2IO WILSON. [Vol. IV. 



teloblast. It lies between A and X, anterior, ventral, and 

 somewhat to the left hand of the latter (Fig. 3). ^ is now 

 approximately of the same size as B, and the embryo is bilater- 

 ally symmetrical, though not absolutely so, since both the pro- 

 teloblasts are slightly displaced to the left, F somewhat more 

 so than X. 



The gastrulation is strictly epibolic, the posterior lip of the 

 blastopore is formed by the progeny of F, the second pro- 



II 



U 



Fig. 3. — View of a later stage from the lower pole, showing the two pro-teloblasts, 

 X and Y, the four macromeres, and the edge of the cap of micromeres. 



teloblast, and the stomodaeum is developed from the micromeres 

 that close in the blastopore. 



The principal interest of the development, from our present 

 point of view, centres in the fate of the pro-teloblasts. From 

 these two cells the entire ventral plate arises, its anterior cells 

 from V, its posterior cells from X. From V arise the mesoblast- 

 bands,from X the neural plates, the seta-sacs, and other structures 

 still Mtidetcrmined. Y first divides into two equal cells (the 

 primary mesoblasts), and X soon does the same, first, however, 

 invariably separating two smaller cells (Fig. 4). The bilateral 

 symmetry of the embryo is now conspicuous, though the slight 

 displacement of the teloblasts to the left still remains. Each of 

 the primary mesoblasts now separates off a somewhat smaller 

 cell at its outer side, thus forming a transverse series of four 



