96 



HOLMES. 



[VOL. I. 



sixteen-cell stage. The lower tier of cells resulting from the 

 latter division are the trochoblasts or turret cells of Conklin. 

 A third quartette of ectomeres is given off in a left-handed 

 spiral, completing the separation of the ectoderm ; at the same 

 time a laeotropic division occurs in the second quartette. The 

 form of the egg which now contains twenty-four cells is almost 

 spherical ; there is a large cleavage cavity which is not, how- 

 ever, in any way homologous with that of the two-cell stage. 

 The macromeres are much smaller than is usual among the 

 eggs of the mollusca ; in fact they scarcely exceed in size the 



U--A- 



D 



C 



Fn.. i. - Kgg of eight cells. 



cells of the third quartette. They are quite conspicuous, 

 however, on account of the yolk granules they contain which 

 give them a bright golden yellow color. 



The resting period which occurs at the twenty-four-cell stage 

 is broken by the dexiotropic cleavage of the upper tier of the 

 second generation of ectomeres (2a l , 2b l , etc.); and, at nearly 

 the same time, the posterior macromere D, by a dexiotropic 

 division, gives off the cell ^d which is destined to produce the 

 mesoblastic bands. The ventral moiety of this division, com- 

 pared with the mesoblast cell, is small, and, as far as it could 

 be observed, remained undivided. The remaining macromeres 

 do not divide until a somewhat later period. The ventral tier of 

 the second quartette next divides dexiotropically ; there are now 

 four groups of four cells each belonging to the second genera- 

 tion of ectomeres, one cell in each group being above and one 

 below, while there is a right and a left cell at the same level. 



