Book - Outline of Comparative Embryology 1-11

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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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This historic 1931 embryology textbook by Richards was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.
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Part One General Embryology

Chapter XI Egg and Embryonic Membranes


The eggs of all animals are covered with one or more membranes and in these coverings there is a wide range of variation due largely to the environment and to the rate of development of the eggs. Those which are surrounded by water develop rapidly and are produced in great numbers, as are eggs of marine invertebrates, and are scantily provided with coverings. Those which take a long time for development,

as birds’ eggs, or those which are exposed to unfavorable conditions, as insect eggs, have more and thicker egg membranes.

Egg membranes are classified with reference to their origin. Primary egg membranes are formed by the egg itself, secondary by the follicle cells around the egg, and tertiary by the uterus or oviduct. All three types of membrane are sometimes found covering one egg, as in one of the cockroaches, Periplaneta.

If only one egg covering occurs, this can be secondary or tertiary, for example the egg of Distoma which is covered by a secretion from the oviduct. Only in exceptional cases are the egg membranes cellular as the zona radiata of Taenia, the spindle—like egg-shell of Echinorhynchus, the chitinous indented shell of Hydra, and the layers covering the ascidian egg. These membranes are really embryonic membranes. In the case of Hydra there are two membranes which are formed after cleavage from the ectodermal cells.of the developing embryo.

The eggs of some animals have been described as naked. Although they may possess none of the three membranes described above as primary, secondary, or tertiary, still they are not actually naked. We now know that all eggs possess a cortical layer, the outer part of which has been called a plasma membrane. This structure is really a delicate surface layer of cytoplasm which cannot be distinguished from that which underlies it but which is physically different from it. The presence of the layer has been demonstrated by micro-dissection experiments (Chambers and others) which show that the surface of the egg is firm and somewhat elastic, offering resistance to mechanical injury. Plas molysis experiments have shown that this layer is a semi-permeable membrane and that it plays an important role in regulating the exchange which takes place between the cell and its surroundings.

In the animal kingdom as a whole there is an astonishing variability in the manner in which egg coverings appear. In sponges, the development of the egg within the adult body makes membranes unnecessary. They are lacking also in Hydrozoa, Siphonophora, and Anthozoa, although some coelenterates do have them. In some of the lamellibranchs, as Drezssensza, no membranes are present, and the eggs of some others, as M ytilus, have them in the beginning of their development and later

fiG 145 Cross section of egg of Hydra arzsea (Redmwn from Korsehelt and Heidor, after Ehrenberg and Brauer )

cc, ectoderm, en. endoderm with volk granules, im , inner membrane, om. outer membrane

throw them off. In some animals, as echinoderms, the eggs are without membranes when shed into the water but the fertilization membrane is formed as soon as the sperm enters the egg.


Vitellme Membrane. In many and perhaps in all eggs, there is a vitelline or fertilization membrane. This membrane is the most constant of the primary membranes, though it is often difficult to see before fertilization. Early workers had thought that the fertilization membrane was formed at the moment of fertilization, for, in many eggs, as for example in those of echinoderms, it separates from the egg immediately after fertilization. The presence of this membrane, often some distance from the egg, is an index of fertilization. Its origin is not clearly known. PRIMARY MEMBRANES 211

It has been thought by some to represent a precipitation reaction, by others to be merely the outer layer of

the egg which has been lifted away, or to be a substance produced at the time of fertilization. The separation of the vitelline membrane may be due to several causes, as follows: the egg itself may lose water and shrink, the membrane may swell, or the substance between the egg and membrane may absorb water. The space so formed between the egg and its fertilization membrane is the perivitelline space. (See fig. 7 ) Its size varies considerably in different species.

In some eggs the vitelline membrane is apparently lacking and the egg is covered directly by a chorion or secondary membrane as in the cephalopods. In many eggs it is present before the sperm touches the egg, though the evidence that all eggs possess it is not conclusive. There are many vertebrates in which it has not been recognized, although for some representatives of nearly all classes of vertebrates it has been described. In these animals it is frequently termed the zona pellucida or, if it is traversed by pores, the zona radiata. It is possible that the vitelline membrane of the ripe vertebrate egg may be the result of the fusion of the primary and secondary membranes of an earlier stage.

Amphioxus is described as having a thin vitelline membrane formed near the end of the growth period. When the egg is shed into the water, another membrane, the perivitelline, is formed inside the first, a space intervening fie 146 Successive stages in the

between the two. ‘But immediately c,l,,e.:/ne,l?f:,l,'::":,,°fC31:‘:la:““(:'}:r"'§‘$$.‘f

upon the entrance of the sperm, the facher.) 212 EGG AND EMBRYONIC MEMBRANES

inner membrane hardens and the two fuse. Thus is formed the fertilization membrane which is separated from the egg by the perivitelline space.

The vitelline membrane is usually structureless, transparent, and devoid of canals. It is found throughout all the animal phyla, although, it may be, ndt universally. Often it forms very quickly at the moment of fertilization as just mentioned, but there are cases in which it appears more gradually. In the latter instance, as in Cycles, the attachment of the egg to the ovarian wall remains as the micropyle. A micropyle is not always present in the vitelline membrane, for sometimes it is unnecessary, since in some eggs fertilization may take place before its formation and in others it is porous and easily penetrable by the sperm. There are, however, some invertebrates, as cephalopods, in which the vitelline membrane is lacking.

The true vitelline membrane is a part of the egg itself, but it proves difficult in some eggs to determine whether the membrane which has been designated the vitelline is really primary or secondary. Confusion also seems to exist regarding the origin of the zona radiata, the membrane which lies next to the vitelline membrane. In some eggs, as in selachians and in Petromyzon, the zona radiata lies inside it, and in others, as in the teleosts, the zona radiata lies outside it. This membrane derives its name from the radiations which are seen in cross section and which in the vertebrates at least are due to fine canals.

To classify properly the vitelline membrane and zona radiata of a particular form as belonging to the primary membranes we must know their origin, and this may be very difficult to ascertain. A review of the text—books will show that these names have been given to diverse coverings in many cases without any knowledge of their origin. Hence it will be clear that the membranes as so named are not homologous in all forms. The question is not of great importance, but is considered only because of confusion in the terminology.

It is obvious that the egg itself, if not surrounded by follicle cells, must secrete its own membrane. However, if follicle cells are present they take a greater or less part in forming the coverings of the egg.

Zona Radiata. The zona radiata occurs in some echinoderms, worms, and molluscs, but there is much doubt as to its origin in these forms. In some cases it is evident that the membrane is a product of the egg itself.

Lillie has shown that the zona radiata in the Nereis egg is not a membrane at all but merely the outer layer of the egg protoplasm, and that it consists of closely crowded alveoli. “The zona radiata,” he says, “is in fact a coarse emulsion or foam structure. The jelly is formed by the extrusion or the diffusion of the alveolar contents of the cortical PRIMARY MEMBRANES 213 layer through the vitelline membrane.” The zona radiata of Nereis thus secretes the jelly which surrounds the egg. The structures which seem to be radiations are the collapsed walls of the alveoli. The socalled zona radiata of Nereis is thus not homologous to the membrane of that name in other forms.

The zona radiata has a wide distribution among vertebrate eggs and is found in all classes. It is in many cases doubtful whether this layer is formed from the outer part of the egg or from the follicle epithelium. In cases where there are two or more egg coverings it probably arises from the egg itself. When the zona radiata arises later than the egg membrane outside it, it must be formed from the egg, as Eigenmann has shown for Fundulus.

The purpose of the zona radiata in many vertebrates is clear. Through the delicate canals of this layer, protoplasmic processes of the follicle cells _ penetrate. Nutrient substances from these cells are 5-f

passed through the processes to the egg. The pur— fe pose of the zona radiata is thus that of a nourishing ' I membrane. In the vertebrates it is of considerable v.m. — thickness during the development of the egg. After y the growth of the egg the layer often diminishes B

greatly in size. Fro 147. A portion

In the shark’s egg (Balfour, Giacomina) the zona radiata develops inside the vitelline membrane which is here a true primary membrane, being formed before the follicle cells themselves. The zona in this form is a provisional egg covering for the development of the egg in the ovary. When the egg development is complete, it becomes a thin membrane.

Contrasted with the provisional zona radiata of the selachians is the permanent layer of that name

of the egg follicle and surface layer of the egg of Scyllmm cam'cula. (Redrawn from Korschelt and Hcider, after Balfour.)

f.e., follicular epithelium; s.f., secondary follicular epithelium; v.m., vitelline membrane; y., yolk; z.r., zona radiata.

of cyclostomes, teleosts, and ganoids. In these fishes the zona radiata is often divided into two layers or at least there are two radially striped layers which have the same origin. In some of the fishes as Perca there is a. gelatinous layer outside of it through which run cell processes that connect it with the follicle cells. This and other similar structures frequently present on the surface of the teleost egg would seem to be ehorionic membranes. However, Eigenmann has shown that in Fundulus they are derived from the egg itself.

Although the origin of the zona radiata from the egg has been determined for the selachians and for Fundulus, more work must be done 214 EGG AND EMBRYONIC MEMBRANES

to ascertain what part the follicle cells may have in the formation of this layer in other fishes.

In the bony fishes, since the egg membrane, which is often sticky, is present before fertilization, the micropyle must be formed in a different manner from‘ that mentioned above for many of the invertebrates.

Eigenmann has described its formation

in Pygosteus in which each micropyle ' is due to a long process of one of the follicle cells that reaches to the egg and passes through the zona radiata.

The eggs of amphibians and reptiles have a vitelline membrane which later forms a zona radiata. The formation of this layer has been clearly described by Rctzius for one of the lizards, Lacerta

fiG 148 Section through the fol- _ _ _ hole and outer layers of the egg of vzmdzs. In this form the early follicle 1S

fi:'l§:rfi)“”“”l” (‘Um K°“’°h°” and one—layered. In it cells of two sizes may

fc’ follicular eI,,the1,um, L gala. be distinguished, large cells, the centers tmous layer. ov. ovarml epithelium: of which are filled with round nuclei, §f,‘,,‘,;,,‘§{°°“‘°“‘°“"" ' ’°"" ' ' ' ‘°““ and smaller cells with oval nuclei. The egg itself has a delicate surface layer, the first anlage of the zona radiata. This membrane, as in the selachians and Fundulus, is formed from the egg itself. During the growth of the egg the zona radiata grows thicker, the large follicle cells become pear shaped and send processes to the egg. Some of the cells remain round and lodge between the stalks of the pear-shaped cells. The small cells take up a more distal position, and meanwhile the processes of the larger ones become branched to penetrate the plasma of the egg. The follicle now thickens and the zona takes on its characteristic radiate structure. finally the follicle cells diminish in size and the follicle itself is thinner. , fiG 149 Mwropyle formation The nuclei of the large cells lose much of mus “mamas their chromatin as the cells are stretched fc , follicular epithelium, t r , more closely over the egg. The zona ‘h°°”' ‘°““’““’ Y’ y°‘‘‘' “' 2°“ _ _ radiata. radiata, in which two layers can now be distinguished, is likewise thinner. The nourishing function of the follicle is thus clear, for it is evident that substances are passed to the egg through the long processes of the follicle cells which penetrate the zona radiata. It is seldom that the ormation of the membrane is as clear as

in this form. (fig. 150.) !'-?«"?3"-:'o' fiG. 150. The origin of the egg membranes of Lacerta viridis. (Redmwn from Buchner. pfter Retzius.) For description, see text. 216 EGG AND EMBRYONIC MEMBRANES

In the bird’s egg the primordial follicle contains the ovum surrounded by a one—ce1led layer of cubical follicle cells, the granulosa. The zona radiata is later formed between the ovum and the follicle cells and there has been considerable discussion as to its origin. Bartelmez thinks that it is formed from the follicle cells; at any rate it has a function similar to that described for the reptiles. The follicular cells are connected to the ovum by delicate strands of protoplasm which pass through Fm_151_ Primordialoggfollicle the zonaradiata.Nutrimentdiffusesthrough from ovary of hen. (Redrawn from these protoplasmic bridges, there being no Lime’ after H°u') evidence of the passage of solid food particles. In the bird the primary vitelline membrane formed immediately

Fro. 152. Section through an ovarian egg and follicle of the egg of a pigeon. (Redrawn from Lillie.)

ex.. theca externa; g.. granulosa; g.v., germinal vesicle; in., theca interns; p., peripheral protoplasm; s~s, stalk of the follicle; z.r.. zona radiate. SECONDARY MEMBRANES 217

over the egg as in other animals is a structureless membrane usually considered as from the egg itself, though some workers have thought it to be a product of the follicle cells. It is very diflicult to differentiate the vitelline membrane from the zona radiata and the former may be in part the anlage of the latter, as in the case of Lacerta which we have just described.

