ESSENTIAL FEATURES OF SEXUAL PROCESS 1353 



the countless hordes of spermatozoa which may be in the neighbour- 

 hood of a given ovum only one as a rule enters. As soon as the sperma- 

 tozoon has entered the ovum a tough membrane is rapidly formed round 

 the latter, so preventing the entrance of any further spermatozoa. 

 The head of the spermatozoon enters the egg, while the tail atrophies 

 and disappears. The head of the spermatozoon enlarges and assumes 

 the character of a nucleus, the dense mass of chrornatin breaking up 

 first into a thread and then into the characteristic number of chromo- 

 somes (Fig. 557). The egg now contains two nuclei or pro-nuclei, exactly 

 similar in appearance, one derived from the male and the other belong- 

 ing to the egg itself . The two nuclei approach one another and join. In 

 many cases there is an apparent fusion of the substance of the two 

 nuclei. In others the chromatin filaments of male and female simply lie 

 side by side, forming a complete nucleus with the somatic number of 

 chromosomes. Fertilisation is rapidly followed by cell division. Each of 

 the chromosomes splits longitudinally, half going to each of the daughter 

 cells, and this process is repeated throughout the succeeding divisions 

 which result in the formation of the new individual. Thus every 

 cell of the body contains a nucleus of which exactly one half is paternal 

 and the other maternal in origin. In ascaris it is often possible, in 

 the first few divisions of the fertilised ovum, to distinguish in the 

 daughter nuclei the chromatin filaments derived from the male from 

 those derived from the female. 



The strong impetus to cell division given by the process of fertilisa- 

 tion has naturally aroused much curiosity as to its intimate character. 

 It might be thought that for cell division to take place a normal 

 number of chromosomes is essential. As against this explanation 

 may be adduced the fact that in many animals parthenogenesis occurs. 

 The female pro -nucleus may, under certain conditions of environment 

 or nutrition, start dividing and give rise to an embryo, each cell of 

 which contains only half the normal number of chromosomes. In 

 other cases of parthenogenesis only one polar body is extruded, or the 

 second polar body joins again with the female pro-nucleus. In either 

 case the ovum contains a nucleus, with a normal number of chromo- 

 somes, which divides and produces an individual resembling that 

 resulting from the union of ovum and spermatozoon. It has been 

 suggested that the impetus to division is given by the entry of the 

 spermatozoon itself. In the series of divisions which precede the 

 formation of the female pro-nucleus the centrosome of the ovum 

 generally disappears, whereas, in the formation of the spermatozoon, the 

 centrosome persists and forms the middle part of the spermatozoon. 

 In many cases the centrosomes divide in the spermatozoon itself, so 

 that this contains two centrosomes when it enters the egg. These 

 two centrosomes then become the centres of attraction spheres. 



