698 PROCEEDINGS OF THE ACADEMY OF [Oct., 



last-formed cells are minute and were at first difficult to detect, being 

 overhung by their parent cells. The four cells Id^, Ic^, la^ and Ib^ 

 correspond both in origin and subsequent fate to the " trochoblasts " 

 of annelids and to the "turret cells" of mollusks, and will be termed 

 "trochoblasts," since the cleavage resembles that of the annelids 

 rather than that of the mollusks. 



While the trochoblasts are forming, lA and IC are in division (figs. 

 11 and 12), giving rise to two other members of the second quartette. 

 The division of the two macromeres is nearly simultaneous, though C 

 is slightly in advance. The two micromeres 2c and 2a are nearly 

 equal in size to the two anterior cells of the first quartette. 



Coincident with the division of lA and IC is the division of X (2d), 

 the first step in the fourth cleavage (figs. 11, 12 and 13). X buds off 

 a cell of about a third of its own diameter, low down on the right side 

 (x^, figs. 12 and 13). Closely following the formation of x* is the leio- 

 tropic division of IB to form 2b, the last member of the second quar- 

 tette, and the dexiotropic division of 2D to form the first member of 

 the third quartette, 3d (figs. 13 and 16). Thus the macromere of 

 the D quadrant has now obtained a lead of one division over the macro- 

 mere of the B quadrant, and this lead is maintained as far as the for- 

 mation of the fifth quartette. 3b is the smallest member of the second 

 quartette, while 3d will prove to be the largest member of the third. 

 Soon after 3d the other members of the third quartette appear one by 

 one, 3c and 3a equal in size, and last 3b, the smallest of the four, which 

 does not appear until after the formation of the mesoblast cell 4d 

 (figs. 13 to 20). 



Meanwhile divisions arc occurring in the first and second quartettes. 

 The divisions of the former are dexiotropic and concern the four larger 

 cells only. The left products are nearly alike in size ; smaller than the 

 right products in the posterior quadrants, nearly equal to them in the 

 anterior. There is thus formed a flat cap of eight cells (fig, 14), four 

 of which lie radially and four interradially ; the former, la^--, Ib^-^, Ic^-^, 

 Id^-^, correspond in origin and very probably in fate to the "intermedi- 

 ate girdle cells" of Nereis (Wilson, 1892), and they will hereafter be 

 called such. The four remaining cells, following the same terminology, 

 will be called "stem cells." While these last divisions are in progress 

 and the third quartette forming, 2a and 2c each buds off a small cell at 

 its apex (figs. 15 and 16, 2a^ and 2c^). According to the rule of altei- 

 nating cleavage this division should be leiotropic; in fact it is nearly 

 equatorial, though the division of 2b, which occurs later, is strongly 

 dexiotropic. The ovum represented in figs. 14, 15 and 16 is the only 