In mammals, the origin of the vitelline membrane is uncertain. Some authors say that there is no true vitelline membrane; the transparent zona pellucida next the egg is thus referred to by others. However, if

’ ,lusaUu\‘\ , Ilralnm gunulocum

zona Plxl.\L€.\a.I.

co uldul ' \ i..'.3.i.i1u..i.' vilclluicb \\,g,qg. ‘mime: uunm a nun In unlrun mam rune

fiG 153 An ovarian follicle approaching maturity (From Patten )

this layer is secreted by the follicle cells, as some workers believe, it is of course a secondary membrane. The zona pellucida may be fairly thick, and in the mole, pig, rabbit, and sheep it takes on a radiate appearance and hence becomes a zona radiata.

The term zona radiata is unfortunate in the mammals, for outside this structure is a cellular layer of radially elongated follicle cells called the corona radiata and confusion is apt to exist between these two.


Secondary membranes are formed by the follicle cells which surround the ovum and are therefore the product of maternal tissue. These membranes, which are truly chorionic in nature, may take on a variety of 218 EGG AND EMBRYONIC MEMBRANES

forms, depending upon the activity of the follicle cells. The distribution of the chorion is more restricted than that of the vitelline membrane, but some eggs which lack the latter possess a chorion. It may arise as a cuticular secretion of the follicle cells forming a delicate membrane which later thickens to assume the character of a chitinous-like shell, as in the insect egg. On the other hand, it may be formed by direct transformation of the follicle cells themselves. Its substance is, according to Tichomirow, chemically like keratin

F 154 A t 1' th fll l - - - . epitlieelium WltlIlJ0tll1;Ol:lh?)r)0: oof lDc‘:ct8': and much “Cher In nitrogen than 15

ms bwolor (Redrawn from Korschelt chitin,

and Heme’) Both primary and secondary mem branes are often present before fertilization takes place, and since they are usually impenetrable to the sperm they are perforated by one or more micropyles situated at or near one of the poles of the egg. In nemerteans and in most molluscs the micropyle is near the lower pole, but in cephalopods and also in echinoderms, insects, and fishes, it is near the upper pole. In the Orthoptera it may be either at the lower pole or at the side of the egg, and in Oedzpoda there is a circle of 30 to 40 micropyles at the lower pole.

The chorion of insect eggs is very different in different species. In the simplest cases it is smooth, one-layered, and structureless. Its surface may have elevations or may be divided into polygonal areas so that it looks like an epithelium. In certain forms, as in the locust, the follicle cells send out processes to these polygonal areas of the chorion around which a shell is secreted. After formation of the shell, the cell processes are withdrawn.

Porous canals through which the egg obtains air often .. _ penetrate the chorion. In Ranatra and N epa, water fiG 155 An scorpions, in which the eggs are sunken into plant iii stalks, the chorion is drawn out at the end of the seven processes

. . . h h egg into delicate processes which serve to aereate the §{,b‘§, s‘(’R':d:::,l,:

egg_ from Korschelt

The micropyle is formed in a characteristic manner and Held”) in insects as has been described in the locust M econema vamans. One of the follicle cells sends a long conical process down to the egg. This process remains during the secretion of the chorion and then is drawn back into the follicle. The chorionic pores referred to are formed in the same way. Also the micropyle of the bony fishes is formed in a SECONDARY MEMBRANES 219

similar manner, but as has been earlier mentioned, the membrane perforated in this case is a zona radiata and not a chorion. (See fig. 191.)

In Octopus the chorion is drawn out into a stalk which becomes twisted and serves to anchor the eggs. Among the chitons there are different types of chorions. Some of the eggs have a knobbed surface

and some are covered with needle-like processes. Many actinian eggs also have sharp processes.

Fro. 156. The egg membranes of Meconema varians. (From Korschelt and Heider.) ch., chorion; f., follicular epithelium; m., micropyle.

The myxinoids have a characteristic shell. At each pole of the egg are attached stiff anchoring filaments which end in lobed umbrella-like expansions. At the germinal pole of the egg the shell is marked by a furrow-like ring so that a lid is formed which the animal pushes off at the time of hatching. There is considerable disagreement about the

fiG. 157. The upper part of the egg of Bdellostama stouti. (Rcdrawn from Ziegler, after Doflein.)

A, section showing nucleus and above it the micropyle between the bases of the processes. B. Opereulum and processes.

origin of this shell. Cunningham says that it is the product of the egg itself, and Doflein derives it from the follicle cells. The membrane is usually considered as a true chorion.

In Petromyzon the inner egg membrane is covered with a mucous layer which causes the egg to hang onto stones. This membrane is formed through transformation of the follicle cells themselves. In some 220 EGG AND EMBRYONIC MEMBRANES

of the ganoids also there is a similar adhesive membrane formed from the follicle cells. Under this layer are two membranes, the chorion and the vitelline membrane.

The follicle cells of the ascidians show a special type of modification. five envelopes are formed by these cells and thus we have a unique example of an egg covered by five secondary membranes. When the ascidian egg is still young it is surrounded by a primary follicular epithelium derived from undifferentiated ovarian cells. Beneath this epithelium is a band of structureless membrane. These flat epithelial cells multiply and become cubical. Some of them find their way inside the epithelium and form an inner layer called the test—cell layer while certain of them wander into the egg. The cells of this layer do not give rise to the test of the adult ascidian as was formerly supposed to be the case. As the egg matures, these test cells degenerate and later are found in a gelatinous mass over the egg.

In the solitary ascidians the follicle layer of the egg takes on a special character. These cells, instead of remaining cubical, increase in size and become highly vacuolated, resembling a foam structure. They also develop processes which support the egg and enable it to float.

In addition to the follicular membrane and the test-cell layer, two other membranes are formed around the ascidian egg. Beneath the two layers already described, the follicle cells secrete a structureless membrane called the chorion. Still another membrane is formed between the basement membrane and the follicle cells. This is the external pavement epithelium and it may be regarded as another layer of follicle cells. There are thus five membranes over the ripe ascidian egg. Next the egg is the gelatinous layer of test cells, and over this are the following layers, the chorion, the foam-like follicle epithelium, the external pavement epithelium, and the basal, follicle membrane. Since all these membranes are derived from the follicle cells they are secondary membranes.

In the Dipnoi the primary membranes are covered by the gelatinous tertiary membranes, and secondary membranes are absent. In the frog’s egg there is a thin but tough membrane around the vitelline membrane and this is secreted by the follicle cells. Over this is the jelly formed by the oviduct, so the frog egg has all three types of membranes. In the urodeles this secondary membrane appears to be lacking. In reptiles and birds the chorion seems to be entirely lacking, for the egg white and shell are tertiary membranes.

From the descriptions just given it will be seen that the chorion has a variety of forms and that its special characteristics are adaptations to the needs of the different species.

Comparable to the chorion in a certain sense is the mammalian SECONDARY MEMBRANES 221

follicle which is here described in some detail. Although of course it cannot be considered as a membrane, still it is formed by the follicle cells. As described in an earlier chapter, the mammalian ovum is highly specialized because of the intra-uterine development of the embryo. There is only a very small amount of yolk present in the egg as contrasted with the large amount in the eggs of birds and reptiles. Secondary and tertiary membranes are entirely lacking, but the Graafian follicle

Fm 158 The developing ovum and follicle cells of the cat (Redrawn from Buchner) See text for description.

which surrounds the egg is a product of the follicle cells. (fig. 153.) In earlier times mammals had eggs similar to those of reptiles and birds, for the monotremes, the simplest mammals, possess an egg with shell and egg albumen. In the entire animal kingdom the Graafian follicle is probably the most specialized structure produced as a modification of the follicle cells. In the middle of the follicle is a cavity filled with liquid, known as the liquor folliculi. The ovum itself, surrounded by the zona pellucida and the zona radiata, is attached to the Wall of the Graafian follicle by a group of cells known as the discus proligerus. The cells of the discus 222 EGG AND EMBRYONIC MEMBRANES

are continued into the cells of the follicle wall known as the membrana granulosa, which consists of many layers. Outside the membrana granulosa the ovarian connective tissue is differentiated into two investing layers, the theca folliculi. The outer of these is densely fibrous, and the inner cellular, penetrated by small blood vessels. Buehner describes the growth of the Graafian follicle in the cat (fig. 158), where it begins as a simple follicle in which the ovum is surrounded by a cellular layer, the granulosa. The cells about the ovum multiply rapidly, and part of them lose their connection with the upper surface of the egg to wander out to form a second loose ring around it. The rings separate sharply from each other and each becomes many-layered, while a cavity known as the antrum appears between them. This cavity becomes filled with a liquid which is secreted into it and quickly enlarges. The outer ring of follicle cells is crowded to the periphery and forms the stratum granulosum. The ovum itself at first bulges slightly into the cavity of the follicle, and later as its surrounding cells are undercut protrudes farther into it, its attachment to the follicle being new a delicate stalk of cells, the discus proligerus. When the egg is ready to escape, me. 159. Follicle and portion of cyto- this narrow sta‘k is broken, and plasm and nucleus of ovum of a rabbit. (Re the follicle ruptures, the egg passes drawn from Buchner.) _ through the tube into the uterus. The follicle now undergoes a transformation and becomes the corpus luteum. Sometimes the follicle cells immediately around the egg are found clinging to it after its discharge.

The mammalian ovum and its surrounding follicle are thus seen to be highly specialized. The presence of only a very small amount of yolk and the preparation for the attachment of the ovum to the uterus are all adjustments to the intra-uterine development of the embryo. The role of the follicle cells as a nourishing membrane has been definitely determined in the dog by experiment. The follicle cells are filled with mitochondria. When lecithin is injected into a dog these mitochondria increase in number. However, if the dog is allowed to become hungry, the mitochondria decrease. They seem to act as store houses for the TERTIARY MEMBRANES 223

developing egg. The identification of lecithin or of a similar substance has been observed by Russo in the protoplasmic bridges which penetrate the zona radiata.


Tertiary membranes, unlike primary and secondary, are formed after the egg has left the ovary. They are secreted by the oviduct or uterus or by special glands connected with them. Tertiary membranes are of various kinds, gelatinous, horny and chitinous, or cocoon-like. The type of membrane shows adaptive correlation to the needs of the developing embryo. The gelatinous type of covering is frequently seen in fishes and in amphibians and also in many invertebrates, as Geryonia, and Sagitta. Chitinous shells cover the eggs of the phylopods, Apus and Branchipus, and a horny capsule covers the shark’s egg. Under the tertiary membranes there may be a primary or secondary membrane or both for in some eggs all three types are present.

In the cephalopods the tertiary membranes show different modifications. The egg of Sepia and its surrounding albumen is covered by a black leathery capsule, but in Loligo the eggs, each in a firm envelope, are arranged in a gelatinous mass known as “dead men’s fingers.” The flatworms have cocoons which are formed in the ovary. There are various kinds. Branchiobdella has a stalked capsule containing one egg. The rhabdocoele turbellarian cocoon contains several eggs. In this capsule and in that of the trielad Dendrocoelum there are a great number of yolk cells included beside the egg and these serve to nourish it. The trematode capsule contains one egg and is either stalked or provided with a lid or both may be present. The polyelad capsule is similar to that of the trematode but there are several eggs in one cell.

Coeoons are sometimes formed by special skin glands. Thus in the lumbricids and in the Hirudinea the cocoon which has a ring-like form is secreted by a special skin gland and drawn off over the anterior end of the worm. In it is deposited albumen for the nourishment of the embryo.

