CELL-NUCLEUSFERTILISATION 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 



