CELL-NUCLEUS—FERTILISATION 231 
the chromosomes are constant in qualities 
and properties from one cell generation to 
another. (3) Owing to the mode of division, 
and distribution of the chromosomes at a 
nuclear division, the two daughter nuclei are, 
to all appearance, exactly alike, each is the 
reflected image of the other. Subsequent 
dissimilarities in size, and likewise in other 
respects, are not excluded, but these are 
almost certainly of secondary importance. 
What does all this mean? We cannot as 
yet give a complete answer to the question, 
but a consideration of the events connected 
with the differentiation of the sexual cells 
will perhaps serve to throw some light on the 
problems involved. 
In the first place, we have seen that the 
sexual act consists essentially in the fusion 
of two nuclei. How, then, can we reconcile 
this with the circumstance that the number 
of chromosomes is constant in the cell nuclei ? 
For it is evident that the nucleus of each 
fertilised egg must contain twice as many 
chromosomes as those present in the nucleus 
of each of the fusing gametes. 
The solution of this problem is furnished 
by a most remarkable nuclear division which 
is invariably intercalated somewhere in the 
series of nuclear divisions that intervene 
between the first formation of the embryo 
at fertilisation and the final production of 
sexual cells which closes the life cycle of the 
organism (Fig. 26, 1-6,). 
