114 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



The two-layered gastrula is, as a rule, developed from the blasto- 

 spliere by invagination (einbolic invagination). In this process the one 

 half (sometimes distinguished by the larger size and more granular 

 nature of its cells) of the cell wall of the blastosphere is pushed 

 in upon the other half (fig. 108), and on the narrowing of the 







FIG. 108. A, Blastosphere of Amphioxus ; B, invagination of the saint; C, gastrula, invagi- 

 nation completed ; O, blastopore (after B. Hatschek). 



aperture of invagination (blastopore, mouth of gastrula) becomes the 

 endodermal layer (JiypoUast) lining the gastrula cavity. The outer 

 layer of cells constitutes the ectoderm or epiblast. This mode of 

 formation of the gastrula, which is very common, is found, e.g., in 

 Ascidians, and amongst the Yertebrata in Amphioxus (fig. 108). 



More rarely the gastrula arises by delamination. This process 

 consists of a concentric splitting of the cells of the blastosphere 

 into an outer layer (epiblast), and an inner (hypoblast) (fig. 109). 



FIG. 109. Transverse sections through three stages in the segmentation of Geryonia (after 

 H. Fol.) A, stage with thirty-two segments, each segment is divided into an external 

 finely granular protoplasm (ectoplasm) and an inner clearer layer (endoplasm) ; B, later 

 stage ; C, embryo after delamination; with ectoderm slightly separated from the endoderm, 

 which is composed of large cells surrounding the segmentation cavity. 



c l"he central cavity of the gastrula in this case is derived from the 

 original segmentation cavity, and the gastrula mouth is only 

 secondarily formed by perforation. This method of development 



