Book - Outline of Comparative Embryology 1-8

From Embryology
Embryology - 30 Mar 2020    Facebook link Pinterest link Twitter link  Expand to Translate  
Google Translate - select your language from the list shown below (this will open a new external page)

العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt    These external translations are automated and may not be accurate. (More? About Translations)

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

Online Editor 
Mark Hill.jpg
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.
History Links: Historic Embryology Papers | Historic Embryology Textbooks | Embryologists | Historic Vignette | Historic Periods | Historic Terminology | Human Embryo Collections | Carnegie Contributions | 17-18th C Anatomies | Embryology Models | Category:Historic Embryology
Historic Papers: 1800's | 1900's | 1910's | 1920's | 1930's | 1940's | 1950's | 1960's | 1970's | 1980's
Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Part One General Embryology

Chapter VIII Mesoderm Formation

Mesoderm is the layer between the outer epidermis and the midgut epithelium. It produces the muscles, coelome, and the structures belonging to the coelome, all the body and intestinal musculature, the gonads, the kidneys (except the Malpighian vessels) and the anlagen of connective tissue and blood vessels.

The concept mesoderm lacks a genetic unity throughout the animal kingdom and is really a collective name, since the mesoderm has a number of different origins within the same group. In spite of the lack of genetic unity of mesoderm, the anlagen of mesodermal structures have a common origin in single great groups of the rnetazoa. There is a definite homology in the mesoderm of all forms with a spiral type of cleavage, as turbellarians, nemertines, molluscs, annelids, and also the arthropods. The mesoderm of all these forms is endodermal, arising from the endoderm, and therefore the anlagen of the mesoderm bands are in genetic unity. It is also true that the ectomesoderm or mesoderm arising from the ectoderm of many forms is homologous. It is furthermore probable that the enterocoeles of echinoderms, Enteropneusta, and chordates are of common origin.


We will now consider the different kinds of mesoderm formation. Since the mesoderm is preceded by the ectoderm and endo(ler1n there are only these two possible origins for the third layer. We will consider first the eetomesoderm.

Ectomesoderm arises from the primary ectoderm and usually produces mesenchymatous muscle cells and connective tissue. This kind of mesoderm occurs in the flatworms, rotifers, nematodes, annelids, and molluses. Although the coelenterates are usually considered as two layered, there is often present a third layer, mesogloea, which, though not cellular, may be considered as eetomesodermal. In sponges, the dermal layer contains both ectoderm and mesoderm (using the term in a doubtful sense) and consists of (a) outer epithelium, (b) canal system except the flagellated chambers which are not included here, (c) connec— tive tissue, and (d) sex cells.

In coelenterates a distinction can hardly be made between ectoderm and mesoderm. Although hydroids have only two layers, still the interstitial cells which are at the bases of the epithelial cells and which later form the gonads may be considered as forerunners of the mesoderm.

In the Actinozoa, the distinction between ectoderm and endoderm is more sharply defined than in hydroids. In the Alcyonaria, there is often connective tissue with skeletal structure, and this layer is ectodermal. The ectoderm becomes many layered, and a jelly substance is secreted between the cells of the deeper layers which become separated from each other. Thus two layers are formed, an outer epithelium and an inner gelatinous layer which may be thought of as representing ectomesoderm.

In the ctenophores the mesoderm becomes more highly differentiated histologically. A definite mesodermal layer is here present and this layer is now known to be ectomesodermal in origin. Kowalevsky and Chun found that the mesoderm originates from ectoderm in the region of the gut. (See fig. 14B.) This is of particular interest because in forms with the spiral type of cleavage the larval mesoblast originates in relation to the blastomeres which produce the stomodacal invagination. In the polyclads, according to Surface, the ectomesoderm produces chiefly musculature of the pharynx.

