W. G. VauNcime — Mnhryolixjy of Eustylochus. 291 



Through the center of this circle or ring of chromosomes a large 

 number of central fibres pass directly from one aster to the other. 

 Figs. 23, 24 and 25 show the successive stages in this process. 



I have already described the way in which the nucleoli disappear. 

 The portion of the reticulum of the pronuclei which is not to 

 develop into the chromosomes disappears about the same time as the 

 nuclear membranes. 



In Eustylochvs and Flanocera the pronuclei ordinarily do not 

 fuse, and their membranes may disappear while they are still quite 

 far apart, so that the spindle contains for a time two well separated 

 groups of chromosomes, derived from the egg and spermatozoon 

 respectively (Fig. 24). At a later stage we can no longer dis- 

 tinguisli these groups. I have also found a few instances like the 

 one mentioned by Francotte, where the cbromospmes of one pro- 

 nucleus developed and the membrane disappeared while the other 

 pronucleus was still intact. 



The minute structure of the centrospheres, aster rays and spindle 

 fibres does not ajjpear to differ from that already described in the 

 polar spindles. In length the cleavage spindle does not exceed that 

 which is often attained by the first polar spindle, but it is much 

 thicker and the asters are generally larger. 



When the chromosomes finally divide each splits lengthwise, form- 

 ing two U-shaj^ed pieces which are drawn toward the opposite poles 

 of the spindle (Fig. 26). An indistinct cell-plate, consisting of a 

 thickening of the central fibres at their middle point, is then gener- 

 ally noticeable. 



Meanwhile the centrosomes in each pole of the spindle have 

 divided, the derived centrosomes usually separating in a direction 

 transverse to the axis of the spindle (Fig. 28). They lie near the 

 middle of the centrosphere, not toward the outside or part most 

 removed from the equator of the spindle, and therefore much nearer 

 to the chromosomes than occurs in Tlialassetna as described by 

 Griftin (12). I have not observed the small asters within the centro- 

 sphere described by this author and by Coe (1). When the ^^^ has 

 divided, the chromosomes swell up into vesicles in which nucleoli and 

 a reticulum appear, and the vesicles in each blastomere fuse into a 

 resting nucleus, but the process so closely resembles the formation of 

 the female pronucleus that its description and ilhxstration Avould be 

 superfluous. 



