Book - Outline of Comparative Embryology 1-7

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Richards A Outline of Comparative Embryology. (1931)
1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types

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Part One General Embryology

Chapter VII Endoderm Formation

I. METHODS OF GASTRULATION

After the formation of the blastula, which in its simplest condition is a spherical mass (usually hollow) of more or less undifferentiated

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fiG 65 Two stages in the gastrulation of Terebratulma septentnonalts (After Conklm.)

cells, the next stage in the development of the embryo is the formation of a gastrula. The gastrula develops from the blastula by the differentiation of two layers of cells, an outer or cctodermal layer, and an inner or endodermal layer. From the eetoderm develops the nervous system and the outer parts of the future animal, and from the endoderm comes the lining of the alimentary canal.

There are several different types of gastrula formation, depending partly on the type of blastula preceding the gastrula. We shall consider each of these types and describe its method of formation.

A. EMBOLIC GASTRULATION

The best-known type of gastrula is the embolie or invaginate gastrula. Gastrulae of this kind occur in embryos with total cleavage, resulting in cells of either equal or slightly unequal size. The blastula resembles

a. hollow sphere whose wall, except in vertebrates, has a. thickness of one cell. A blastoeoele cavity is often, though not always, present inside the sphere, but sometimes it is quite restricted, as in Terebratulina, a. brachiopod which has a thick-walled type of coeloblastula. The cells of the blastula are often wedge shaped, with the wedge narrowing inwardly.

98 EMBOLIC GASTRULATION 99

In one part, the vegetative half, the cells are typically larger than elsewhere, and sometimes markedly so.

The first sign of gastrulation is a slight flattening of the vegetative region. This flattening gradually deepens and the resulting gastrula has the appearance of a rubber ball which has been pushed in at one point by pressure with a finger. The inpushing of the cells at the vegetative pole continues until this invaginated layer lies close to the cells of the outer wall. In this way a two-layered gastrula is formed and two cell layers are differentiated. The outer or ectodermal layer covers the gastrula and is continuous with the inner or endodermal layer at the

fiG. 66. Gastrula of amphioxus. (After Cerfontaine.) Oriented with respect to the future embryonic axis.

point of invagination. Between the two layers there remains for a while a cavity which is the remnant of the segmentation cavity or blastocoele. The blind cavity, which communicates with the outside through a single blastoporal opening and which is formed by the inturned cells as the lip of the depression gradually narrows, is the primitive gut cavity, or the archenteron.

It will be seen at once that the conditions in an invaginate gastrula closely resemble those of the adult coelenterate which has two cell layers and whose gut has a single opening to the outside. However, it is to be noted that in only a few coelenterates are the gastrulae formed by invagination, and that the same result in these forms is attained in another way.

The mechanism which causes invagination has been attributed by different workers to different causes. Rhumbler regarded it as an active 100 ENDODERM FORMATION

attempt at migration by the cells of the vegetative half, a process which in the last analysis is to be attributed to chemotactic influences. If this explanation is correct, then the migration of the endodermal cells must be connected with a change in surface tension at the boundary between the endoderm and the blastocoele content, especially in the region of the cells which undergo invagination.

Some of the causes which play a part in the invagination may be the following: (a) Differences in the rate of growth between the ectodermal and endodermal portions of the blastodcrm by which a lateral pressure is exerted on the endodermal plate. (b) Resistance of the closely pressed egg membrane, so that the surface increase of the blastodcrm is possible only internally. This factor is probably not often effective, for in many cases the egg membrane appears pushed away by the pcrivitelline fluid and the blastocoele is not an empty cavity but is filled with fluid which would have to be displaced. (c) The constantly decreasing fluid of the blastocoele which perhaps exerts a sucking effect on the endodermal plate. We must at the present time regard this factor as of doubtful occurrence and significance.

The size of that part of the old segmentation cavity or blastocoele which remains after gastrulation may be very small, as in amphioxus, or it may be quite extensive, as in the echinoderms in whose embryos the primitive gut is relatively small. The fate of the fluid or gelatinous content of the blastocoele is uncertain; some authors think that it is resorbed by the endoderm cells. If this process could be satisfactorily demonstrated it would be regarded as a causal factor in gastrulation.

A monaxial structure, with the axis from the middle of the blastopore opening to the opposite pole, is characteristic of the early gastrula. In many forms the gastrula becomes later bilaterally symmetrical as the blastopore becomes narrowed and the gut laterally compressed. In most coelenterates the monaxial structure remains.