In the gastropods the cocoon is large though the eggs themselves are small, for they are surrounded by albumen. In Limax, Helix, and Paludina there is only one egg in a cocoon. In some gastropods only one egg is deposited but in other cases there are a number, all but one of which are utilized to contribute to the growth of the embryo formed from that one. (This is the phenomenon called by some “cannibalism.”) Some insects also have cocoons which are tertiary membranes as contrasted with the larval cocoon formed for the pupa of the butterfly. Periplaneta has a cocoon which is carried by the female attached to the 224 EGG AND EMBRYONIC MEMBRANES

posterior end of the abdomen. In this cocoon the eggs are laid each enclosed in the vitelline membrane and the chorion. H ydrophilus, the water beetle, has a round cocoon in the form of a web. The female places the cocoon on the under side of a leaf to which it is attached by a stalk. There are fofty-five to fifty eggs in palisade-like arrangement within the cocoon nourished from the substance of the web.

The selachians, when not viviparous, have tough egg membranes. The albumen and the horny capsule are deposited about the egg while it is in the oviduct. The capsule differs in different species. In the sharks its four corners are drawn out into fibers which permit the egg to cling to water plants. Occasionally several eggs are found in one capsule.

Water-living animals frequently have eggs with gelatinous coverings. They are laid singly in the urodeles and in Ceratodus. In many other animals, however, the eggs are laid in gelatinous masses of varying sizes which may be irregular (cephalopods and frogs), plate-like (some polyclads, gastropods and insects), or rope-like (some nemerteans and annelids).

A gelatinous covering over the egg is typical of the amphibia. In many of the amphibia there are special modifications of these tertiary envelopes. An interesting example of this kind is that of a tropical anuran. At the time of egg laying the oviducal secretion which surrounds the eggs is beaten up by the hind legs of the parents into a fine foam containing many air bubbles. This foam is sometimes placed on the surface of the pond and floats about or it may also be placed in a wet hollow on the ground. Some of the frogs deposit the eggs in a mass of jelly on one or more leaves. Sometimes the single leaf is bent by the copulating frogs into a funnel which retains its shape by means of the adhesive jelly around the eggs. Some of the Dipnoi (Ceradotus and Lepidosiren) have a gelatinous tertiary envelope similar to that of the amphibians. In Lepz'dosz'ren this layer may be very thin or very well developed.

In birds there are a series of tertiary membranes. The hen’s egg may be described as typical. Immediately around the yolk is deposited a spirally wound layer of egg albumen, the edges of which are twisted into the chalazae, stretching from the yolk to the poles of the egg. About this layer is deposited another layer of egg albumen and over the entire mass the shell is formed. The shell membrane is double and the two layers are separated at the blunt end of the egg to form an air chamber. The shell membrane lies immediately below the shell, which is porous and brittle and impregnated by calcium salts, even to the extent of 98 per cent of the substance. The tertiary membranes of birds are in general like those of the hen’s egg, the differences which occur being due to INVERTEBRATES 225

size, shape, and color of the shell. The egg that is relatively largest is that of Apteryz, for in this species it weighs one-fourth of the body weight of the parent.

The eggs of reptiles are much like those of birds, but the chalazae are lacking. In some forms, as in the lizards and snakes, the shells are more like parchment, little calcareous matter being deposited. The egg-shells of crocodiles and of some turtles are hard like those of birds.

Tertiary membranes are entirely lacking in the mammals with the exception of the primitive monotremes. The egg of Echidna is much like a bird’s egg but the egg—shell is formed of keratin and not of calcareous salts. The mammals have adapted themselves to a terrestrial life by the development of a series of embryonic membranes, as the birds and reptiles have done with egg membranes, but in the higher group it has been found necessary to forsake entirely the devices which were adequate for the lower forms. The membranes of the embryos are considered in the second part of this chapter.



In addition to the membranes which properly are classed with the egg itself the animal kingdom possesses numerous cases of accessory membranes which are developed by the embryo. These are in the nature of cellular sheets which by various invaginations, foldings, outpushings, etc., cover the embryo or parts of it. They are especially well developed in the vertebrates and to that group most of the attention is paid by embryologists. Nevertheless, among the invertebrates there are a good many illustrations of the development of different embryonic membranes. Most common are the amnion and the membrane which is formed in connection with it, the serosa. But there is also an invertebrate yolk sac, surrounded by a yolk membrane, and even a placenta occurs in a prevertebrate group, the ascidians of the family Salpidae. Of course this latter structure is very rudimentary. It is really nothing but a tubercle or knob of rather specialized maternal tissue which becomes richly vascular and to which the zooid already budded off attaches itself for nourishment.

Of the invertebrates proper certain molluscs show the development of the yolk sac. The amnion occurs more or less developed in the pilidium larva of the nemerteans, in the developing Polygordius, in the echinopluteus when it is undergoing a metamorphosis, in the larva of insects and of the scorpions. Some of these groups also show the possession of 8. serosa in connection with the amnion. 226 EGG AND EMBRYONIC MEMBRANES

The Yolk Sac

The true yolk sac can occur only in forms having an extreme telolecithal type of egg structure. Among the invertebrates the cephalopods

fiG. 160. Various stages in the development of 0. squid, showing the relations of the yolk sac. (Modified from Harmon and Gardiner.)

B and C. dorsal views; D, sagittal section. a., arm rudiment; {., funnel; m., mouth; o.c., optic cup; y.ex., external yolk;, internal yolk.

are the only forms which show a well-developed yolk sac. As is described in Chapter V, the cleavage of the protoplasmic cap of the cephalopod THE AMNION 227

egg results in the formation of a series of blastoccnes arranged about a germinal area consisting of blastomeres. These blastocones separate entirely from the blastoderm and rapidly extend themselves over the surface of the yolk underneath the growing blastoderm. They constitute a yolk membrane enclosing the large mass of yolk. Over them the blastoderm gradually extends and when the embryo proper is formed it becomes marked off by a groove from the yolk sac, as it may now be properly called. The yolk sac takes on the appearance of an appendage which it maintains until the development of the embryo is far advanced. Actually the yolk mass becomes divided into two portions, one of which is contained within the body of the embryo and constitutes the internal yolk sac in contrast with the external one which has just been mentioned. The yolk masses of the two are joined at all times, however. The further growth of the embryo results in proportionate decrease in the size of the external yolk sac which is gradually included within the body of the young animal. Morphologically this structure represents a precocious median development of the mass of tissue which produces the arms of the squid and is comparable to the developing foot of other molluscs. Strictly speaking it is not merely an embryonic appendage, for all of it is finally withdrawn into the body of the growing animal. A somewhat similar ventral protrusion from the foot of the snail, Helzlr, was described by Lankester and while not a true yolk sac it is indicative of the tendency so markedly illustrated by the squid.

The Amnion

The first structure in the animal kingdom which may be designated as an amnion is seen in the metamorphosis of the pilidium larva of the nemerteans which is described on page 153 (figs. 103 and 104). On the flattened under surface of the helmet-like pilidium, four amniotic invaginations form. They consist of 'ciliated ectoderm and other deeper parts and are known as the imaginal discs. Their continued inward growth upward and over the alimentary tract finally results in their fusion. The inner parts, or the imaginal discs, form the outer covering of the little worms while the remaining portions of the coalesced invagination make up the amnion which is a temporary covering entirely lost with metamorphosis.

The second appearance of the amnion is seen in the metamorphosis of the annelid, Polygordius. This structure superficially resembles that of the pilidium, for here also four invaginations appear arising from a circular groove which is known as the amniotic fold. It serves as a covering for the folded portion of the worm which remains in this con228 EGG AND EMBRYONIC MEMBRANES

dition until the time of metamorphosis when, by severe contraction, the worm becomes straightened out and the amnion has no further function.

An amniotic invagination is described in the metamorphosis of the echinopluteus of the sea-urchin. It occurs on the left side of the larva as a pit which deepens and broadens until it comes in contact with the wall of the hydrocoele. The tissue which lies between the amniotic in

fiG. 161. Transverse sections of embryos of Clytia. (Redrawn from Folsom after Lecaillon.)

A, through germ band at gastrulation; B, through germ layers and amniotic folds; C, stage later than B; D, through germ layers. a., amnion; a.c., amniotic cavity; c., coelome sac; e.. ectodcrm; i., inner layer; g., germ bond; n., neuroblasts (primitive nervous cells); 59., serosa.

vagination and the hydrocoele undergoes important changes and is involved in the formation of the so-called echinus rudiment. Into the amniotic cavity this rudiment and especially the tube feet develop.

In the scorpion, according to Brauer, an amnion is formed shortly after the two-layered stage shown in fig. 39. At the edge of the blastederm a sheet of cells arises and grows backward and presently a second layer is formed over the first. The outer is the serosa and the inner the amnion. Underneath these the embryo develops.

The amnion of invertebrates reaches its highest development among the insects, and of these the Coleoptera show it most clearly. There are THE AMNION 229

really two types of development with respect to the relations of formative area. and of yolk shown in the insects. In the one type, called by

some “overgrown,” the germ band retains its original position in the blastoderm and the folds of the amnion and serosa arise at its sides and

fiG. 162. Diagrammntie sagittal sections of Calopteryx. (From Folsom, after Brandt.)

A-D, illustrate invaginution of germ band; E-F, illustrate rex olution of embryo. a., anterior pole; a.c., amniotic cavity; am., amnion; z1iit., antenna; b. blastodcrm, d., dorsal; g.. germ band; h., head end of germ band; 1., laliriuin, l‘-l“, thoracic legs; m., mandible; mx., maxilla; p., posterior pole; 5., sorosa; v., ventral; y , yolk.

grow over it. This occurs in the Coleoptera, and it is for this reason that the formation of the amnion is most clearly shown there. In the other type, called “invaginated,” the germ band invaginates at one end of the egg, and is finally carried into a cavity formed within the yolk mass; after the closure of the opening of the invagination the formative portion develops entirely within the egg. Due to this invagination the ventral 230 EGG AND EMBRYONIC MEMBRANES

surface of the developing embryo comes to face the dorsal side of the egg and a subsequent process of revolution is necessary before the embryo at length resumes its original orientation. This type occurs in the Odonata. The relations of the folds of the amnion are shown in fig. 116, which illustrates the development of Hydrophilus, and also in fig. 161, which shows the formation of the amnion in the beetle Clytra. Folds of the blastoderm arise at the side of the germ band and continue their growth until they meet above it and fuse. The membrane thus formed directly above the germ band is the amnion, and the outer one, the serosa. The formation of the amnion and serosa in Calopterya: is shown in fig. 162.

Students of the embryology of the insects are not entirely in agreement in regard to the morphological relations of the embryonic membranes found throughout the group and several types of classification have been proposed. It hardly seems necessary to consider the various classifications or the points upon which they have been based. Serosa and amnion occur together in the Coleoptera, Lepidoptera, Diptera, Dcrmoptera, Isoptera, and Odonata. An amnion only is found in Xenos, a strepsipteran parasite, and even it is vestigial in such forms as M usca and Drosophila. A serosa only is formed in a few of the Hymenoptera including the honey bee, wasps, and ants.


