SYSTEMATIC EMBRYOLOGY. 105 



embryonic form produced by this process is known as a gastrula. 

 The process by which it originates is known as embolic invagination, 

 or shortly invagination. Of the two layers of which it is formed the 

 inner one (c) is known as the hypoblast and the outer (d) as the 

 epiblast, while the pore leading into its cavity lined by the hypoblast 

 is the blastopore (a). The cavity itself is the archenteron (6). 



(2) The cells of the blastosphere may divide themselves by a 

 process of concentric splitting into two layers (fig. 56 s). The two layers 

 are as before the epiblast and hypoblast, and the process by which they 

 originate is known as delamination. The central cavity or archen- 

 teron (F) is in the case of delamination 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 embryonic 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 (1) distinctly marked 

 off from the epiblast cells during 

 the segmentation ; and (2) very 

 much more bulky than the epiblast 

 cells. The bulkiness of the hypo- 

 blast cells necessitates a modifica- 

 tion of the normal process of embolic 

 invagination, and causes another 

 process to be substituted for it, viz. 

 the growth of the epiblast cells as FIG. 57. TRANSVERSE SECTION THROUGH 



a thin layer over the hypoblast. THE OVUM OF EDAXES DURING AN EABLT 



mi /c fi-,\ i STAGE OF DEVELOPMENT. (After Kowa- 



Ihis process (fig. 57) is known i evs ky.) 



as epibolic invagination. The ep. epiblast; ms. mesoblastic bacd; 



point where the complete enclosure h y- h yp blast - 



of the hypoblast cells is effected is known as the blastopore. All 



intermediate conditions between epibolic and embolic invagination 



have been found. 



In delaminatiou, when the segmentation is not uniform, or when 

 a solid morula is formed, the differentiation of the epiblast and 

 hypoblast is effected by the separation of the central solid mass of 

 cells from the peripheral cells (fig. 58 A). 



In the case of epibolic invagination as well as in that of the type 

 of -delamination just spoken of, the archenteric cavity is in most 

 cases secondarily formed in the solid mass of hypoblast (fig. 58 B). 



In ova with a partial segmentation there is usually some modi- 

 fication of the epibolic gastrula. 



