Gastrula without separate food-yelk. Hologastrula. I. Primitive Segmentation. Archiblastic ova. Bell-gastrula (archigastrula.) _a._ Many lower zoophyta (sponges, hydrapolyps, medusæ, 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). II. Unequal Segmentation. Amphiblastic ova. Hooded-gastrula (amphigastrula). _a._ Many zoophyta (sponges, medusæ, corals, siphonophoræ, 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). II. Partial Segmentation. Meroblastic ova. Gastrula with separate food-yelk. Merogastrula. III. 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. 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._