Embryonic membranes are especially well developed in the vertebrates where they serve for protection, nutrition, and respiration. These are the yolk sac, amnion,serosa or chcrion,allantois, and its derivative, the placenta. Since the name “chorion” is also correctly used for the secondary membrane secreted by the follicle cells, it is confusing to use this term for an embryonic membrane of different origin. We shall therefore always designate this embryonic membrane as serosa although it is quite customary among mammalian embryologists to use “chorion” in both senses. A

For an introduction to the nature of the embryonic membranes of the vertebrates and of the parts they play, the conditions as found in the chick may be taken as a type. Of course these conditions apply particularly to the Sauropsida; however, they may be looked upon as providing a point of departure for the entire study. They have often been described, so a brief reference here is all that is necessary. The first membrane to appear in the chick is the yolk sac. As the blastoderm grows over the yolk its lower edge consists after gastrulation of primary ectoderm. At an early stage the endoderm and later the mesoderm may be seen back of the edge gradually making their way over the yolk. The VERTEBRATES EXCEPT MAMMALS 231

mesoderm divides into two layers, and the investments of the yolk are first the splanchnopleure (endoderm and mesoderm), then the extra Fra 163 Diagrams of the extraemhryomc membranes of the chick (From Duv9.l’s Atlas)

A, 2-day embryo, B, 3-day embryo a , air space, of , ammotlc fold, a c , amniotic cavity, all , allantois, alb , albumen, e , embryo e e c , extruembryomc eoelome, ect , ectoderm, end , endoderm, In , mesoderm, ser. serosn; s , shell, sm . shell membrane, so. soma.topleure. sp , splanchnopleure, v , vltellmo membrane, y , yolk, y s . yolk sac

embryonic coelome, and last the somatopleure (mesoderm and ectoderm). The anterior and posterior parts of the gut are the first to develop 232 EGG AND EMBRYONIC MEMBRANES

a cellular floor. The midgut remains open to the yolk for some time. As the embryo becomes constricted off from the yolk the two ends of the gut enlarge further, and the extent of the opening of the midgut is decreased. finally, the latter is connected to the yolk only by a narrow


flu 164 (‘ontmuation of fig 163 A, 5—day embryo, B, 14-day embryo

am , amnion, all e . allantoie cavity, all s , allantolc stalk, u , umbilical stalk neck called the yolk stalk. At the yolk stalk the inner and the extraembryonic splanchnopleure are continuous. The absorption of yolk material through the yolk stalk probably does not occur directly but is brought about by the vitelline circulation. Deep yolk sac septa which are folds of the splanchnopleure grow down into the yolk, and glandular VERTEBRATES EXCEPT MAMMAIS 233

absorbing cells aid in its digestion and in passing it to the vitelline circulation. The yolk mass decreases toward the end of the incubation period, and finally the entire structure is drawn into the body.

During the early growth of the blastoderm over the yolk the amnion and the serosa are being formed. The amnion arises by folds of the extraembryonic somatopleure near the anterior region of the embryo. first to form is the head fold of ectoderm and endoderm, for the mesoderm has not yet reached this region, and this is joined by lateral folds of somatopleure so that the entire region is covered. Later a similar tail fold covers that region and is continued forward until it meets the anterior lateral folds. The fusion of these folds encloses the embryo in an ectodermal cavity, the amniotic cavity. The union of its two folds has caused the somatopleure to double back on itself and thus to enclose the embryo in two membranes of similar nature although of inverted order. The inner membrane or amnion consists of ectoderm continuous with that ‘over the embryo and of somatic mesoderm. The outer membrane or serosa consists of somatic mesoderm and of ectoderm continuous with the outer covering of the yolk. Between the amnion and the serosa is the extraembryonic coelome which is connected with the body cavity of the embryo. The serosa is merely a continuation of the somatopleure which has made its way nearly around the yolk. It also covers the allantois which later develops within the extraembryonic coelome. Thus it comes to line the shell and covers the embryo and the other three membranes.

The allantois arises as a diverticulum from the hind gut within the body of the embryo. As development proceeds it extends out into the extraembryonic coelome, its connection with the gut being designated as the allantoic stalk. Since it arises from the gut, it has necessarily a lining of endoderm and an outer layer of mesoderm, that is, it is an extension of the splanchnopleure. It grows very rapidly and unites with the serosa so that there is now a double layer of mesoderm in its outer wall, splanchnic on the inside and somatic from the serosa on the outside, and over all is the ectoderm of the serosa. The rapid growth of the allantois carries it first over the amnion, with which it fuses, and then over the yolk sac. finally, pushing the serosa before it, it extends even over the albumen, which has become dense from loss of water, forming an albumen sac which is lined by the ectoderm of the serosa. The allantois has a rich blood supply in a network lying close to the porous shell, through which the exchange of gases is made easy. The chief function of the allantois is respiratory but it also serves as a receptacle for the excretory wastes from the kidney of the embryo. Shortly before the chick hatches, its beak breaks through into the air space of 234 EGG AND EMBRYONIC MEMBRANES

the egg, pulmonary respiration begins, and the circulation of the allantois becomes slower and slower, until at hatching time the dried allantois remains inside the shell.

With conditions as found in the chick as a starting point, the development of the various vertebrate membranes may be appropriately taken up.

The Yolk Sac

The yolk sac is the only accessory structure which may be looked upon as of general occurrence among the vertebrates. It will be recalled that all vertebrate eggs show a distinction between yolk pole and animal

fiG. 165. Embryo of toad fish showing large yolk sac attached.

pole. (See Chapters IV and V.) The eggs are markedly telolecithal whether the cleavage is holoblastic or meroblastic. The relation between discoidal and holoblastic eggs is discussed and illustrated earlier (fig. 51) where it is shown that the condition of the large yolk-laden endoderm cells of the bilateral type is gradually converted into the yolk mass of the discoidal type. The latter is incapable of segmentation, yet the homology between them is complete. The yo1k—laden endoderm cells which make up the ventral portion of the enteron and serve as a source of food supply for the growing embryo may be regarded as the antecedent of the yolk sac of the meroblastic vertebrates. They do not constitute a true yolk sac, but the mass of cells is the equivalent of an internal one and corresponds in a general way to the outer sac of fishes, reptiles, birds, and mammals. For all of these it is an important nutritive structure.

In myxinoids, elasmobranchs, and teleosts, in all of which the eggs are extremely telolccithal, a well-developed yolk sac is characteristic. THE AMNION 235

Before the blastoderm spreads over the yolk the embryo has formed and has become folded 0E. The yolk sac, therefore, is formed by the extraembryonic blastoderm which grows over the yolk and it is attached to the embryonic body by a stalk, narrow in elasmobranchs but wide in teleosts. The yolk sac has an area vasculosa that is rich in blood vessels by which the food material of the yolk is brought to the embryo.

Although it is an outgrowth of the gut the connection of the yolk sac with that structure may be interrupted early in development as is found among the teleosts (for example the salmonids), in which case the absorption of the yolk is brought about exclusively by the vitelline vessels. In addition to its nutritive function, the yolk sac often acts as a respiratory organ. It functions in this manner in the lower vertebrates whcre an exchange of gases takes place between the blood vessels of the yolk sac and the surrounding water and also in the early stages of higher vertebrates where the exchange takes place between its blood vessels and the medium surrounding the egg. Even in mammals the yolk sac may have a respiratory function. In the mono- ‘ tremes it still retains its yolk and hence its nutritive function. In the placental mammals, however, it is devoid of yolk but may still be functional in early stages when it is quite vascular and helps to absorb fiG 166 Embryo of Torpedo nourishment from the maternal tissues. §l;?,Y,‘“§sb?:“:f0r,;°:

Structurally the yolk sac is developed in from the posterior part of the about the same manner throughout the embm’ (After zlegler’) lower classes of vertebrates. A distinction is to be observed in the appearance of the mesoderm of the elasmobranchs, as Torpedo, and in teleosts, as the salmon. In the former the mesoderm is an undivided layer which grows out peripherally from the posterior portion of the embryo and then anteriorly about the edge of the blastodisc. In the latter case the mesoderm splits into somatic and splanchnic layers which grow laterally from the embryonic area and carry the body cavity as the extraembryonic coelome out over the yolk.

The Amnion

In the Sauropsida (reptiles and birds), we have the first appearance of the amnion, serosa, and allantois. This group and the mammals make up the Amniota, which possess the amnion and are contrasted with the Anamnia which lack it. Since most of these latter lay their eggs in a 236 EGG AND EMBRYONIC MEMBRANES

fluid medium, usually water, the developing embryos are not subjected to injury through violent motion or through changes in temperature or in humidity, and thus no special covering of the embryo is necessary. All the higher vertebrates either lay their eggs in a shell or hold them in the mother’s body during the embryonic period and some kind of protection is heeded to prevent the embryo from drying or from pressure or stress that might reach it. Therefore these embryos are protected by two membranes, the amnion and the serosa. When fully formed the amnion has always an inner layer of ectoderm and an outer layer of mesoderm.

Of the Amniota the reptiles are the most primitive. It is interesting to note that in them the amnion develops earlier than in the birds at a time before the head has become prominent and before the mesoderm

fiG 167 Diagrammatxe transverse section through a (‘helonian embryo (Cl€m"l1I8)(Redrawn from Kerr. based on a figure of Mitsukuri.)

has split into its two layers. In the Chelonia the amnion is at flrst formed of ectoderm and endoderm, the mid-dorsal part being of solid ectoderm. Later after the mesoderm has split it is drawn up into the amniotic folds and the endoderm takes no further part in the formation of the amnion. This is different from the condition in the chick in which the amnion is formed from the somatopleure except in the proamnion region where there is no mesoderm. (A proamnion region is characteristic of reptiles in general.) The broad ectodermal band in the roof of the amnion in the Chelonia has its counterpart in a small area known as the scro-amniotic isthmus in the amniotic roof of the chick. This latter may be considered as the remains of the broader layer of the reptiles. The amniotic fold in the Chelonia begins at the head end and proceeds backward until the whole embryo is covered. By the backward growth of this flap there is formed a long tunnel-like cavity covering the embryo and extending posterior to it. Later this posterior extension becomes partly obliterated and the cavity is closed. This again is quite THE AMN ION 237

different from the formation of the amnion in birds in which a head fold, lateral fold, and a tail fold take part. In some reptiles, however, as the chameleons, amnion formation is similar to that of birds, from lateral folds around the embryo, and is not due to the elongation of one an 5 fiG 168 A and B Chelonnn blastodorms showing stages in the formation of the amnion A, Cheli/dra, B, Clemmys (Redrawn from Kerr after Mitsukuri)

a . amnion showing neural groove beneath it, a f , edge of amniotic fold, a t . amniotic tunnel.

terior fold. Since the amnion appears earlier in the reptiles than in birds, and its cavity is formed before the embryo is marked off from the yolk, there is no need for the amniotic folds to grow upward as in the chick. The development of the amnion in the birds as a group offers nothing different in principle from the type already described, that is, from this conditions in the chick. The amnion of the mammals presents new features, however. Their character has already been discussed in the chapter on the formation of the mammalian embryo, Chapter X, to which the student is referred, and the chief facts regarding it are again mentioned a little later in this chapter under the head of the membranes of the mammalian embryo.

The Allantois

The allantois appears first in reptiles and is developed in all amniota. It always originates as an outgrowth of the hind gut and has therefore an endodermal lining and an outer layer of mesoderm. In amphibians it is represented by the urinary bladder. In reptiles the outgrowth has been described as solid, but it has also been maintained that the contracted space in which it develops causes it to be collapsed and so to take on the appearance of a solid structure. In birds it has a three-fold function. It increases in size, probably on account of the large amount of excretory Wastes collected during the embryonic life in the shell. Its large size and its position next to the air space in the end of the egg and just inside the shell are correlated with its new use as a respiratory organ. Also, pushing the serosa before it, it forms a double-layered sac over the albumen. This sac is thus lined with ectoderm. Since it absorbs the albumen, the allantois thus has a nutritive function, although of course this is of less importance than the similar function of the yolk sac.

The allantois of mammals acquires no further function. It aids the trophoblast in the formation of the placenta which is for the mammal an embryonic organ of nutrition, respiration, and the removal of excretion. It thus becomes the most important embryonic membrane of mammals, for through the participation of its blood vessels it has the chief part in the formation of the placenta.

