Fig. 42.—Gastrulation. A, Gastrula of a Zoophyte (Gastrophysema). (After Häckel.) B, Gastrula of a Worm (Sagitta). (After Kowalevsky.) C, Gastrula of an Echinoderm (Uraster). (After A. Agassiz.) D, Gastrula of an Arthropod (Nauplius). (After Häckel.) E, Gastrula of a Mollusk (Limnæus). (After Rabl.) F, Gastrula of a Vertebrate (Amphioxus). (After Kowalevsky.) In all, d, indicates the intestinal cavity; o, the primitive mouth; s, the cleavage-cavity; i, the endoderm, or intestinal layer; e, the ectoderm or skin-layer.
Fig. 43.—Gastrula of a Chalk Sponge. (After Häckel.) A, External view. B, Longitudinal section. g, digestive cavities; o, mouth; i, endoderm; e, ectoderm.
Fig. 44.—Prophysema primordiale, an extant gastræa-form. (After Häckel.) (A). External view of the whole animal, attached by its foot to seaweed. (B). Longitudinal section of the same. The digestive cavity (d) opens at its upper end in the mouth (m). Among the cells of the endoderm (g) lie amœboid egg-cells of large size (e). The ectoderm (h) is encrusted with grains of sand, above the sponge spicules.
Here, then, we leave the lower forms of Metazoa in their condition of permanent gastrulæ. They differ from the transitory stage of other Metazoa only in being enormously larger (owing to greatly further growth, without any further development as to matters of fundamental importance), and in having sundry tentacles and other organs added later on to meet their special requirements. The point to remember is, that in all cases a gastrula is an open sac composed of two layers of cells—the outer layer being called the ectoderm, and the inner the endoderm. They have also been called the animal layer and the vegetative layer, because it is the outer layer (ectoderm) that gives rise to all the organs of sensation and movement—viz. the skin, the nervous system, and the muscular system; while it is the inner layer (endoderm) that gives rise to all the organs of nutrition and reproduction. It is desirable only further to explain that gastrulation does not take place in all the Metazoa after exactly the same plan. In different lines of descent various and often considerable modifications of the original and most simple plan have been introduced; but I will not burden the present exposition by describing these modifications[15]. It is enough for us that they always end in the formation of the two primary layers of ectoderm and endoderm.
The next stage of differentiation is common to all the Metazoa, except those lowest forms which, as we Have just seen, remain permanently as large gastrulæ, with sundry specialized additions in the way of tentacles, &c. This stage of differentiation consists in the formation of either a pouch or an additional layer between the ectoderm and the endoderm, which is called the mesoderm. It is probably in most cases derived from the endoderm, but the exact mode of its derivation is still somewhat obscure. sometimes it has the appearance of itself constituting two layers; but it is needless to go into these details; for in any case the ultimate result is the same—viz. that of converting the metazoön into the form of a tube, the walls of which are composed of concentric layers of cells. The outermost layer afterwards gives rise to the epidermis with its various appendages, and also to the central nervous system with its organs of special sense. The median layer gives rise to the voluntary muscles, bones, cartilages, &c., the nutritive systems of the blood, the chyle, the lymph, and the muscular tube of the intestine. lastly, the innermost layer developes into the epithelium lining of the intestine, with its various appendages of liver, lungs, intestinal glands, &c.
I have just said that this three or four layered stage is shared by all the Metazoa, except those very lowest forms—such as sponges and jelly-fish—which do not pass on to it. But from this point the developmental histories of all the main branches of the Metazoa diverge—the Vermes, the Echinodermata, the Mollusca, the Articulata, and the Vertebrata, each taking a different road in their subsequent evolution. I will therefore confine attention to only one of these several roads or methods, namely, that which is followed by the Vertebrata—observing merely that, if space permitted, the same principles of progressive though diverging histories of evolution would equally well admit of being traced in all the other sub-kingdoms which have just been named.