Illustration: Figure 198Fig. 198. Diagram of a Gastrula.(From Gegenbaur.)a.mouth;b.archenteron;c.hypoblast;d.epiblast.
Fig. 198. Diagram of a Gastrula.(From Gegenbaur.)a.mouth;b.archenteron;c.hypoblast;d.epiblast.
The differentiation of the epiblast and hypoblast may commence during the later phases of the segmentation, but is generally not completed till after its termination. Not only do the cells of the blastoderm become differentiatedinto two layers, but these two layers, in the case of a very large number of ova with but little food-yolk, constitute a double-walled sack—the gastrula (fig. 198)—the characters of which are too well known to require further description. Following the lines of phylogenetic speculation above indicated, it may be concluded that the two-layered condition of the organism represents in a general way the passage from the protozoon to the metazoon condition. It is probable that we may safely go further, and assert that the gastrula reproduces, with more or less fidelity, a stage in the evolution of the Metazoa, permanent in the simpler Hydrozoa, during which the organism was provided with (1) a fully developed digestive cavity (fig. 198b) lined by the hypoblast with digestive and assimilative functions, (2) an oral opening (a), and (3) a superficial epiblast (d). These generalisations, which are now widely accepted, are no doubt very valuable, but they leave unanswered the following important questions:(1) By what steps did the compound Protozoon become differentiated into a Metazoon?(2) Are there any grounds for thinking that there is more than one line along which the Metazoa have become independently evolved from the Protozoa?(3) To what extent is there a complete homology between the two primary germinal layers throughout the Metazoa?
Ontogenetically there is a great variety of processes by which the passage from the segmented ovum to the two-layered or diploblastic condition is arrived at.
These processes may be grouped under the following heads:
1. Invagination. Under this term a considerable number of closely connected processes are included. When the segmentation results in the formation of a blastosphere, one half of the blastosphere may be pushed in towards the opposite half, and a gastrula be thus produced (fig. 199, A and B). This process is known as embolic invagination. Another process, known as epibolic invagination, consists in epiblast cells growing round and enclosingthe hypoblast (fig. 200). This process replaces the former process when the hypoblast cells are so bulky from being distended by food-yolk that their invagination is mechanically impossible.
Illustration: Figure 199Fig. 199. Two stages in the development of Holothuria tubulosa, viewed in optical section.(After Selenka.)A. Stage at the close of segmentation. B. Gastrula stage.mr.micropyle;fl.chorion;s.c.segmentation cavity;bl.blastoderm;ep.epiblast;hy.hypoblast;ms.amœboid cells derived from hypoblast;a.e.archenteron.
Fig. 199. Two stages in the development of Holothuria tubulosa, viewed in optical section.(After Selenka.)A. Stage at the close of segmentation. B. Gastrula stage.mr.micropyle;fl.chorion;s.c.segmentation cavity;bl.blastoderm;ep.epiblast;hy.hypoblast;ms.amœboid cells derived from hypoblast;a.e.archenteron.
There are various peculiar modifications of invagination which cannot be dealt with in detail.
Illustration: Figure 200Fig. 200. Transverse section through the ovum of Euaxes during an early stage of development, to shew the nature of epibolic invagination.(After Kowalevsky.)ep.epiblast;ms.mesoblastic band;hy.hypoblast.
Fig. 200. Transverse section through the ovum of Euaxes during an early stage of development, to shew the nature of epibolic invagination.(After Kowalevsky.)ep.epiblast;ms.mesoblastic band;hy.hypoblast.
Invagination in one form or other occurs in some or all the members of the following groups:
The Dicyemidæ, Calcispongiæ (after the amphiblastula stage) and Silicispongiæ, Cœlenterata, Turbellaria, Nemertea, Rotifera, Mollusca, Polyzoa, Brachiopoda, Chætopoda, Discophora, Gephyrea, Chætognatha, Nematelminthes, Crustacea, Echinodermata, and Chordata.
The gastrula of the Crustacea is peculiar, as is also that of many of the Chordata (Reptilia, Aves, Mammalia), but there is every reason to supposethat the gastrulæ of these groups are simply modifications of the normal type.
