KARYOKINESIS AND ITS RELATION TO FERTILIZATION. 261 
reduced, compared to the nucleus of the original sperm mother- 
cell. The two nuclear structures meeting in the egg-cell are 
therefore, according to van Beneden, not quite complete nuclei, 
and herein lies also the reason, as I understand, why van 
Beneden describes these nuclear structures, not as “ nuclei,” 
but as “ pronuclei.”” Compare also what is further stated below 
as to the number of chromatic loops. 
Since both pronuclei in the egg-cell become incorporated, 
they are complemental one to the other, so as to form a com- 
plete nucleus, without it being necessary that they should 
materially fuse. Both, on the contrary, appear to separate in 
the karyokinesis accompanying the first segmentation. Thus 
E. van Beneden arrives at a clear explanation of fertilization 
in the words previously quoted, “ Remplacement par certains 
éléments dérivés du gonocyte male des parties éliminées par 
Voeuf lors de la formation des globules polaires et des 
couches périvitellines.’” I think, therefore, that EH. van 
Beneden’s conception should be described as the “nuclear 
replacement theory.” 
A third view, which we will call briefly the ‘‘ Pure nuclear 
theory,” has quite recently been put forward by Kultschitzky 
(115, 116). It rests, it should be pointed out, on the results 
of E. van Beneden, which Kultschitzky is able to corroborate 
in all particulars. Kultschitzky adopts E. van Beneden’s view, 
that fertilization is complete at the moment when the two 
pronuclei become perfect. In keeping with the sense of the 
word “ Befruchtung,’ by which is understood that action 
which the male substance exerts on the egg-cell, he prefers to 
say that fertilization occurs at the moment when the “ male” 
pronucleus is completely formed. 
In his conception of the act of fertilization, Kultschitzky ex- 
cludes van Beneden’s more detailed definition “ replacement.” 
He does not get so far as this. The significance to be attached 
to the formation of the directive corpuscles he leaves entirely 
aside, since he is dealing with the definition of that which we 
have to call “Fertilization.” I believe I have correctly ex- 
pressed the difference which exists between van Beneden’s and 
