matter, but in one case the notochord is described as being formed from the middle streak of the whole thickness of the roof, and even in the frog such a procedure may be experimentally instigated by subjecting the embryos to the influence of cane sugar and other substances.

A comparison of these processes in the small-yolked and the large-yolked types shows that :

1. The blastoderm of the large-yolked corresponds to the animal region of the small-yolked egg, the yolk to the vegetative part, and that the edge of the blastoderm in the former is equivalent to the boundary between animal and yolk cells in the latter.

2. In both this bounding line becomes in its entirety the lip of the blastopore (except where the ventral lip is absent), the posterior point of the edge in the large-yolked being equivalent to the dorsal lip of the small-yolked, the anterior point to the ventral lip.

3. In both the germinal layers are laid down during the bilateral closure of this blastopore, the notochord stretching in front of the dorsal lip, the mesoderm springing from the lateral lips in two sheets which are continuous with one another behind the ventral lip.

1355 T

130

THE GERMINAL LAYERS

VI

4. The principal points of difference are two. First, the closure of the blastopore in Elasmobranchs, Myxinoids, and Teleostei, is effected in two periods ; during the first the overgrowth is almost confined to the dorsal lip and produces the material for

-piQ. 78 - Formation of the germ-layers in the Axolotl.

A, Sagittal section after completion of the blastopore and rotation of the egg.

B, Transverse section of the same stage. i. u a c Dorsal part of a transverse section of a later stage, n.ch., notochord;

d.l.', dorsal hp ; v.l, ventral lip ; mes.v., mesoderm formed ^Wentral hp ; mei.l, dorsal mesoderm ; mes.2, ventral mesoderm (from the yolk-cellh pushed into the segmentation cavity) ; end., endoderm.

the formation of the embryo ; in the second the yolk is gradually covered by an extension of the blastoderm in which the lateral and anterior margins are alone concerned. Secondly, in these cases a part only of the blastoporic hp is involved in the formation of the embryo, the lateral and ventral lips remaining wholly extra-embryonic.

VI

THE GERMINAL LAYERS

131

Gymnophiona

In this group the egg is so laden with yolk that in it segmentation nearly approaches the meroblastic type and results in a blastoderm lying on a partially divided yolk. This blastoderm consists of a superficial epithelium of columnar cells, covering

Fig. 79.- Formation and closure of the blastopore in the Gymnophiona. A-D, Surface views of the blastoderm of Hypogeophis. The lateral lips are seen to meet behind, and so form the ventral lip ; y.p., yolk-plug (after Brauer). e. Embryo of Ichthyophis lying on the partially segmented yolk which is still uncovered by the blastoderm. (After the brothers Sarasin.)

several irregular layers of scattered cells which are more abundantly supplied with yolk. The cavities between these cells are equivalent to the ordinary segmentation cavity. Below these again is the yolk, divided at its surface into cells, and containing nuclei scattered through its substance. Immediately round the blastoderm the surface of the yolk is also partially segmented. At one point- the posterior middle point- of the edge of this

I 2

132

THE GERMINAL LAYERS

VI

blastoderm the dorsal lip appears (Fig. 80) ; it exhibits the usual radiate arrangement of cells. The lip quickly grows back and so produces a long archenteron which comes to open into

80.- Formation of the germ-layers in Hyi)0!7eoi)te. (After Br auer.) A-c Sagittal sections of three successive stages, d, Transverse section Lough^he blastopore and yolk-plug {y-v.) ; s.c, ^^tlX /f into which in B and o the archenteron {aroh.) opens ; U., dorsal hp , l.L, lateral lip ; and vX., ventral Up.

the segmentation cavity in front. The roof of the archenteron which seems to be derived entirely from the superficial layer of the blastoderm, consists of a plate of columnar cells, its floor of the partially segmented yolk.