Viviparity among the vertebrates is especially related to the functions of the allantois and the adaptations which it makes, but other embryonic membranes are also employed in bringing about this mode of caring for the embryo. Viviparity occurs in all the vertebrate groups, beginning with the fishes, except the ganoids, lung fishes, and birds. It is possible that it may have been characteristic of some fossil ganoids, but probably the birds because of their habit of flight have never been able to develop this method of caring for their young. There are many parallel methods of relating the embryo to the maternal tissues, and functional placentae are not confined to the mammals. In all forms below the mammals the embryo is chiefly nourished by the yolk sac even though other structural modifications supply additional sources of food. Development within the body of the mother always serves the same function, protection in THE ALLANTOIS 239

the struggle for existence. Among the elasmobranchs there are several genera in which there is intra—uterine development. In Acanthias the typical egg membranes are formed but the horny egg-shell is thin and disappears in later stages. The uterine wall contributes to the nourishment of the embryo by supplying the albuminous fluid which is enclosed in the egg-shell. In this form the lining of the uterus during later stages of development is thrown into folds which are brought into contact with the yolk sac. In Torpedo the folds are prolonged in trophonemata, nutritive threads which reach the pharynx of the embryo through its spiracles. In M ustelus laevis the folds of the uterus fit into grooves in

Fm. 169. Yolk sac placenta of Muslclus lacvis. (Rcdmwn from Corning after Joh. Muller.)

u., wall of uterus; y.o., yolk sue.

the yolk sac and, as both are highly vascular, they form a functional yolk sac placenta. A placenta used in this broad sense means any structure which brings about, by means of an abundant vascular supply, an intimate relation between maternal and embryonic tissue, for the purpose of conveying nourishment from the one to the other. Here the yolk sac functions in this manner, but in the Amniota it is usually the allantois.

The teleosts include a number of viviparous forms. In some the eggs are developed in the ovary itself where vascular processes on the surface contain the eggs and act as placentae. In others the embryos develop in the enlarged oviduct _or uterus. Sometimes (Scorpaenidae) the maternal body merely holds the embryo which obtains its food supply entirely 240 EGG AND EMBRYONIC MEMBRANES

from the yolk. In other cases (Anableps) the yolk sac sends out villi to the uterine wall.

Viviparity is uncommon among the amphibia. In one of the salamanders (Salamandra atra), however, development takes place in the oviduct but usually only one of the eggs reaches maturity for the others break up and are absorbed by the outer gills of the developing embryo. The embryo has additional nourishment from the blood which escapes from the uterine wall.

fiG 170 Embryo of (‘hulcides tndactylus (Redrawn from Kerr after Giaconiine)

A, 7-mm embryo, B, 15-16-mm embryo seen from apical pole all, allantois, pl, allantoic placenta (foetal portion) . y s , yolk sac

The reptiles include a number of viviparous forms. In some, the blind worm and the viper, there is a thin shell and the embryo is simply held in the uterus, the lining of which remains unchanged. In the lizard, Chalczdes ocellatus, the shell breaks and finally disappears, the uterine wall becomes vascularized, and 1S thrown into folds which fit other folds on the yolk sac. Both the allantois and the uterus adjoining it are highly vascularized but the placenta-like folds develop only on the yolk sac. A type of placenta more nearly approaching that of the mammals is found in still another lizard, Chalczdes trzdactylus. The condition of its allantois and yolk sac may be regarded as transitional from that typical of the Sauropsida to that of the mammals, for both function as placenta and projections from both vascularized sacs interlock with MAMMAIS 241

similar projections from the uterus. But of the two the allantoic placenta is the more highly developed. Thus the true allantoic placentae are not limited to the mammals but appear also among the reptiles.

Evidently the problem of relating the embryo to the maternal tissues has been solved in similar ways, although employing different structures in the different groups of the vertebrates. In the mammals the wall of the uterus is highly developed as a special adaptation of the maternal tissue to accomplish this function. In other forms the oviduct sometimes takes care of the need so far as the mother is concerned, and even the ovary may serve in this capacity. On the part of the embryo, the yolk sac of lower forms often carries on the function, while in the higher forms it is taken over by the allantois.


Of the embryonic membranes of mammals the greatest interest is in connection with the allantois and the placenta which it helps to form. The yolk sac and amnion are also present, however, and show features which are new. In the entire early development of mammals there is a rather general lack of structural homology with that of other classes of vertebrates, and although similar results are reached convergently so far as the structure of the embryonic membranes is concerned, their methods of origin in some forms is very different.

There are two great types of development found in the mammals. The monotremes are unlike all the others in that their eggs are large and yolk-laden and their cleavage discoidal. The animals are oviparous, the development of the embryo taking place within the egg-shell, as in the case of the Sauropsida. Cleavage, germ-layer formation, and the development of the embryonic membranes is comparable to these processes in that group. The membranes are of the same kind and are probably formed in the same manner.

The marsupials are the lowest mammals which have intra—uterine development. The young are born in an immature condition, and are placed in the abdominal pouch where they become attached to the teats of the mammary glands, Here the immature period is completed. Since at birth the young are so miniature, the allantois, which in the higher vertebrates functions as a medium for obtaining nourishment from the uterus, is usually small. The yolk sac contains no yolk, though it is large and well developed. Its wall opposite the embryo contains only endoderm, and the extraembryonic coelome is absent in this part of the blastoderm. During uterine life there is usually a yolk sac placenta.

The marsupials include a number of forms which taken together make up a series similar to that described for the lower vertebrates and illus242 EGG AND EMBRYONIC MEMBRANES

trate the change from a yolk sac placenta to an allantoic placenta. The first of the series is the opossum, Didelphys, in which the yolk sac has 9, nutritive function. This membrane makes connection by means of folds

. on: 077777;!‘ 6

Fm. 171. A, diagram of a section through monotremo embryo; B. diagram of a. section through marsupial embryo. Ectodcrm, solid line; mesoderm, dotted line; endoderm, dashed line.

a., amnion; a.c., amniotic cavity; 3.11., allantois; e., embryo; e.e.c.. extraembryonic coelome; sen, serosa; y.s., yolk sac.

with the uterine wall. The latter secretes a nourishing fluid which is absorbed by the trophoblast and the endoderm of the yolk sac, and thus is transported to the embryo. In Dasyurus we have a further step THE YOLK SAC 243

in the series. The allantois is still small and the yolk sac large. The trophoblast forms a syncytium with pseudopodial processes which attack the uterine wall and engulf uterine blood vessels. The nourishment is passed on to the yolk sac which may be correctly called a yolk sac placenta, for the connection between the uterine wall and the yolk sac is very close. Parameles illustrates the last step in the series. In this form the yolk sac is large and probably has some function in absorbing nourishment. The allantois, however, grows larger, develops a richer blood supply, and reaches the trophoblast. The uterine wall, at the place where the trophoblast touches it, develops a syneytium and becomes vascular. The trophoblast disappears and the blood vessels of the allantois and the uterus come into close contact. There are no true villi as in the higher mammals. This is the first case we have met of a true allantoic placenta and the only one known for the marsupials. It differs from all others in that the syncytium is entirely of uterine origin and does not develop from the trophoblast as it does in those mammals having a discoidal, deciduate placenta.

The development of the eggs of the true placental mammals involves the formation of a blastocyst with differentiated trophoblast and inner cell mass as is described in Chapter X. The embryo resembles a hollow vesicle with a drop hanging from one spot on its inner surface. The inner cell mass is the formative or embryonic portion. The trophoblast is non-formative and produces only ectoderm of the serosa. After this stage the inner cell mass or embryonic knob differentiates into two portions, an upper eetodermal layer from which will be derived the ectoderm and amnion of the embryo and a lower endodermal layer from which will be derived the enteric canal, the yolk sac, and the allantois. The embryo proper at this stage is called the embryonic shield and consists of a layer of ectoderm covering a layer of endoderm. (See fig. 144.)

Jenkinson has offered an interesting suggestion about the significance of the early development of the mammal. He compares the blastocyst stage with the later stage of the reptile or bird in which the yolk sac has become completely covered and the serosa contains the embryo in the amnion with its yolk sac and allantois attached. In the mammals also there is a stage of a closed sac, the blastocyst, which contains in the inner cell mass all the material for the embryo and its membranes. J enkinson regards this as a precocious separation of the materials of the mammalian embryo due to the loss of the yolk on the part of the ovum.

The Yolk Sac

The yolk sac has the same layers as that of the lower forms, but in the placental mammals it arises in a different way and is functional 244 EGG AND EMBRYONIC MEMBRANES

only in early stages, if at all. The endodermal layer is formed from the innermost layer of the inner cell mass. These cells proliferate and spread around the inside of the trophoblast until they form a closed sac just as would have happened had a central yolk mass been present and they had grown around it. Sometimes the cavity is very small and the yolk sac correspondingly restricted, as in man, when it forms a very small vesicle owing to its slow growth. In other cases the endoderm is quite closely applied to the inner side of the trophoblast and a large yolk sac is thus formed. In no case, however, does it contain yolk. It is therefore an insignificant structure and the function which it serves in the lower

Fro. 172. Diagram of a. section of rodent embryo. p., placenta; other letters as in fig. 171.

forms is presently taken over by the allantois. The completion of the yolk sac comes only when in later stages the mesoderm is formed between the endoderm and the trophoblast.

The formation of the mesoderm begins in the embryonic area and some time elapses after its separation into its two layers before it grows out into the space between the endoderm and the trophoblast of the vesicle; but at length it pushes out from the embryonic area and grows between these two layers. An early split in this mesodermal layer completes the yolk sac which thus consists of endoderm and splanchnic mesoderm. The vitelline circulation develops to provide the usual vascular supply.

In many of the higher forms it is merely a vestige of the membrane through which the embryos of simpler forms obtain their food. In some cases, however, it retains something of its earlier function in that in THE YOLK SAC 245

early stages it absorbs nutrirnent from the uterus. In these cases it becomes a yolk sac placenta. When this occurs, as in the ungulates, camivores, and rodents, the yolk sac is large and its blood supply is rich and comparable to that of the chick. In marsupials, in which the uterine development is of short duration, the yolk sac placenta often functions until birth.

In rodents the yolk sac is always large and invagination of its upper wall by the embryo gives apparent inversion of the germ layers. This may take place early in development as in the mouse and guinea pig, or

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later as in the rabbit and squirrel. In some cases the lower wall of the yolk sac is never formed. In others it disappears along with the trophoblast, and the cavity of the yolk sac is in communication with the cavity of the uterus. The upper wall of the yolk sac absorbs the material secreted by the uterus and its supply of blood vessels carries this to the embryo. Thus the yolk sac acts as an accessory placenta.

In the rabbit the yolk sac, which contains a nourishing fluid, is very large and in early stages practically fills the blastodermic vesicle. A rich supply of blood vessels develops in the side of the yolk sac which is toward the embryo. The opposite side contains no mesoderm and fuses with the trophoblast. Later the yolk sac shrivels and since its outer wall is fused with the trophoblast it remains as a mushroom-shaped 246 EGG AND EMBRYONIC MEMBRANES

membrane. The two walls of the shrunken vesicle thus come close together and later they fuse. Since the inner wall of the yolk sac was highly vascularized its blood vessels continue to supply a large part of the blastoderm. The rest of the blastoderm is fed by blood vessels of the allantois which help form the placenta. The yolk sac of the rabbit is probably a more important nutritive organ than the allantois. The vitelline artery ends in the sinus terminalis and the blood is returned to the heart by the vitelline veins.

In the Insectivora the yolk sac is often large in early stages and its blood vessels may begin to form a yolk sac placenta as in Tupaija. In

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Fm. 174. Diagram of 11 section of embryo of insectivore. u., wall of uterus; other letters as in fig. 171.

some insectivores the mesoderm never covers the lower wall of the yolk sac (Sores: and Talpa). This organ is unimportant in later stages.

The yolk sac circulation in most mammals is of importance in early stages only, while the allantois is being formed. Later, when the allantois has established its relation to the uterine wall, there is a retrogression of the yolk sac. This is true of man, in which the yolk sac becomes a solid strand of cells extending; out into the body stalk or umbilical cord. In some forms, as the insectivores, the yolk sac remains until birth.

The Amnion

The formation of the amnion is due to the development of the upper portion of the inner cell mass. It is accomplished by one of the two THE PLACENTA 247

methods described in Chapter X. In the one case, there may be a fold or a series of folds of the extraembryonic somatopleure similar to those of the chick, as in most ungulates and some insectivores. In the other case, the cavity may arise in situ in the embryonic knob or between it and the trophoblast, and thus may never open into any other cavity, as in the mouse, guinea pig, bat, other insectivores, and many primates, including man.