2. Delamination. Three types of delamination may be distinguished:a.Delamination where the cells of a solid morula become divided into a superficial epiblast, and a central solid mass in which the digestive cavity is subsequently hollowed out (fig. 201).
Illustration: Figure 201Fig. 201. Two stages in the development of Stephanomia pictum, to illustrate the formation of the layers by delamination.(After Metschnikoff.)A. Stage after the delamination;ep.epiblastic invagination to form pneumatocyst.B. Later stage after the formation of the gastric cavity in the solid hypoblast.po.polypite;t.tentacle;pp.pneumatocyst;ep.epiblast of pneumatocyst;hy.hypoblast surrounding pneumatocyst.
Fig. 201. Two stages in the development of Stephanomia pictum, to illustrate the formation of the layers by delamination.(After Metschnikoff.)A. Stage after the delamination;ep.epiblastic invagination to form pneumatocyst.B. Later stage after the formation of the gastric cavity in the solid hypoblast.po.polypite;t.tentacle;pp.pneumatocyst;ep.epiblast of pneumatocyst;hy.hypoblast surrounding pneumatocyst.
b.Delamination where the segmented ovum has the form of a blastosphere, the cells of which give rise by budding to scattered cells in the interior of the vesicle, which, though they may at first form a solid mass, finally arrange themselves in the form of a definite layer around a central digestive cavity (fig. 202).c.Delamination where the segmented ovum has the form of a blastosphere in the cells of which the protoplasm is differentiated into an inner and an outer part. By a subsequentprocess the inner parts of the cells become separated from the outer, and the walls of the blastosphere are so divided into two distinct layers (fig. 205).
Although the third of these processes is usually regarded as the type of delamination, it does not, so far as I know, occur in nature, but is most nearly approached in Geryonia (fig. 203).
The first type of delamination is found in the Ceratospongiæ, some Silicispongiæ (?), and in many Hydrozoa and Actinozoa, and in Nemertea and Nematelminthes (Gordioidea?). The second type occurs in many Porifera [Calcispongiæ (Ascetta),Myxospongiæ], and in some Cœlenterata, and Brachiopoda (Thecidium).
Illustration: Figure 202Fig. 202. Three larval stages of Eucope polystyla.(After Kowalevsky.)A. Blastosphere stage with hypoblast spheres becoming budded off into central cavity. B. Planula stage with solid hypoblast. C. Planula stage with a gastric cavity.ep.epiblast;hy.hypoblast;al.gastric cavity.
Fig. 202. Three larval stages of Eucope polystyla.(After Kowalevsky.)A. Blastosphere stage with hypoblast spheres becoming budded off into central cavity. B. Planula stage with solid hypoblast. C. Planula stage with a gastric cavity.ep.epiblast;hy.hypoblast;al.gastric cavity.
Delamination and invagination are undoubtedly the two most frequent modes in which the layers are differentiated, but there are in addition several others. In the first place the whole of the Tracheata (with the apparent exception of the Scorpion) develop, so far as is known, on a plan peculiar to them, which approaches delamination. This consists in the appearance of a superficial layer of cells enclosing a central yolk mass, which corresponds to the hypoblast (figs.204and214). This mode of development might be classed under delamination, were it not for the fact that the early developmentof many Crustacea is almost the same, but is subsequently followed by an invagination (fig. 208), which apparently correspondsto the normal invagination of other types. There are strong grounds for thinking that the tracheate type of formation of the epiblast and hypoblast is asecondary modification of an invaginate type(videVol.II. p.457).
Illustration: Figure 203Fig. 203. Diagrammatic figures shewing the delamination of the embryo of Geryonia.(After Fol.)A. Stage at the commencement of the delamination; the dotted linesxshew the course of the next planes of division. B. Stage at the close of the delamination.cs.segmentation cavity;a.endoplasm;b.ectoplasm;ep.epiblast;hy.hypoblast.
Fig. 203. Diagrammatic figures shewing the delamination of the embryo of Geryonia.(After Fol.)A. Stage at the commencement of the delamination; the dotted linesxshew the course of the next planes of division. B. Stage at the close of the delamination.cs.segmentation cavity;a.endoplasm;b.ectoplasm;ep.epiblast;hy.hypoblast.
