132 



BIOLOGICAL LECTURES. 



clear. The cells which correspond to the primary trochoblasts 

 are the smallest of all, and the slowest to divide. In Crepidula 

 and in other gasteropods these four cells are known as " turret- 

 cells," but for the sake of uniformity in this paper I will call 

 them primary trochoblasts. They remain undivided, or at most 

 divide only once up to a very late cleavage stage. During 

 these stages, however, they undergo a remarkable growth, so 

 that finally, instead of being the smallest, they are the largest 

 cells in the umbrella. It is the destiny of the primary trocho- 

 blast, however, which is of special interest to us. On this 



c 



FIG. 24. Three successive stages, representing the relative position of derivatives of the secondary 

 trochoblast q r s t in several forms. Diagram approaches most nearly the condition seen in 

 Ampkitrite. 



point Conklin says : " In four species of Crepidula, at least the 

 two anterior ones form a portion of the preoral velum (proto- 

 troch), and this is probably true of the two posterior ones also." 

 In other words, the primary trochoblasts in Crepidula have 

 probably the same destiny as those of the eight genera of 

 annelids mentioned on page 130. 



We may now consider the behavior of the secondary tro- 

 choblasts in Crepidula those three cells of the ideal 32-cell 

 stage, most of whose descendants in Ampkitrite, Clynienella, 

 and Arenicola complete the prototroch by filling three of the 

 gaps between its separate parts. In Crepidula the secondary 



