510 BULLETIN OF THE 



this is very similar to what I have observed on hardened eggs of Limax, 

 and since Hertwig's observations were made on living eggs there can be 

 little doubt that the condition I have represented in Fig. 68 is entirely 

 conformable to that which existed before the eggs were hardened. The 

 figure which Hertwig gives represents these structures as being more 

 angular than those 1 have figured, and I have detected no appreciable 

 difference in density between the narrower and broader ends. On the 

 other hand, Hertwig has figured the outlines as of equal distinctness 

 throughout, even when the pronuclei are in close proximity to each 

 other. For Limax I must repeat that, while the outline of each pronu- 

 cleus is not interrupted or effaced, it is much less boldly expressed at and 

 near its more pointed extremity. I doubt if this indicates a difference in 

 the composition of the two extremities of the nucleus. May it not be 

 that the density of the surrounding protoplasm changes with its distance 

 from the central point of attraction, in such a manner that it more 

 closely approaches the refringency of the pronucleus at the smaller ex- 

 tremity of the latter than it does at its more obtuse end 1 Sagitta and 

 Limax afford the only cases, so far as I recall, where this peculiar shape 

 of the pronuclei has been observed. 



Hertwig's observations on the formation and copulation of the pronu- 

 clei in mollusks afford several interesting points of comparison with 

 Limax. Thus, in the case of Mytilus, he says that they melt together, 

 and then become indistinct. In a pteropod (Tiedemannia), soon after 

 the first appearance of the two pronuclei, there was seen emerging from 

 the male pronucleus, in the part of the yolk not obscured by yolk 

 granules, a filament which followed when this pronucleus advanced to 

 meet the female pronucleus. Meantime the clear protoplasm increased 

 in extent about the animal pole of the egg, and thus was brought to view 

 successively more and more of the filament, which remained visible till 

 the time of the first segmentation. Hertwig interprets this as being 

 the filament of a spermatazoon, which penetrates at the vegetative pole, 

 and remains at first concealed by the yolk granules of that portion of 

 the egg. The head of the spermatazoon (i. e. the nucleus of the sper- 

 matic cell) gives rise to the sperm- vacuole, and the filament remains to 

 be dissolved in the yolk during and after the first segmentation. The 

 pronuclei in this case reach dimensions proportionate to the same 

 structures in Limax. By the time they have attained this relatively 

 great size, a large number of nucleoli have been formed (ausgeschieden) 

 in them. 



The next change which overtakes the nuclei does not seem to be the 