Illustration: Figure 204Fig. 204. Segmentation and formation of the blastoderm in Chelifer.In A the ovum is divided into a number of separate segments. In B a number of small cells have appeared (bl) which form a blastoderm enveloping the large yolk-spheres. In C the blastoderm has become divided into two layers.
Fig. 204. Segmentation and formation of the blastoderm in Chelifer.In A the ovum is divided into a number of separate segments. In B a number of small cells have appeared (bl) which form a blastoderm enveloping the large yolk-spheres. In C the blastoderm has become divided into two layers.
The type of some Turbellaria (Stylochopsis ponticus) and that of Nephelis amongst the Discophora is not capable of being reduced to the invaginate type.
The development of almost all the parasitic groups,i.e.the Trematoda, the Cestoda, the Acanthocephala, and the Linguatulida, and also of the Tardigrada, Pycnogonida, and other minor groups, is too imperfectly known to be classed with either the delaminate or invaginate types.
It will, I think, be conceded on all sides that, if any of the ontogenetic processes by which a gastrula form is reached are repetitions of the process by which a simple two-layered gastrula was actually evolved from a compound Protozoon, these processes are most probably of the nature either of invagination or of delamination.
The much disputed questions which have been raised about the gastrula and planula theories, originally put forward by Haeckel and Lankester, resolve themselves then into the simple question, whether any, and if so which, of the ontogenetic processes by which the gastrula is formed are repetitions of the phylogenetic origin of the gastrula.
It is very difficult to bring forward arguments of a conclusive kind in favour of either of these processes. The fact that delaminate and invaginate gastrulæ are in several instances found coexisting in the same group renders it certain that there are not two independent phyla of the Metazoa, derived respectively from an invaginate and a delaminate gastrula[119].
The four most important cases in which the two processes coexist are the Porifera, the Cœlenterata, the Nemertea, and the Brachiopoda. In the cases of the Porifera and Cœlenterata, there do not appear to me to be any means of deciding which of these processes is derived from the other; but in the Nemertea and the Brachiopoda the case is different. In all the types of Nemertea in which the development is relatively not abbreviated there is an invaginate gastrula, while in the types with a greatly abbreviated development there is a delaminate gastrula. It would seem to follow from this that a delaminate gastrula has here been a secondary result of an abbreviation in the development. In the Brachiopoda, again, the majority of types develop by a process of invagination, while Thecidium appears to develop by delamination; here also the delaminate type would appear to be secondarily derived from the invaginate.
If these considerations are justified, delamination must be in some instances secondarily derived from invagination; and this fact is so far an argument in favour of the more primitive nature of invagination; though it by no means follows that in the invaginate process the steps by which the Metazoa were derived from the Protozoa are preserved.
It does not, therefore, seem possible to decide conclusively in favour of either of these processes by a comparison of the cases where they occur in the same groups.
The relative frequency of the two processes supplies us with another possible means for deciding between them; and there is no doubt that here again the scale inclines towards invagination. It must, however, be borne in mind that the frequency of the process of invagination admits of another possible explanation. There is a continual tendency for the processes of development to be abbreviated and simplified, and it is quite possible that the frequent occurrence of invagination is due to the fact of its being, in most cases, the simplest means by which the two-layered condition can be reached. But this argument can have but little weight until it can be shewn in each case that invagination is a simpler process than delamination; and it is rendered improbable by the cases already mentioned in which delamination has been secondarily derived from invagination.
If it were the case that the blastopore hadin all typesthesame relation to the adult mouth, there would be strong grounds for regarding the invaginate gastrula as an ancestral form; but the fact that this is by no means so is an argument of great weight in favour of some other explanation of the frequency of invagination.
The force of this consideration can best be displayed by a short summary of the fate of the blastopore in different forms.
The fate of the blastopore is so variable that it is difficult even to classify the cases which have been described.
(1)It becomes the permanent mouth in the following forms[120]:Cœlenterata.—Pelagia, Cereanthus.Turbellaria.—Leptoplana (?), Thysanozoon.Nemertea.—Pilidium, larvæ of the type of Desor.Mollusca.—In numerous examples of most Molluscan groups, except the Cephalopoda.Chætopoda.—Most Oligochæta, and probably many Polychæta.Gephyrea.—Phascolosoma, Phoronis.Nematelminthes.—Cucullanus.
