SYSTEMATIC EMBRYOLOGY. 



129 



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 

 modification of the epibolic gastrula. 



Many varieties are found in the animal kingdom of the types 

 of invagination and delamination just characterized, and in not 

 a few forms the layers originate in a manner which cannot 

 be brought into connection with either of these processes. 



CJJ ^^sr^f^ A B 



FIG. 59. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel. ) 



A. Stage when the four hypoblast cells are nearly enclosed. 



B. Stage after the formation of the mesoblast has commenced by an infolding of 

 the lips of the blastopore. 



ep. epiblast ; inc. mesoblast ; bl. blastopore. 



The mesoblast usually originates subsequently to the two 

 primary layers. It then springs from one or both of the other 

 layers, but its modes of origin are so various that it would be 

 useless to attempt to classify them here. In cases of invagination 

 it often arises at the lips of the blastopore (fig. 57 and 59), and 

 in other cases part of it springs as paired hollow outgrowths of 

 the walls of the archenteron. Such outgrowths are shewn in 

 fig. 60, B and C at pv. The cavity of the outgrowths forms the 

 body cavity, and the walls of the outgrowths the somatic and 

 splanchnic layers of mesoblast (fig. C. sp. and so.). The archen- 

 teron is in part always converted into a section of the permanent 

 alimentary tract; and the section of the alimentary tract so 

 derived is known as the mesenteron. There are however 

 usually two additional parts of the alimentary tract, known as 

 B. II. 9 



