572 LILLIE. [Vol. VIII. 



ment of the cells is caused not so much by a rotation of the 

 cells when formed, as by the direction of the spindles during 

 the division. The same thing has been noted by Wilson. 



After the divisions of the macromeres are completed, «', b,^ 

 c, ' and d ' divide at different times, giving rise to eight apical 

 cells, alternating with one another in the typical way. During 

 the division of the apical cells a spindle forms in d,^ and a 

 small cell is segmented off on the right side of the vege- 

 tative pole {x,^ Fig. 3). 



I must remark here on the unsuitability, in this case and 

 possibly in others, of the terms macromere and micromere. If 

 we use the terms according to their etymological significance, 

 a,- b,~ ^,2 and d~ are the macromeres, and A, B, C, and D the 

 micromeres ; functionally the reverse is the case, inasmuch as 

 A, B, C, and D, like the macromeres of Nereis, contain the 

 entoderm. 



The cell d- corresponds exactly to the cell d- or X of 

 Wilson, which is the "first somatoblast." In the case of Nereis 

 this cell forms the ventral plate and the mid-dorsal region as 

 far as the prototroch.^ In Unio it forms the shell-gland, 

 and I think, that I can show, that in addition it forms the 

 foot. 



In general the apical pole lags far behind the vegetative pole 

 in the further divisions, so that in the fifty-cell stage there are 

 still only sixteen apical cells. This fact has made it impossible 

 for me to follow the divisions of the apical pole cells to the 

 late stage to which Wilson has followed them in Nereis, and 

 Conklin ^ in Crepidula. 



Let us now fix our attention upon the vegetative pole. 

 Another division of A, B, C, and D takes place, giving rise in a 

 right-handed spiral to the third generation of micromeres, 

 a,Z b,l> c,Z and dz respectively. Shortly afterwards a fourth 

 division of D takes place. This division is very unequal, the 

 vegetative part D being much smaller than the part posterior 



1 Conklin, Preliminary Note on the Embryology of Crepidula fornicata and of 

 Urosalpinx cinerea. Johns Hopkins Univ. Circulars, X, no. 88. 



2 This is true of the younger stages only ; later the products of the first somato- 

 blast become separated from the prototroch. 



