KARYOKINESIS. 35 



division to the two daughter cells? However this phenomenon may be explained, 

 the fact that there is a single nucleolus in each germ nucleus before their union, and 

 that there is a single nucleolus in each half of the dual nuclei during the cleavage, 

 is additional evidence that the halves of these dual nuclei actually represent the 

 germ nuclei. 1 



Such a case as that of Crepidula indicates that the apparently single vesicular 

 nucleus of the resting stage may really be double in character, and the fact that out 

 of such a nucleus there may arise in the anaphase and telophase dual daughter 

 nuclei shows that the germ nuclei may still preserve their individuality, though no 

 trace of such separateness may be apparent at other periods. Further, it is possible, 

 even in advanced stages of the cleavage to determine with considerable probability 

 which part of a double nucleus is derived from the egg and which from the sperm, 

 the egg half always lying nearer the animal pole than the sperm half (see text 

 figures V-XVI). 



This independence of the germ nuclei during the cleavage of Crepidula is funda- 

 mentally like the observations of Hacker ('92, '95) and Ruckert ('95) on Cyclops; 

 here also the germ nuclei do not completely fuse throughout the early cleavage, their 

 independence being most clearly shown in the telophase. Ruckert also finds evi- 

 dence of a similar independence of the germ nuclei in the figures of Fol ('79) on 

 Toxopneustes and of Bellonci ('84) and Kolliker ('89) on Siredon. Some of these 

 figures referred to furnish very doubtful evidence. For example only one of 

 Fol's figures (pi. VII, fig. 7) shows a dual nucleus, while the figure in Kolliker's 

 textbook (fig. 36) is most probably a case of the indentation of the nuclear mem- 

 brane opposite the centrosomes, a thing which frequently happens in the early pro- 

 phase. Bellonci's figs. 1 and 20 show an indentation on one side of the nucleus 

 which may correspond to a division between the germ halves, but none of his fig- 

 ures, with the possible exception of fig. 20, show a fusion of the chromosomal vesi- 

 cles into two separate groups. Coe ('99) figures an indented nucleus in the telophase 

 of the first cleavage of Cerebratulus (see his fig. 40) which probably represents the 

 incomplete fusion of the germ nuclei in this animal. With the exception of Hacker 

 and Ruckert, none of the authors named call attention to these indented nuclei or 

 suggest their possible significance, and I think it may fairly be said that Crepidula 

 affords the most satisfactory and convincing evidence of the independence of the 

 germ nuclei which has yet been discovered. 



These observations are intimately related to the important discovery of Herla 

 ('93) and Zoja ('95) that the egg and sperm chromosomes of Ascaris remain inde- 

 pendent at least as late as the twelve cell stage, and this discovery was anticipated 



1 More recent experimental work on Crejndula egg has shown that when the chromosomal vesicles 

 are prevented from fusing a single nucleolus usually appears within each ; in general, one nucleolus 

 is found within each nuclear vesicle, and the fact that two are so generally found in the telophase is 

 probably due to the fact that at this stage the nucleus consists of two vesicles, whereas the more com- 

 plete fusion of these vesicles in later stages may lead to the formation of a single nucleolus. Such a 

 view would bring the size and number of nucleoli into relation with the size and number of nuclear 

 vesicles present at any stage. 