The Allantois

The allantois arises as a diverticulum from the gut in the same manner as in the chick. In the rabbit this diverticulum extends out into the extraembryonic eoelome and soon comes into relation with the trophoblast. In many rodents, as the mouse and guinea pig, it is often small and its endoderm may not grow beyond the body of the embryo. In Tarsius and in man the vesicle never extends freely into the extraembryonic eoelome but is enclosed as a vestigial structure in the body stalk. This latter is an enlargement of the primitive connection between the embryo and the trophoblast, a connection which is never lost. It is regarded as the equivalent of the allantoic stalk of other forms.

The allantois of lower mammals assumes the function of nutrition but retains its cavity which may serve as a receptacle for urinary wastes. The size of the cavity is progressively reduced until in some forms it disappears completely with the inner endodermal layer and only its mesodermal portion remains. This part becomes highly vascularized and, together with the trophoblast, forms the embryonic part of the placenta, a structure which comes into relation with the circulatory system of the mother and so takes care of the functions of respiration, nutrition, and excretion. Thus the mesodermal portion of the allantois accomplishes the function which in lower forms the entire structure serves.

The Placenta

A placenta is any organ which brings about an intimate relation and vascular connection between maternal and foetal tissues. The formation of a yolk sac placenta has been described for some of the fishes and reptiles and is again found among the marsupial mammals. Among the higher mammals, however, it is always the allantois which brings about this relation between the embryo and the mother. In these forms, the actual contact is made by the trophoblast, as part or all of it comes into relation with the uterine walls. It is the allantoic vessels, however, which, through contact with the trophoblast, furnish the foetal part of the

placental circulation. _ The placenta is necessarily a compound structure. Its embryonic 248 EGG AND EMBRYONIC MEMBRANES

parts are trophoblast (sometimes lined with mesoderm) and the allantois with its blood vessels. Its maternal part is the vascular uterine wall. The foetal and maternal parts may be merely in apposition or they may be in such close relation that they are actually fused. In both cases an intimate physiological relation results. The blood vessels of the allantois (umbilical arteries and veins) are brought close to those of the uterus, making possible an exchange of substance by diffusion. The embryo is thus able to obtain oxygen and food materials and also to dispose of its waste products. There is never any direct connection between foetal and maternal blood vessels.

The placentae of mammals are of two general types, deciduate and non—deciduate. The non—deciduate placenta is sometimes called a semiplacenta. The name deciduate was given to indicate the loss, at birth, of the lining of the uterus. Although this loss is not significant in all socalled deciduate forms, the name has still been retained. The following brief classification of types of placentae of the Placentalia is partly based on the conception of Jenkinson. The types are closely related to the method of implantation of the ovum in the uterine wall. Of implantation there are three types, central, eccentric, and interstitial, as mentioned in Chapter X. Implantation is central if the embryo becomes attached superficially to the uterine wall; eccentric if it is enclosed by folds of the uterus; or it is interstitial if the embryo burrows down into the uterine wall. The classes of placentae are as follows:

The non—deciduate type is that in which the surface of the trophoblast develops villi which fit into crypts or depressions in the uterine wall. In the simplest cases, the uterine epithelium persists in these crypts and at birth the villi are drawn out without damage to the uterus. In other cases, the uterine epithelium is destroyed and there is a closer union between foetal and maternal tissues. This type of placenta is characteristic of the ungulates, but is also found in the Cetacea, Edentata, Sirenia, and Lemuroidea except Tarsius.

The second type is the deciduate in which the uterine epithelium is always destroyed by the trophoblast. Foetal and maternal blood are in close relation. There are two subdivisions of this type as follows:

(a) Zonary deciduate placenta. The maternal blood circulates in capillaries of the uterus. The placenta, which is zonary or band shaped, is composed of foetal and maternal tissue in very close relation. At birth part of the wall of the uterus is destroyed and there is a true decidua. This type of placenta is characteristic of the Carnivora. (b) Discoidal deciduate placenta. The trophoblast destroys not only the uterine epithelium but also the subepithelial tissue. The maternal blood circulates in lacunae or spaces in the trophoblastic wall. These spaces NON-DECIDUATE PLACENTA 249

represent maternal blood vessels which have been engulfed by the trophoblast and they are the chief part of the placenta contributed by uterus. Blood is the principal maternal tissue lost at birth, and the deciduae are thin and more or less degenerate. The portion of the trophoblast which is involved in the placenta is disc-shaped. Placentae of this type are found among rodents, insectivores, bats, in Tarsius, monkeys, and man. These three types of placentae require a more detailed description.

Non—decz'duate Placenta. The non—deciduate placenta may be illustrated by that of the ungulates. In these forms the allantois enlarges rapidly and occupies the length of the trophoblast. The surface of the trophoblast forms villi into which extend the mesoderm and blood vessels of the allantois. These may be single or branched. They may be scattered over the surface of the trophoblast (diffuse placenta, as pig);

Fm. 175. Diagram of embryo of ungulate contained in uterus. Wall of semen has been cut, showing embryo in section. c.. cotyledon; ed., edge of cut in serosa.

they may be situated in groups or cotyledons (cotyledonary placenta) when they are also branched (ruminants), or there may be a combination of the two types, both cotyledons and simple villi being interspersed (intermediate placenta). Also the cotyledons may be grouped in a band around the trophoblast.

The trophoblastic villi fit into crypts or corresponding depressions in the uterine wall. When the villi are in groups or cotyledons, the corresponding groups of crypts on the maternal side are called maternal cotyledons. These latter are in some cases stalked and the foetal cotyledons are then concave.

In the difiuse placenta, the uterine epithelium persists as the lining of the crypts and at birth the villi are withdrawn with no injury to the uterus. In the cotyledonous placenta the uterine epithelium, except that of the glands, degenerates and there is a much closer union of maternal and foetal tissues. Consequently the two portions of a placenta of this type separate with some difficulty.

In the diffuse placenta the uterine and trophoblastic epithelia are in contact, but in the cotylcdonary placenta the contact of the tropho250 EGG AND EMBRYONIC MEMBRANES

blastic epithelium is with a highly modified and vascular subepithelial connective tissue. In the latter case, however, there is no true fusion of foetal and maternal tissue as is true of the deciduate placentae. The maternal connective tissue and trophoblastic villi are both rich in capillaries. Nourishment is obtained by the trophoblast in two Ways. Both the villi and the trophoblastic cells at their base absorb nourishment (“uterine milk”) from the maternal tissues, and extravasted blood and

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F10. 176. Cross sections of placentae. A, pig; B, cow; C, out. (Modified from Grosser.)

d.. degenerating uterine epithelium; 0., embryonic capillary; m., maternal capillary; mes., mesoderm of serosa; s., epithelium of serosa; u., uterine epithelium; v., villus.

cell débris from the crypt walls are ingested by pseudopodia from the cells of the trophoblast. Absorption and respiration also are brought about by diffusion between the maternal and foetal capillaries. The blood in these vessels is separated by very thin layers of tissue only. The placental layer consists, on the foetal side, of the capillary walls, connective tissue, and trophoblast, and on the maternal side of connective tissue, eapillary walls and, in the diffuse placentae, of uterine epithelium also. Thus diffusion between foetal and maternal blood vessels is easily carried on. DECIDUATE PLACENTA 251

In the allantoic fluid of ungulates are found bodies of queer shapes called hippomanes. These are formed as accumulations of coagulable material from the uterine milk and become saturated with calcium oxalate crystals. They push, pocket-like, into the trophoblastic wall between the cotyledons and later are pinched off into the cavity of the allantois, surrounded by trophoblast and allantoic wall.

Fm. 177. Embryo of sheep enclosed in embryonic membranes and showing cotyledonous placenta characteristic of many ungulates. (Redrawn from Corning after 0. Schultze.)

a., amnion; c., cotyledon; e., atrophied end of blastodermie vesicle; u., umbilical cord.

Deciduate Placenta. In deciduate placentae the union of maternal and foetal tissues is closer than in placentae of the non-deciduate type. In deciduate placentae, part or all of the trophoblast, called the trophoderm, becomes specialized and, by the aid of enzymes, erodes the uterine wall. At birth the lining of the uterus is removed as part of the placenta. In the zonary deciduate type, this lining or decidua is in close union with the foetal blood vessels, so that maternal and foetal tissues are removed together. In the discoidal deciduate type, however, the 252 EGG AND EMBRYONIC MEMBRANES

maternal blood, which circulates in spaces in the trophoblast, is the chief tissue lost from the uterus, for the deciduae are thin and degenerate.

The placenta of the zonary deciduate type occurs only in the Carnivora. This placenta may in some respects be considered as intermediate between the naon-deciduate and the discoidal deciduate types. In the Carnivora, the trophoblast destroys the uterine epithelium (but not the subepithelium) and eats its way into the wall of the uterus. Here trophoblastic villi invade the connective tissue of the uterus and come in contact with the maternal capillaries which are much richer than in the nondeciduate placenta. Foetal capillaries grow into the villi and foetal and maternal blood are thus brought close together. This placenta is therefore formed by fusion of foetal and maternal tissues. The trophoblast, connective tissue, and the capillaries of the allantois contribute the embryonic part, and the capillaries and connective tissue of the uterus contribute the maternal part. The maternal blood is separated from the foetal blood only by these structures. The placenta is at first thin, but becomes thicker owing to the elongation and branching of the villi and the growth of the connective tissue. When birth takes place the blood vessels and connective tissue of the uterus are removed along with the foetal vessels and the connective tissue. It is possible that this placenta may have originated from the non-deciduate type by a closer association of the maternal and

pm 173_ Embm, 0f dog foetal tissues after the erosion of the uterine

showing band-shaped placenta wall_ 11 te ' t' fth ' . . . . ((:R:iui*(:a.wrtisfil'(dxl)1 Co:§ill.:.1)vorcs The placenta of the dzscozdal deczduate type

differs from the carnivorous placenta in that the cavities in which the maternal blood circulates are not maternal

capillaries but broad laeunar spaces which have formed in the walls of the trophoblast itself. The tissue of the placenta is almost entirely of foetal origin. The placenta of the mouse may be taken as typical of this type. In this form the trophoblast erodes the uterine epithelium and attaches itself to the subepithelial tissues of the uterus. It then develops lacunar spaces in its wall. Later after the allantois has developed and has reached the somatopleure its capillaries DECIDUATE PLACENTA 253

grow in between the lacunae of the trophoblast. Further growth of the placenta is brought about by the thickening of this wall. Maternal blood from ruptured vessels of the eroded uterine wall circulates in the lacunae. The maternal and foetal blood, however, are separated from each other only by the walls of the trophoblastic lacunae, a small amount of connective tissue, and the walls of the capillaries themselves. As these lacunae are in such close relation to the allantoic capillaries, diffusion is easily possible between them. The placenta of the mouse is typical of the deciduate placentae of the discoidal type. Since maternal blood

fiG. 179. of embryo of carnivore enclosed in uterus. Band—shuped placenta IS represented as if transparent, showing embryo in section below. Letters as in fig. 171.

circulates in lacunar spaces in the trophoblast one can readily understand why the uterus loses only blood and a slight amount of connective tissue at birth.

The discoidal deciduate placenta of the Anthropoidea is similar to that of the rodents.

When a human trophoblast reaches the uterus it bores its way into the wall and implantation is thus interstitial. The trophoblast becomes differentiated into two layers, plasmotrophoblast, which is a plasmodial covering without cell walls but containing many nuclei, and an inner cytotrophoblast or layer of Langhans, which consists of distinct cubical cells. The plasrnotrophoblast at its place of attachment to the uterus erodes the uterine epithelium and dissolves the wall. The outer trophoblast has a spongy character and the spaces become filled with maternal 254 EGG AND EMBRYONIC MEMBRANES

blood from the eroded uterine wall. Later the spongy network is transformed into villi which are surrounded by the trophoblastic spaces that now have the character of lacunae. These villi are at first solid, with the syncytial layer on the outside and cells within except in late stages when the cellular layer disappears. Some of the villi branch freely into the lacunar spaces, but others anchor the trophoblast to the uterine wall. The lacunae are lined everywhere, even on the maternal side, by a syncytial layer. Later the mesoderm of the serosa wanders into the villi which are now branched and which become vascularized by the allantoie blood vessels. These so—called villi are very different from the

Fro. 180. Diagram of human embryo 1n utero. d.r., decidua reflcxa; c.u., cavity of uterus, um., umbilical cord.

villi of ungulates, which are projections from the wall of the trophoblast. They are not villi at all in this sense but are more like the foetal capillaries of rodents which are covered by the trophoblast into which they have pushed. They are merely the irregular walls of the lacunae of the trophoblast.