Since the work on cell lineage, the development of annelids and molluscs is more definitely understoo(l, and we now know that the part of the mesoderm which is to become the mesenchyme of the trochophore arises from the ectoderm. In all forms with spiral type of cleavage, the first three quartettes form the ectoderm. Wilson has shown for polyelads that the mesoderm cells are formed by division of the cells of the second quartette, and that the third quartette probably also shares in the formation of mesoblast. The mesoderm formed in this way is the larval mesoblast, the adult mesoderm arising from cell 4d as in molluscs and annelids and therefore being endodermal in origin.

The relations in other flatworms, nernerteans and nematodes, is not clear. In rotifers the entire mesoderm appears to be larval mesoblast and some of this has been shown to be ectodermal. There are no mesoderm bands and the gonads come from primary endoderm and may represent the mesoderm bands of related forms.

In a number of the annelids the ectomesoderm arises definitely from three cells of the third quartette, 3a, 3c, 3d. The anlage is thus asymmetrical, being paired on the dorsal side, while on the ventral only the left forms it. In some molluscs the ectomesoderm arises exclusively from the third quartette. In Trochus (fig. 33, 34, Table p. 56) it arises from 3c and 3d from the dorsal cells of the quartette. In other forms the second quartette also contributes. Part of the body musculature of the annelids is also known to be of eetodermal origin. The presence of ectomesoderm in arthropods is not clearly shown.

Ectomesoderm produces connective tissue and larval musculature of the trochophore larva. For annelids and molluscs it seems to be a provisional tissue degenerating upon metamorphosis. It is doubtful whether the ectomesoderm produces excretory organs, but the kidneys of some molluscs are known to be of ectodermal origin.

We have seen that ectomesoderm occurs in many coelenterates and also in many groups which are near the trochophore type; these may be collectively called Prostomia. Endomesoderm appears to be lacking in the Molluscoidea. It seems striking that the entire group called Deuterostomia (composed of Enteropneusta, Echinodermata, and Chordata) shows scarcely any ectomesoderm. Thus we recognize two great stems of the animal kingdom, in so far as mesoderm formation is concerned.


We will now consider the formation of the endomesoderm, which includes all the mesodermal structures in the embryo that come from endoderm. Endomesoderm may arise as bands of cells, as hollow coelomic pouches or through the coming together of mesenchyme cells. We shall consider each of these cases separately under the following heads: (a) Teloblastic mesoderm band formation. In these cases paired mesoderm bands take their origin from two primitive mesoderm cells as in annelids and molluscs. (b) Secondary or derived mesoderm band formation. Those cases belong here in which paired mesoderm bands are laid down without primitive mesoderm cells. They are often modifications of the first type, since they occur among forms derived from annelids and molluscs. Examples are arthropods and cephalopods. (c) Enterocoele formation. The formation of coelomic sacs by outfolding from the primitive gut characterizes this type. (d) Coelome formation by solid ingrowth. Obviously this case is closely related to the foregoing type. (e) Mesenchymatous coelome formation. The coelome is formed through the coming together of mesenchyme cells.

It might seem natural to consider mesenchyme structures as different from mesoderm. Mesenchyme is, however, not a genetically uniform structure, but only a histological conception. It may arise by migration from various places of origin: (a) from the coelome walls, (b) from the primitive gut, (c) from the blastoderm of the coeloblastula, or (d) from the reduction of the mesoderm band, as in molluscs. Mesenchyme cells may come together and form a coelome as in the collar and body coelome of some tornaria. The multiplicity of the origin of mesenchyme shows that there is no unity in the term.

1. Teloblastic Mesoderm Band Formation

In cases of teloblastic band formation, a pair of originally solid mesoderm bands is formed by the budding of two terminally lying primitive mesoderm cells. This type of mesoderm formation occurs in forms with the spiral type of cleavage.

