112 



GASTROPODA. 



for instance, one of the four macromeres is markedly smaller than the 

 others, although the cleavage, in other respects, follows the usual 

 course (Fig. 41 A). At the four-celled stage in A'plysia, two blasto- 

 meres are distinguished by their smaller size, a difference which can 

 be recognised in the later stages also (Fig. 41 B). Although the two 

 smaller macromeres are still visible at this stage (B, III. and IV.), yet 

 in later ontogenetic stages, only the two larger ones are still distinct, 

 and these are apparent until grown over by the micromeres (epibolic 

 gastrulation, Kay Lankester, Chap, xxvi., Lit. Xo. 29 ; Manfredi, 

 No. 72, Blochmann, No. 8). Another Opisthobranch, Acera, re- 

 sembles Aplvsia in this respect (Rabl, No. 91 ). 



Fig. A2.—A-E stages of cleavage in Nassa mutabilis (after Bobretzky from Balfour's 

 Text-book). A-C, formation of the macromeres, on which, in I), four, and in E a 



large number of micromeres lie. 



The first stages of cleavage, in Nasso, mutabilis, are very striking 

 and peculiar (Bobretzky, No. 11). The egg contains a large 

 amount of food-yolk, and the formative protoplasm is aggregated at 

 the animal pole, over which the polar bodies are situated. An 

 equatorial and a vertical furrow, the former near the animal pole, 

 appear simultaneously, and divide the ovum into three segments, 

 two smaller blastomeres which are produced by the vertical furrow 

 and one large brown sphere, minus a nucleus and consisting entirely 

 of yolk-material (Fig. 42 A). The two blastomeres thus rest upon 

 this sphere somewhat like a germ-disc, except that the yolk has in 

 tli is case not attained to any great size. This condition soon dis- 