A special type of gastrulation occurs in Lumbricus and the ascidians. Here the blastula is a placula having a two-layered, plate-like structure. Gastrulation is brought about by the hemispherical inbending of the placula which then rounds out so that a complete sphere is formed. In some cases there is an active increase by epiboly in the ectodermal cells preceding invagination. This may be one of the causes for the invagination in placulae, but on the other hand the process may be due to the same causes that lie back of typical invagination.

Invagination occurs in the following forms: some actinians, some syphopolyps; Sagitta, Pedicellina, brachiopods; Phoronis, nemerteans; echinoderms; some arthropods, as crayfish, Palimonetes; some insects; many annelids, as Polygordius, Podarke, Eupomotus; some molluscs, as EPIBOLIC GASTRULATION 101

Chiton, Dentalium, Dondersia; lamellibranchs, as Cyclas, Pisidium, Unio, Ostrea; gasteropods, as Paludina, Planorbis; Balanoglossus, amphioxus; the vertebrates in general.

It will be observed that this type of gastrulation is typical of primitive forms. Hence it is most striking that very few eoelenterates belong to this group. Invagination with a preceding placula includes the following: some nematodes, some ascidians, some Lumbricidae. Many forms are transitional between the type of pure invagination and the Dlacula type.

B. EPIBOLIC GASTRULATION

In the invaginate or embolic gastrula just described, the blastomeres were approximately equal in size. In many other embryos, however,

fiG. 67. Gastrulation by epiboly in Crepidula. (After Conklin.)

there is a great difference in size between the small cells at the upper or animal pole and the large cells at the lower or vegetative pole. The small cells are called micromeres, and the large ones which contain much yolk and are few in number are called the macromeres. In the blastulae of embryos which have undergone this kind of cleavage, the cleavage cavity is small or absent.

It is obvious that there is no room for invagination of endoderm cells 102 EN DODERM FORMATION

in embryos with practically no cleavage cavity. Gastrulation, which cannot be embolic, occurs in the following way. There is a multiplication of the small cells at the animal pole and an overgrowth or migration of these cells down over the larger cells at the vegetative pole. These large yolk-filled cells seem to take no active part in the process. The free edge of the growing cap is called the blastopore, and the protruding yolkfilled cells, the yolk plug. When the overgrowth by the small cells is complete, the blastopore usually closes and the large cells are enclosed. The small, overgrowing cells are ectodermal and the large yolk cells endodermal. A gut cavity is not present at first but is formed later by a splitting and solution of the yolk in the solid endoderm cells. The cause of epiboly, or gastrulation of this kind, has usually been ascribed to the activity of the micromeres, but there is no evidence of a lack of activity on the part of the large, inwardly migrating macromeres.

The part of the gut so formed is known as the mesenteron, for it is the middle portion of the entire tract. The anterior end, or stomodaeum, and the posterior end, or proctodaeum, are formed by ingrowth of the ectodermal cells which later unite with the mesenteron so that there is a continuous lumen through the gut.

There are many transitions between epibolic and embolic gastrulation. If in the embolic gastrula the endoderm cells are larger and the lumen of the invagination correspondingly smaller, then the gastrulation approaches the epibolic type.

Epiboly is very widespread and occurs in all cases where the eggs are sharply telolecithal and where there are, as a result, size differences of the cells at the animal and vegetative poles.

Epibolic gastrulation occurs in ctenophores (fig. 14B), rotifers, turbellarians, some molluscs, some annelids, and many crustaceans including many parasitic copepods.

C. POLAR INGRESSION

1. Restricted Unipolar Ingression

In some cases of gastrulation there is neither a sharp invagination nor an epibolic overgrowth. In these cases there is a total, unequal cleavage, resulting in an embryo with a micromere cap resting on a few large yolk-laden cells. The micromeres continue to multiply, and a blastula is formed of cells of very unequal size; in it a well—developed blastocoele is present. The large cells, never many, the common anlage of mesoderm and endoderm, reach the interior not by invagination or epiboly, but through their own active inward migration.

The mechanism by which the large cells get into the interior is little MANY-CELLED UNIPOLAR INGRESSION 103

understood, and the term polar ingression is not adequately applied. This method of gastrulation is often not sharply separated from either of the preceding types.

The following forms have gastrulation by restricted unipolar ingression: some holoblastic Crustacea (some schizopods and Cladocera), Ascaris megalocephala and other nematodes, and some molluscs (Patella).

