No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 389 



sible for the great difference in the period at which the division 

 of the trochoblasts occurs. 



It might be urged that if the large size of the trochoblasts 

 were the cause of their early division, in forms with a rela- 

 tively small amount of yolk, they ought to continue to divide, 

 since they grow more rapidly than perhaps any other cells 

 of the Q.^^. In Crepidula these cells are at first "much the 

 smallest cells of the entire ^gz'' \ finally they become, with 

 the exception of the yolk cells, the largest cells of the &gg\ 

 and their greater relative size is due not only to the fact that 

 the other cells are getting smaller as division proceeds, but 

 their absolute bulk is greatly increased. Yet neither in Crepi- 

 dula nor in any other gasteropod has more than one cleavage 

 of these cells been observed, while in Ischnochiton, accord- 

 ing to Heath ('99), and in Amphitrite, Lepidonotus, and Clyme- 

 nella among annelids, according to Mead, they stop dividing 

 entirely after the second cleavage. The cessation of the 

 cleavage of these cells is doubtless connected, as Conklin 

 maintains, with their prospective destiny. They soon acquire 

 cilia and become a part of a specialized organ. This necessi- 

 tates a special modification of their structure, which, setting in 

 at an early period, checks the tendency to division. It is well 

 known that when the differentiation of a cell is carried very far 

 it generally ceases to divide, or if it does divide, its specialized 

 structure in great part disappears and it returns to a more 

 embryonic condition. And we probably have in the cessation 

 of the division of the trochoblasts simply an exhibition of this 

 rule. The trochoblasts in Planorbis, after they have divided, 

 become much flattened or thinned out. In eggs stained with 

 silver nitrate they are almost transparent, while the cells of the 

 cross are stained brown, causing this structure to appear in 

 conspicuous contrast to the cells between the arms. With the 

 growth of the trochoblasts the arms of the cross become more 

 narrow, as if they were pressed together at the sides. They 

 thus take a somewhat darker stain and stand out in a more 

 conspicuous manner than before. The further history of the 

 trochoblasts will be described in the section on the cell lineage 

 of the prototroch. 



