220 PRINCIPLES OF ANIMAL BIOLOGY 



separated; the protoplasm is a convex disc resting on top of the yolk 

 mass. In such eggs the cleavage is limited to the protoplasmic portion, 

 as shown in Fig. 172. The yolk is never divided at any stage. Cleavage 

 does not follow such a regular scheme as in homolecithal eggs. A blastula 

 is formed, but the blastocoele is only a narrow space between the disc of 

 cells above and the undivided yolk below (Fig. 173). Scattered cells 

 may, however, rest upon the yolk at the bottom of this cavity. 



Like the cleavage of the fish egg is that of the birds and reptiles, whose 

 eggs are also strongly telolecithal. 



Cleavage in Centrolecithal Eggs. — Cleavage in the eggs of insects 

 and others of the same type is at first limited to division of the nucleus. 

 The nucleus, ordinarily situated in a small mass of cytoplasm in the middle 

 of the egg, divides repeatedly while the cytoplasm is everywhere unseg- 

 mented. The daughter nuclei migrate toward the periphery of the egg, 

 accompanied by small portions of the central mass of cytoplasm, and 

 arrange themselves in a layer at the surface. Then division of the cyto- 

 plasm takes place between the nuclei (see Fig. 162). For a long time, 



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Fig. 174. — Gastrulation in Amphioxus, early (A) and late (B) stages, ap, animal pole; 

 ar, archenteron; h, blastocoele; hp, blastopore; ec, ectoderm; en, endoderm. 



however, the cytoplasmic divisions do not pass entirely around their 

 respective nuclei, so that the inner ends of the "cells" at the periphery 

 are continuous with the mass of the egg. 



Gastrulation.^ — When the blastula is well formed the wall of cells on 

 one side of it begins to be turned in, gradually obliterating the blastocoele. 

 This inturning, or invagination, of the wall of the blastula is styled gas- 

 trulation, and the end product a gastrula. The process differs in blastulas 

 of different forms. 



Gastrulation in Homolecithal Embryos. — The simplest form of in- 

 vagination takes place in those animals whose eggs have a small amount 

 of yolk evenly distributed. The fish-like Amphioxus and the holothurian 

 Synapta are classical examples (Figs. 174 and 175). The vegetative pole 

 of the blastula becomes flattened, and then turned inward, much as a 

 hollow rubber ball might be indented on one side by pressure. The in- 

 vagination proceeds until the inturned cells are in contact with those of 

 the opposite side. The blastocoele is thus completely obliterated. The 

 two-layered embryo thus formed is called a gastrula. The outer layer 



