Endoderm

“ It is clear, both from their origin and history, that the layers or masses of cells called ectoderm, endoderm and mesoderm have not the same value in different animals; indeed, it is misleading to speak of three layers. At the most we can only speak of two, for the mesoderm is formed after the others, has a composite origin, and has no more claim to be considered an embryonic layer than has the rudiment of the central nervous system, which in some animals, indeed, appears as soon as the mesoderm. ”

“ If the endoderm-cells also contained germ-plasm it would be hard to understand why the germ-cells never arise from them, since their situation offers much better conditions for their further development than that of the ectoderm-cells. It would also be hard to understand why such a circuitous route was chosen as that exhibited by the migration of the young germ-cells into the endoderm. Something must be lacking in the endoderm that is necessary to make a cell into a germ-cell: that something is the germ-plasm. ”

“ We indeed are entitled to say that the ectoderm of Echinus shows “self-differentiation” with regard to the endoderm; it acquires its mouth, for instance, as has been shown by experiment, even in cases where no intestine is present at all (Fig. 10) ; but ectoderm and endoderm both are formatively dependent on the intimate and the material organisation of the blastoderm. ”

“ These two most general and most radically-opposed functions become in the Polyzoa, much more clearly marked-off from each other: at the same time that each of them becomes partially divided into subordinate functions. The endoderm and ectoderm are no longer merely the inner and outer walls of the same simple sac into which the food is drawn: but the endoderm forms a true alimentary canal, separated from the ectoderm by a peri-visceral cavity, containing the nutritive matters absorbed from the food. ”

“ The Endoderm.—The cells of the endoderm of a tentacle are long and quite slender (Fig. 31) . At their bases they are vacuolated quite like the cells of the ampulla and the canal of the sensory clubs. They contain a well-formed nucleus with a nucleolus. In their distal half small light bodies with a dark center are very evident. These bodies are evidently a secretion. ”

“ Farther development of the mesoderm. Formation of the body-cavity We have seen that by means of an invagination extending throughout the entire length of the primitive band a layer of cells is produced which soon spreads out on the inner side of the band and thus forms a second lower (inner) layer (Fig. 539, C) . From this inner layer is separated at the anterior and posterior ends of the primitive band, the endoderm, which lies in direct contact with the invaginations of the proctodæum and stomodæum. The remainder, by far the most extensive part of the inner layer, is the mesoderm. ”

“ He finds that the endoderm arises from an anterior and a posterior rudiment derived from the “endoblast,” that many of the cells of these rudiments wander into the yolk, and that the mesenteric epithelium becomes reinforced by cells that migrate from the yolk. K. Escherich (1901) , after a new research on the embryology of the muscid Diptera, claims that the fore and hind endodermal rudiments arise from the blastoderm by invagination, and are from their origin distinct from the mesoderm. ”

“ Now, degrees of differentiation imply adaptation to the environment and the endoderm, which is least differentiated, is least able to meet vicissitudes. “Evolutionally speaking, it is a very old-fashioned and out-of-date ancestral relic which causes man an infinity of troubles. Practically all public-health activities have been directed toward overcoming the difficulties which arise because man carries about this antediluvian sort of endoderm.” Prior to the age of sixty the breakdown of organs of endodermic origin causes most deaths ”

“ We have next to describe the marginal system. The vascular lamellæ mentioned above in every case connected endoderm of one cavity with endoderm of another; those of the margin have the noteworthy difference that they run from endoderms of some part of the [32] gastro-vascular system to ectoderm of the surface. The outermost cells of the endodermal lamellæ make direct connection with the ectodermal cells, without the usual intervention of a layer of gelatine.
The marginal lamella of Charybdea lies, as the name implies, just on the bell margin where the edge is curving round into the velarium. ”

“ Through this step in segmentation, I conclude, the determinants of all the ectoderm-cells become separated from those of the endoderm-cells; the determinant architecture of the ids must be so constructed in such species that it can be segregated at the first egg-cleavage into ectodermal and endodermal groups of determinants. ”

“ The body consists of two layers, an ectoderm and endoderm; between these two layers there is a mesoglœa of jelly-like consistency; in the simplest cases there are no cells in the mesoglœa, but secondarily endodermal cells may migrate into it. Stinging cells are generally present.
There are two divergent types of structure. The more primitive one is the sessile tubular hydroid, which may be compared to a gastrula furnished at one end with a crown of tentacles surrounding the opening of the enteron, and fixed at the opposite end. ”

“ It is true they are not quite so simple as Götte [74] figures them, for they not only consist of ectoderm and germ-cells, but, according to Julin [75] , the endoderm is arranged in two layers—the germ-cells and a layer which forms during development a strong muscular coat; and in the second female form the egg-cells are surrounded by a tolerably thick granular tissue. ”

“ In the majority of Coelenterates the nematocysts are confined to the ectoderm, but in many Anthozoa, Scyphozoa, and Siphonophora they are found in tissues that are certainly or probably endodermic in origin. It has not been definitely proved in any case that the cnidoblast cells that form these nematocysts have originally been formed in the endoderm, and it is possible that they are always derived from ectoderm cells which migrate into the endoderm. ”

“ Germ-cells can thus only arise from the cells of the germ-track, and in no other way, no matter whether the germ-track lie in the ectoderm, as in the Hydromedusæ, or in the endoderm, as in true jellyfishes (Acalephæ) and the Ctenophores, or in the mesoderm, as in many higher groups of animals. ”

“ The tissues of the bud become differentiated into ectoderm and endoderm, and the endoderm of the bud becomes secondarily continuous with that of the parent, but no part of the parental endoderm contributes to the building up of the daughter-polyp. ”

