The Development of Ischnochiton. 581 



Gastropods, Annelids and Chitons, in which the cell lineage has been 

 carefully followed, the prototroch is in part formed from the first 

 division of the first quartette. Conklin has already emphasized the 

 fact that there is almost certainly a homology between the trocho- 

 blasts of Gastropods and Annelids, and the development of the proto- 

 troch of Ischnochiton gives greater strength to the view. In fact 

 throughout the development, not only in the behavior of the trocho- 

 blasts, but of other cells, we have many striking resemblances that 

 argue strongly for a genetic relationship between Annelids, Gastro- 

 pods and Chitons. 



In Nereis, according to Wilson, not all the substance of the 

 trochoblasts enters into the formation of the prototroch. On the 

 other hand Mead finds that it does in Amphitrite and Ch/menella, 

 as has Treadwell for PodarJce and Child for Arenicola. In Gastro- 

 pods Conklin finds that the two anterior groups of trochoblasts enter 

 into the velum, and possibly this may be said of the posterior also. 

 In Ischnochiton the entire substance aids in the formation of the 

 prototroch, but they are not the only locomotor cells from 

 the first quartette^). 



Kowalevski ('83) in his figs. 9 and 13 of the development of 

 Chiton polii shows the 16 cell stage very similar to Figs. 8 and 9 of 

 Ischnochiton. Without doubt he represents the trochoblasts and their 

 parent cells, the second quartette and macromeres. In the transition 

 from the eight to the sixteen cell stage he determined the origin of 

 the second quartette, but was unable to decide as to the origin of 

 the cells e, e' etc. (trochoblasts). However, it appeared more probable 

 that in the rhythm of segmentation they arose by the division of the 

 cells corresponding to the first quartette, a view which is undoubtedly 

 correct. 



Metcalf ('93) represents as 16 cell stage in the development of 

 Chiton squamosus and C. marmoratus in which the second quartette 

 forms in the usual manner, but the cell which forms as the first pro- 

 duct of the first quartette arises in a dexio tropic direction which is 

 the reverse of what we find in Polyclades, Annelids and dextrally 

 twisted Gastropods ^). 



1) In accordance with Mead I shall term the cells forming from 

 the first quartette and corresponding to the trochoblasts of Annelids, 

 the primary trochoblasts. 



2) Professor Metcalf kindly informs me that his conclusion was 

 based on a few eggs which showed a distinct bulging on the lower 



