276 BULLETIN OF THE 



ably the band of interzonal filaments, but the author failed to discover 

 that it was composed of filaments, and in general overlooked all the finer 

 details of structure which the use of reagents would have made apparent. 



But these are enough to show how frequently had been noticed a 

 change in the appearance of the nucleus before segmentation. Aside, 

 however, from this peculiarity and the almost concurrent testimony as 

 to a lengthening of the nucleus by those who believed in its division, 

 little advance was made in a knowledge of the nuclear changes. 



a. Segmentation. — The peculiar metamorphoses which the substance 

 of the nucleus undergoes in the formation of the spindle figure, and the 

 accompanying changes which manifest themselves in the surrounding 

 protoplasm of the yolk just prior to cleavage, like most discoveries, have 

 been only gradually comprehended. Yet the advances in the intimate 

 knowledge of these changes within the last half-decade seem marvellous. 



In the light of recent studies on maturation and impregnation, we 

 are now able to say that most of the earlier descriptions of the lengthen- 

 ing, constriction, and ultimate division of the nuclear structure of the 

 egg just before the first segmentation rest upon the observation of some- 

 thing else than the germinative vesicle. In most cases it has been either 

 the nucleus or the amphiaster of the first cleavage sphere that has been 

 seen and wrongly considered as the dividing germinative vesicle. The 

 class of observations in question, then, deals not with the phenomena of 

 maturation, but with those of cell division, and is therefore properly 

 considered in this connection. But where, on the other hand, observa- 

 tions have been less connected, and a division has been inferred from the 

 presence and close approximation of two nuclear structures, it may be 

 that in some cases the pronuclei have been mistaken for the resultants 

 of division. Such is probably the case, to cite a single example, with 

 the observations of Kolliker ('43, pp. 77, 78, Taf. VI. Figs. 5, 6) on 

 Ascaris dentata. 



Of those who have seen something more than an elongation of the 

 nuclear structure, it seems that among zoologists Ratzel was the earliest ; 

 but as his observations probably relate to the germinative vesicle rather 

 than to the primary cleavage nucleus, they will be considered under the 

 head of " Maturation." 



Another observer, who has given evidence of having seen in animal 

 cells the structure which is now generally known under the name of 

 ''nuclear spindle," is the Russian embryologist, Kowalevsky ('71, p. 13). 

 As is well known, this naturalist early made use of sections in his em- 

 bryological studies of invertebrates. The first unequivocal view of the 



