The Development of Isehnochiton. 



597 



great as these differences are there is essentially the same arrangement 

 of cells in each case; the apicals lie at the animal pole and in the 

 angles between them lie the intermediate cells, while at their outer 

 extremities lie the trochoblasts : also, in both, the intermediate cell 

 cleaves leiotropically forming a cell in each quadrant that lies in the 

 gap between the groups of trochoblasts. It is this cell in Isehnochiton 

 which forms the accessory trochoblast, and it is important to note that 

 the division forming each is leiotropic. 



Fig. C. The cross in Isehnochiton^ 

 Umbrella (Heymons, fig. 20) and Am- 

 phitrite (Mead, fig. 24). Cells of like 

 origin are designated by the same 

 devices; rosette series ("Annelid cross"), 

 blank ; basal cells of cross (intermediate 

 girdle cells of Annelids) and tip cells, 

 stippled; accessory primary trochoblasts, 

 shaded and stippled ; primary trocho- 

 blasts, ciliated. 



In the diagram of Isehnochiton (C, a) the accessory trochoblasts 

 are comparatively small cells, lying one in each quadrant on the left 

 of the primary trochoblasts. The basal cells, the largest of the first 



