400 HOLMES. [Vol. XVI. 



than that of the others in the same tier, and the basal cell of 

 the cross which arises from it, being, therefore, of the same size 

 as the other basals, divides approximately at the same time. 

 We have, therefore, in the different character of the division of 

 liT in Crepidula and Planorbis, the beginning of the difference 

 in the course of development of the posterior arm of the cross in 

 the two forms. We may be unable to explain the differential 

 character of this cleavage of \d^. Why one out of four proto- 

 plasmic cells, identical, so far as we can determine, in size and 

 structure, should divide much more unequally than the others 

 is certainly not apparent. Nevertheless it is desirable to find 

 the precise point where the histories of structures in different 

 forms begin to diverge, although we are unable to discover the 

 antecedent phenomena which give the direction to the different 

 lines of divergence. The earlier cleavage of the posterior tip 

 cell in Crepidula is probably connected with its larger size. 

 Its cleavage is radial, and each of the daughter-cells divides 

 again in a radial direction, giving rise in all to six cells in the 

 posterior arm. The cause of these successive longitudinal 

 divisions may possibly be the growth of the posterior trocho- 

 blasts and the cells lying below them, which at this period 

 have reached a considerable size. The enlargement of these 

 cells would naturally subject the arm to pressure at the sides 

 and give the cells a longitudinal elongation. A comparison of 

 Conklin's Fig. 49 with Fig. 53 shows that the trochoblasts in 

 the latter figure are larger, and the basal and tip cells longer 

 and narrower. The shape of the tip cells just before their 

 division is not figured, but it is probable that they have become 

 more or less compressed like the basals, after the stage shown 

 in Fig. 51. However, after their division they have been 

 narrowed to about one-half their former diameter (Fig. 53). 

 The posterior tip cell in Planorbis has quite a different history. 

 In the first place it is no larger than the tip cells of the other 

 arms, but the marked difference it presents from the cor- 

 responding cells in Crepidula is that it never divides. It 

 increases enormously in size and becomes transparent. Its 

 shape changes entirely ; at first it is elongated transversely to 

 the arm of the cross ; gradually, as it enlarges, it becomes 