(2)It closes in the position where the mouth is subsequently formed.Cœlenterata.—Ctenophora (?).Mollusca.—In numerous examples of most Molluscan groups, except the Cephalopoda.Crustacea.—Cirripedia (?), some Cladocera (Moina) (?).
(3)It becomes the permanent anus.Mollusca.—Paludina.Chætopoda.—Serpula and some other types.Echinodermata.—Almost universally, except amongst the Crinoidea.
(4)It closes in the position where the anus is subsequently formed.Echinodermata.—Crinoidea.
(5)It closes in a position which does not correspond or is not known to correspond[121]either with the future mouth or anus.—Porifera—Sycandra.Cœlenterata—Chrysaora*, Aurelia*.Nemertea*—Some larvæ which develop without a metamorphosis.Rotifera*.Mollusca—Cephalopoda.Polyzoa*.Brachiopoda—Argiope, Terebratula, Terebratulina.Chætopoda—Euaxes.Discophora—Clepsine.Gephyrea—Bonellia*.Chætognatha.Crustacea—Decapoda.Chordata.
The forms which have been classed together under the last heading vary considerably in the character of the blastopore. In some cases the fact of its not coinciding either with the mouthor anus appears to be due simply to the presence of a large amount of food-yolk. The cases of the Cephalopoda, of Euaxes, and perhaps of Clepsine and Bonellia, are to be explained in this way: in the case of all these forms, except Bonellia, the blastopore has the form of an elongated slit along the ventral surface. This type of blastopore is characteristic of the Mollusca generally, of the Polyzoa, of the Nematelminthes, and very possibly of the Chætopoda and Discophora. In the Chætognatha (fig. 209B) the blastopore is situated, so far as can be determined, behind the future anus. In many Decapoda the blastopore is placed behind, but not far from, the anus. In the Chordata it is also placed posteriorly to the anus, and, remarkably enough, remains, in a large number of forms, for some time in connection with the neural tube by a neurenteric canal.
The great variations in the character of the gastrula, indicated in the above summary, go far to shew that if the gastrulæ, as we find them in most types, have any ancestral characters, these characters can only be of the most general kind. This may best be shewn by the consideration of a few striking instances. The blastopore in Mollusca has an elongated slit-like form, extending along the ventral surface from the mouth to the anus. In Echinodermata it is a narrow pore, remaining as the anus. In most Chætopoda it is a pore remaining as the mouth, but in some as the anus. In Chordata it is a posteriorly-placed pore, opening into both the archenteron and the neural canal.
It is clearly out of the question to explain all these differences as having connection with the characters of ancestral forms. Many of them can only be accounted for as secondary adaptations for the convenience of development.
The epibolic gastrula of Mammalia (videpp.215and291) is a still more striking case of a secondary embryonic process, and is not directly derived from the gastrula of the lower Chordata. It probably originated in connection with the loss of food-yolk which took place on the establishment of a placental nutrition for the fœtus. The epibolic gastrula of the Scorpion, of Isopods, and of other Arthropoda, seems also to be a derived gastrula. These instances of secondary gastrulæ are very probably by nomeans isolated, and should serve as a warning against laying too much stress upon the frequency of the occurrence of invagination. The great influence of the food-yolk upon the early development might be illustrated by numerous examples, especially amongst the Chordata (videChapterXI.).
If the descendants of a form with a large amount of food-yolk in its ova were to produce ova with but little food-yolk, the type of formation of the germinal layers which would thereby result would be by no means the same as that of the ancestors of the forms with much food-yolk, but would probably be something very different, as in the case of Mammalia. Yet amongst the countless generations of ancestors of most existing forms, such oscillations in the amount of the food-yolk must have occurred in a large number of instances.
The whole of the above considerations point towards the view that the formation of the hypoblast by invagination, as it occurs in most forms at the present day, can have in many instances no special phylogenetic significance, and that the argument from frequency, in favour of invagination as opposed to delamination, is not of prime importance.
