The lowest metazoa we know—namely, the lower zoophyta (sponges, simple polyps, etc.)—remain throughout life at a stage of development which differs little from the gastrula; their whole body consists of two layers of cells. This is a fact of extreme importance. We see that man, and also other vertebrates, pass quickly through a stage of development in which they consist of two layers, just as these lower zoophyta do throughout life. If we apply our biogenetic law to the matter, we at once reach this important conclusion. “Man and all the other animals which pass through the two-layer stage, or gastrula-form, in the course of their embryonic development, must descend from a primitive simple stem-form, the whole body of which consisted throughout life (as is the case with the lower zoophyta to-day) merely of two cell-strata or germinal layers.” We will call this primitive stem-form, with which we shall deal more fully later on, the gastræa—that is to say, “primitive-gut animal.”
According to this gastræa-theory there was originally in all the multicellular animals one organ with the same structure and function. This was the primitive gut; and the two primary germinal layers which form its wall must also be regarded as identical in all. This important homology or identity of the primary germinal layers is proved, on the one hand, from the fact that the gastrula was originally formed in the same way in all cases—namely, by the curving of the blastula; and, on the other hand, by the fact that in every case the same fundamental organs arise from the germinal layers. The outer or animal layer, or ectoderm, always forms the chief organs of animal life—the skin, nervous system, sense-organs, etc.; the inner or vegetal layer, or entoderm, gives rise to the chief organs of vegetative life—the organs of nourishment, digestion, blood-formation, etc.
In the lower zoophyta, whose body remains at the two-layer stage throughout life, the gastræads, the simplest sponges (Olynthus), and polyps (Hydra), these two groups of functions, animal and vegetative, are strictly divided between the two simple primary layers. Throughout life the outer or animal layer acts simply as a covering for the body, and accomplishes its movement and sensation. The inner or vegetative layer of cells acts throughout life as a gut-lining, or nutritive layer of enteric cells, and often also yields the reproductive cells.
The best known of these “gastræads,” or “gastrula-like animals,” is the common fresh-water polyp (Hydra). This simplest of all the cnidaria has, it is true, a crown of tentacles round its mouth. Also its outer germinal layer has certain special modifications. But these are secondary additions, and the inner germinal layer is a simple stratum of cells. On the whole, the hydra has preserved to our day by heredity the simple structure of our primitive ancestor, the gastræa (cf. Chapter XIX).
In all other animals, particularly the vertebrates, the gastrula is merely a brief transitional stage. Here the two-layer stage of the embryonic development is quickly succeeded by a three-layer, and then a four-layer, stage. With the appearance of the four superimposed germinal layers we reach again a firm and steady standing-ground, from which we may follow the further, and much more difficult and complicated, course of embryonic development.
SUMMARY OF THE CHIEF DIFFERENCES IN THE OVUM-SEGMENTATION AND GASTRULATION OF ANIMALS.
The animal stems are indicated by the letters a–g: a Zoophyta. b Annelida.
c Mollusca. d Echinoderma. e Articulata. f Tunicata. g Vertebrata.
| I. Total Segmentation. Holoblastic ova. Gastrula without separate food-yelk. Hologastrula. | I. Primitive Segmentation. Archiblastic ova. Bell-gastrula (archigastrula.) | a. Many lower zoophyta(sponges, hydrapolyps, medusæ, simplercorals). b. Many lower annelids (sagitta, phoronis, many nematoda, etc., terebratula, argiope, pisidium). c. Some lower molluscs. d. Many echinoderms. e. A few lower articulata (somebrachiopods, copepods: Tardigrades,pteromalina). f. Many tunicata. g. The acrania(amphioxus). |
| II. Unequal Segmentation. Amphiblastic ova. Hooded-gastrula (amphigastrula). | a. Many zoophyta (sponges, medusæ, corals, siphonophoræ, ctenophora). b. Mostworms. c. Most molluscs. d. Many echinoderms(viviparous species and some others). e. Some of the lower articulata (both crustacea and tracheata). f. Many tunicata. g. Cyclostoma, the oldest fishes, amphibia, mammals (not including man). | |
| II. PartialSegmentation. Meroblastic ova. Gastrula with separate food-yelk. Merogastrula. | III. Discoid Segmentation. Discoblasticova. Discoid gastrula. | c.Cephalopods or cuttlefish. e. Many articulata, wood-lice,scorpions, etc. g. Primitive fishes, bony fishes, reptiles,birds, monotremes. |
| IV. Superficial Segmentation. Periblastic ova. Spherical-gastrula. | e. The great majority of the articulata (crustaceans, myriapods, arachnids, insects). |
Chapter IX.
THE GASTRULATION OF THE VERTEBRATE[[20]]
[20] Cf. Balfour’s Manual of Comparative Embryology, vol. ii; Theodore Morgan’s The Development of the Frog’s Egg.
The remarkable processes of gastrulation, ovum-segmentation, and formation of germinal layers present a most conspicuous variety. There is to-day only the lowest of the vertebrates, the amphioxus, that exhibits the original form of those processes, or the palingenetic gastrulation which we have considered in the preceding chapter, and which culminates in the formation of the archigastrula (Fig. 38). In all other extant vertebrates these fundamental processes have been more or less modified by adaptation to the conditions of embryonic development (especially by changes in the food-yelk); they exhibit various cenogenetic types of the formation of germinal layers. However, the different classes vary considerably from each other. In order to grasp the unity that underlies the manifold differences in these phenomena and their historical connection, it is necessary to bear in mind always the unity of the vertebrate-stem. This “phylogenetic unity,” which I developed in my General Morphology in 1866, is now generally admitted. All impartial zoologists agree to-day that all the vertebrates, from the amphioxus and the fishes to the ape and man, descend from a common ancestor, “the primitive vertebrate.” Hence the embryonic processes, by which each individual vertebrate is developed, must also be capable of being reduced to one common type of embryonic development; and this primitive type is most certainly exhibited to-day by the amphioxus.