The two primitive mesoderm cells arise always from the 4d and thus must be considered as primary endoderm, since the ectoderm is complete with the third quartette and whatever comes from blastomeres arising

fiG. 83. Mesoderm of the snail, Physa. (Redrawn from Korschelt and Heider, after Wierzejski.) The four larger cells M“, M", M“, and M“ and three pairs of micromeres m‘, m1, m3, give rise to the endomeeoderm.

later is to be regarded as endodermal. The two primitive mesodermal cells are more nearly like endoderm than ectoderm in appearance, size, and yolk content. This intimate relation of 4d to primary endoderm is shown by the fact that, besides the primitive mesoderm, 4d often produces other cells which contribute to the formation of the midgut.

In most forms the third quartette is given off dexiotropically and 4d by laeotropic division. The next division of cell 4d, now called M, is again dexiotropic or simply bilateral and the two daughter cells are symmetrical. Cell M, originally in the region of the endoderm plate, early sinks inside the remnant of the cleavage cavity and cells M1 and M 2 become symmetrically arranged. By these first divisions, small cells, enteroblasts which take part in midgut formation, are formed. An equal division of M1 and M 2 takes place and thus four large dorsal cells are formed, the two median ones of which, M 1’ and M?’-’, become the primitive cells of the mesoderm bands. The two other large cells in some forms (pulmonates) have been said to give rise to the primitive kidney of the trochophore, A series of divisions of primitive mesoderm cells now follows and two rows of small cells are formed. These become surrounded by the ectodermal products of the first quartettes.

In many yolk-poor eggs, as in oligochaetes, mesoderm bands are formed very early before the beginning of gastrulation, and lie in the blastocoele at the blastula stage. The mesoderm bands are at first dorsal and produce cells ventrally. With changes of body form they go through changes of position. The bands are given off horizontally and become lengthened parallel to the prototroch. (See p. 127.) Since the

fiG. 84. Embryo of Crepidula. at the time of the closure of the hlastopore and the formation of the stomodaeum. In addition mesodormal cells (In) and the gut cavity are to be seen. (After Conklin.)

middle of the embryo is taken up by the gut they must bend, so both become curved and form a right and a left curve lying in a horizontal plane. When later the blastopore closes and pushes ventrally, the bands shift and lengthen ventrally. The original large mesodermal cells soon divide into smaller ones.

In annelids, the mesoderm bands break up later into segments which hollow out and become coelomic sacs. From their walls develop the trunk and gut musculature, gonads, and nephridia. Ectodermal cells, however, often take part in the formation of the nephridia.

In molluscs, mesoderm bands do not elongate as in annelids. Early in development they are lost in the mesenchyme and, according to one author, produce only connective tissue and musculature while the anlagen of the definitive kidney, pericardium, and gonads are from the ectoderm. Subsequent authors indicate that, in some molluscs, kidney, gonads, and pericardium come from the mesoderm, bringing the later development of mesoderm bands of molluscs more nearly in line with that of the annelids.

2. Secondary or Derived Mesoderm Band Formation

In the arthropods, as in the annelids, we find a pair of mesoderm bands. At first solid and sometimes single layered, these bands later

fiG 85 Diagrams of the development of Trochophorn showing the shift in position of the mesoderm bands (After Korsehelt and Heider)

g, gut, t , teloblast cells, t b , teloblastic mesoderm bands

become segmented and coelomic cavities develop in them. We cannot doubt the homology of the mesoderm bands of arthropods and annelids, but the method of formation of these bands is very different. In the few-celled embryo of the annelids, as we have seen, the combined anlage of the entomesoderm sinks into the cleavage cavity as two cells, the paired mesoderm cells, and from them mesodermal bands develop. These bands arise through teloblastic growth, by successive cell proliferation from the sides of the original mesoderm cells. In the arthropods, on the other hand, the anlage of the mesoderm bands is many layered from the beginning. From the moment that they may be recognized as separate cell groups no original mesodermal cells are observed and no teleblastic cells increase. Gastrulation in most of the arthropods is in the form of a many-celled solid ingrowth. The ingrowing cells form the primitive entoderm which spreads itself under the ectoderm as a so-called under layer since it pushes up to the opposite side of the central yolk mass. Now there is a division into an endodermal cell mass and into paired mesoderm bands. These latter spread out and divide into segmented sections.

