THE CELL ORIGIN OF THE PROTOTROCH. 



135 



stippled cells). These four cells divide into sixteen, all of 

 which enter the prototroch ; so far, therefore, the resemblance 

 is complete. The secondary trochoblast also arises in the 

 32-cell stage, precisely as it does in Amphitrite, Clymenella, and 

 Arenicola (Fig. 8, cell shaded with lines). All of the deriva- 

 tives of this cell enter the prototroch in IsclmocJiiton, and all 

 but one very minute cell product do so in the annelids named 

 (Figs. 10-12). 



The essential features of Heath's account can be most effect- 

 ively presented by means of diagrams. Fig. 25 represents a 

 lateral view of the prototrochal cells of one side in three stages 



FIG. 25. Represents trochoblast as seen on left side in Ischnochiton (Heath). Three stages shaded 

 like previous drawings. Unshaded cells are not part of the prototroch in the other forms 

 examined. See p. 132. 



of development, and the various cells are shaded like those in 

 Fig. 24 (AmpkitriU). Diagram A, Fig. 25, shows the condition 

 of the trochoblasts at about the 32-cell stage. It shows two 

 of the four groups of primary trochoblasts (stippled), each 

 group consisting at this period of two cells, as in Ampkitrite, 

 etc., Figs. 8 and 24, A. Between these cells lies the trocho- 

 blast marked q, r, s, t. The clear cell marked x is a trochoblast, 

 and its two derivatives in each quadrant enter the prototroch, 



