1. Total Segmentation. Holoblastic ova. Gastrula without separate food-yelk. Hologastrula.
1.1. Primitive Segmentation. Archiblastic ova. Bell-gastrula (archigastrula.) a. Many lower zoophyta (sponges, hydrapolyps, medusae, simpler corals). b. Many lower annelids (sagitta, phoronis, many nematoda, etc., terebratula, argiope, pisidium). c. Some lower molluscs. d. Many echinoderms. e. A few lower articulata (some brachiopods, copepods: Tardigrades, pteromalina). f. Many tunicata. g. The acrania (amphioxus).
1.2. Unequal Segmentation. Amphiblastic ova. Hooded-gastrula (amphigastrula). a. Many zoophyta (sponges, medusae, corals, siphonophorae, ctenophora). b. Most worms. 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).
2. Partial Segmentation. Meroblastic ova. Gastrula with separate food-yelk. Merogastrula.
2.3. Discoid Segmentation. Discoblastic ova. Discoid gastrula. c. Cephalopods or cuttlefish. e. Many articulata, wood-lice, scorpions, etc. g. Primitive fishes, bony fishes, reptiles, birds, monotremes.
2.4. Superficial Segmentation. Periblastic ova. Spherical-gastrula. e. The great majority of the articulata (crustaceans, myriapods, arachnids, insects).
CHAPTER 1.9. THE GASTRULATION OF THE VERTEBRATE.*
(* Cf. Balfour's Manual of Comparative Embryology volume 2; 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 (Figure 1.38). 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.
It must, therefore, be our next task to make a comparative study of the various forms of vertebrate gastrulation, and trace them backwards to that of the lancelet. Broadly speaking, they fall first into two groups: the older cyclostoma, the earliest fishes, most of the amphibia, and the viviparous mammals, have holoblastic ova—that is to say, ova with total, unequal segmentation; while the younger cyclostoma, most of the fishes, the cephalopods, reptiles, birds, and monotremes, have meroblastic ova, or ova with partial discoid segmentation. A closer study of them shows, however, that these two groups do not present a natural unity, and that the historical relations between their several divisions are very complicated. In order to understand them properly, we must first consider the various modifications of gastrulation in these classes. We may begin with that of the amphibia.