The Development of Ischnochiton. (343 



their formation the same relations as the same cells in the anterior 

 quadrants and for a certain time pursue the same course of devel- 

 opment yet in time their paths diverge. 



The studies of Crampton ('96) and Holmes ('97) on the devel- 

 opment of sinistral Gastropods serve to show that the positions held 

 by blastomeres are not the result of simple mechanical forces or 

 conditions and the above facts concerning the position of cells lead to 

 the belief that function is not dependent upon location. So that we 

 are compelled to seek for the causes of the perfectly coordinated 

 regular series of events which occur throughout development in those 

 subtle intrinsic forces whose nature is as yet not subject to analysis. 



5. Relationship between the Annelid and Chiton 

 Trochophore. 



Upon the theory that the progenitor of the trochophore was a 

 quadriradial organism I believe that it is possible not only to under- 

 stand many of the transformations undergone by the developing Chiton 

 embryo but to show that both the Chiton and Annelid trochophore 

 are constructed upon essentially the same type. Commencing at the 

 stage in the Chiton development represented by a, diagram E, the first 

 quartette composes the head vesicle and a portion of the prototroch 

 ■while the second and third quartettes alternating comprise the lower 

 hemisphere. In the further development the head vesicle never ceases 

 to consist wholly of first quartette cells maintaining as a whole the 

 same axial relations; but the cells in the lower hemisphere suffer 

 profound modifications from their original positions. Until the blasto- 

 pore has shifted within 30 ° of the prototroch the second and third 

 quartettes remain in contact with it on one side and with the proto- 

 troch on the other and there is no reason to believe that this condition 

 of affairs does not exist when the mouth has reached its permanent 

 position (diagram E, d, e). And while the second and third quartettes 

 shift their axial positions they also exhibit marked differences in their 

 rate of growth. The second quartette of the anterior quadrant pro- 

 bably enters into the formation of the stomodaeum ; these cells in the 

 right and left quadrants lengthen without increasing in width; while 

 the posterior band (first somatoblast) increases its length along the 

 ventral surface in all probability 90 ° at the same time enlarging its 

 dorsal area. The third quartette anteriorly {Ä and B quadrants) 

 during this period remain small while the cells in C and D increase 

 greatly and occupy the positions indicated in the diagram. 