VI

THE GERMINAL LAYERS

133

The process of overgrowth is not limited to the dorsal lip, but extends to the immediate right and left. Surface views (Fig. 79, a-d) show that the transversely placed li]p soon becomes crescentic, and that the horns of the crescent then grow not only backwards, but towards the middle line as well, approaching one another until they meet and so form what is the ventral lip of the now circular blastopore. In section it is seen that there is a slight ingrowth at the lateral and at the ventral lips of a plate of cells continuous with the similarly formed jolate

Â¥iG. 81.- Transverse sections of HypogeopMs showing the differentiation ot the root of the archenteron into notochorcl (nxh.) and mesoderm, and

Z fXf (Iftttrarxg"' ^"'^-^

which forms the roof of the archenteron in front ; beneath the plate is a slit-like space, also, of course, archenteric ; in the midst of the blastopore is the projecting typically Amphibian yolk-plug.

But m spite of this resemblance there is a very serious difference between the ventral lip of the Gymnopliiona and that of ail other Anamnia. For while in the latter the whole of the edge of the blastoderm or small-celled area is converted into a blastoporic hp, the posterior point being the dorsal, the diametrically opposite anterior point becoming sooner (in small-yolked eggs) or la er (m large-yolked eggs) the ventral lip, and while consequently the whole of the vegetative surface of the egg is covered up when the blastopore closes, in the former the anterior and

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THE GERMINAL LAYERS

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a large part of the two lateral edges take no iiart in this process, which is confined to the posterior and immediately adjacent portions of the edge ; this small portion gives rise to the dorsal and two lateral lips, which latter by their fusion produce the â–  remarkable similitude of the ventral lip of other forms. As a result the vegetative hemisphere is still uncovered when the blastopore has become circular (Fig. 79, d, e). The importance of this fact for the correct understanding of the relations of the blastopore to the blastoderm in the Amniota cannot possibly be over-emphasized.

To return to the germinal layers. The superficial layer is now the ectoderm. The roof of the archenteron becomes divided into a median strip- the notochord, and two lateral sheets- the

mesoderm which are continuous with one another behind the

yolk-plug by means of the cell-plate invaginated at the lateral and ventral hps (Fig. 81). The mesoderm has in fact precisely the same relations as in other Anamnia at this stage. The notochord passes back into the dorsal lip. No additions are made to either notochord or mesoderm from any other source. The roof of the gut (endoderm) is completed by upgrowth and ingrowth of vegetative cells underneath the midcUe layer.

Amniota

Whereas in the Anamnia the blastoporic lip appears at the edge of the blastoderm, in the Amniota it hes wholly within the latter. The blastopore leads into an archenteron, and with the formation of these structures the materials for the germinal layers are laid down. Only in the more primitive forms is the archenteric cavity well developed ; usually it is much reduced and represented only by the ' neurenteric ' passage or ' chordacanal In primitive forms the upper and lower layers are still united at the point where the blastopore and archenteron arise, and both layers may perhaps be said to share in their formation ; but in most cases all these parts are derived from the upper layer of the blastoderm alone, the subsequent fusion with the lower layer being purely secondary. The edge of the blastoderm, which is entirely independent of the blastopore, grows steadily over the surface of the yolk, finally enclosing it at the vegetative pole.

i

IPiG 82 - Three stages in the formation of the blastopore at the hinder end of the embryonic shield of a Reptile {PlaUjdacUjlus). Sm'face views. (After Will.)

P. 1S5

II

VI

THE GERMINAL LAYERS

136

The Reptiles will be considered first as the whole process is far clearer in them than in the other two groups.

Rbptilia

There is distinguishable in the blastoderm at the close of segmentation a circular or oval area placed excentrically towards the posterior end ; this area is the embryonic shield. The upper layer of the blastoderm consists of cyHndrical cells in the embryonic shield, of flat cells in the surrounding region ; below it is the segmentation cavity. The lower layer is an irregular sheet of scattered rounded cells, not arranged at present in an epithelium, and is constantly being reinforced by the addition of cells from the nucleated yolk beneath. Between the lower layer and the yolk is a shallow cavity, the subgerminal cavity. In some forms, such as Platydactylus and Lacerta, there is one point in the margin of the embryonic shield where upper and lower layers are continuous ; this is the primitive plate, and it is situate at what will be the hinder end (Fig. 83, a). The lower layer cells before long arrange themselves in a flat epithelium. Meanwhile a depression has appeared in the primitive plate ; this is the beginning of the archenteron, and its anterior margin is the dorsal lip of the blastopore. Seen from the surface (Fig. 82) the dorsal lip presents the appearance of a transverse rim bounding a groove at the hinder edge of the embryonic shield. The rim rapidly becomes crescentic, the horns of the crescent turn back, meet, and fuse behind the primitive plate which now corresponds exactly to the Gymnophionan yolk-plug.

