ESSENTIAL FEATURES OF SEXUAL PROCESS 1211 



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 ana 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 

 chemical in character. By alteration of the medium in which the eggs are 

 contained, Loeb has succeeded in imitating exactly the changes which 

 normally need the entrance of a spermatozoon for their occurrence. On 

 immersing the eggs in a weak solution of formic or lactic acid, a membrane 

 is formed. The eggs are then taken from the acid, placect in concentrated 

 sea- water for a short time, and then removed to ordinary sea- water. Division 

 rapidly occurs with the production of a normal larva. He suggests that the 

 spermatozoon brings with it ferments, or other chemical substances, which 

 excite the egg- nucleus and cytoplasm in the same way as the chemical 

 measures adopted for this artificial induction of segmentation. 