A third possible method of deciding between delamination and invagination is to be found in the consideration as to which of these processes occurs in the most primitive forms. If there were any agreement amongst primitive forms as to the type of their development this argument might have some weight. On the whole, delamination is, no doubt, characteristic of many primitive types, but the not infrequent occurrence of invagination in both the Cœlenterata and the Porifera—the two groups which would on all hands be admitted to be amongst the most primitive—deprives this argument of much of the value it might otherwise have.
To sum up—considering the almost indisputable fact that both the processes above dealt with have in many instances had a purely secondary origin, no valid arguments can be produced to shew that either of them reproduces the mode of passage between the Protozoa and the ancestral two-layered Metazoa. These conclusions do not, however, throw any doubt upon the fact that the gastrula, however evolved, was a primitive form of the Metazoa; since this conclusion is founded upon the actualexistence of adult gastrula forms independently of their occurrence in development.
Illustration: Figure 205Fig. 205. Diagram shewing the formation of a Gastrula by delamination.(From Lankester.)Fig. 1, ovum; fig. 2, stage in segmentation; fig. 3, commencement of delamination after the appearance of a central cavity; fig. 4, delamination completed, mouth forming atM.Infigs.1, 2, and 3,Ec.is ectoplasm, andEn.is endoplasm. In fig. 4,Ec.is epiblast, andEn.hypoblast.E.andF.food particles.
Fig. 205. Diagram shewing the formation of a Gastrula by delamination.(From Lankester.)Fig. 1, ovum; fig. 2, stage in segmentation; fig. 3, commencement of delamination after the appearance of a central cavity; fig. 4, delamination completed, mouth forming atM.Infigs.1, 2, and 3,Ec.is ectoplasm, andEn.is endoplasm. In fig. 4,Ec.is epiblast, andEn.hypoblast.E.andF.food particles.
Though embryology does not at present furnish us with a definite answer to the question how the Metazoa became developed from the Protozoa, it is nevertheless worth while reviewing some of the processes by which this can be conceived to have occurred.
On purelyà priorigrounds there is in my opinion more to be said for invagination than for any other view.
On this view we may suppose that the colony of Protozoa in the course of conversion into Metazoa had the form of a blastosphere; and that at one pole of this a depression appeared. The cells lining this depression we may suppose to have been amœboid, and to have carried on the work of digestion; while the remaining cells were probably ciliated. The digestion may be supposed to have been at first carried on in the interior of the cells, as in the Protozoa; but, as the depression became deeper (in order to increase the area of nutritive cells and to retain the food) a digestive secretion probably became poured out from the cells lining it, and the mode of digestion generally characteristic of the Metazoa was thereby inaugurated. It may be noted that an intracellular protozoon type of digestion persists in the Porifera, and appears also to occur in many Cœlenterata, Turbellaria,&c., though in most of these cases both kinds of digestion probably go on simultaneously[122].
Another hypothetical mode of passage, which fits in with delamination, has been put forward by Lankester, and is illustrated byfig. 205. He supposes that at the blastosphere stage the fluid in the centre of the colony acquired special digestive properties; the inner ends of the cells having at this stage somewhat different properties from the outer, and the food being still incepted by the surface of the cells (fig. 205, 3). In a later stage of the process the inner portions of the cells became separated off as the hypoblast; while the food, though still ingested in the form of solid particles by the superficial cells, was carried through the protoplasm into the central digestive cavity. Later (fig. 205, 4), the point where the food entered became localised, and eventually a mouth became formed at this point.
The main objection which can be raised against Lankester’s view is that it presupposes a type of delamination which does not occur in nature except in Geryonia.
Metschnikoff has propounded a third view with reference to delamination. He starts as before with a ciliated blastosphere. He next supposes the cells from the walls of this to become budded off into the central cavity, as in Eucope (fig. 202), and to lose their cilia. These cells give rise to an internal parenchyma, which carries on an intracellular digestion. At a later stage a central digestive cavity is supposed to be formed. This view of the passage from the protozoon to the metazoon state, though to my mind improbable in itself, fits in very well with the ontogeny of the lower Hydrozoa.