During the backgrowth of the horns of the crescent, which are the lateral blastoporic lips, the cavity of the archenteron has rapidly extended until it reaches the anterior end of the embryonic shield (Fig. 83) ; the cavity is broad. The roof consists of a layer of columnar cells which at the dorsal lip turn over in the ordinary way into the cells of the upper layer. The floor is in front distinct from the lower layer, and here it consists of a single layer of cubical cells ; behind the dorsal lip - in the primitive plate- it is much thickened, and from this thickenmg there proceeds backwards a narrow tongue of cells between the upper and the lower layers.

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THE GERMINAL LAYERS

VI

A transverse sectign (Figs. 84, a ; 86) through the blastopore shows the mass of cells of the primitive plate flanked on each side by a projecting blastoporic lip and sending out between the upper and lower layers two lateral sheets of cells.

Fig. 83. - Sagittal sections of the blastopore and archenteron in the Gecko Platydadylus. (After Will.) a-e, Successive stages ; jj.^p., primitive plate ; pd., lower layer or paraderm ; s.g.c, subgerminal cavity ; arch., archenteron ; d.l, dorsal lip ; y.p., yolk-plug ; mes.v., mesoderm formed at the ventral lip.

The resemblance between these structures and those in the Amphibian, and particularly the Gymnophionan egg when the blastopore has become circular, is sufficiently obvious. The dorsal and lateral lips (there is no ventral lip in the Reptiles) clearly correspond in the two cases ; the mass of cells in the primitive plate embraced by these lips is the yolk-plug ; the

Fio. 84. - Four successive transverse sections through the blastopore and archenteron of Plalydactylus. (After Will. )

A, Posterior section through the yolk-plug {y.f.) ; l.l, lateral lip ; 2^(1. , lower layer ; mes., mesoderm springing from the lateral lips.

B is more anterior, just behind the dorsal lip.

c is just in front of the dorsal lip, where the floor of the archenteron {arch.) is still intact, and

D more anterior, where the archenteron communicates with the subgerminal cavity.

c

arc?-..

c

138

THE GERMINAL LAYERS

VI

cavity of invagination is the archenteron in which floor corresponds to floor and roof to roof ; lastly, the sheets of cells projecting beneath the upper layer at the sides of and behind the blastopore are the equivalents of the mesoderm formed at the lateral and ventral lips in the Amphibia.

From this comparison it follows of course that cells which are the morphological equivalents of the yolk-cells of the Amphibia are to be found in the upper layer of the Reptihan blastoderm. That layer, therefore, cannot be termed the ectoderm until the process of invagination is complete.

The floor of the archenteron now fuses throughout with the lower layer, and as soon as the fusion is completed perforations

-PTP 86 - Transverse section of the blastopore and yolk-plug (y.p.) of thTS^itoise (Trionyx). (After Mitsukuri.) U., lateral lip ; > m/so^erm produeed at the lateral lips ; pel., lower layer not yet detaehed from the yolk (stippled).

begin to appear in tte fused layers (Figs. 83, E ; 84, e). They seem to be unable to keep pace with the general gro^vth of the blastoderm and to become first stretched and then fenestrated. But to whatever causes the perforation may be due, the floor of the archenteron with the underlying lower layer completely disappears, and the archenteron then communicates freely with the subgerminal cavity. The roof of the archenteron is now inserted by its edges into the surrounding lower layer.

The median strip of the roof next thickens to form the notochord (Fig 85), and separates from the two lateral portions which then become the mesoderm. The notochord passes posteriorly into the dorsal lip. the plates of mesoderm into the latei^hps of the blastopore, and here the latter are perfectly contmuous with the mesoderm produced at the sides of and behind the

I

Fig 87*.- Area pellucida of the lien's egg. a, After 12 hours , b, After 18 hours' incubation, as seen by transmitted light. J5r.£/., prnnitive groove ; n.ch., notochord ; pr.am., pro-amnion.

