KARYOKINESIS AND ITS RELATION TO FERTILIZATION. 231 
E. van Beneden leaves it doubtful whether any portion of 
the egg-protoplasm takes a share in the formation of the 
directive corpuscles. Fol, Hertwig, and Schultze, as we have 
seen, regard this as being the case. If no protoplasm is con- 
cerned we can compare the whole process only to the division 
of a nucleus, and not to cell division. 
Finally, it must be remembered that with one exception, to 
which we shall refer below, at least two directive corpuscles 
are extruded.! 
We now have arrived at the fundamental fact that what 
remains of the germinal vesicle and spot after the extrusion of 
the directive corpuscles is the female pronucleus or egg-nucleus 
of Hertwig, which is destined to fuse with the sperm-nucleus. 
It is thus evident that from the original germinal vesicle im- 
portant portions have to be eliminated before copulation is 
possible. This follows in the clearest way from E. van Beneden’s 
account. 
The latter found, namely, that the extrusion of the directive 
corpuscle in Ascaris occurs, without exception, just when the 
spermatozoon has entered the yolk. Although the spermatozoon 
lies in the immediate neighbourhood of the germinal vesicle, yet 
it does not fuse with it till both directive corpuscles have been 
extruded. It thus appears that the formation of them is a 
necessity in the chain of the whole process of fertilization. 
We shall see, later on, what significance is to be attached to 
the directive corpuscles. 
We will now turn to the question of the more minute 
processes in the rusion of the male and female pronuclei. 
The earlier workers, with the exception of Flemming, did not 
go outside the simple idea of a ‘ fusion,” and no further 
account was taken of the several constituents of the fusing 
nuclei (see the older statements of O. Hertwig, Fol, &c.). 
1 As being worthy of notice, we must here mention the fact that Biitschli 
was originally of opinion that the whole of the structure discovered by him, 
and named “ Richtungsspindel,” was extruded. Hertwig and Fol were the 
first to discover the true state of affairs in Echinoderms, the directive 
corpuscles of which H. van Beneden had recognised. 
VOL, XXX, PART 3—-NEW SER, Q 
