KARYOKINESIS AND ITS RELATION TO FERTILIZATION. 288 
in regard to the minuter processes in the act of fusion he made 
no further advance than the first discoverer of it; only it must 
be mentioned that he observed a thread figure in the seg- 
mentation nucleus, i.e. in the nucleus resulting from the 
fusion. But besides the fusion of the nuclear structures, 
Nussbaum lays quite as much stress on the fusion of the 
protoplasmic portions of the egg-cell and spermatozoon. 
In his memoir he says, in reference to this: “ Therefore 
the fertilization in Ascaris megalocephala consists in the 
conjugation of two cells, the protoplasm of which, with 
all the structures derived from it, mixes, and the nuclei, after 
the extrusion of the directive corpuscles, unite to become the 
nucleus of the fertilized egg.” He states his view in the same 
way in the two memoirs already cited (147, 148). 
Nussbaum (149) arrived at a still more important conclu- 
sion in Leptodera nigrovenosa, Schneider. Here the two 
pronuclei, male and female, take up a position parallel to the 
long axis of the egg, and so fuse with one another lengthwise. 
But the first embryonic cell-division passes across the long axis 
of the egg, and thus across the plane of fusion of the two 
pronuclei. It therefore follows that each nucleus of the 
first two segmentation spheres contains one half 
of the sperm-nucleus and one half of the egg- 
nucleus. Nussbaum expressly recognised the importance of 
this observation to the question of heredity. All the cells 
of our body arise from the first two segmentation spheres ; 
it is almost to be taken for granted that in the later 
divisions of these two segmentation spheres, as well as of their 
successors, each contains an equal quantity of male and of 
female constituents, even although we are no longer able to 
follow the distribution of the two nuclear constituents in the 
later processes of segmentation. 
The process of the copulation of the nuclei is treated by E. 
van Beneden (‘ Recherches sur la Fécondation,’ &c., 23), much 
more minutely and in greater detail than any of his prede- 
cessors. His researches! on Ascaris megalocephala and 
According to a letter of E. van Beneden. 