P. 139

VI THE GERMINAL LAYERS 139

blastopore (Figs. 84, a, b ; 86). The mesoderm thus exhibits all the relations which it has in the Anamnia.

The Uning epithelium of the alimentary canal (endoderm) is derived from the lower layer, which grows in from the sides below the mesoderm and notochord (Fig. 85, c, d). From this layer the gut is subsequently folded off, the remainder being yolk-sac epithelium. In several cases the lip of the blastopore is not the only source of origin of notochord and mesoderm, both receiving additions in front, and the mesoderm at the sides also, from the lower layer.

Fig. 87. - ^Formation of the primitive streak and groove of the chick by proliferation of cells of the upper layer. Transverse sections.

A, At 10 hours. There is at present no sign of the primitive groove ; the lower layer {'pd.) takes no part in the proliferation.

B, At 15 hoiu's. The primitive groove has appeared. It is occupied by a projecting mass of cells, tlie yolk-plug {y.'p.), and bounded by the lateral hps {U.). The proliferated cells spread out on cacli side as the lateral sheets of mesoderm {mes.).

The conditions observed in the Birds are very readily derived from and very easily understood in the light of those which obtain in the Reptiles.

There appears in the posterior region of the blastoderm a proliferation of cells in the upper layer (Fig. 87, a) ; this rapidly extends in the median line, and along it there appears a narrow groove. The cell proliferation is the ' primitive streak ', the groove the "primitive groove" (Fig. 87*).

Fig. 90. - ^Anterior (a) and posterior (b) halves of a sagittal section through the primitive streak and associated structures of the sparrow. (After Schauinsland.) There is a sUght cavity, archenteron, below the dorsal Hp (d.i.), and a well-marked ventral lip {vL). n.ch., notochord ; p.s., primitive streak ; 7nes.v., mesoderm behind the ventral lip ; p.a., lower layer.

Fig. 91. - Transverse section of the anterior end of the blastoderm of the chick "at 15 hours showing the formation of anterior notochord (n.ch.) and mesoderm (mes.) directly from the lower layer {end.) ; ec., ectoderm.

This primitive groove is simply an elongated and laterally compressed blastopore. In front of the anterior end - the dorsal lip - the notochord is produced (Figs. 88, 89) ; to right and left of the notochord are the sheets of mesoderm which, springing from the sides - the lateral lips - of the groove (Fig. 87, b), are continued into one another behind its posterior end, where there may be an actual ventral lip (Fig. 90). The archenteric cavity has, however, in most cases disappeared, though a vestige of it is sometimes to be seen (Fig. 90) . Between the sides of the groove- which still exhibit the structure characteristic of blastoporic hps, is merely a mass of cells- representative of the yolk-plug (Fig. 87, b) - fused with the lower layer. The so-called ' neurenteric canal ' , which appears later, is the sole remnant of the archenteron together with the communication which we have seen to become established between it and the subgerminal cavity in Reptiles. The primitive streak and groove invariably originate in the upper Icayer, fusion with the lower layer being merely secondary ; only after the germ-layers have been formed can the upper layer be described as ectoderm.

The notochord and mesoderm receive increments in front from the lower layer (Fig. 91).

The gut (endoderm) is formed as in Reptiles.

Mammaiaa

In the Monotremata there is a long archenteron with a much reduced lumen produced from the upper layer. The blastopore is an elongated ' primitive groove '. The notochord and mesoderm have the usual relations to these structures. The interpretation put by Wilson and Hill on their observations - namely, that the dorsal lip and archenteron are derived from the ' primitive plate ' while the primitive streak and groove are of distinct origin - is probably erroneous. We may accept Assheton's explanation that the ' primitive plate ' of the authors is simply the point of final enclosure of the yolk by the blastoderm, a precociously rapid process in this form, and that archenteron and primitive groove are, as everywhere else, parts of one and the same structure (Fig. 92).

We are still in ignorance of the formation of the germinal layers in Marsupials, though we may hazard the conjecture that the embryonic area of the blastocyst wall will be found to behave like the embryonic shield in Reptilia, that a blastopore and archenteron will be developed near its posterior edge in connexion with which the notochord and mesoderm will arise in the usual way, that the archenteron will break through into the subgerminal cavity below the lower layer, and that this layer will give rise to the gut.

