No. 2. J 



THE MESOBLAST-BAJVDS IN ANNELIDS. 



211 



cells which lie at the surface and form the posterior lip of the 

 blastopore. Each of the ^'s now separates off a smaller cell 



Fig. 4. — View corresponding to that given in Fig. 3, after the division of the pro- 

 teloblasts. F, Y, the primary mesoblasts; X, X, the two primary posterior teloblasts. 



in front, then a small cell at its outer side, and finally divides 

 longitudinally into two equal parts. At this period (Fig. 5) the 



II 



Pr— vo 



II 



Fig. 5. — View from the ventral side of a still later Nereis embryo. The ventral 

 plate consists of fourteen cells (two small lateral cells are not shown). Bp, the blas- 

 topore; Pr, micromeres from which the prototroch is almost immediately afterwards 

 developed. 