2. Many-Celled Unipolar Ingression

In the ciliated blastulae of the coelenterates occurs a second form of gastrulation by polar ingrowth. In it there is an elongated, oval or sausageshaped, free-swimming stage (called a planula) which is really a blastula with a large blastocoele and with thickened cells at the posterior pole. Here there takes place the migration of cells to a position within the blastula which is responsible for a second type of unipolar ingression. This migration extends from the pole over the nearby portion of the embryo. The cells which first migrated into the blastocoele lie isolated in the jelly contents but, later,

when migration takes place en masse, Fm. ex. Endoderm formation in ‘the a plugof cells turns inward from the fgiggigfi 1;;j’;m”“,§,§r;$g;‘“;n';*8',§:§g;';; posterior pole and gradually fills the after Patten.)

blastocoele. The migration of cells in ward is not limited to the vegetative pole, but reaches to neighboring parts. There is no interruption in the continuity of cells at the hinder pole, and no gap in the outer superficial cell layer. At the end stage of the gastrulation, there is an embryo with an outer superficial epithelial layer and with a solid cell mass filling the interior of the larva. This stage is variously called a stereogastrula or a parenchymella.

The formation of the gut cavity follows later by dehiscenee. A gradually widening split occurs in the interior of the endoderm cell mass and later breaks through to the outside, forming the mouth. During this process, the endodermal cells become arranged in a layer.

It will be seen that there is a close relation between gastrulation by invagination and gastrulation by polar ingression. Both methods might be regarded as modifications of one process. In the invagination process a closed epithelial layer is shifted by infolding to the interior, and in the ingrowth process the cells of the vegetative half pass inward as a solid cell mass, later taking on an epithelial character, and develop104 EN DODERM FORMATION

ing a lumen. Metchnikoff concluded that the polar ingression was brought about by a simple migration of cells without differential division.

There are transitional types between gastrulation by invagination and gastrulation by polar ingrowth. In many forms, as in Aurelia aurita

fiG. 69. Endodenn formation by unipolar ingression in the plrmuln of Aequoria according to Claus. (Redruwn from Korschelt and Heider.)

and Eupomotus, there are invaginate gastrulae in which the gut lumen is reduced to a very narrow split. If we think of the originally solid invaginated mass as broken up into cellular parts, we have a picture of

unipolar ingression. DELAM INATION 105

Examples of unipolar ingression are found in the following forms: hydroids which have free medusae and whose eggs produce ciliated swimming blastulae (Tima and Obelia), some anthomedusae, some scyphomedusae, and some leptomedusae. This type of gastrulation is widespread in arthropods.

3. Multipolar Ingression (Apolar Ingtession)

A process similar in many respects to the two types of polar ingrowth just described and perhaps related to those types is that of multipolar ingression. It differs from the preceding only in the inward movement of a small number of cells from several places instead of a single one on the surface of the blastula. It might be spoken of as apolar ingrowth as distinguished from that which proceeds from a single restricted portion of the blastula. However, since the processes are so similar to those already described in this section it seems desirable to assign this type to the same division even if it presents some departures from the other cases placed here.

Multipolar ingression occurs in coeleblastulae when cells, few in number at first, pass inside the embryo from various places on the surface. The blastocoele is in this way filled up with endoderm. The resulting stage is a solidly built mass of cells like a morula. It may be called a pseudo-morula, differing from the typical morula in that there is no differentiation of cells in the latter _ Fm. 70. Multipolar ingreswhereas here the endodermal cells, now a ?‘§:d‘?a$:”°;::f:‘§’Zg;;:fl“Z:‘§ solid mass, have already arisen by an inward Heider. after Metchnikofi.) movement of some of the outer cells. The gut cavity appears later inside the endoderm. Cases of this sort of endodermal formation seem to be rare. The best investigated case is that of Aeginopsis (Metchnikoff).

D. DELAMINATION

In the cases already studied, there was a separation of the inner or endodermal layer from the vegetative pole, or from the vegetative half of the embryo. There is, however, another type of endodermal formation, common in coelenterates, in which there is no polar localiza106 ENDODERM FORMATION

tion of endoderm-forming cells, for in these the endoderm comes from diffused parts of the embryo. The endoderm may arise in one of the following ways: by migration into the coeloblastula from visible spots in the embryo, by cutting off from the cell wall of the coeloblastula by division, or, when the end stage of cleavage is a solid embryo, by histological differentiation of an epithelial outer layer from the parenchymous inner mass. There are many types which belong here. It will be observed that multipolar ingrowth might from some points of view be included under this heading. The term delamination has been used to bring together processes of different kinds and is no longer a definite term of definite meaning. Many of the forms intergrade with each other through transitional stages. It is best to consider, under the head of delamination, all cases of endoderm formation which are not derived from invagination, epiboly, or polar ingression (including multipolar ingression).

