26 Researches on the Fecundation of Animals. [January, 
is very clear when fecundation appears in the form of 
conjugation, or even in the type in which it occurs in the 
Vorticellae. A Vorticella, a, resolves itself into a macrospore, 
M (female gonocyt), and a microspore, m (male gonocyt). 
Another Vorticella, b, is similarly split up into M and m : 
M then coalesces with m, and m with in. There are formed 
thus two renovated individuals, A and b, but there is no 
increase in the number of individuals. After fecundation, 
as well as before, there are only two Vorticellae. Fecunda- 
tion as it is performed in the Metazoa follows the same plan. 
An ovum, A, is divided into a female gonocyt, g, and in 
polar vesicles, g ; a spermacyt is split up into a cytophoral 
part, c, and a spermatozoid, s. We then see that g unites 
with s to form a rejuvenated cell, which is the pristine em- 
bryonic cell. Theoretically c and g might produce a second 
cell ; but in reality these elements perish, so that after 
fecundation, in place of two cells capable of further division, 
there is only one. It is clear that here an increase has no 
more occurred than in the case of the Vorticellae. It would 
seem that the power of cells to increase by division is 
limited ; then comes a moment when they can divide no 
longer unless rejuvenated by the process of fecundation. 
In animals and in plants the only cells capable of being thus 
rejuvenated are the ova, and the only ones which can effect 
this change are the spermacyts. All other parts of the indi- 
vidual are doomed to death. Fecundation is the condition 
of the continuity of life. By it (or in it) the generator es- 
capes death. I have taught this hypothesis since 1876, 
and it finds new supports in the study of Ascaris megalo- 
cephala. 
It would be interesting to examine whether a vegetable 
form could be produced indefinitely by cuttings without a 
slow decline setting in, or without the individuals becoming 
enfeebled [horticultural experience points in this direction] ; 
whether a species can maintain itself in cases where fe- 
cundation does not from time to time interrupt the series of 
asexual reproduction. 
By the hypothesis of the hermaphroditism of cells, par- 
thenogenesis may be explained if the formation of the polar 
vesicles does not take place, or if these elements after their 
formation reunite with the female gonocyt. In the hope of 
finding this conclusion confirmed I suggested to the Natural 
History Section of the Belgian Academy, in 1876, to propose 
as a prize-question the enquiry whether in parthenogenesis 
the ovum evolves polar vesicles. It is certain that, in most 
cases in which parthenogenesis has been observed, this mode 
