SECOND PEBIOD OF DEVELOPMENT. 



FORMATION OF THE GASTRULA, AND THE CLOSING or 



THE GASTRULA-MOUTH (FlGS. 17-34). 



Kowalevsky describes how the round blastula first 

 becomes oval, then through flattening of the one wall 

 and invagiiiation of the same takes the shape of a flat 

 two-layered embryo. My observations of this process 

 do not differ in important points from his, and the 

 segmentation cavity, which Kowalevsky describes 

 even after the invagination as a small slit, according 

 to my observations disappears entirely, so that the 

 two layers touch one another immediately. 



After the invagination follows the closing of the 

 gastrula-mouth, 1 whereby the embryo, according to 

 Kowalevsky, gradually takes the shape of "a some- 

 what elongated hollow globe." The embryo is now 

 covered, according to him, with cilia. Further, it is 

 stretched to a yet greater length, and the gastrula- 

 mouth, considerably narrowed, becomes excentric, 



1 Blastopore. 



56 