This indeed is what occiurs in the Placental Mammals, the only diflference being that here the embryonic area is from the first enclosed in the sac of the trophoblast as part of the embryonic knob. This knob, as we have already seen, is, together with the lower layer, differentiated from the original inner mass.

The embryonic area (Fig. 92*), derived from the embryonic knob, behaves precisely as the embryonic shield of the upper layer in Reptiles, giving rise to an archenteron and blastopore ; this event is, however, postponed until after the amnion has been formed.

trek a

Fig. 132. Diagiain of the egg of Ornithorhynchus after formation of the germinal layers. (After Assheton's modification of Wilson and Hill.) x, the point at which the blastoderm has finally enclosed the yolk ; here the upper layer (double line) and lower layer (broken line) are continuous with one another and with the yolk. This is the ' primitive plate ' of Wilson and Hill, a to p, primitive streak ; a, anterior end (dorsal lip) ; p., posterior end. In front of a. is the archenteron (arch.), behind p. the mesoderm of the ventral lip {mes.v.).

Fig. 92.- Embryonic shield of the dog. (After Bomiet.) In the embryonic shield, where the cells are columnar, the nuclei are more closely packed than in the surrounding trophoblast, where the cells are flat. At tne posterior end is a notch, the blastopore (lower end in the fagure).

When the archenteron has been developed it behaves in the manner we are already acquainted with. Its floor fuses with the lower layer, and then the two break away so that the archenteron comes to communicate with the subgerminal or yolk-sac cavity (Fig. 93). The notochord is differentiated out of its roof, the mesodermal sheets pass into the lateral lips and are

Fig. 93. - a. Longitudinal section of the embryonic shield and blastopore of the bat, VesperlUio. (After Van Beneden.) The archenteron (arch.) has broken through into the subgerminal cavity [s.g.c.) or cavity of the blastocyst. Below tlie dorsal lip (d.l.) is the blastopore (so-called neurcnteric canal), and behind this the yolk- plug {ij.j}.). (With this should be compared Fig. 138, which shows a human embryo in the same stage.)

B, Transverse section showing the origin of the notochord [n.cli.) from the roof of the rudimentary archenteron in the mouse. The floor of the archenteron has already disappeared, mes., mesoderm ; 'pd., lower layer. Above is the ectoderm of the medullar}^ plate.

continuous with one another behind the blastopore. Accessory notochordal and mesoblastic material is proliferated in front from the lower layer. After this the lower layer is endoderm, and gives rise to the gut and yolk-sac, after growing in from the sides underneath the notochord.

The archenteron may be well developed (as in VesperUlio), but more usually is reduced to a narrow canal, the ' chorda-canal ' or, so called, ' ncurenteric ' passage.^

^ Neurenteric passage means properly the communication between the medullary tube and the hmd end of the archenteron. See below, chap. vii.

The Relation between the Amniote and the Anamnian Blastopore

The facts we have now reviewed will have made it evident that there are certain features common to the separation of the germinal layers in all Vertebrates.

Thus in all cases the material for the germ-layers is laid down during an overgrowth and ingrowth of cells which takes place at the lip of the blastopore during the formation and closure of the latter. This closure is always bilaterally symmetrical, beginning at the dorsal lip and taking place most actively there, less actively at the lateral lips, and least of all at the ventral lip. It leads to the formation of a bilateral archenteron, the extent of which is greatest anteriorly, least posteriorly. The layer that now remains outside is the ectoderm. The notochord is differentiated out of the roof of the archenteron in the middle line in front of the dorsal lip, while the mesoderm sheets which flank the notochord pass back to the lateral lips and are confluent with one another behind the ventral lip.

A, 1-3, The closure of the blastopore in such a form as the frog ; 1, 2, before, 3, after rotation of the egg. The blastoderm, or small-celled area, is heavily stippled. Its whole edge, which becomes the lip of the blastopore, is represented by a thick continuous line, d.l., dorsal, v.l., ventral lip.