Delamination occurs in those forms whose embryos are set free late in development either from spore sacs or from eggs with membranous and gelatinous coverings. Many forms classified here belong to the intergrading or mixed type of endoderm formation.

1. Coeloblastic or Mixed Delamination

When, in a coeloblastula, paratangential division planes which are differential in character cut off single cells into the interior of the blastoeoele and these subsequently become arranged into a layer, coeloblastic delamination may be said to occur. These inner cells constitute the endoderm and at length fill the blastocoele. By dehiscence within them the gut subsequently arises. In many cases of this mixed delamination, there is a combination of the origin of endoderm by cell division and by multipolar migration. Many of the hydroids, of which Hydra is the best example, show this mixed delamination. Geryonia, the actinians, and some syphomedusae have coeloblastic delamination.

2. Morula Delamination

In morula delamination, also called secondary delamination, cleavage results in the formation of an embryo without a blastocoele, that is, in one solid from the first. Early in cleavage there occur both radial and paratangential division planes with no histological differentiation resulting between the inner and outer cells. An embryo arising in this manner is a true morula in which a cleavage cavity never develops. Haeckel thought that the morula, the stage of solid aggregates of blastomeres, always preceded the blastocoele. But it is now well known that a MORULA DELAMINATION 107

blastocoele cavity appears in some forms as early as the 4-cell stage, and that true morulae, as the above, are formed only in certain cases. In multipolar ingrowth and in coeloblastic delamination, the pseudo fiG 71. Hydra embryo (Redrawn from Korschelt and Heider. after Brauer)

A, showing early migration of some endoderm cells inward and the origin of others by cell division. B, a later stage in which the endoderm fills up the cauty

morula follows the blastocoele stage instead of preceding it, differing from Ha.eckel’s description of a typical embryo.

In morula delamination, differentiation of layers follows the solid stage of undifferentiated cells. In Clava (fig. 62) the cells at the surface 108 EN DODERM FORMATION

take on a prismatic character and lose their polyhedral shape. An undulating line of separation is first formed between the outer or ectodermal cells and the inner or future endoderm cells. The basement membrane is later differentiated, and the inner cells connected with it become epithelial. The cells adjoining them lose their yolk content and take positions among the outer endodermal cells. The innermost cells of the yolk mass finally liquefy and are resorbed, thus giving rise to the gastrula cavity. The formation of a definite endoderm is accomplished, and the planulae, now ciliated, swarm out.

Morula delamination occurs also in

Trachymedusae, Siphonophora, in alcyonians, Stauromedusae, and Cubomedusae.

3. Syncytial Delamination

Syncytial delamination occurs in those coelenterates whose embryos show a similarity to the superficial cleavage of arthropods. In these embryos, only a few cell boundaries may be seen, and there is a solid syncytial inner mass, instead of a morula stage. The ectoderm becomes

fiG. 72. Endoderm formation separated from the inner mass, which in

3;affggaogmgfgjgggigglzfiggg; part forms the endoderm and in part

layers by difierentiation. (Redrawn gradually is used up. ‘mm K°“°h°” '”‘d Held” me’ In Turritopsis there is at first total Hum) cleavage, but in later stages the blastomeres flow together and form a syncytium. In this case the syncytial nuclei at the surface multiply, cell boundaries cut in and a delicate basement membrane is formed. Differentiation of the endoderm comes later, when its cell boundaries begin to appear. The primitive endoderm cells are irregularly disposed, but later, when a split forms inside, representing the gut cavity, they are definitely formed into an epithelial layer. In the hydrocorals, according to Hickson, there is no separation into blastomeres and the embryo is a syncytium from the first, recalling the cleavage of insects. Nuclear fragmentation of the first cleavage nucleus has been described in these forms as the source of the nuclei of the plasma islands, which arise, according to this account, from regenerating chromatin particles. About the plasma islands cell boundaries appear, and later the ectoderm becomes differentiated as a layer and development proceeds.