In some few arthropods there has been observed a special place of ingrowth for the mesoderm. In the crustaceans, Astacus and Asellus, this special place is the region of the forward blastopore lip; in Peripatus, at the hinder blastopore region. Certain crustacea deviate from the general scheme given for mesoderm formation in the annelids, because their cleavage is more nearly total. In these forms (e.g., Branchipus) there are paired mesoderm cells or cells which because of their position and fate may be compared to paired mesoderm cells. They are similar to the annelid type.

A teloblastic method of mesoderm growth is found in the isopods. Here there is at the hinder end a cross row of eight teloblasts which through division.produce the mesoderm of the metanauplius germ band. There is here only a distant relation to the annelid type as the conditions are entirely different.

In the cephalopods there is no trace of the spiral type of cleavage and cleavage is typically discoidal. The growth at the edge of the germ disc leads to the formation of the endoderm. Later a cell ingrowth in the hinder part of the germ disc takes place and this produces mesoderm and gonad cells. Thus we see that the arthropods and cephalopods are sharply contrasted to the annelids, for in many arthropods and in the cephalopods the mesoderm originates from groups of cells rather than from a single mesodermal cell anlage.

A consideration of the mesoderm formation of the Enteropneusta will lead us naturally to the other chordates. The Enteropneusta form an ancestral group through which the echinoderms are brought into connection with the chordates. In this small group different types of coelome formation may be observed, the type of simple enterocoele formation and the type of coelome formation by the coming together of single mesenchyme cells. Since all these types of mesoderm formation belong in such a small genetic group of animals, they evidently must belong together. Let us consider first the coelome of Balanoglossus. Here there are, corresponding to the three body regions, three coelomic cavities; the head, unpaired but showing by its two pores and incomplete mesentery a double nature, the collar, and the body coelornes. The latter coelome consists of paired spaces separated by a median mesentery. The head coelome is known to be formed by the cutting off of a vesicle at the forward end of the primitive gut, the end opposite the blastopore. The water vesicle of the tornaria larva is the anlage of the head coelome.

There are several descriptions of the method of formation of collar and body coelome. In those forms of Balanoglossus which have no tornaria, but develop directly, two pairs of gut diverticula become_ the collar and body coelomes. In another form these two pairs of sacs are cut off behind from the first-formed coelomic vesicle, the water vesicle. Here in a form without tornaria there is a similarity to the formation of the enterecoele in most echinoderms which also form an originally forward vesicle through the fusion of three pairs of coelomic sacs, the forward enterocoele, the hydrocoele, and the hinder enterocoele.

In the New England tornaria the collar coelome is formed by paired cell proliferations at the side of the stomach, the trunk F“, 86. Diagram of the coelome coelome by solid evaginations of the intes- forrxxzmtion of Balanoalossus Kowalevtine. In the tornaria from the Bahamas, §aft'e‘;':n§°’s°h°lt and on the other hand, these coelomic spaces are formed at a considerable distance from the gut by aggregations of single mesenchyme cells. In one form they arise from the wall of the acorn coelome.

There are thus, in this one group of Enteropneusta, three types of coelome formation: (a) through diverticula of the gut, (b) through cell proliferation, (c) through aggregations of mesenchyme cells.

The second type is easily derived from the first, as solid cell proliferation is one step beyond a diverticulum. The third type differs, but may be considered to fall into line if we think of the cells of a solid proliferation as having lost their connection and become scattered mesenchyme cells.

3. Enterocoele Formation

In the eehinoderms and also in amphioxus the coelome is formed by outpocketings from the alimentary canal, that is, by the formation of

an enterocoele. We must first consider the inesoderm formation of echinodeiins.