Another view has been put forward by myself in the chapter on the Porifera[123], to the effect that the amphiblastula larva of Calcispongiæ may be a transitional form between the Protozoa and the Metazoa, composed of a hemisphere of nutritive amœboid cells, and a hemisphere of ciliated cells. The absence of such a larval form in the Cœlenterata and higher Metazoa is opposed, however, to this larva being regarded as a transitional form, except for the Porifera.
It is obvious that so long as there is complete uncertainty as to the value to be attached to the early developmental processes, it is not possible to decide from these processes whether there is only a single metazoon phylum or whether there may not be two or more such phyla. At the same time there appear to be strongarguments for regarding the Porifera as a phylum of the Metazoa derived independently from the Protozoa. This seems to me to be shewn (1) by the striking larval peculiarities of the Porifera; (2) by the early development of the mesoblast in the Porifera, which stands in strong contrast to the absence of this layer in the embryos of most Cœlenterata; and above all, (3) by the remarkable characters of the system of digestive channels. A further argument in the same direction is supplied by the fact that the germinal layers of the Sponges very probably do not correspond physiologically to the germinal layers of other types. The embryological evidence is insufficient to decide whether the amphiblastula larva is, as suggested above, to be regarded as the larval ancestor of the Porifera.
Homologies of the germinal layers. The question as to how far there is a complete homology between the two primary germinal layers throughout the Metazoa was the third of the questions proposed to be discussed here.
Since there are some Metazoa with only two germinal layers, and other Metazoa with three, and since, as is shewn in the following section, the third layer or mesoblast can only be regarded as a derivative of one or both the primary layers, it is clear that a complete homology between the two primary germinal layers does not exist.
That there is a general homology appears on the other hand hardly open to doubt.
The primary layers are usually continuous with each other, near one or both (when both are present) the openings of the alimentary tract.
As a rule an oral and anal section of the alimentary tract—the stomodæum and proctodæum—are derived from the epiblast; but the limits of both these sections are so variable, sometimes even in closely allied forms, that it is difficult to avoid the conclusion that there is a border-land between the epiblast and hypoblast, which appears by its development to belong in some forms to the epiblast and in other forms to the hypoblast. If this is not the case it is necessary to admit that there are instances in which a very large portion of the alimentary canal is phylogenetically an epiblastic structure. In some of the Isopods, for example, the stomodæum and proctodæum giverise to almost the whole of the alimentary canal with its appendages, except the liver.
The origin of the Mesoblast. A diploblastic condition of the organism preceded, as we have seen, the triploblastic. The epiblast during the diploblastic condition was, as appears from such forms as Hydra, especially the sensory and protective layer, while the hypoblast was the secretory and assimilating layer; both layers giving rise to muscular elements. It must not, however, be supposed that in the early diploblastic ancestors there was a complete differentiation of function, but there is reason to think that both the primary layers retained an indefinite capacity for developing into any form of tissue[124]. The fact of the triploblastic condition being later than the diploblastic proves in a conclusive way that the mesoblast is a derivative of one or both the primary layers. In the Cœlenterata we can study the actual origin from the two primary layers of various forms of tissue which in the higher types are derived from the mesoblast[125]. This fact, as well as generalà prioriconsiderations, conclusively prove thatthe mesoblast did not at first originate as a mass of independent cells between the two primary layers, but that in the first instance it gradually arose as differentiations of the two layers, and that its condition in the embryo as an independent layer of undifferentiated cells is a secondary condition, brought about by the general tendencytowards a simplification of development, and a retardation of histological differentiation[126].
The Hertwigs have recently attempted (No.271) to distinguish two types of differentiation of the mesoblast,viz.(1) a direct differentiation from the primitive epithelial cells; (2) a differentiation from primitively indifferent cells budded off into the gelatinous matter between the two primary layers.
It is quite possible that this distinction may be well founded, but no conclusive evidence of the occurrence of the second process has yet been adduced. The Ctenophora are the type upon which special stress is laid, but the early passage of amœboid cells into the gelatinous tissue, which subsequently become muscular, is very probably an embryonic abbreviation; and it is quite possible that these cells may phylogenetically have originated from epithelial cells provided with contractile processes passing through the gelatinous tissue.
