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.z 



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, 2 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 



