314 MEAD. [Vol. X. 



centrosome and rays may now be seen at the inner end of the 

 second maturation spindle (Fig. 2). In the connecting fibres 

 there are equatorial thickenings, Zwischenkorper {J). 



The nine chromosomes left in the o.g'g after the expulsion of 

 the second polar globule become vesiculated so that the female 

 pronucleus has the form of a cluster of vesicles, in the midst 

 of which lies the centrosome, with numerous long rays running 

 out from it. As the female pronucleus moves back toward the 

 male the rays become gradually less numerous. They con- 

 verge to the center of the cluster in the manner represented in 

 Fig. 3. At this time the male pronucleus is large and is pro- 

 vided with its two centrosomes, from which enormous rays 

 extend to the periphery of the o.^'g in every direction (Fig. 3), 

 In stages like that represented in Fig. 4, the last traces of 

 radiation connected with the female pronucleus may usually be 

 seen. Often one can still trace the outlines of the component 

 vesicles of the latter, though they are closely pressed together 

 into a single oval nuclear mass. 



By the time the pronuclei meet, the vesicles of the female 

 pronucleus have apparently fused together. I can find no 

 trace of centrosomes, or rays, connected with it, although 

 those of the male pronucleus have become even more distinct. 

 The two pronuclei meet, become closely applied to each other, 

 and then the two male centrosomes, always found at the foci 

 of two huge systems of protoplasmic rays, move far apart, 

 greatly elongating the segmentation nucleus. In the latter, at 

 all times, the two pronuclei may be distinguished as separate 

 structures (Fig. 5). The chromosomes next assume their 

 definite shape and arrange themselves in the equatorial plate. 

 The segmentation nucleus always contains numerous nucleoli 

 which during karyokinesis, are dropped out into the cytoplasm, 

 where they ultimately degenerate. They are usually, if not 

 always, found only in the larger of the two cells. 



By the time the equatorial plate is formed, the centrosomes 

 have again divided, and have begun to move apart, anticipating 

 the cleavage into four cells. At this time also a peculiar lobe 

 begins to make its appearance upon the lower hemisphere of 

 the &g%, always exactly opposite the polar globules. Fig. 6 



