390 HOLMES. [Vol. XVI. 



Although only the anterior pair of trochoblasts in Planorbis 

 enters into the formation of the prototroch, the term " trocho- 

 blasts " has been applied to the posterior pair as well. The 

 latter form a portion of the head vesicle. As this structure 

 may be considered as consisting mainly of the enlarged poste- 

 rior portion of the prototroch, it would scarcely be incorrect to 

 call these cells trochoblasts also. 



The Division of the Third Quartette. — The cells of the 

 third quartette are large and elongated in a meridional direc- 

 tion. Their cleavage is almost exactly radial, as in Limax. 

 In Crepidula, according to Conklin, "the direction of the 

 cleavage is nearly radial, though after the cleavage has occurred 

 it is seen to be plainly laeotropic in 3^, -^b, and 3^, and dexio- 

 tropic in 3^, i.e., the cleavage is nearly bilateral on the posterior 

 end of the ovum." Yet the spindle in ^d is sometimes 

 laeotropic, as Conklin adds, and the nuclei may show this 

 relation to each other, while " the cell body may show reversal 

 of cleavage." "This," to quote the same author again, "is 

 but another illustration of the fact that bilaterality first appears 

 on the posterior side of the egg, that it is due to the change 

 in direction of one out of four cells, and that it is not per- 

 fect when it first appears, but is merely a deviation from the 

 spiral type toward the bilateral." In Planorbis the two cells 

 on the posterior side of the Qgg, 3^ and 3^, divide before the 

 anterior ones, 3^ and 3^, but I have been unable to find any 

 constant deviation from the radial direction of their cleavage. 

 A radial cleavage may, however, be considered an approach 

 toward the bilateral type, and we may view the earlier division 

 of the cells 3^ and 3^ as the first foreshadowing of bilateral 

 cleavage. 



The lower cells of the third quartette are somewhat smaller 

 than the upper ones and have their long axis horizontal, while 

 the long axis of the upper cells is still radial. In Neritina and 

 Umbrella the cleavage of the cells of this quartette is also 

 nearly radial, and the lower cell is the smaller, as is also the 

 case in Crepidula, and, according to Kofoid's and Meisen- 

 heimer's figures, in Limax. In Physa, according to Wierzejski, 

 the cleavage is also meridional and unequal, but in the anterior 



