236 



MEAD. 



[Vol. XIII. 



(X, and x""), are among the first to divide (Fig. 20, also blue). 

 Hereafter I shall speak of these cells or their descendants as 

 the soviatic-plate, for they give rise in Amphitrite not only to 

 the ventral plate of the trunk, but to the lateral and dorsal 

 areas as well. The secondary trochoblasts also divide early: 

 a^'^, b'^-^, and c^''' (Figs. 21, 22). 



Soon after the cleavage of the rest of the cells takes place. 

 A^, Bi, and C^, whose products, A^, B^, C^ and «", b"", c*, with 

 those of D^, give rise to the entoderm (stippled in figures), 

 are among the last to divide. 



It is essential to obtain a clear idea of the orientation at the 

 ideal 64-cell stage, for with reference to it the future cleavage 

 will be described, one group of cells at a time. In the follow- 

 ing table the cells are arranged in groups from the animal to 

 the vegetative pole: 



64 



