KARYOKINESIS AND ITS RELATION TO FERTILIZATION. 259 
shows distinctly the four chromatic threads; the other (8) 
shows them only indistinctly, and later on its chromatin is 
only to be seen in the form of numerous isolated granules. 
The segmentation-cells arising from the sphere B in all the 
generations always showed these conditions during division, 
while from sphere a two spheres with four loops arise, a, and 
B,; B, behaves later on like.B, and A, like a. Boveri rightly 
regards this as a strong support of the Nussbaum-Weismann 
doctrine of the separateness of sexual cells, since he regards 
the one, or at most two cells, with distinct loops, as the 
primitive form of “ sexual cells.” 
If we now, in conclusion, try to realise the gain to the 
theory of fertilization from facts recently brought forward, the 
progress will best be rendered evident if we quite briefly review 
the various fertilization theories which have been sug- 
gested from time to time. 
One has always understood by “ fertilization” that influence 
of the male generating substance which renders the female 
generating substance—the egg-cell—completely fit for de- 
velopment. So expressed, the explanation holds also for 
parthenogenetic eggs, for we know that in a series of partheno- 
genetic developments a male generative substance must from 
time to time enter. Here, obviously, a simple fertilization 
suffices for a whole series of generations. 
We know that Spallanzani (188) already in his day disproved 
the theory of the fertilizing influence of the ‘ Aura semi- 
nalis,’ and since his renowned researches it is established 
that, in order to attain a fertilization, a direct material contact 
between male and female generating substance is necessary. 
A considerable advance was made in 1843 by Martin Barry’s 
celebrated discovery (14) that the spermatozoa penetrate into 
the interior of the egg. It was evident that the fertilizing 
influence was closely connected with the spermatozoon; it 
became quite evident, through the researches of O. Hertwig, 
E. van Beneden, Fol, Selenka, and others, that a single sper- 
matozoon is sufficient in order to render fertilization perfect ; 
indeed, that the penetration of more spermatozoa is injurious. 
