34 KARYOKINESIS. 



the same position in which it was last seen, fig. 82, text figs. VII and VIII. In 

 this groove the new central spindle for the next cleavage lies and in the following 

 division each half of the double nucleus is divided equally, frequently showing a 

 double chromosome plate in the metaphase. In the succeeding anaphase and telo- 

 phase each nucleus is again plainly double, being separated by a partition wall into 

 two parts. The dual character of the cleavage nuclei has been observed in the 

 telophase of every cleavage cell up to the 29-cell stage and in many cells up to the 

 60-cell stage, text figs. V-XVI. 



It is very probable that the halves of these double nuclei descend in unbroken 

 continuity from each of the germ nuclei and for the following reasons: 



1. In the first and second cleavages the nuclear halves are distinct at all 

 stages except during the metakinesis, and the relative positions of these halves cor- 

 respond to those of the germ nuclei. Even in later cleavages the relative positions 

 of the nuclear halves indicate that the one lying nearest the animal pole is probably 

 from the egg nucleus and the other from the sperm, text figs. V-XVI. 



2. In the first, second, third and fourth cleavages, and probably in all, the 

 central spindle when first formed in the early prophase, lies in a groove between the 

 nuclear halves, and hence in the only plane in which it could lie if the nuclear 

 halves are to be equally divided. Since successive cell divisions in Crepidula alter- 

 nate in direction, it follows, if the plane of nuclear division is always at right angles 

 to the plane of contact between the two halves, that the nuclei or nuclear spindles 

 must rotate at every cycle of division. This actually occurs, as a glance at the text 

 figures will show; the rotation usually occurs in each nuclear cycle before the pro- 

 phase but sometimes as late as the metaphase. 



3. In certain abnormal cases blastomeres are found with two entirely separate 

 nuclei in the resting stage ; in other cases two entirely separate mitotic figures lie 

 side by side in the same cell and in one such case, text fig. IX, there are thirty 

 chromosomes in each of these spindles, the same number which is found in each 

 of the germ nuclei. 



4. Finally there is always a single nucleolus in each of the germ nuclei before 

 their union, and in all of the cleavages, so far as I have observed, there are two and 

 only two nucleoli present in the telophase, but during the resting period, particularly 

 if it be prolonged, they may fuse into a single one. In view of current teaching 

 with regard to the significance of the nucleolus this persistence of a definite number 

 of nucleoli in each telophase is a somewhat surprising fact and may possibly indi- 

 cate that there is a persistence of some structure which may act as a center for 

 the formation of the nucleolus in each cell generation. 1 Since the nucleolus itself 

 is dissolved at the beginning of each mitosis, may not some achromatic structure, in 

 which or around which the new nucleolus is formed, persist and be transmitted by 



1 Montgomey ('99) has compiled tables showing the number of nucleoli in the egg cells of 170 

 or more genera representing almost every phylum in the animal kingdom. As a result of this work he 

 concludes that the number is not constant for a species, that it does not depend upon the amount of 

 yolk, mode of cleavage nor upon the manner of deposition of the egg, and that the facts do not warrant 

 an attempt to explain the factors limiting the number of nucleoli. 