There is considerable doubt as to whether all the cases classified as SYNCYTIAL DELAMINATION 109

syncytial belong here, as different authors give different accounts for the same material and describe many doubtful processes. Some of their

fiG 73. Development of Turruopsts. (Redrawn from Korschelt and Helder, after Brooks and Rlttonhouse )

A. morula. B, fusion 01 blastomeres, C, young planula Wltl) definitely formed ectoderm.

material may be improperly fixed, and a reinvestigation may furnish more complete knowledge of their development. Many hydroids probably belong in this group which develops by

syncytial delamination. l 10 ENDODERM FORMATION

II. THE DISCOGASTRULA A. VERTEBRATES

All the forms of gastrulae studied so far have eggs in which there is not a great quantity of yolk. In those forms, however, which have a discoblastula, a mass of yolk occupies the greater part of the egg and

Fm 74. Gastrulation in Petromyzan fluvumlis (Redrawn from Ziegler, after Goette) A, B. beginnings of gastrulation, C, formation of an anlage of the medullary tube.

the protoplasmic part is limited to a small disc at one end. The inert yolk is incapable of division and the protoplasmic disc alone undergoes segmentation. Gastrulation usually takes place by the inturning of the edge of the disc and the spreading out of this inturned layer between the protoplasmic disc and the yolk mass. This inturned layer is the endoderm and the two—layered embryo thus formed is a. discogastrula. VERTEBRATES 1 1 1

The discogastrula lies on top of the yolk and spreads around it. The edge of the embryo is the blastopore.

In order to understand this process of gastrulation thus briefly described it will be necessary to take up at greater length the gastru |IIIIIIIlIn.,,’ __....§ 4"""lIIn\:

fiG. 75. Series of diagrams showing gastrulation. (After Ziegler.)

A, B. sections through embryos of an amphibian or a dipnoan in the stage of the circular blastopore; C, gastrula of a hypothetical intermediate between Amphibia and Amniota; D, gastrula of a reptile. y., yolk plug; ec., eetoderm; bl., blastocoele; g., gastrocoele; m., medullary plate; d.. dorsal lip of the blastopore; s., subgerminal cavity.

lation of the frog (figs. 22, 51, and 74-76); although its egg has total, unequal cleavage and gastrulation is therefore chiefly embolie, the study of its egg enables us to interpret much better the conditions in a true discogastrula. In the lower or vegetative part of the frog’s egg, the white portion, there is an accumulation of yolk material; the animal half or dark colored part of the egg, however, is not entirely devoid of it. I12 ENDODERM FORMATION

Because of the small amount of yolk the animal half divides more rapidly than does the vegetative half. Cleavage thus results in an embryo with large and small blastomeres, the larger ones serving to force the cleavage cavity nearer the animal pole. Between the pigmented and the lighter portions of the egg is the gray crescent. The bilateral arrangement of the egg substances is shown in some amphibia, but the appearance of the so-called gray crescent which is similar to that of the ascidians cannot always be made out.

fiG. 76. Development of embryo of Rana showing closure of blastopore, formation of neural folds, and development of body form.

Invagination begins as a slit-like groove at the border of the gray crescent somewhat below the equatorial region. The large amount of yolk in the vegetative region prevents an equal invagination. The slit—like invagination is the dorsal lip of the blastopore. The invagination gradually extends to each side of the gastrula and the groove becomes crescent shaped.

Previous to the invagination a change in the distribution of the black pigment has occurred, for the small black cells have started to move down over the white cells and the black pigment extends below the equator. It will be seen at once that overgrowth, or epiboly, as well as invagination, plays a part in gastrulation of the frog’s egg. VERTEBRATES 1 13

The crescent groove follows the border of the pigment zone as the latter gradually extends over the yolk and represents the dorsal side of the egg. The ventral hp of the blastopore appears later, when invagination takes place there, and the blastopore then becomes circular. The dorsal lip advances over the yolk more rapidly than does the ventral, probably on account of the greater amount of yolk in the latter region. The gullet, formed by invagination, is of a circular, s1it—like character, and the yolk mass extends as a plug in the primitive gut lumen. As the

. B Bio 77. Embryos of Rana A at time of first indication of gill slit, B, Just before appearance of external gills at time of hatching

ring-shaped blastopore grows together, owing to the greater downgrowth of the dorsal lip, it draws down on the yolk plug and finally becomes a small median, slit-like furrow. This process of the growing together of the edges of the blastopore is concrescence We may then regard concrescence as a third factor in the gastrulation of the frog.

At first the dorsal, and later the ventral, blastopore lips become differentiated with distinct ectodermal and endodermal layers. The cells of the yolk mass, which have extended as a plug in the blastopore, become pushed within as the blastopore grows together. We might thus 1 14 ENDODERM FORMATION

consider this kind as a solid cell ingrowth, perhaps a fourth factor in gastrulation.

