KARYOKINESIS AND [TS RELATION TO FERTILIZATION. 245 
appeared meanwhile. Four of the loops pass to each pole, as 
constituents of one of the two daughter-nuclei, and then 
cell-division follows. Zacharias describes the course of events 
in the same way as van Beneden, excepting that the latter 
has overlooked the facet that in each of his pronuclei the 
fusion between the male and female chromatic elements 
had already taken place when they began their transformation 
into thread structures. ; 
In the second example the process occurs, according to 
Zacharias, in the following way :—LHither all four chromatin fer- 
tilizing granules (two male and two female) are enclosed together 
in a common nuclear membrane, or the two male by themselves 
and the two female by themselves. When all four are enclosed in 
a membrane the division into microsomes takes place, followed 
by the mixture of these, together with the formation of a thread 
structure, the four loops, longitudinal splitting, and so on, as 
usual. Zacharias considers as the act of fertilization the 
mingling of the microsomes so that one can no longer distinguish 
male and female elements. When the two male granules and 
two female granules are enclosed separately, then matters go 
on in the way E. van Beneden has described as being the rule, 
without any exceptions, and the further course of events fur- 
nishes the proof. Zacharias found that, as van Beneden 
states, each of the now sexually differentiated pronuclei 
can become matured in the way described, but then not 
merely does a simple exchange of threads occur, as van Beneden 
figures, whereby the male is always distinguishable from the 
female, but both pronuclei fuse before the formation of dis- 
tinct thread-loops into a simple microsome stage, so that it is 
no longer possible, in the later stages, to separate the male and 
female elements from one another. Van Gehuchten also 
upholds the fusion of the male and female nuclear elements, 
although he thinks the first of the modifications described by 
Zacharias cannot occur. 
O. and R. Hertwig (96) also came to the conclusion that 
the egg-nucleus and the sperm-nucleus must completely per- 
meate (‘‘ eindringen’’), as a simple exchange of separate chro- 
