1262 PHYSIOLOGY 



rule enters. As soon as the spermatozoon has entered the ovurn, 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 chromatin 

 breaking up first into a thread and then into the characteristic number 

 of chromosomes (Fig. 572). The egg now contains two nuclei or pro-nuclei, 

 exactly similar in appearance, one derived from the male and the other 

 belonging 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 chromo- 

 somes. 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 con- 

 tains 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 fertilisation 

 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 chromo- 

 somes 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 chromosomes, 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 sperma- 

 tozoon 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 attrac- 

 tion spheres. They diverge, and between them is formed an achromatic 

 spindle, along the equator of which the chromatin filaments of male and 

 female pro-nuclei arrange themselves. It is doubtful however how far 

 the centrosome can be regarded as a permanent cell structure. In echino- 

 derm eggs, various modes of treatment will lead to the appearance of attrac- 

 tion spheres in the cytoplasm, and even to division of the non-fertilised 

 egg. Loeb has suggested that the action of the spermatozoon is essentially 