Four factors may be said to enter into the gastrulation of the frog’s egg: invagipation, epiboly, concrescence, and a solid cell ingrowth.

a. Selachians. We may now contrast the selachian egg with that of the frog. In the former, a protoplasmic disc rests on the top of the yolk mass, and there is thus a sharp division between protoplasm and yolk, instead of a gradual transition as in the frog. The disc—shaped protoplasmic mass of the selachian becomes furrowcd by cleavage planes, but the yolk mass remains unsegmented. That part of the endodermal mass of the frog which we have called the yolk plug must be represented in the selachian by the uncleaved yolk mass. Although the yolk does not cleave, in it are to be found scattered nuclei of various origins, but these nuclei appear to take no further part in development. Some of the scattered nuclei may be metamorphosed sperm nuclei, for polyspermy exists in the selachians. A part of the nuclei originate from the peripheral cleavage

fiG. 78. Median section through the gastrula of Torpedo (After Ziegler)

cells and are called the periblast cells. These periblast nuclei are comparable to the eephalopod blastocones; that is, they are blastomeres not cut off from the yolk mass by cleavage.

In the selachians the cell mass of the germ disc forms a many-layered embryo which lies in a depression in the surface of the yolk. The cells of this mass are at first round, indifferent cells, not arranged in an epithelial layer. Between the cell mass and the surface of the depression a space known as the germ cavity appears which may be regarded as the cleavage cavity. It does not extend equally under the embryo, but is pushed toward one side which corresponds to the hinder end of the future embryo.

The first change in the germ disc of the selachian is the formation of an epithelial layer from the superficial cells which become flattened and broadened. This layer begins to form at the hinder part of the embryo where the cleavage cavity is larger and the germ disc thinner and spreads from this region over the whole germ disc.

Gastrulation begins as a depression at the hinder edge of the germ disc where the cells begin to turn under. Thus is formed the primitive gut cavity, lying between the inturned cells and the surface of the yolk VERTEBRATES 1 15

which has previously become covered with so-called periblast. The invaginating cells are formed from the round cells of the blastoderm, spreading as an iris-like ingrowth from the edge of the germ disc and of the growing epithelial endoderm. The invaginated gastrula cavity is at first crescent shaped, comparable to that of amphibians, but with an axial cavity in addition to the peripheral cavities of the crescent. The edge of the entire germ disc may be thought of as a blastopore.

fiG 7‘) G1-«itriilitiam of the salmon. Salmo salar; ll. growth of the bl i-itoderm of the salmon, (‘, emlirvonal shield stage, the germ ring covering one-li ilf of the yolk (After Ziegler )

The anterior edge of the blastopore becomes extremely active and begins to grow down over the yolk. finally it grows entiiely over the yolk, passing over the vegetative pole, and reappearing behind the embryo as the ventral lip of the blastopore. The yolk thus becomes entirely covered and the blastopore eventually closes.

b. Teleosts. In the teleosts the process of gastiulation is essentially similar to that in the selachians. The formation of the blastopore begins at the posterior edge of the germ disc where invagination bi iiigs about the origin of the primitive gut. There is a slight invagination at the lateral and anterior edges also, and the yolk is finally enclosed by the blastoderm, the anterior margin becoming the ventral lip of the blaste1 16 ENDODERM FORMATION

pore. In the elasmobranchs, teleosts, and also in the myxinoids, there are two periods in the closure of the blastopore. During the first period the overgrowth is practically limited to the dorsal lip, and the material for the formation of the body of the embryo is produced. During the second, the lateral and anterior margins of the blastoderm gradually cover the yolk. Thus only the dorsal lip takes part in the formation of the future embryo, while the lateral and ventral lips are entirely extraembryonic.

c. Reptiles and Birds. In all the vertebrates thus far described, the Anamnia, the blastoporic lip appears at the edge of the blastoderm. In the reptiles and birds, which have embryonic membranes and hence belong to the Amniota, the blastoporic lip lies wholly within the blastoderm. The archenteric cavity is well developed only in primitive forms and is usually represented only by the neurenteric canal. In most cases

fiG. 80. Gastrula of pigeon. (After Patterson.)

Thirty-six hours after fertilization. Section through the posterior portion of the blastodisc showing the blastopore and its dorsal lip, d.b.

the upper layer or future ectoderm alone gives rise to the blastopore and archenteron.