B, 1-3, Three similar stages in such a form as Lepidosiren, where the ventral lip is absent. Only that part of the edge of the blastoderm which becomes converted into a blastoporic lip - namely, the posterior and immediately adjacent parts - is indicated by the thick continuous line, d.l, dorsal lip. '

c, 1, 2, The condition seen in the Gymnophiona, where still less of the edge of the blastoderm- only a small part at the posterior end, represented by the thick line- becomes the lip of the blastopore, but the lateral lips swing back, meet, and fuse to form the ventral lip, v.l. Thus the yolk (white) remains uncovered.

D, 1, 2, The Amniote blastopore. The heavily stippled area is the embryomc shield, the central portion only of the Amniote blastoderm but the equivalent of the whole blastoderm of the Anamnia. From the posterior part of its margin a blastoporic lip is formed (d.l, dorsal lip) and by the bending back and union of the lateral lips a ventral lip (v I ) as m the Gymnophiona. i' v

The lightly stippled area outside this represents the extra- embryonic Fâ„¢,"? blastoderm; which is equivalent to the yolk-cells immediately surrounding the blastoderm of the Gymnophiona ^ xlr^l^^-^l the unsegmented yolk (white). Thus the blastopore of the f^t lTfC formed inside its blastoderm, but at the edge of what is equiva

Th« blastoderm, namely, the embryonic shield. ^

lUe yolk is finally covered later on by the growth of the blastoderm

k2

148

THE GERMINAL LAYERS

VI

So far there is general agreement. There is, however, a very serious difference between the two great groups of Vertebrates in respect of the reh^.tion of the blastoporic lip to the blastoderm - the cap of cells produced at the end of segmentation in a largeyolked egg or the area of small cells in a small-yolked egg - for in the Anamnia the blastopore arises from the edge of this blastoderm (Fig. 94, a), while in the Amniota it arises inside it (Fig 94, d). By the help of the Gymnophiona, however, the second condition may without difficulty be derived from the first.

In the Gymnophiona (Fig. 94, c) (1) the blastoderm is an oval area of columnar cells resting upon and surrounded by a partially segmented yolk. (2) Only a part of the edge of the blastoderm is converted into a blastoporic lip, namely, a small region at the posterior end. Here a dorsal lip is formed and lateral lips quickly follow ; the lateral lips then turn back, encircling a small area of the yolk, behind which they meet and fuse to form a ventral lip to the now circular blastopore. In this process the anterior margin of the blastoderm is wholly unconcerned. (3) The archenteron opens into the segmentation cavity, notochord and mesoderm are derived from its roof, the endoderm from the yolk-cells which lie in its floor. The notochord stretches in front of the dorsal lip ; the mesoderm sheets springing from the lateral lips are continuous with one another behind the ventral lip.

As a result of this peculiarity in the formation of the ventral lip the yolk remains uncovered. In all other Anamnia, however, where the ventral lip is developed from the anterior edge of the blastoderm, the yolk is necessarily covered up by the closure of the blastopore.

We turn now to the Amniota, to the Reptiles for instance, and find (1) that the embryonic shield is a circular or oval area of columnar cells resting upon a lower layer, and surrounded by a zone of flattened cells. (2) At the posterior margin of this embryonic shield upper and lower layers are continuous. Here a dorsal lip is formed and lateral lips quickly follow ; the lateral lips turn back encircling a small area of the outer zone of cells- where these are continuous ^\'ith the lower layer- behind which they meet and fuse to form (a virtual, in some cases an actual)

VI

THE GERMINAL LAYERS

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ventral lip to the now circular blastopore. In this process the anterior margiji of the embryonic shield is wholly unconcerned. (3) The archenteron opens into the subgerminal cavity, notochord and mesoderm are derived from its roof, the endoderm from the lower layer. The notochord stretches in front of the dorsal lip, the sheets of mesoderm springing from the lateral lips are continuous with one another behind the ventral lip.

It seems clear, then, that the embryonic shield of the Amniota is the representative of the blastoderm of the Gymnophiona (and of all Anamnia), while the marginal zone of the upper layer, together with the lower layer with which it is at one point - ^the primitive plate - still united, represents the yolk-cells or nucleated yolk.