In eehinoderms, niesoderm formation is in general like that of Enteropneusta, by an evagination of the ceeloinic sac fiom the primitive gut. At the same time iiiesenchyine cells migrate into the blastocoele. Mesonchyme formation may begin very early, even before the beginning of gastrulation, or may not begin until later. Following this process there occurs in echiiioids and in Comatula, at the place which will become the top of the

Fm 87 Diagram of the me_ primitive gut, a cell migration into the gelati lgiimg forniation Km Cgnlnatulad nous blastocoele. By the time gastrulation is ra n rom orse e 1; an - Hefdenvvufter Scehger ) completed, cell migration from the top of the

gut has already been going on for some time. There are two types of coelome formation in the echinodeims. Com atula is an example of one type. In this form the hydiocoele develops

V e h CT2 mg 0 C2 3. mg i A B C fiG 88 Metamorphosis of the coelome sacs in the eehinoids (A, B. redrawn from Korschelt and Heider after Theel, C, redrawn from Korsehelt and I-Ieider after Bury)

a, anus, C1, left anterior enterocoele, 02, left posterior enterocoele, en, right anterior enterocoele, crz, right posterior enterocoele, h, left hydrocoele anlage, 1. intestine, mg, stomach, in, ectodermal mouth invagmatien, oe, oesophagus.

as an outgrowth of the forward part of the gut and the enterocoele as

outgrowths from the posterior part of the gut. The hydrocoele is the anlage of the ambulacral system. COELOME FORMATION BY SOLID INGROWTH 131

The other type of coelome formation in the echinoderms is illustrated by the echinoids. In these forms coelomic spaces arise from the division of an unpaired coelomic pouch given off at the top of the primitive gut. This vesicle soon divides into right and left halves, each of which divides again producing anterior and posterior right and left enterocoeles. The anlage of the hydroeoele or ambulacral system develops from the left forward enterocoele sac.

In chaetognaths and the brachiopods also we find enterocoele formation. Amphioxus must also be considered since its mesoderm is formed as an enterocoele, paired diverticula originating on the dorsal side of the embryo and later surrounding the gut. Hatschek thought that there were primitive mesoderm cells in amphioxus, and such cells are figured in many text-books, but they do not exist. The enterocoelous diverticula of the alimentary canal occur shortly after gastrulation when the embryo is lengthening. The notochord forms on the dorsal side of the alimentary canal by folding and is cut off lengthwise. At the sides of the notochord are the two Fm. 89. Cross section of an grooves at the anterior end of which paired inS:‘0‘:l“:)i;‘cff evagmations occur. These evaginations are tion with the enterou. (Rethe anlagen of the segments and increase in “"d number as new ones are formed behind.

They become completely separated from the gut and form paired coelomic sacs which are the primitive segments of the embryo.

4. Coelome Formation by Solid Ingrowth

In many cases the coelomic sacs are formed by solid outgrowth from the gut. As already mentioned, the collar and body cavities of the New England tornaria originate by outgrowths from the gut. There are similar cases among the eehinoderms, for in Ophiothrix fragz'l2's the two coelome anlagen have at first no lumen, and also among the brachiopods a similar condition prevails. In the tunicates likewise mesoderm originates as a solid outgrowth from the primitive gut.

The mesoderm formation of vertebrates may also be considered to belong to this type. In the embryo of Triton there is at each side of the notochord a small region of the gut from which coelomic sacs take their origin. These sacs appear as two many-layered cell masses which push in at the sides between the ectoderm and the entoderm. A split occurs in each mass and thus is begun the separation between the outer somatic 132 MESODERM FORMATION

and inner splanchnic layers of the mesodermal sacs. The vertebrates do not show the division of the mesodcrm into primitive segments from the first, as does amphioxus, but the segmentation appears later in the form of ring-shaped invaginations. Under the dorsal lip of the blastepore the invagination is deep and elsewhere it is hollow. The mesodcrm formed near the dorsal lip is called the gastral mesodcrm, and the mesoderm formed all around the blastopore itself is called the peristemial mesodcrm.

fiG. 90. Gastrula of Rana fusca. (Redrawn from Ziegler, after Schwink.) ch, chorda anluge; ec, ectederm; en, endoderm. n1, mesodcrm, g, gastral cavity.

