CHAPTER in 



FERTILISATION 



THE MOMENT of fertilisation is the conventional point of origin from which 

 to date the existence of a new individual. We have seen in the last chapter 

 that in fact many processes which are most important for the developmg 

 embryo occur before fertilisation, during the maturation of the egg. It 

 cannot be denied, however, that fertilisation is, normally at least, the 

 most crucial event within the continuous series of changes by which the 

 new creature comes into being. It is not a simple occurrence, at which 

 there is only one happening of importance; but its two important phases 

 succeed one another quite quickly, and although they can be dissociated 

 from one another in experiments, they are normally closely bound up 

 with each other, so that fertilisation appears as a single, though complex, 



event (Fig. 3-i)- 1-1, 



It would be out of place to discuss here the many and various mechan- 



FlGURE 3.1 



Fertilisation in the annelid Urechis. In 1 a sperm is entering the egg at the 

 bottom left. In 2 the egg has formed a fertilisation cone and the germinal 

 vesicle is breaking down; 3, second polar body division and sperm aster; 

 4, egg and sperm nuclei approaching one another— polar bodies at animal 

 pole; 3, union (conjugation) of male and female nuclei; 6, first cleavage 

 division. (After Belar.) 



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