SYSTEMATIC EMBRYOLOGY. 



127 



process by which they originate is known as delamination. 

 The central cavity or archenteron (F) is in the case of delamina- 

 tion the original segmentation cavity ; and not an entirely new 

 cavity as in the case of invagination. By the perforation of the 

 closed two-walled vesicle resulting from delamination an embry- 



Fig.l 



FIG. 56. DIAGRAM SHEWING THE FORMATION OF A GASTRULA BY DELAMINATION. 



(From Lankester.) 

 Fig. i. Ovum. 



Fig. 2. Stage in segmentation. 



Fig. 3- Commencement of delamination after the appearance of a central cavity. 

 Fig. 4. Delamination completed, mouth forming at M. 

 In fig. i, i and 3 EC. is ectoplasm, and En. is entoplasm. 

 In fig. 4 EC. is epiblast and En. hypoblast. 



onic form is produced which cannot be distinguished in structure 

 from the gastrula produced by invagination (fig. 56, 4). The 

 opening (M] in this case is not however known as the blastopore 

 but as the mouth. 



When segmentation does not take place on the regular type 

 the processes above described are as a rule somewhat modified. 

 The yolk is usually concentrated in the cells which would, in 

 the case of a simple gastrula, be invaginated. As a consequence 

 of this, these cells become (i) distinctly marked off from the 

 epiblast cells during the segmentation ; and (2) very much 

 more bulky than the epiblast cells. The bulkiness of the 