In the chick the blastoderm or germ disc lies on top of the yolk mass, as in the fish. The endoderm is formed by an inturning of the germ disc at the posterior margin and by ingrowth from the inturned portion. At the posterior part of the germ disc the primitive streak is formed by a linear thickening of the upper layer of the blastoderm. The endodermal layer in the chick gradually grows around the yolk and the yolk sac is thus formed. The ectoderm does not directly cover the yolk sac, however, as here conditions are complicated by the presence of embryonic membranes, the amnion, which encloses the embryo, the allantois, and the chorion.

In the chick the primitive streak represents the fused lips of the margin of invagination and therefore is to be considered as the closing blastopore. In the primitive streak the germ layers are fused, the differentiated part of the embryo being formed in front of it, the anus at the posterior end, and the neurenteric canal at the anterior end. We must o

d-b- v.b.

i J fiG 81 Dlagram to Illustrate dxfierence between the blastopores of Annmnm and Ammcte. (After Jenklnson )

a—c Closure of blastopore 1n frog or sxmxlar form Stlppled reglon 1s the blastoderm. The edge of the blnstoderm represents the edges of the blastopore d-—f Ventral hp of blustopore xs absent m such a. form as Lemdoszren The heavy solld lxne represents the portlon whmh forms the lips of the blastopore g, h Only a. small part at the postenor end becomes the blastopore as In the Gymnophlona. The yolk remams uncovered 1. J The condxtlon of the blastopore 1n Ammota The embryomc shleld IS darkly stlppled and Is equwalent to the entxre blastoderm of the Anamma The ammote blustoderm extends further as mdxcated by the hght stxpplmg 11 8 EN DODERM FORMATION

therefore consider the primitive streak of the bird as an elongated blastopore since the relations are similar to those found, for example, in the frog.

In the reptiles there IS a so-called embryonic shield at the posterior end of the blastoderm. This IS a circular or oval area of columnar cells resting upon a lower layer and surrounded by a region of flattened cells.

\

B C

fiG 82 Bali'our’s diagrams showing homology between bliistopore and the primitive streak (After Ziegler )

At the posterior margin of the embryonic shield the two layers come together A depression in the primitive plate appears here which is the beginning of the archenteron, and its anterior margin is the dorsal lip of the blastopore Lateral lips which are formed later turn back and fuse, in some cases forming a vertical or even actual ventral lip, although cases occur in which there is no ventral lip at all.

A mass of primitive cells surrounded by the dorsal and lateral lips SCORPIONS 1 19

corresponds to the yolk plug. It will be seen that cells which are the morphological equivalents of the yolk cells of the Amphibia are thus to be found in the reptilian blastoderm. Here, as in the birds, the blastopore arises inside the blastoderm and not from its edge as in the Anamnia. The comparison of the Anamnia and Amniota can best be understood by observing the development of the gymnophionan egg. Here the blastoderm is an oval layer of columnar cells resting upon a partly segmented yolk. Only a part of the edge of the blastoderm at its posterior end is converted into a blastoporic lip which is at first dorsal. Later the lateral lips turn back around a small part of the yolk, and then join to form the ventral lip. The anterior part of the blastoderm thus plays no part in the formation of the blastopore. In this form the yolk remains uncovered. We have seen that, in other Anamnia, the ventral lip develops from the anterior edge of the blastoderm and the yolk is necessarily covered where_the blastopore closes.

The embryonic shield of the Amniota must be represented, in the Gymnophiona, by the blastoderm, and the marginal zone of the amniotes, together with the lower layer with which it is connected at the primitive plate, must be represented by the yolk cells or nucleated yolk. It seems natural to suppose that in the transition from forms like the Gymnophiona to the higher vertebrates, as the yolk material in the egg increased in amount, segmentation has become restricted, not to the blastoderm alone as in the fishes, but to the blastoderm and those adjacent and subjacent cells which in the Gymnophiona are partly segmented from the yolk.

These points may make it clear why the blastopore of the amniote is formed inside the blastoderm but at the edge of the embryonic shield which is really equivalent to the anamnian blastopore.

B. SCORPIONS

The discoidal type of gastrulation is not limited to the vertebrates alone, for the scorpion and the eephalopod eggs are also of this type. Gastrulation is of course dependent on the structure of the egg, and an egg with a protoplasmic disc resting on a large yolk mass will belong to this type, regardless of whether it is an invertebrate or a vertebrate egg.

The scorpion egg (fig. 39) is rich in yolk and the cleavage of the egg is discoidal. In young stages there is found a small, one-layered cap on the yolk. The blastoderm spreads gradually from this point and advances over the yolk. Scorpion eggs, since they have cleavage of this type, are comparable to eggs in which there is superficial cleavage with early formation of the blastodisc on one portion of the egg only. The scorpion condition is probably an extreme case and is to be traced back to the 120 ENDODERM FORMATION

type which is widespread among the arachnids. The scorpion egg, therefore, represents a modification of the type common to its group (as is also indicated by its viviparous mode of development).