The conversion of non-embryonic connective-tissue cells into muscle cells in the higher types has been described, but very much more evidence is required before it can be accepted as a common occurrence.
In addition to the probably degraded Dicyemidæ and Orthonectidæ, the Cœlenterata are the only group in which a true mesoblast is not always present. In other words, the Cœlenterata are the only group in which there is not found in the embryo an undifferentiated group of cells from which the majority of the organs situated between the epidermis and the alimentary epithelium are developed.
The organs invariably derived, in the triploblastic forms, from the mesoblast, are the vascular and lymphatic systems, the muscular system, and the greater part of the connective tissue and the excretory and generative (?) systems. On the other hand, the nervous systems (with a few possible exceptions) and organs of sense, the epithelium of most glands, and a few exceptional connective-tissue organs, as for example the notochord, are developed from the two primary layers.
The fact of the first-named set of organs being invariably derived from the mesoblast points to the establishment of the two following propositions:—(1)That with the differentiationof the mesoblast as a distinct layer by the process already explained, the two primary layers lost for the most part the capacity they primitively possessed of giving rise to muscular and connective-tissue differentiations[127],to the epithelium of the excretory organs, and to generative cells. (2) That the mesoblast throughout the triploblastic Metazoa, in so far as these forms have sprung from a common triploblastic ancestor, is an homologous structure.
The second proposition follows from the first. The mesoblast can only have ceased to be homologous throughout the triploblastica by additions from the two primary layers, and the existence of such additions is negatived by the first proposition.
These two propositions, which hang together, are possibly only approximately true, since it is quite possible that future investigations may shew that differentiations of the two primary layers are not so rare as has been hitherto imagined.
Ranvier[128]finds that the muscles of the sweat-glands are developed from the inner part of the layer of epiblast cells, invaginated to form these glands.
Götte[129]describes the epiblast cells of the larva of Comatula as being at a certain stage contractile and compares them with the epithelio-muscular cells of Hydra. These cells would appear subsequently to be converted into a simple cuticular structure.
It is moreover quite possible that fresh differentiations from the two primary layers may have arisen after the triploblastic condition had been established, and by the process of simplification of development and precocious segregation, as Lankester calls it, have become indistinguishable from the normal mesoblast. In spite of these exceptions it is probable that the major part of the muscular system of all existing triploblastic forms has been differentiated from the muscular system of the ancestor or ancestors (if there is more than one phylum) of the triploblastica.In the case of other tissues there are a few instances which might be regarded as examples of an organ primitively developed in one of the two primary layers having become secondarily carried into the mesoblast. The notochord has sometimes been cited as such an organ, but, as indicated in a previous chapter, it is probable that its hypoblastic origin can always be demonstrated.
Illustration: Figure 206Fig. 206. Epibolic gastrula of Bonellia.(After Spengel.)A. Stage when the four hypoblast cells are nearly enclosed.B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore.ep.epiblast;me.mesoblast;bl.blastopore.
Fig. 206. Epibolic gastrula of Bonellia.(After Spengel.)A. Stage when the four hypoblast cells are nearly enclosed.B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore.ep.epiblast;me.mesoblast;bl.blastopore.
The nervous system, although imbedded in mesoblastic derivates in the adults of all the higher triploblastica, retains with marvellous constancy its epiblastic origin (though it is usually separated from the epiblast prior to its histogenic differentiation); yet in the Cephalopoda, and some other Mollusca, the evidence is in favour of its developing in the mesoblast. Should future investigations confirm these conclusions, a good example will be afforded of an organ changing the layer from which it usually develops[130]. The explanation of such a change would be precisely the same as that already given for the mesoblast as a whole.
The actual mode of origin of various tissues, which in the true triploblastic forms arise in mesoblast, can be traced in theCœlenterata[131]. In this group the epiblast and hypoblast both give rise to muscular and connective-tissue elements; and although the main part of the nervous system is formed in the epiblast, it seems certain that in some types nerves may be derived from the hypoblast[132]. These facts are extremely interesting, but it is by no means certain that any conclusions can be directly drawn from them as to the actual origin of the mesoblast in the triploblastic forms, till we know from what diploblastic forms the triploblastica originated. All that they shew is that any of the constituents of the mesoblast may have originated from either of the primitive layers.
