THE SPIRAL TYPE OF CLEAVAGE. 



251 



Fig. 21. — Arenicola. Descendants of 

 cross cells. 



for close comparison. The cross does not consist of the same 

 cells in the two groups. As Conklin has shown, the cross of 

 Crepidula arises mostly from the cells corresponding to the 

 so-called "intermediate girdle cells" of the polychaete, which 

 there form a relatively small portion of the pre-trochal region. 

 On the other hand, the cross 

 of the polychaete is composed 

 of the cells which Conklin calls 

 the "rosette series," and these 

 certainly do not form as large a 

 part of the pre-trochal region in 

 Crepidula as in the polychaete 

 (Figs. 21, 22). 



Another good illustration of 

 the lack of correspondence be- 

 tween origin and fate is found 

 in a comparison of the proto- 



troch formation in different forms, e.g., in Nereis, Arenicola, 

 Capitella, Sternaspis, Crepidula, and Ischnochiton. In all these 

 forms except Sternaspis, which has no prototroch, the details 

 of the prototroch formation are different, although the cells 



which form the prototroch arise 

 from the same general region 

 of the egg, and, where the divi- 

 sions are spiral, of course by 

 similar divisions. In Sternaspis, 

 cells corresponding to the so- 

 called primary trochoblasts in 

 Amphitrite, etc., are formed, 

 but are simply a portion of the 

 ectoderm. Again, in Nereis 

 and Capitella the stomatoblasts 



Fig. 22. — Crepidula. Upper pole at late stage, ■'' ^^ 



showing space occupied by cells correspond- (" OeSOphagoblaStS ) arC mCUl- 

 ing to annelid cross (after Conklin). ^^^^ ^^ ^^^ SCCOUd qUartCttC, 



while in Arenicola they belong to the third, and in Ischnochiton 

 to both second and third. The difference in position of larval 

 mesoblast, or its absence in some cases, indicates independent 

 modification. Numerous other examples of the same kind 



