CELL-NUCLEUS—FERTILISATION 235 
of future development on the part of the 
offspring. 
Naturally, the whole story of the nucleus 
in its relation to heredity is a very long one, 
and in this brief sketch it has not been possible 
to attempt more than to indicate, in the 
barest outline, a few of the most important 
features of meiosis and of fertilisation. 
Meiosis has, however, a further claim on 
our attention, inasmuch as it has served as 
the starting-point for some of the most strik- 
ing morphological developments in the whole 
series of higher plants. 
It has been seen that sexual cells cannot, 
as a rule, arise until after the nuclei have 
undergone meiosis. It might, perhaps, be 
expected that immediately the meiotic phase 
is over, the four cells which result from it 
would at once become sexual gametes. In 
animals this commonly is the case—for in the 
male animal the four sperms arise by the direct 
transformation of the cells and nuclei that have 
just passed through meiosis. In the female 
the same is true, for the ripe egg, together 
with the three transitory polar bodies, form 
the corresponding female gametes. Of these, 
however, only the egg is normally functional. 
In plants, on the other hand, the four cells 
formed at meiosis never differentiate directly 
into sexual cells—at least no instance of their 
doing so is yet known. Often a long series 
of cell generations intervenes between meiosis 
and the formation of gametes. The four cells 
