KARYOKINESIS AND ITS RELATION TO FERTILIZATION. 237 
case of the male pronucleus, that the substance of the female 
pronucleus increases during its formation, probably at the 
expense of the surrounding protoplasm. E. van Beneden says, 
in regard to this (1. ¢., p. 287) :—“ Le but de l’élimination que 
se fait dans les globules polaires ne peut donc étre de diminuer 
la quantité de substance chromatique du nucléole de Pout : 
cette expulsion ne peut-étre congue comme une épuration.” 
A very remarkable fact is the perfect equality of the two pro- 
nuclei. E. van Beneden distinctly points this out. Kults- 
chitzky (Il. c.) has succeeded in discovering some new and 
interesting details about this. He found each pronucleus to 
be provided with a nucleolus. In a number of cases he found 
two nucleoli in each pronucleus, rarely three; but the number 
of nucleoli was always the same in the two pronuclei. 
The perfect equality of the sperm- and egg-nuclei was also 
recognised by Strasburger (191) in plants; on the other hand, 
Platner (160) found in Arion empiricorum a striking 
inequality. 
Not only does the nucleus of the egg-cell extrude elements, 
but, at the same time that the directive corpuscles are formed, 
some of the protoplasmic substance of the egg is also separated, 
which becomes arranged in two peripheral layers or envelopes 
round the rest of the egg (‘‘ couches périvitellines,” see fig. 14) ; 
these take no share in the further process of development. E. van 
Beneden points out that in other eggs the separation of a 
similar substance, “ liquor vitellinus,”’ is already established, 
and he compares the “ couches périvitellines” of the egg of 
Ascaris with this. 
When the two pronuclei have been formed from the sperm 
remnant and the remnant of germinal vesicle, then, according 
to the statements of most authors, the true process of fertiliza- 
tion occurs—the fusion of the two pronuclei to form a seg- 
mentation-nucleus. In his description of this so-called 
*‘ fusion,” E. van Beneden differs completely from all other 
writers. According to him, this important act takes place as 
follows:—The varicose chromatin thread of each of the two 
pronuclei, which have in the meantime come to lie close 
