THE FORMATION OP THE GERM-LAYERS. 



113 



appears, the yolk-sphere fusing with one of the blastomeres (Fig. 42 

 B) ; at the four-celled stage, produced by a triple segmentation of the 

 large sphere and division into two of the small blastomeres, it re- 

 appears (C). The yolk-sphere, which at this stage is distinct from 

 the blastomeres, again fuses with one of the cleavage-spheres, and it 

 thus happens that, the eight-celled stage (D) does not essentially 

 differ from the usual condition (Fig. 40 C) except for the fact that 

 one of the macromeres is specially large, the greatest mass of the 

 yolk having accumulated in it. The further cleavage seems to take 

 place in a regular manner and in a way similar to that above 

 described. Finally, here also, a large number of very small micro- 

 meres lie like a disc or cap upon the four macromeres (Fig. 42 E)~ 

 At a later stage, in Nassa, there is one large cell which is specially 

 distinguished from the rest. While the other cells divide further 

 it remains, on account of its large amount of yolk, almost unchanged. 

 It represents a kind of food-yolk which, in a much more specialised 

 form, will be found again in the Cephalopoda. 



The preponderance of one macromere over the three others is found 

 to a striking degree in Purpura (SELENKA, No. 115) and in Uro- 

 salpinx (BROOKS, No. 17 ; CONKLIN, No. 24), forms which in their 

 ontogeny seem to resemble Nassa. 



3. The Formation of the Germ-layers. 



The first appearance of the germ-layers in a few forms has already 

 been alluded to in connection 

 with the phenomena of cleav- 

 age, but in other forms these 

 layers arise in a somewhat 

 different way, their origin in 

 some cases being so differently 

 described by authors that this 

 point calls for special atten- 

 tion. 



Gastrulation is attained in 

 different ways in accordance 

 with the variations in cleav- 

 age. In the simplest cases, 

 e.g., Planorlis and Patella, 

 a blastula with a compara- 

 tively large cleavage-cavity 

 arises (Fig. 43). The vegetative pole of the blastula is formed by the 



FIG. 43. Blastula-stage of Patella (after 

 PATTEN). The ciliated tuft (at the cephalic 

 pole) and the ciliated ring are already 

 indicated. 



