38 KARYOKINESIS. 



The separation of the chromosomes coincides in point of time with the flow of 

 the interfilar substance of the spindle to the spheres. The chromosomes move to- 

 ward the poles until they come into contact with the spheres and even spread around 

 them to a certain extent, figs. 59, 66, 67. Such a fact is irreconcilable with the 

 theory that the chromosomes are moved solely by the contraction of the spindle 

 fibres, as Wilson ('95) and Griffin ('99) have pointed out, and suggests that the 

 movements of both interfilar substance and of chromosomes may be due to the 

 chemotropic attraction of spheres and centrosomes, as Strasburger ('93) maintains. 



When the chromosomes have reached the borders of the sphere at the end of 

 the spindle they do not enter into the sphere but spread somewhat over its surface 

 figs. 59, 66, 67. In this position the chromosomes are rapidly transformed into 

 vesicles, which grow larger and larger. These vesicles then fuse together and the 

 nucleus becomes an apparently single vesicle, though divided by a partition wall as 

 described above (p. 34). A reticulum of chromatin is then formed within the daughter 

 nuclei, which probably arises from the walls of the chromosomal vesicles, and on 

 each side of the partition wall there appears a single nucleolus, fig. 60. While these 

 chromosomal vesicles are in contact with the sphere, the latter frequently becomes 

 pear-shaped with the pointed extremity toward the chromosomes, fig. 67. In all 

 cases the daughter nuclei have processes which extend partially around and even 

 into the spheres, figs. 60, 81. Gradually, however, the processes disappear as the 

 daughter nuclei increase in size and the latter finally become rounded on the side 

 next the spheres, figs. 61 and 68. The significance of these processes of the nucleus 

 which project into the spheres is not far to seek. ' The daughter nuclei are at this 

 stage increasing their achromatic substance at a great rate, and the form of these 

 nuclei at once suggests that this substance is absorbed in large part from the 

 spheres. The nuclei of the growing egg cells of Dytiscus, as described by Korschelt 

 ('89), are similar in form, and perhaps in function, to this stage of these cleavage 

 nuclei. 



Lillie ('99) has observed that just before the "inner sphere" begins to expand, 

 after the second maturation division in Unto, it is three-quarters surrounded by the 

 chromosomes, and he suggests that there may be at this time a diffusion of chromatin 

 into the sphere, the interior of which stains more darkly than before. According to 

 my interpretation of the similar phenomenon in Crepidula, the chromosomes are at 

 this time absorbing substances from the spheres ; not until much later does the 

 "inner sphere '' or centrosome again stain more deeply. 



During this rapid growth of the daughter nuclei the spheres decrease some- 

 what in size (cf. figs. 60 and 61, also 67 and 68), in spite of the fact that at this time 

 sphere substance is collecting into the spheres from the astral radiations so that 

 the decrease in the size of the spheres is not so great as it would otherwise be. 



The chromatin reticulum which is formed in the daughter nuclei gives place in 

 the next prophase to chromatic granules connected together by linin threads, figs. 

 61, 62, 70. 



In early stages of the prophase, when the centrosomes are just moving into 



