THE EARLY HISTORY OF THE OVUM. 



211 



displaced from the centre, so that it comes to lie immediately 

 beneath the protoplasmic pole. The layer of cells formed by 

 this segmentation is 



SET 



known as the blasto- 

 derm. In the teleosts 

 where protoplasm and 

 deutoplasm are sharply 

 distinct, the protoplas- 

 mic portions alone are 

 segmented, the food 

 yolk remaining undi- 

 vided, and the segmen- 

 tation cavity here lies 

 between the blastoderm and the deutoplasmic mass. 



The little that is known of the development of the eggs of 

 the monotremes shows that in their features of segmentation 

 they are closely like the reptiles and birds, they are mero- 

 blastic ; but the other mammals present several peculiarities in 

 their segmentation, which can best be considered after a descrip- 

 tion of the process of gastrulation in other vertebrates. 



In the amphibia and similar forms the segmentation cavity 

 is so small that it would be impossible for the larger yolk-loaded 



FlG. 216. Early segmentation of hen's egg, 

 after Duval. e, ectoderm ; /, lower layer cells ; 

 s, segmentation cavity ; w, white yolk ; y, yel- 

 low yolk. Only a small part of the egg shown; 

 compare Fig. 212. 



FlG. 217. Diagram of the process of gastrulation in Amblystoma ; the cells 

 to be invaginated as entoderm shaded, a and b, front and side views of the beginning 

 of the invagination, the primitive groove beginning in b', c, a later stage with longer 

 primitive groove ; in d, the process is nearly complete, a small patch of entode/tw 

 being seen behind (yolk plug in the ' anus of Rusconi '). 



cells to become invaginated in the typical manner (p. 5), and 

 so a modification of the process, not easily described, takes place, 

 the result however being the production of the two-layered gas- 

 trula. In a few words the process may be outlined as follows : 

 Since the large cells cannot be pushed inside the smaller ones, 



