FERTILISATION 



73 



(2) The union of spermatozoon and ovum is very in- 

 timate ; the nucleus of the spermatozoon and the reduced 

 nucleus of the ovum approach one another, combining to 

 form a unified nucleus. 



(3) The ovum centrosome disappears before fertilisation, 

 but a centrosome is introduced or evoked by the spermato- 

 zoon. It divides into the two which play an important 

 role in the segmentation of the fertilised ovum. 



(4) When the combined or segmentation nucleus begins 

 the process of development by dividing, each of the two 

 daughter nuclei which result consists partly of material 



11 



m 



Fig. 36.— -Fertilisation of egg-cell. — After Fol. 



I. Shows a minute hillock of protoplasm rising from the ovum towards 



the approaching spermatozoon. 

 II. Shows how the head of the successful spermatozoon has entered the 

 ovum. 

 III. The tail is nipped off when the head has entered. A pellicle — 

 the fertilisation membrane — is seen around the o\'um. 



derived from the sperm nucleus, partly of material derived 

 from the ovum nucleus. In other words, the union is 

 orderly as well as intimate, and the subsequent division is 

 so exact, that the qualities marvellously inherent in the 

 sperm nucleus (those of the male parent), and in the ovum 

 nucleus (those of the mother animal), are diffused through- 

 out the body of the offspring, and persfst in its reproductive 

 cells. 



(5) The spermatozoon may be able to enter the egg only 

 through pre-existing apertures or micropyles, or it may be 

 restricted to a particular region of the egg where the cyto- 

 plasm is not too heavily charged with yolk ; in the simplest 

 cases, as, for example, in annelids, echinoderms, and 



