294 HOLMES. [Vol. XVI. 



have cut the ventral cross furrow at almost a right angle. After 

 the fourth quartette is formed, owing to the shifting of the small 

 cells at the vegetal pole, the cross furrow would be cut at an 

 oblique angle. Viewed from the animal pole, the cross furrow 

 has been turned several degrees in a laeotropic direction. This 

 rotation is obviously a result of the dexiotropic cleavage of the 

 macromeres. Of course, if we take the small cells at the vege- 

 tative pole as fixed, we might consider that the rest of the &gg, 

 the fourth quartette and the ectomeres, had rotated in an oppo- 

 site, or dexiotropic, direction. In an Q.%g like that of Crepidula, 

 in which the homologues of these small cells form the greater 

 part of the bulk of the egg, it is natural to regard the furrows 

 between these cells as fixed, and to speak of a laeotropic rota- 

 tion of the ectoblast rather than a dexiotropic rotation of A, B, 

 C, and D. Whichever pole of the egg we regard as fixed, — and 

 this is the essential point, — it is certain that the rotation in the 

 two forms has taken place, in accordance with their reverse 

 types of cleavage, in opposite directions. 



The next cleavage in the cross occurs in the basal cells at 

 about the sixty-four-cell stage. This division is nearly radial, 

 though slightly laeotropic. It is an interesting fact that we 

 have here a violation of the rule of alternation of spirals 

 exactly where it first occurs in Crepidula and Neritina, but 

 with this difference, that in Planorbis the two successive cleav- 

 ages are laeotropic, while in Crepidula and Neritina they are 

 dexiotropic. 



The apical cells i^'', i^'*, etc., next divide. There seems to 

 be a tendency to a dexiotropic cleavage in these cells, but the 

 direction of the division appears to be more or less inconstant. 

 To the extent that this cleavage may be considered dexiotropic, 

 it is in accordance with the rule of alternation of spirals. This 

 division marks the close of the period of definite spiral cleavage 

 in the cells of the cross. The subsequent cleavages, to which 

 this may be considered a transition, are all of the bilateral type. 

 With the completion of this division the number of cells in the 

 egg reaches 104. In Crepidula this cleavage is laeotropic, and 

 Heymons mentions the fact that in Umbrella these cells divide, 

 but says nothing concerning the direction of the cleavage. The 



