No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 395 



next cleavage occurs in the basal cells of the arms of the cross, 

 and takes place in a radial direction. Each arm of the cross 

 now contains three cells, besides the tip cells, which belong to 

 the second quartette. This division is in the same direction as 

 the preceding division of the basal cells, so there occurs a sec- 

 ond exception to the rule of alternation of spirals. It is a note- 

 worthy fact, also, that the direction of this division is at right 

 angles to the corresponding cleavage in Crepidula. In Crepi- 

 dula, when there are three cells in each arm of the cross, the 

 two cells behind the small tip cells are much elongated trans- 

 versely to the long axis of the arms, and it is natural that their 

 cleavage should be at right angles to their longest diameter. 

 The middle cells of the arms in Crepidula divide first ; this 

 division is followed by the cleavage of the cells at the apical 

 pole of the Q%g, and soon afterward the division of the basal 

 cells takes place. In Planorbis it is the apical cells that first 

 divide ; then the basal cells divide radially, so that before the 

 longitudinal splitting of the arms occurs each arm has one more 

 cell than in Crepidula ; a radial division, lengthening the arms 

 in Planorbis, takes the place of a transverse division, splitting 

 the arms in Crepidula. Moreover, the cells lying just behind 

 the tip cells of the cross, which in Crepidula divide first, in 

 Planorbis, except in the anterior arm of the cross, never divide 

 again. 



The inner median cell of the anterior arm of the cross next 

 divides transversely and marks the beginning of the splitting 

 of the arms of the cross. After a short interval the splitting 

 of the lateral arms also begins ; the cleavages are slightly 

 oblique, the lines of division pointing toward the anterior end 

 of the cross. This nearly transverse division occurs only in 

 the basal cells and the ones lying next to them, the inner 

 median cells. The basal cell in the anterior arm undergoes no 

 further divisions. Its history will be described later. The 

 transverse splitting in the anterior arm at this stage is, there- 

 fore, limited to a single cell. The posterior arm of the cross, 

 as in Crepidula, remains undivided throughout its entire history. 

 With the longitudinal division of the basal cell above described, 

 the history of the cleavages in the posterior arm is completed. 