The villi are at first over the whole surface of the trophoblast (diffuse placenta), but later, those nearest to the cavity of the uterus degenerate so that here the surface is smooth. The villi of the other side remain giving rise to a diseoidal-shaped placenta.

The uterine covering over the trophoblast is known as the decidua reflexa or decidua capsularis. The part of the uterus where the embryo is attached is known as the decidua basalis or serotina, and the opposite wall the decidua Vera. In man the decidua capsularis comes in contact with the decidua vera in the fifth month and the uterine cavity is obliterated. Later the capsularis becomes non-vascular and gradually DECIDUATE PLACENTA 255

disappears so that the trophoblast is in contact with the decidua Vera. This in turn, becomes degenerate, as only the inner portion of the uterine lining remains. The decidua basalis, on the other hand, takes an important part in the formation of the placenta. The uterine capillaries increase and maternal blood from this region supplies the placenta. In late stages, the basalis is reduced to a very thin membrane.

When birth takes place the amnion is ruptured, the amniotic fluid escapes and contractions of the uterus expel the embryo, the break taking place across the degenerate deciduae. Later, there is discharged an afterbirth, which consists of the placenta with the thin layer of decidua basalis and the attached membrane composed of fused amnion, serosa, and deciduae capsularis and vera. It is thus evident that the

fiG. 181. e.h., capillary of the decidua basalis; e.v., capillary of a villus; i, inter-villous cavity

filled with maternal blood; L., Langhan's cells (cytotrophoblast); m., mesoderm of the serosa; s., syncitial layer (plasmotrophoblast); v.f.. fixation or anchoring villus.

only maternal tissue which is lost is uterine blood and a small amount of connective tissue representing the thin layer of the degenerate lining of the uterus. The largest part of the placenta is of foetal origin, but maternal blood accompanies it.

In some of the monkeys, the placenta is double. There is, of course, only one umbilical cord, but the two placentae are connected by blood vessels. In the orang and gibbon there is a single placenta. The lower monkeys have no decidua reflexa.

The following outline lists the characteristics of the embryonic membranes and placentae in the different orders of mammals. In many cases the embryology of only a few forms has been investigated. It is probable that further investigation may show that some animals do not follow in all respects the usual types found in their orders. Indeed, it may be found that some show quite aberrant types, as is true of the lemur, Tarsius. EMBRYONIC MEMBRANES OF MAMMALS




Yon: SAC

Like bird, contains yolk on left side of embryo

Large well-developed, but

contains no yolk No extraembryonic coelome in blastoderm opposite embryo During uterine life there is yolk sac placenta with exception of Pera— melee

Small functional only in

early stages, when it is vascular


Has sero—amniotic connection and is probably formed :13 in Sauropeida

Probably formed by folds

Inner cell mass moves upward pushing trophoblast away The folds from edge of embryonic shield form amnion





As in artiodactyls

As in artiodact) ls

Large amnion of Orca obIiteratc-s exttaembry onic cavity

Amnion obliterates extra embryonic coelome in Elephas


Well-developed. on right side of embryo

Small, since young are born immature

Large, occupying most of uterus Goat embryo 2 inches long has allantoia 2 feet long Hippomanea in allantoic fluid

As in artiodactyls

In Halicore is large. as in 8|’tl0d8.Cl.\lS Hippomanea in allantoic fluid



Found only in Peramelas Trophoblaat not involved in its production, uterine wall develops vascular ayncytium


Oviparoifa egga with hard shell Young fed milky secretion from primitive abdominal mammary glands

Series of forms through Dtdelphys. Dan/uriia, and Perameles showing transfer of placental function from yolk sac and allantoia

.\on—deciduate Most ruminants cotyledonary Pig diffuse unbranched villi Deer hippopotamus earrel, girafle intermediate

l\on-deciduate placenta Hfirse branched difiused V] l

l\on-deciduate placenta With diffuse, branched villi

‘Ion-deciduate W itli branched difiuae villi arranged in band around tropboblast (Really zonary non-deciduate )

fuse villi \ot classified yet as to deciduate or nondeciduate but villi seem to be like those of articdactyls

Central implantation

Membranes incompletely described

Zonary placenta Vlltll dif Embryology not Sufi ciently investigated







Primates Lemuroidea


Large yolk sac probably functional in early stages


Large Invagination of upper wall gives apparent inversion of germ layers

Lpper wall acts as accessory placenta That of rabbits is more important nutn tive organ than allant ois

Functional onl stage Repla tois

in early by allian Large Yolk sac placenta formed in Tupaija Mesoderm restricted to upper wall in Talpa and Sore: Persists until birth

Rabbit amniotic posterior

Formed from series of folds

Large amniotic cavity oblitcrating extraembryonic coelomc

folds formed after embryonic shield has taken superficial place on blastoderm \/louse, rat giunea pig closed sac inside inner cell mass

Vespertillw confluence of irregular spaces in embryonic knob forms amnion

Extremely large Grown between 5 olk sac and trophoblast completely lining serosa


Often small and endoderm does not grow out beyond body as in mouse and giinea pig

Replaces yolk sac

Hedgehog forms a closed Groves out prominently cavity in embryonic knob dwarfing yolk sac

Talpa Sore: Tupaua formed by folds

Small yolk sac early disappears (Tarnus small vascular)

Small yolk sac with cavity and vascular in early stages

Tarsms formed by folds

Amniotic cavity formed in inner cell mass

Very large fills extraembryonic space Tarsms small outgrowth of gut remains in base of umbilical cord

Small contained in stalk of umbilical cord

Zonary deciduate

May be zonary oval bellshaped or difluse Marius non-decidiiate with simple diffuse villi Tatusm modfied discoidal deciduate

Discoidal deciduate

Discoidaldeciduate saucer

shaped or bell shaped

Discoidal deciduate usuallv concave In hedgehog is decidua reflexa like that of man but formed differently In mole acontradeciduate condition (allantoic capillaries separated at birth from placental trophoblast which is absorbed by uterine walls)

Non-deciduate diffuse (Tarsius discoidal deciduate)

Deciduate. discoidal

Central implantation

Little known about mem branes of edentates Central implantation in Ta!usm

Implantation is central in rabbit eccentric in mouse. interstitial in guinea pig and gopher

Implantation eccentric (in some forms)

Implantation central

Implantation interstitial




Germ 1' or- Types of ma— Cleavage

Typra of Blaatulae


Ty pea of Formation



«Xe =Occutrence in the Phylum

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Larval Forms



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Annelida. . . . . . . . . . . . %% Polychaeta . . . . . . . . X Oligochaeta . . . . . . . .

Echinodermata..... 9%! i

Asteroidea . . . . . . . Echinoidea . . . . . . Ophiuroidea. . . _ Holothuroidea . . . . . .

Mollusca . . . . . . . . . . . Amphineura. . . . Pelecypoda . . . . . . Gaateropoda . . . Cephalopoda . . . . . . .

Arthxopoda . . . . . . . . . .




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Cruatacea_ . . . . . . . . . Bx-anchnopoda. . . . Ostracoda . . . Copepoda . . . . . . Eucopepoda. . . . . Cimpedia . . . . Amphipodn. . . . Schizopoda .. Decapoda . . . . . .

Myriapoda. .

Insecta . . . . . . . . . . Coleoptcra . . .

Arachmda . . . .. Ax-aneida . Scorpionida. . . .

Chordata . . . . . . . . . . fié Enteropncusta Tumcata . . . Cephalochordata X Vercebrata .. .....

Cyclostomata Petromyzontm Myxinoidea .

Elaamobranchii. . .

Ganoidea. . . .. .

Teleoatei . . . . . .

Amphibia . . . . . .

Reptilia . . . . . .

Aves . . .

Mammalia . . X X


x# X xxxxxx xxxxx x

Larva. pupa, and others

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X Tomnria. Tadpole and others

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X. 260




(None: This table does not include the forms listed in certain chapters in Part Two, in which the systematic position is given in the text.)

Phylum Porif’era

Class 1. Calcarea Order 1. Homocoela. Leucosolenia Order 2. Heterocoela, Grantia, Sycandra Class 2. Hexactinellida Euplectella

Class 3. Demospongia

Order 1. Tetraxonida Geodia, Cliona

Order 2. Monaxonida Spongella

Order 3. Keratosa Euspongia

Phylum Coelenterata

Type 1. Cnidaris. Class 1. Hydrozoa

Order 1. Anthomedusae (Tubularia) Hydra, Hydractimea, Pennaria, Bougainvillia, Clava, Clavellina, Turritopsis, Eudendrium, Tubularia, Margelis, Nemopsis, Sarsia, Podocoryne, Protohydra, M icrohydra, Pol:/podium

Order 2. Leptomedusae Obelia, Campanularia, Plumularia, Sertularia, Clytia, Cory/morpha M onocaudis, Gcmothyrca, Tima, Aequoria

Order 3. Trachymedusae Campanella, Geryonia, Trachynema, Persa, Liriope, Aeginopsis, Gombmmus, Haleremita

Order 4. Narcomedusae Cunina, Cunocantha, Aegina

Order 5. Hydrocorallinae M illepora, Stylaster

Order 6. Siphonophors. Physophom, Nanomia, Diphyes, Physalia, Porpita, Velella, Halistemma

Class 2. Scyphozoa.

Order 1. Stauromedusae Tessara, Lucemaria

Order 2. Peromedusae Pericolpa, Periphylla ANIMAL CLASSIfiCATION 261

Order 3. Cubomedusae Charybdea

Order 4. Discomcdusae Aurelia, Pelagia, Cassiopea, Cg/anea, Ulmaria, Stomolophus, Polyclonia

Class 3. Anthozoa (Actinozoa) Subclass 1. Alcyoniaria

Order 1. Stolonifera (Tubiporidae) Tubipom

Order 2. Alcyonacea Alcyonium

Order 3. Gorgonacea Corallium

Order 4. Pennatulacea Pennalula, Ifenilla

Subclass 2. Zoantharia (Hexacoralla)

Order 1. Edwardsiidae

' Edwardsia

Order 2. Actinaria Metridium, Halcampa, Paranemonia, Bunodes, Sagartia, Tealia, Bicidium, Epizoanthus

Order 3. Madreporaria Sclerophylla, Oculi/La, Aslrangirc, Aslrca, Faira, Fungia, Madrepora, Porites, Astroides, Mcandrina

Order 4. Antipatharia Antipathes, Gerardia

Order 5. Zoanthidea

Order 6. Cerianthidea Cerirmlhus

Type 2. Ctenophora Class 1. Tenmculata Cullianira, Plcurobrachia, IIrmm'phora, Mnemiopsis, Bolina, Cestus Class 2. Nuda Bertie, I dyia Phylum Platyhelminthes Subphylum 1. Platoda Class 1. Turbellaria Order 1. Rhabdocoelida M icrostoma, Vortex, M onosceles, M onops, Stenostoma

Order 2. Tricladida Poly/scelis, Planaria, Dendrocoelzmz, Bipalizzm, I2’(.’elloum, Gunda, Poly chaerus, Syncoelidium, Phagocata Order 3. Polycladida Leptoplana, Stylochus, Thysanozoon, Discocoelis, Planocera, Yungia Class 2. Trematoda. Order 1. Monogenea (Polystomea, Heterocotylea) 262 APPENDIX TO PART ONE

Polystomum, Sphyranura, Epibdella, Gyrodactylus, Diplozoon, Tristoma, Microcotyle Order 2. Digenea (Distomeae)

Distomum (Fasciolaria), Monostomum, Bilharzia, Clonorchis, Paragonimus

Class 3. Cestoda. Caryophyllaeus, Archigetes, Ligula, Tetrarhynchus, Echinobothrium, Acanthobothrium, Bothriocephalus, Taenia, Anoplocephala, M oniezia, Dipylidium

