MUSEUM OF COMPARATIVE ZOOLOGY. 319 



with observations ou Limax that these thickenings represent the cell 

 plate, and that the young nuclei have been overlooked, nevertheless, if 

 this is not the case, there must evidently be a want of synchronism in 

 the events of cleavage in the eggs of these two gasteropods, for at a 

 nearly corresponding stage of segmentation in Limax (Fig. 90) the 

 halves of the nuclear plate are already far apart. That, however, which 

 seems to render the first supposition almost certain, is the fact that the 

 testimony of other observers who have carefully studied these stages leads 

 to the same coc elusion. I will call attention to the statements of Hert- 

 wig (75, pp. 409, 410, Fig. 25) and Bobretzky (76, p. 101, Fig. 25), 

 and to the fact that Biitschli's own figures of Succinea (Taf. IV. Fig. 19), 

 Cucullanus (Taf. III. Fig. 21), and Nephelis (Taf. I. Fig. 12) point to 

 the same conclusion. It is to be noticed further, in regard to the last- 

 mentioned figure and its explanation, that the author places himself in 

 an ambiguous position, unless " ^er7^platte," in the statement, "Die 

 sogennante Kernplatte ist gebildet," is a misprint for ^e^/platte.* 



The author informs us that he has not observed the separation of the 

 halves of the nuclear plate, from which I conclude that he has not ob- 

 served any stages showing the halves of the plate, for he has given no 

 such figures, and a direct observation of the migration is hardly to be 

 thought of in such opaque eggs. The new nuclei are also in this case, 

 without doubt, formed by a differentiation of the halves of the nuclear 

 plate which have migrated into the ends of the spindle. Where they 

 have already appeared (Taf. IV. Figs. 14, 19) one sees clearly that the 

 fibres which join them are again swollen in the middle and have become 

 dark and lustrous. I am at a loss to understand the figures given by 

 Biitschli in this connection. They correspond in no way, as far as re- 

 gards the shape and appearance of the thichened portions, with anything I 

 have seen. If the fibres of the lower half of the spindle represented in 

 Fig. 19 were slightly thickened in the middle, and the upper half were 

 entirely absent, the figure would very closely correspond to what I have 

 many times seen in Limax. The nuclei increase in size and remain 

 united by the interzonal filaments when the segmentation is otherwise 

 completed. The author did not observe what became of the cell plate. 



The phenomena presented by Rotifera agree very well with the obser- 



* Since writing the above I observe what had escaped my notice before, — that 

 Biitschli has expressed, in his explanations of Taf. IV. Fig. 13, a doubt as to this 

 structure being after all a nuclear plate. It seems to me that, with the mateiial at 

 command, he might have expressed himself even more decidedly in favor of its being 

 a cell plate. 



