No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 399 



together. Thus the arms of the cross would be subjected to a 

 lateral pressure, which would tend to give the cells an elonga- 

 tion in a radial direction. In fact, as the trochoblasts increase 

 in size, the arms of the cross actually do become narrower, as 

 may readily be seen by comparing the figures of the earlier and 

 later stages in the history of this structure. In Crepidula the 

 trochoblasts are relatively much smaller than in Planorbis, and, 

 at the time that the divisions above described occur, have not 

 attained a sufficient size to exert much influence upon the arms 

 of the cross. Thus the proximate cause of the difference in the 

 direction of the division of the basal cells of the cross in the two 

 forms seems very probably to lie in the different character of 

 the trochoblasts. The cause of these differences in the trocho- 

 blasts may, as has been suggested above, lie partly, at least, in 

 the relative size of the first quartette of ectomeres in the two 

 forms, which, in turn, is largely dependent on the amount of 

 yolk in the ^gg. 



Are there any facts which throw any light on the different 

 behavior of the cells in the posterior arm of the cross in the 

 two forms } It will be remembered that in both Crepidula and 

 Planorbis the posterior arm always remains undivided ; i.e., 

 it consists of but a single row of cells. In Crepidula the 

 development of this arm of the cross lags behind the others, 

 the first basal cell not dividing until a considerable time after 

 the others. Why is this .-* The explanation appears to lie in the 

 fact that the cell kT''^ is smaller than the other three basals. 

 Conklin, I believe, does not mention the fact, but in all of his 

 figures this cell is uniformly represented as smaller than the 

 others (Figs. 23, 25, 26, 29-31), in one case (Fig. 31) the dif- 

 ference being very marked. Further back than this it is to be 

 noted that the division of the cell from which the basal cell kT'^ 

 arose seems to be delayed, a fact which would indicate the 

 smaller size of this cell, although this is not otherwise notice- 

 able. It is certain, however this may be, that this cell icV' 

 divides more unequally than the others, and the principal cause 

 of the delayed cleavage of the posterior basal must, therefore, 

 be sought in whatever agency gives rise to the unequal division 

 of \d^. In Planorbis the division of this cell is no more unequal 



