388 HOLMES. [Vol. XVI. 



by a bilateral division. At a late period the anterior trocho- 

 blasts in Planorbis become so shifted as to lie in very nearly 

 the same position as in Crepidula. It is probable that there is 

 no other group of cel^ vi^hich presents, in different mollusks, 

 such a remarkable degree of variation, both in the time and in 

 the direction of their cleavage. Yet they have essentially the 

 same fate not only in mollusks, but also in annelids. There is, 

 I believe, no reasonable escape from the conclusion expressed 

 by Conklin, that the trochoblasts in annelids and mollusks are 

 truly homologous. Having the same origin, position, and fate 

 in both these groups, the evidence of their homology is as com- 

 plete as ontogeny can furnish. 



In most gasteropods the trochoblasts are of small size. This 

 is especially the case in Neritina, Umbrella, and Crepidula, and, 

 according to Conklin, they are small also in Urosalpinx and 

 Fulgur. In the pulmonates, however, their size is larger. In 

 the preceding cases the cells of the first quartette are very 

 small in relation to the macromeres and they divide unequally, 

 the outer cells being much smaller than the apical ones. In 

 Li7nax agrestis (Kofoid) and in Planorbis the cells of the first 

 quartette are not only much larger than in the above forms, but 

 they divide into almost equal parts. In the twenty-four-cell 

 stage of both forms the trochoblasts have about the same size 

 as the other cells of the Q.%g. Their bulk is, therefore, vastly 

 greater than that of the corresponding cells in Umbrella or 

 Crepidula. Has not the relatively large size of the trochoblasts 

 in Limax and Planorbis some causal connection with their pre- 

 cocious divisions .-' Is not their size dependent largely on the 

 size of the cells of the first quartette, and this, again, upon the 

 relatively small amount of yolk in the q%^ ? It seems probable 

 that the amount of yolk in the egg may indirectly influence 

 the size of the trochoblasts and the time at which they divide. 

 It is not contended that this is the only factor in the case. It 

 does not explain, for instance, why the cells of the first quartette 

 divide almost equally in some cases and very unequally in 

 others. But when we compare the eggs of Unio, Planorbis, 

 and Limax with those of Umbrella and Crepidula, it is difficult 

 to resist the impression that yolk is, in great measure, respon- 



