220 W. WALDEYER. 
3. The sperm-nucleus (male pronucleus) is not the head of a 
spermatozoon. 
The two pronuclei are morphologically new structures, 
but substantially the female pronucleus originates from the 
substance of the egg-cell; how far the substance of the 
germinal vesicle takes a share in it, E. van Beneden leaves 
undecided in his first memoir. Substantially, too, the sperma- 
tozoon shares in the formation of the male pronucleus. Van 
Beneden finds that the head of the spermatozoon always lies 
close to the cortical zone of the egg, and that a part of the 
head of a spermatozoon extends into the cortical layer of the 
egg-cell. Since then the male pronucleus arises in this cor- 
tical layer, E. van Beneden is of opinion “ que le pronucleus 
superficiel se forme au moins partiellement aux depens de 
la substance spermatique.” 
Van Beneden further observed that both nuclei, even if they 
touch, still remain quite distinct from one another. He, how- 
ever, recognises, as already said, a fusion (conjugation) of the 
two nuclei. 
As we see, these two authors differ from one another in not 
unessential points, and also leave many important questions un- 
decided. O. Hertwig cannot make up his mind to identify the 
germinal spot quite certainly with thelateregg-nucleus. Again, he 
has not directly observed the act of entrance of the spermatozoon 
into the egg and the continuous transformation of it into the 
sperm-nucleus ; just as little has he observed the act of fusion 
in flagrante. The conclusion must here be supplied by 
direct observation. So, too, is it with van Beneden; by him 
the details of the formation of the two pronuclei remain 
undecided ; nor has he directly observed the act of fusion. In 
the recognition of the spermatic element in the male pro- 
nucleus he is behind O. Hertwig; however, he saw more 
correctly what happened to the germinal spot, and Hertwig 
himself soon afterwards relinquished his view that the germinal 
spot becomes the egg-nucleus. Now, it is no wonder that such 
a fundamental process as the behaviour of the male and female 
nuclear elements in the egg-cell was not discovered at once and 
