KARYOKINESIS AND ITS RELATION TO FERTILIZATION. 219 
chiefly from the fact that the appearance of a second nuclear- 
like structure, passing in from the periphery of the egg, 
regularly takes place a few minutes after the fertilization of 
the egg, and further, that in some cases he saw a fine thread 
passing away direct from this nucleus, and projecting from the 
surface of the yolk. 
Finally, O. Hertwig observed that the two nuclei approached 
one another, and lay close together ; and in other preparations 
he saw later on, only a single nucleus, which, however, was 
larger than the two. He concludes, therefore, that the two 
nuclei—the “ Egg-nucleus” and the “ Sperm-nucleus,” as he 
ealls the nucleus arising from the spermatozoon—fuse with one 
another. The nucleus arising from this fusion Hertwig ‘calls 
the “Cleavage nucleus” (first segmentation nucleus), since 
the latter is that which divides immediately after the process 
of fertilization, and becomes the parent nucleus for all the 
nuclei in a developing organism. 
Contemporaneously with Hertwig’s memoir there appeared 
E. van Beneden’s first observations of this subject (20). A 
further contribution (21) followed in 1876. The former (20) 
was quite independent of Hertwig, and dealt with the 
egg of the rabbit, whereas O. Hertwig had worked on that 
of the sea urchin. E. van Beneden saw two nuclear-like 
structures arise, which he describes as “ pronucleus-central, s. 
femelle,” and “ pronucleus périphérique, s. miale.”” He also 
noticed a conjugation of these two pronuclei, and regarded this 
as the act of fertilization. (‘Le premier noyau embryonnaire 
résulte de union de ces deux pronuclei, ce premier noyau est 
le product d’une véritable conjugaison entre un élément male 
(pronucleus périphérique) et un élément femelle (pronucleus 
central’). 
E. van Beneden differs from O. Hertwig in the following 
points : 
1. The egg-nucleus (female pronucleus) is not the persistent 
germinal spot. 
2. Of the whole germinal vesicle no morphological element 
remains. 