“ The ectoderm rarely consists of more than one layer of cells: these are divisible by structure and function into nervous, muscular and secretory cells, supported by interstitial cells. The endoderm is generally also an epithelium one cell in thickness, the cells being digestive, secretory and sometimes muscular. Reproductive sexual cells may be found in either of these two layers, according to the class and sub-class in question. ”

“ All animals higher than these have been termed Coelomata; they possess a distinct body-cavity in addition to the stomach. Huxley went further than this, and the most profound suggestion in his paper is the comparison of the two layers with those which appear in the germ of the higher animals. The consequences which have flowed from this prophetic generalization of the ectoderm and endoderm are familiar to every student of evolution. ”

“ In 1859 Huxley made an important step towards the explanation of the nature of these layers by comparing them with the ectoderm and endoderm of the Hydrozoa; while the brilliant researches of Kowalevsky on the development of a great variety of invertebrate forms formed the starting point of the current views on this subject.
Fig. 198. Diagram of a Gastrula. (From Gegenbaur.)
a. mouth; b. archenteron; c. hypoblast; d. epiblast. ”

“ The inner layer, now turned outwards, cannot resist the influence of the water, and the outer layer, now turned inwards, cannot effect digestion; in short, one cannot be transformed into the other, and we must therefore conclude that both are specialized, that they no longer contain the complete germ-plasm, but only the specific determinants of ectoderm and endoderm respectively. ”

“ Moreover, in their psychical development, certain insects are equal, or even superior, to any other animals, except birds and mammals.
The animal kingdom is primarily divided into two grand divisions, the one-celled (Protozoa) and many-celled animals (Metazoa) . In the latter group the cells and tissues forming the body are arranged in three fundamental cell-layers; viz. the ectoderm or outer layer, the mesoderm, and endoderm. ”

“ Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed mesogloea, secreted by the cell-layers of the body-wall; the mesogloea may be a very thin layer, or may reach a fair thickness, and then sometimes contains skeletal elements formed by cells which have migrated into it from the ectoderm. ”

“ The endodermal spadix (sp.) of the sporosac represents the endoderm of the manubrium; the ectodermal lining of the sporosac (ex.) represents the ex-umbral ectoderm of the medusa; and the intervening layers, together with the sub-umbral cavity, have disappeared. The spadix, as the organ of nutrition for the gonads, may be developed in various ways, being simple (fig. 41, F) or branched (fig. 41, H) ; in Eudendrium (fig. 41, G) it curls round the single large ovum. ”

“ As has been implied in the above account, mesenchyme is usually formed from epithelial mesoderm or from endoderm, or from tissue destined to form endoderm. It is also sometimes formed from ectoderm, as in the Vertebrata at the nerve crest and other places. In some Coelenterata also it appears certain that the ectoderm does furnish tissue of a mesenchymatous nature which passes into the jelly, but this phenomenon takes place comparatively late in life, at any rate after the embryonic period. ”

“ Thus certain embryologists supposed that this layer corresponded to the reproductive organs of primitive animals: others regarded it as the prototype of the organs of locomotion. My embryological and physiological studies on sponges led me to the conclusion that the mesoderm must function in the hypothetically primitive animals as a mass of digestive cells, in all points similar to those of the endoderm. ”

“ Cases of heterotopy, or of organs being developed in a position or a germ-layer other than that in which they originally arose in phylogeny, are not so easy to find. According to Haeckel, the origin of the generative products in the mesoderm is a heterotopic phenomenon, for he considers that they must have originated phylogenetically in one of the two primary layers, ectoderm or endoderm. ”

“ While dealing with our harmonious-equipotential systems as the starting-points of processes of restitution, e.g. in Tubularia, Clavellina, the flatworms, and other instances, we always have regarded cross-sections of the body as constituting the elements of equipotentiality. Now cross-sections, of course, are by no means simple in themselves, but are made up of very different tissues, which are derivates of all three of the original germ layers—ectoderm, mesoderm, and endoderm. ”

“ Nevertheless, we are not justified in concluding that every cell [Pg 5] possesses the power of reproducing the whole. If, with the help of a bristle, we turn one of these polyps outside in like the finger of a glove, and then prevent it turning right again by sticking the bristle transversely through it, it does not live, but soon dies, obviously because the cells of the two layers of the body, ectoderm and endoderm, cannot mutually replace each other, and cannot mutually produce each other. ”

“ On the whole it seems likely that the endoderm is represented in part by the yolk, and in part by those anterior and posterior rudiments which usually form the mesenteron, but that in some Hexapoda the whole digestive tract may be ectodermal. It must be admitted that some or the later work on insect embryology has justified the growing scepticism in the universal applicability of the “germ-layer theory.” Heider has suggested, however, that the apparent origin of the mid-gut from the stomodaeum and proctodaeum may be explained by the presence of a “latent endoderm-group” in those invaginations. ”

“ You know, from the second lecture, what a gastrula of our sea-urchin is. If you bisect this gastrula, when it is completely formed, or still better, if you bisect the gastrula of the starfish, either along the axis or at right angles to it, you get complete little organisms developed from the parts: the ectoderm is formed in the typical manner in the parts, and so is the endoderm; everything is proportionate and only smaller than in the normal case. ”

“ Segmentation is complete and unequal, producing macromeres and micromeres marked by differences in the size and in yolk-contents. The micromeres give rise to the ectoderm; each of the sixteen macromeres, after budding off a small mesoblast cell, passes on as endoderm. A gastrula is established by a mixed process of embole and epibole. ”