No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 38 1 



place, according to Rabl, by the simultaneous division of all 

 of the cells of the &gg. In P. trivolvis the first quartette 

 divides before the others, forming a sixteen-cell stage. Finally, 

 although Rabl says nothing concerning the direction of the 

 cleavage of the cells of the second quartette, his Figs. \2A and 

 \2B indicate that the division was dexiotropic. In P. trivolvis 

 this cleavage is plainly laeotropic, and the cells lie much more 

 nearly in a vertical plane than they are represented in Rabl's 

 figure. There can be no doubt that the first quartette in the 

 forms studied by Rabl is given off in a laeotropic direction. 

 He expressly states this, and it is also indicated by his figures. 

 The second quartette, according to the law of alternation of 

 spirals, should be given off in a dexiotropic direction. If, as 

 Rabl's figures indicate, the second quartette divides dexiotrop- 

 ically, there would result two dexiotropic divisions in immediate 

 succession. It seems more probable that Rabl's figures are mis- 

 leading on this point than that there should be an exception to 

 the law of alternation of spirals at this early period of cleavage. 

 It is not unnatural that, not having in mind any significance 

 attached to this point, an observer should be in error regard- 

 ing it. 



The cleavage of the eggs of Physa and Lymnaea as far as the 

 twenty-four-cell stage is essentially the same in almost every 

 point as that of Planorbis, and there is a remarkable agreement 

 between the cleavage of Planorbis and Limax, as described by 

 Kofoid and Meisenheimer, as far as the cell lineage in the latter 

 forms was carried. The cleavage of the eggs of the pulmonates, 

 so far as they have been studied, seems, in fact, to be charac- 

 terized by several points of marked similarity. The amount of 

 yolk in the eggs is not great ; the ectomeres are large ; there 

 is a recurrent cleavage cavity ; and vacuoles are often formed 

 between the blastomeres. In the twenty-four-cell stage in the 

 above forms the cells have essentially the same relative size and 

 arrangement, and the entomeres are scarcely larger than the 

 other cells of the ^gg. The eggs of pulmonates complete their 

 development in capsules which contain a very large amount of 

 fluid albuminous substance, which serves to nourish the embryo. 

 As the amount of food in the form of albumen is large, it is 