Subphylum 2. Nemertinea Class 1. Nemertea Order 1. Protonemertini Carmella Order 2. Mesonemertini Cephalathrix Order 3. Metanemertini Geomzmertes, Tetmstemma, Amphiporus, Malacobdella, Nectonemertes Order 4. Heteronemertini (Schizonemcrtini) Lineus, M icrura, Cerebratulus, Zygeupolia Phylum Nemathelminthes

Class 1. Nematoda Ascaris, Ancylostoma, Trichinella, filaria

Class 2. Gordiacea Gordius

Class 3. Acanthoccphala Echinorhync/zus

Phylum Trochelminthes

Class 1. Rotifera

Philodina, H ydatina Phylum Molluseoidea Subphylum 1. Podaxonia

Class 1. Phoronida Phoronis, Echiurus, Bonellia

Class 2. Gephyrea Sipunculus

Subphylum 2. Polyzoa

Class 1. Entoprocta. Urnatella, Pedicellina, Loxosoma

Class 2. Ectoprocta

Order 1. Gymnolaemata Gemmellaria, Bugula, flustra, flustrella, Eschara, Crisia, Tubulipora, Alcyanidium, Valkeria, Paludicella Order 2. Phylactolaemata. Plumatella, Pectinatella, Cristatella Subphylum 3. Brachiopoda

Terebratula, Terebratulina, Lingula, Waldheimia, Rhynchonella ANIMAL CLASSIfiCATION 263

Supplementary group of uncertain position, showing some relations to Entero pneusta:

Pterobranchia Cephalodiscus, Rhabdopleura Phylum Chaetognatha Sagitta, Spadella Phylum Annelida Class 1. Archiannelida Polygordius Class 2. Chaetopoda Subclass 1. Polychaeta Order 1. Phanerocephala Nereis, Podarke, Aphrodite, Lepidonotus, Autolytus, Polymie, Diopatra, N othria, Eunice, Syllis, Trypanosyllis Order 2. Cryptocephala Eupomatus, Hydroides, Spiroides, Arenicola, Amphitrite, Terebella, Sqbella Subclass 2. Oligochaeta Order 1. Microdrili (Limicola) Tulnfex, Dcro, Nais, Sparganophilus, Lumbriculus, Ctenodrilus, Branchiobdclla Order 2. Macrodrili (Terricola) Lumbricus, Allolobophora, Diplocardia Class 3. Hirudinea Order 1. Gnathobdellidae Ilirudo, Jllacrobdella Order 2. Rhynchobdollidae Clepsine, Pontobdella, Piscicola Phylum Echinodcrmata Class 1. Asteroidea Asterias, Asterina, Astropecten, Heliaster, Pythonaster, Asteriscus, Culcita, H ippasteria, Ctenodiscus Class 2. Ophiuroidea. Ophiura, Ophiothrzlt, Ophiopholis, Amphiura, Astrophyton Class 3. Echinoidea Cidaris, Arbacia, Toxapneustes, Strcmgylocentrotus, Coelopleurus, Spawngus, Echinocardium, Brissus, Echinarachnius, Clypeasler, M ellita Class 4. Holothuroidea Holothuria, Cucumaria, Psolus, Thyone, Caudina, M olpadia, Synapta Class 5. Crinoidea Rhizocrinws, Pentacrinus, Comatula, Antedon Phylum Mollusca Class 1. Amphineura Order 1. Placophora (Chitonida) A Amicula, Trachydermon, Chiton, I schnochiton, Cryptochiton 264 APPENDIX TO PART ONE

Order 2. Aplacophora (Solonogastres) Proneomenia, Conchoderma, C/Laetoderma, Dondersia Class 2. Scaphopoda Dentalium Class 3. Pelecypoda (Lamellibranchiata, Acephala)

Order 1. Protobranchia N ucula, Leda, Yoldia, Solenomya

Order 2. filibranchia Arca, M ytilus, M odiola, Anemia, Trigonia

Order 3. Pseudolamellibranchia Pecten, Ostrea, M eleagrina, Lima, Pinna

Order 4. Eulamellibranchia Anodonta, Unio, Pisidium, Cg/clas, Cardium, Astarte, Mg/a, Pholas, Teredo, Aspergillum, Venus, M actra, Tellina, Salem, Dreissensia

Order 5. Septibranchia. Silenia, Cuspidaria, Poromya

Class 4. Gasteropoda. Order 1. Prosobranchia. Suborder 1. Aspidobranchia Acmaea, Palella, Haliotis, Margarita, Trochus Suborder 2. Pectinibranchia

Littorina, Strombus, Sycotypus, Busycon (Fulgur), Crepidula, Urosalpinx, Murex, Paludina, Carinaria, Pleurobranchidimn

Order 2. Opisthobranchia

Suborder 1. Tectibranchia Bulla, Philine, Aplysia, Haminea, Umbrella Suborder 2. Pteropoda Cymbiliopsis, H yalaea, Clione Suborder 3. N udibranchia

Doris, Asolidia, fiona, Staurodoris

Order 3. Pulmonata Helix, Bulimus, Limax, Lymnaea, Physa, Planorbis

Class 5. Cephalopoda

Order 1. Tetrabranchia Nautilus

Order 2. Dibranchia

Suborder 1. Decapodn. Spimla, Ommastrephes, Architeuthis, Loligo, Sepia Suborder 2. Octopoda. Octopus, Allopasus Phylum Arthropoda Class 1. Crustacea.

Subclass 1. Entomostraca

Order 1. Phyllopoda Suborder 1. Branchiopoda Branchipus, Artemia, Apus ANIMAL CLASSIfiCATION 265

Suborder 2. Cladocera

Daphnia, Simocephalus, Leptodora, Polyplwmus

Order 2. Ostracoda Cypris

Order 3. Copepoda.

Suborder 1. Eucopepoda. Cyclops, Lemaea Suborder 2. Branchiura.


Order 4. Cirrepedia

Suborder 1. Eucirripedia Balanus, Lepas Suborder 2. Rhizocephala. Sacculina Subclass 2. Malacostraca Superorder 1. Leptostraca

Order 1. Phyllocarida

Nebalia Superorder 2. Arthrostraca

Order 1. Amphipoda Gammarus, Caprella

Order 2. Isopoda Asellus, Porcellio

Superorder 3. Thoracostraca

Order 1. Cumacea. Diastylis

Order 2. Stomatopoda. Squilla

Order 3. Schizopoda. M ysis

Order 4. Decapoda

Suborder 1. Macrura Palaemonetes, Crangon, Penaeus, Lucifer, Astacus, Homarus Suborder 2. Brachyura Dromia, Cancer Class 2. Onychophora. Peripatus Class 3. Myriapodu

Order 1. Pauropoda. Pauropus

Order 2. Diplopoda. J ulus

Order 3. Chilopoda. Lithobius, Scutigera, Scolopendra

Order 4. Symphyla Scutigerella 266 APPENDIX TO PART ONE

Class 4. Insecta. Macrotoma, 9. thysanuran; Hydrophilus, the water beetle; M iastor, the fly; Leptinotarsa, the potato beetle; Copidosoma, parasitic hvmenoptera Class 5. Arachnida Order 1. Araneida , Epeira

Order 2. Scorpionidea Buthus, Euscorpius

Order 3. Phalangidea Phalangium

Order 4. Acarina Lepas, Izodes

Order 5. Pedipalpi Tarantula

Order 6. Palpigradi Kaenenia

Order 7. Solpugida Eremobates

Order 8. Pseudoscorpionida Chelifer

Order 9. Xiphosura Limulus

Order 10. Eurypterida Eurypterus

Supplementary Arachnid groups of doubtful relationship Pantapocla, Tardigradn, Pentastomidea (Linguatulina) Phylum Chordata Subphylum 1. Enteropneusta Balanoglossus, Dol2'choglossu.s, Ilarrimania Subphylum 2. Tunicata Cynthia, (Hana, Molgula, Appcndicularia, Distaplia, Botryllus, Salpa, Pyrosoma, Doliolum Subphylum 3. Cephalochordata Amphioxus, Branchiostoma Subphylum 4. Vertebrate. Series 1. Anamnia Class 1. Cyclostomata Order 1. Myxinoidea M yzine, Bdellostoma Order 2. Petromyzontia Petromyzon, Lampetra Class 2. Pisces Subclass 1. Elasmobranchii Superorder 1. Selachii Squalus, M ustelus, Galeus, Raia, Acanthias, Scyllium, Carcharias, Cestracion, H exanchus, N otodanus, Pristiurus, Torpedo ANIMAL CLASSIfiCATION 267

Superordcr 2. Holocephali

Chimaera Subclass 2. Ganoidea Amia, Acipemer, Lepisosteus (Lepidosteus) Subclass 3. Teleostei Serranus, Ctenolabrus, Bclonc, Salmo, Fundulus, Perca Subclass 4. Dipnoi Cemtodus, I’rotopterus, Lcpidosiren, Class 3. Amphibia. Subclass 1. Stegooephalia Subclass 2. Lissamphibia

Order 1. Apoda ((iym11ophiona.) Dermo]2Izis, Coocilia

Order 2. Urodela

Fam. 1. Proteidae N ecturus, Proteus, Typhlomolge Farm. 2. Sironidac Siren, Pseudobmnchus Farm. 3. Amphiumidac Amphiuma, Cryptobranchus Farm. 4. Salmnandridac

Ambystoma (Amblystoma, Siredon), Diemyctylus, Salamandra, Triton, Plcthmlon, Eurycea (Hpelerpes), Desmognathus

Order 3. Anura Bufo, Rana, Pseudacris (Chorophilus), Ilyla, Pipa, Alytes

Series 2. Amniota Division 1. Sauropsida. Class 4. Reptilia

Order 1. Rhynchocephalia. Sphcnodon

Order 2. Squamata.

Suborder 1. Sauria (Lacertilia) Gecko, Draco, Anolis, Lacerta, Varamts, Iguana Suborder 2. Serpontis (Ophidia)

Python, Boa, T/zamnophis, H ydroplzis, Elaps, Vipera

Order 3. Loricata (Crocodilia) Alligator, Crocodilus, Gavialis

Order 4. Testudinata (Chelnnizi) Testudo, Cizrysemys, Clemmys, Chelone

Class 5. Birds Division 2. Mammalia Subclass 1. Prototheria.

Order 1. Monotremata.

Echidna, Ornithorhynchus Subclass 2. Eutheria Division 1. Didelphia. 268 APPENDIX TO PART ONE

Order 1. Marsupialia Didelphys, Perameles, Dasyurus, M acropus, Phascolomys, Phascolarctus Division 2. Monodelphia (Placentalia) Section A. Unguiculata (Clawed Mammals) ' Order 1. Insectivora Sorex, Talpa, Erinaceus, Tupaija Order 2. Dermoptera Galeopithecus Order 3. Chiroptera Vespertilio, Pteropus, Desmodus, M yotis Order 4. Carnivore. Suborder 1. fissipedia. Canis, Hyaemz, Procyon, Felis Suborder 2. Pinnipedia. Odobalnus Order 5. Rodentis. Lepus, Sciurus, Mus, Camla Order 6. Edentata, Bradypus, Dasypus, Tatusia Order 7. Pholidota. (Lepidota) Manis Order 8. Tubulidentata Org/cteropus Section B. Primates Order 9. Primates Suborder 1. Lemuroidea Lemur, Tarsius Suborder 2. Anthropoidea. Cebus, Ateles, Simia, Gorilla, Cynocephalus, Homo Section C. Ungulata (Hoofed Mammals) Order 10. Artiodactyla Bos, Camelus, Sus, Dicotyles, Hippopotamus, Corvus Order 11. Perissodactyla Equus, Tapirus, Rhinoceros Order 12. Proboscidea. Elephas, Loxodonta Order 13. Sirenia M anatus, Halicore Order 14. Hyracoidra. Procavia (Hyrax) Section D. Cetacea Order 15. Odontoceti Dolphin, Delphinus, Phocaena, Grampus Order 16. Mystacoceti Balaena

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

Cite this page: Hill, M.A. (2020, April 3) Embryology Book - Outline of Comparative Embryology 1-11. Retrieved from

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