In the frog there is a trace of the outpoeketing of the endodcrm to form the mesoderm sacs, for a groove occurs on each side of the notochord so that the notochord endedcrm is separated from the lateral endoderm.

In selachians mesodcrm formation occurs around the entire disc edge. At the anterior region and on the forward parts of the lateral edges the mesodcrm un(ler the blastocoele becomes split and later the blood islands form here. In the more posterior region of the embryo the mesodcrm is formed by ingrowth of cells from mesodcrm forming grooves of the endoderm. One of these grooves is adjacent to the notechordal region and the other is peripheral. From these grooves groups of cells grow into the peripheral blastocoele.

5. Mesenchymatous Coelome Formation

The origin of coelomic sacs by combination of originally free or independent cells seems to occur. It has already been stated that Morgan found this method of mesoderm formation to occur in the tornaria from the Bahamas.

In general the mesoderm formation of Phoromls-, one of the Molluseoidea, is from scattered mesenehyme. Phoronis has an invaginate gastrula. At this stage single endodermal cells migrate into the blastoeoele and become mesenehyme. This migration appears to be especially great in the region of the edge of the blastopore. The mesenchyme cells which are at first scattered in the blastecoele early show an inclination to lie on the inner surface of the ectoderm, and later on the gut endoderm. Thus are formed the somatopleure and the splanchnopleure.

It is doubtful whether or not such a method of mesoderm formation is

fiG. 91. Oblique section through gas prhnltiva We "light infer that Such trula of Phm-om's sahatieri. (Redrawn from

is the case since Morgan’s tornaria Iforst-halt and H_eidor. after Selys-Lomzfrom the Bahamas shows this kind cmnms) Sh°w‘(';§es_;'_"°s°n°hyme cells of mesoderm formation and other

Enteropneusta do not. It may be, however, that a secondary change

has occurred in the matter of mesoderm formation.


A discussion of the origin of the mesoderm involves also a consideration of the origin of the coelome. We might define the coelome as a body cavity lined by peritoneum from which arise the gonads. The tissue of the coelome, as of all other mesoderm, must necessarily arise from the ectodcrm or endoderm. We have already discussed the double origin of the mesoderm, but have not yet considered the origin of the coelomic cavity itself.

The theories of the origin of the coelome are of course hypothetical. There are three main theories, the enteroeoele theory, the gonoeoele theory, and the nephrocoele theory. The enteroeoele theory was first suggested by Leuekart in 1848 in one of his first papers on the coelenterates. According to this theory, the coelome of higher forms is represented by the radial canals and gastric pouches of medusae. The gastric cavity of coelenterates thus corresponds to both the gastric cavity and coelome of worms and higher forms. This original idea was suggested long before Kowalevsky and Metchnikofi found that the coelome of forms such as Sagitta, brachiopods, echinoderms, and amphioxus really develops from gut pouches. The enterocoele or gut-po11ch theory was taken up with enthusiasm by many, including Balfour and Lang. Lang developed the theory, illustrating it by Gunda. He regarded the metamerism of higher animals as resulting from the separation of paired gastric pouches. Each diverticulum of the gut had perhaps originally the task of producing genital products as well as the excretory function. As the diverticula separated from the gut, new structures must be formed to bear the functions of gonoducts or nephridia. Since the ancestry of the vertebrates is so problematical, this theory as applied to them must be largely conjectural. Lang finally gave up this theory for another.

This theory may be true as applied to the entcrocoelous or vertebrate series of animals, but is evidently not true in the old sense as applied to the teloblastic series of animals where the coelome is derived from a pair of pole cells.