The endoderm arises by the differentiation of the cells of the germ disc which lie next to the yolk and form a regular epithelium. The irregular cells which remain between the outer layer or ectoderm and the inner or endodermal layer become arranged into two symmetrical mesodermal bands.

It will be seen at once that in the scorpions, as in the myxinoids, the endoderm does not develop from the edge of the germ disc and thus the edge of the germ disc cannot be regarded as the blastopore.

C. CEPHALOPODS

The cephalopod eggs are unusually rich in yolk and consequently they are considerably larger than are the eggs of other molluscs. There is a considerable difference in egg size throughout the group, but the yolk takes up by far the most of the egg. The entire egg is covered with a thin layer of protoplasm, which becomes thicker to form a disc at the upper or future animal pole. The germ disc thus formed is sharply marked off from the yolk mass.

The cephalopod egg is bilateral even before cleavage sets in. At the part of the germ disc which is to become the anterior end of the embryo, the disc extends farther down toward the equator. Since the cleavage furrows pass into the thin layer of protoplasm surrounding the yolk, the first cleavage cells and later the outer cells of the disc, the blastocones, are not cut off from the peripheral protoplasm. The germ disc spreads out and increases in size, at first chiefly at the expense of the formative yolk with which the blastocones are connected. The germ disc, one cell in thickness and covering only a small part of the egg, becomes thickened at the periphery as it spreads. When the cleavage cells have increased to a considerable number the peripheral blastocones become detached from the germ disc. These cells, according to Vialleton, wander beneath the germ disc and become arranged into a layer which spreads over the food yolk, forming a yolk epithelium. The exact origin of the yolk epithelium seems still to be in doubt. Some say it originates from the nuclei within the yolk, others from cells of the germ disc itself. The origin of the layer need not be of very special concern to us at this time, for the yolk epithelium, though it surrounds the yolk mass, does not become the endoderm.

While the yolk epithelium is forming and the edge of the germ disc is becoming thickened, the outer cells of the disc increase rapidly (fig. 44). This outer layer extends gradually over the whole egg (and thus CEPHALOPODS 121

covers the growing yolk epithelium) and forms the ectoderm. These two layers, the ectoderm and the yolk epithelium, are soon followed by a third or middle layer, which also covers the yolk. There are thus two definite regions in the egg at this time, the germ disc which forms the embryonic rudiment, and the yolk sac composed of three layers.

The function of the yolk epithelium is to form an envelope for the yolk and to make it available to the embryo. In later stages this envelope surrounds the yolk as in the earlier development. There gradually forms an outer yolk sac of part of the yolk not covered by the developing embryo, and an inner yolk sac of that part which is embraced by the embryo’s body; the latter remains always connected to the outer yolk sac, but the connection becomes restricted to a narrow duct. The external yolk sac evidently passes on its contents to the internal sac and then the nutritive material is conducted to the embryo from the inner yolk sac by aid of the yolk epithelium. This seems the most logical method of absorption, for there are no known vessels in the external sac.

The extension of the germ disc over the yolk varies greatly in different cephalopods. In Sepia the germ disc is very small and the yolk sac very large. In forms like Loligo and Octopus the external yolk sac is more reduced and the embryo contains the greater part of the yolk. Grenacher studied a form in which there was scarcely any external yolk sac and at an early stage the small yolk mass was enclosed by the embryonic rudiment.

We have seen how the yolk sac and germ disc are formed and how their outer layers become ectodermal. We have still to describe the method of formation of the endoderm. About the time when the first rudiments of organs appear externally in the embryo, there may be seen, next to the yolk, an epithelial plate of only a few cells. It is the first indication of the enteron. It soon increases in size and becomes sac like, finally separating from the yolk. The yolk epithelium, previously lacking at this point, now grows under the enteron. The origin of the endodermal plate thus described is probably from the thickened peripheral mass at the edge of the germ disc which evidently represents meso-endoderm. The whole process is thus to be regarded as a much modified invagination, and the edge of the germ disc is the blastopore filled by the large yolk plug, which also fills the whole enteric cavity.

Bibliographic Note

Among the more important accounts of the subjects contained in this chapter are the following: Korschelt and Heider, Conklin, Selys-Longchamps, Patten, Claus, Lankester. These works are cited in full in the bibliography on page 406.

1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types

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