In passing from the Gjrmnophiona to the higher Vertebrates we have therefore to suppose that with the further increase of yolk segmentation has become restricted not to the blastoderm alone (as in Fishes), but to the blastoderm and those circumjacent and subjacent cells which in the Gymnophiona are partially segmented from the yolk. In the most primitive Reptiles the lower layer cells are still crowded with yolk and still retain a connexion, in the primitive plate, with the marginal cells of the upper layer. In other Reptiles, in Birds, and in Mammals this primitive connexion is lost, and it is only secondarily, after the formation of the primitive groove and streak, that the upper fuses with the lower layer.

The Gymnophionan condition must in turn be derived from some Anamnian blastopore in the formation of which the anterior edge takes no part, in which consequently no ventral lip is formed. Such a form may be found in Lepidosiren (Fig. 94, b), in which the yolk is less abundant than in the Gymnophiona, but more abundant than in the typical smaU-yolked egg. Here the formation of a blastopore is restricted to the dorsal and lateral Jips. The absence of a ventral lip may be a very primitive feature, smce none is found in Petromyzon.

It may also be noticed that the union of segmentation cavity with archenteron occurs here and there in various Anamnia, sometimes in Eana, and in Petromyzon, thus foreshadowing the condition seen in Gymnophiona and the Amniota.

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THE GERMINAL LAYERS

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In the Anamnia, indeed, the archenteron has a direct relation to the endoderm in that, after notochord and mesoderm have been differentiated, the aUmentary canal is formed from its roof, or floor, or both. But as we pass up the series the archenteric cavity loses this significance, its lumen dwindles and finally disappears, and its function is reduced to the differentiation of notochord and mesoderm alone. The endoderm is then derived from the lower layer cells - ^representative of yolk-cells - ^which line the segmentation cavity.

The same lower layer cells may contribute to the notochord and mesoderm anteriorly, and this, as we have seen, is of constant occurrence in such small-yolked Anamnian types as the Amphibia, and Petromyzon ; not, however, in the large-yolked eggs of Fishes.

The Significance of the Gebminal Layers It will have been repeatedly noticed that the same elementary organ or germ-layer may come into being by different processes. This is true of the front end of the notochord and mesoderm, and still more obviously of the endoderm, for the lining epithehum of the alimentary canal may be derived from the roof only of the archenteron (Elasmobranchs and Teleostei), from the floor only {Petromyzon, Urodela, Ceratodus), from both roof and floor {Rana, Lepidosiren), from the yolk-cells in the floor and from those in the segmentation cavity (Gymnophiona, occasionally Rana), or from the lower layer (yolk-) cells of the segmentation cavity alone (Amniota).

In considering such discrepancies in the mode of origin of homologous structures- and discrepancies of this kind are of common occurrence, not only in development from the egg but also in budding and regeneration- it must be borne in mind that experiment has shown the formation of the embryonic organs- such as the germ-layers- to be dependent on the presence of certain stuffs in the cytoplasm of the ovum, but that these stuffs are not necessarily deposited in the situations which will eventually be occupied by the organs to which they give rise, nor even in the same position in the ova of animals belonging to the same group. Thus they may occupy dissimilar

VI THE GERMINAL LAYERS 151

positions also in the segmented ovum, and again in the later stage which we speak of as gastrulation or the closure of the blastopore. The necessary materials - now cut up into cells - have then to move into their definite positions, and thus we witness the roof of the gut being formed by an upgrowth of yolk-cells, or its floor by a bending down of the roof of the archenteron.

The way in which an organ is developed is not, therefore, necessarily a criterion of its homologies. Homologous structures, that is, those derived, like the alimentary tract of the Vertebrate, from some common ancestral structure, may differ in their origin during individual development. The stuffs on which their differentiation depends are doubtless comparable, but the paths by which that differentiation is achieved may be diverse.

LITERATURE

R. AsSHETON. Professor Hubrecht's paper on the early ontogenetic phenomena in Mammals. Quart. Jotirn. Micr. Sci., 1909.

E. VAN Beneden. Untersuchungen iiber die Blatterbildung, den Chordakanal und die Gastrulation bei Siiugetieren. Anat. Am. iii, 1888.