Hatschek is responsible for the gonocoele theory of coelome formation, suggesting it in 1876. According to this theory, the pole cells of the teleblasts are primordial germ cells, and the germ band, in which the coelome develops, is homologous to a gonad. Lang and others took 11p this theory and assumed that the original ancestral type has essentially the same morphology as a rotifer or flatworm. They suggested that the original gonad cavity was still connected to the outside by the gonoduct. Along with the extensions of the gonad cavity, a corresponding reduction in the parenchyma took place and the walls of the gonad became partly sterile, muscles developing in them. It is thus clear why the gonoduct does not connect directly with the gonad, for since the walls of the gonad are partly sterile, the eggs or sperm fall into the coelome and are taken up by the gonoducts. This theory explains also the reduction of the spaces in the flat worm occupied by the circulatory fluid. These spaces are entirely outside the body cavity and from them are formed the blood vessels. In some annelids there is a sinus-like blood system in at least part of the body and this is regarded by Lang as a primitive condition.

The nephrocoele theory is due chiefly to Ziegler, who regards the coelome as primarily and originally an organ of excretion. This organ consisted of a vesicle, the nephrocoele, and its duct. By its expansion a modification of the coelome is produced.

These three views are not as sharply distinguished from each other as might perhaps seem, since cases may be cited in which the coelome functions in such a manner as to suggest all of them.

Bibliographic Note

Among the more important accounts of the subjects contained in this chapter are the following: Korschclt and Heider. See also note at end of Chapter VII, and Hatschek, Wicrzejski. Buteson. These works are cited in full in the bibliography on page 406. 136




Ctenophora Platyhelminthes




Larger Group




Turbellaria Polyclads Trematodes



Chaetopoda. Gephyrea Phoronidn




AMPHIBLASTULA (Pseudogastrula)

Parenchymula. Actinula. PLANULA

SCYPHISTOMA (Hydratuba) Scyphula Ephyra Strobila.


Zoanthella _ _ _ . _ I . _ . _ Zoanthins. Cydippid larvae

MI’JrLLEn’s Larva Sporocyst, Miricidium, Redia, Cercaria. Cysticercus PILIDIUM Larva of Desor Tnocnopuonn Mitraria Trochophore

Actinotrocha. Cyphonautes (Closely related to trocho phore) (Related to trochophore)


Type Genus

Sycandra raphnnus

Leucosolenia vari abilis Clathrina blancu Tubularia Most Hydrozon Many Actinozon Aurelia

Actinia urticinia In family Cere:Lnthidue

Zoanthidac Order Cydippidae


Cerebratulus Lineus

Phascolosoma. Sipunculus Phoronis Mcmbranopnra


Phylum R Larger Group Larva Type Genus

'l‘ro<:l1clmintl1e.s Rotifora (Supposed to be per sistent trochophore,

but early develop ment is not in ac cord with this view) Cllrzetognzltlla (Fairly direct) Sagitta AI'f_‘1I‘()])0dd. Crustncea NAUPLIUS A few schizopods

A few decapods

Metanauplius, Protozoaea, Zoaea, Calyptopsis, Copepodid, Metazoaea, Mysis, Phyllosoma, Mega— lops, Cypris

Erichthoidina Erichthus (Pseud0- Stomotopod zoaea) Alima Insecta Larva, pupa, caterpillar (eruoiform) Campodeiform Mollusca Gastropoda Ctenophore stage Patella TROCHOPHORE Drcissensia. Lamellibranchia VELIGER Unio, Dreissensia Glochidium Echinodermata Asteroidea BIPINNARIA Asterias Ophiuroidea Ophiopluteus Ophiothrix Echinoidea. Echino- Pluteus Echinus pluteus Holothuroidea Auricularia Synapta ' Crinoidea Pentacrinoid Antedon I’rotochordata Enteropneusta Tornaria Balanoglossus Tunicata. Tadpole oozoid Cyathozoid Ascidiozooid Salpa. Trochozooid


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, March 30) Embryology Book - Outline of Comparative Embryology 1-8. Retrieved from

What Links Here?
© Dr Mark Hill 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G