Illustration: Figure 207Fig. 207. Two transverse sections through embryos of Hydrophilus piceus.(After Kowalevsky.)A. Section through an embryo at the point where the two germinal folds most approximate.B. Section through an embryo, in the anterior region where the folds of the amnion have not united.gg.germinal groove;me.mesoblast;am.amnion;yk.yolk.
Fig. 207. Two transverse sections through embryos of Hydrophilus piceus.(After Kowalevsky.)A. Section through an embryo at the point where the two germinal folds most approximate.B. Section through an embryo, in the anterior region where the folds of the amnion have not united.gg.germinal groove;me.mesoblast;am.amnion;yk.yolk.
For further light as to the origin of the mesoblast, it is necessary to turn to its actual development.
Illustration: Figure 208Fig. 208. Figures illustrating the development of Astacus.(From Parker; after Reichenbach.)A. Section through part of the ovum during segmentation.n.nuclei;w.y.white yolk;y.p.yolk pyramids;c.central yolk mass.B. and C. Longitudinal sections of the gastrula stage.a.archenteron;b.blastopore;ms.mesoblast;ec.epiblast;en.hypoblast, distinguished from epiblast by shading.D. Highly magnified view of anterior lip of blastopore, to shew the origin of the primary mesoblast from the wall of the archenteron.p.ms.primary mesoblast;ec.epiblast;en.hypoblast.E. Two hypoblast cells to shew the amœeba-like absorption of yolk spheres.y.yolk;n.nucleus;p.pseudopodial process.F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.ms.);n.nucleus.
Fig. 208. Figures illustrating the development of Astacus.(From Parker; after Reichenbach.)A. Section through part of the ovum during segmentation.n.nuclei;w.y.white yolk;y.p.yolk pyramids;c.central yolk mass.B. and C. Longitudinal sections of the gastrula stage.a.archenteron;b.blastopore;ms.mesoblast;ec.epiblast;en.hypoblast, distinguished from epiblast by shading.D. Highly magnified view of anterior lip of blastopore, to shew the origin of the primary mesoblast from the wall of the archenteron.p.ms.primary mesoblast;ec.epiblast;en.hypoblast.E. Two hypoblast cells to shew the amœeba-like absorption of yolk spheres.y.yolk;n.nucleus;p.pseudopodial process.F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.ms.);n.nucleus.
The following summary illustrates the more important modes in which the mesoblast originates.
1. It grows inwards from the lips of the blastopore as a pair of bands. In these cases it may originate (a) from cells which are clearly hypoblastic, (b) from cells which are clearly epiblastic, (c) from cells which cannot be regarded as belonging to either layer.
Mollusca.—Gasteropoda, Cephalopoda, and Lamellibranchiata. In Gasteropoda and Lamellibranchiata the mesoblast sometimes originatesfrom a pair of cells at the lips of the blastopore, though very probably some of the elements subsequently come from the epiblast; and in Cephalopoda it begins as a ring of cells round the edge of the blastoderm.
Polyzoa Entoprocta.—It originates from a pair of cells at the lips of the blastopore.
Chætopoda.—Euaxes. It arises as a ridge of cells at the lips of the blastopore (fig. 200).
Gephyrea.—Bonellia. It arises (fig. 206) as an infolding of the epiblastic lips of the blastopore.
Nematelminthes.—Cucullanus. It grows backwards from the hypoblast cells at the persistent oral opening of the blastopore.
Tracheata.—Insecta. It grows inwards from the lips of the germinal groove (fig. 207), which probably represent the remains of a blastopore. Part of the mesoblast is probably also derived from the yolk-cells. A similar though more modified development of the mesoblast occurs in the Araneina (fig. 214).
Crustacea.—Decapoda. It partly grows in from the hypoblastic lips of the blastopore, and is partly derived from the yolk-cells (fig. 208).
2. The mesoblast is developed from the walls of hollow outgrowths of the archenteron, the cavities of which become the body cavity.
Brachiopoda.—The walls of a pair of outgrowths form the whole of the mesoblast.
Chætognatha.—The mesoblast arises in the same manner as in the Brachiopoda (fig. 209).