R. Bonnet. Beitrage zur Embryologie des Hundes. Anat. Hefte, Abt. ix, 1897.

A. Beaueh. Beitrage zur Entwickelungsgeschichte der Gymnophionen. Zool. Jahrb. x, 1897.

Bashfoed Dean. The early development of gar-pike and sturgeon. Journ. Morph. xi, 1895.

Bashfoed Dean. On the embryology of Bdellostoma stouti. Festschr. f. C. von Kupffer, Jena, 1899.

L. F. Hennequy. Embryog6nie de la truite. Journ. de VAnat. et de la Phys. xxiv, 1888.

J. W. Jenkinson. Remarks on the germinal layers of Vertebrates and on the significance of germinal layers in general. Mem,. Manchester Lit. and Phil. Soc. I, 1906.

J. Geaham Keeb. The development of Lepidosiren paradoxa. Quart. Journ. Micr. Sci. xlv, 1901.

K. MiTSUKUEi and C. IsmKAWA. On the formation of the germinal layers in Chelonia. Quart. Journ. Micr. Sci. xxvii, 1886.

J. Ruckeet. Die erste Entwickelung des Eies der Elasmobranchier. Festschr. f. C. von Kupffer, Jena, 1899.

H. ScHAUiNSLAND. Studien zur Entwickelungsgeschichte der Sauropsiden. Zoologica, xvi, 1903.

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11. Semon. Die Furchung und Entwickelung dcr Keimbliitter bei Ceralodiis forstcri. Zool. Forschiivgsreise in Anslralien, 1901.

A. E. Shipley. The development of Pctromyzon jluvialilis. Quart. Joum. Micr. Sci. xxvii, 1887.

J. SoBOTTA. Die Gastrulation von .4»n{aaZw. VerJtaiidl. Anat. Geaellsch. Berlin, 1896.

C. O. Whitman and A. C. Eycleshymer. The egg of Aviia and its cleavage. Journ. Mor]}h. xii, 1897.

L. Will. Die Entwickelungsgescliichte der Ileptilien. Zool. Jahrh. vi, ix.

H. V. Wilson. The embryology of the sea- bass {Scrramts alrarius). Bull. U. S. Fish Commission, ix, 1889.

J. T. Wilson and J. P. Hill. Observations on the development of Ornilhorhynchus. Phil. Trails. Roy. Soc, Series B, cxcix, 1907.

H. E. ZiEGLEE. Beitrage zur Entwickelungsgeschichte von Torpedo. Arch. mikr. -Anal, xxxix, 1892.

i

Fig. 95. - External features of the development of the tadpole of the Frog.

a. Medullary plate, anterior end : the three divisions of the brain are apparent.

h. The same embryo from the posterior end : the sides of the medullary plate pass back on either side of the blastopore. The blastopore is now reduced to a narrow slit by the approximation of the lateral lips ; at the dorsal and ventral lips the aperture is rather wider.

c. Medullary folds and groove, anterior end : the three divisions of the brain are readily seen, and the anterior part of what will be the spinal cord. External to the inner medullary folds are the outer, and these pass in front into the broad gill-plates, in front of which again are the senseplates.

d. Closure of medullary folds, but the suture is still visible : the gill-plate is divided on each side into two, and in front of it is the sense-plate ; behind the gill-plate is a slight constriction.

e. Anterior view of the same embryo : the medullary folds have not quite closed in front. Beneath their anterior end is a depression, the stomodaeum, and on either side of this the sense-plates ; the gill-plates can just be seen behind these.

f. Posterior view of the same embryo : the medullary folds have closed over the dorsal division of the blastopore (neurenteric canal) while the ventral remains as the proctodaeum. The middle region of the blastopore is marked by a very narrow suture.

g. Later embryo from below showing the stomodaeum, in front of the V-shaped sucker, and posteriorly the proctodaeum at the base of the tailstump.

h. Older embryo from the right side. The tail is rather longer, the proctodaeum at its base : the stomodaeum can be seen in front between the two halves of the sucker. At the side of the head in front is the nostril, behind the gill-slits.

i. Older embryo (ready to hatch) with well-developed tail and external gills.

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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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Cite this page: Hill, M.A. (2024, June 1) Embryology Book - Vertebrate Embryology (1913) 6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Vertebrate_Embryology_(1913)_6

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