lo INTRODUCTION TO SEXUAL PHYSIOLOGY 



and Fordham). This phenomenon, which must facilitate fertilisa- 

 tion, is called Chemotoxis. 



At the beginning of the act of fertilisation the head of the 

 spermatozoon comes in contact with the ovum and then passes 

 inward through the wall of the ovum, so that the latter at first 

 contains two nuclei, its own and the head of the spermatozoon. 

 Eventually the nuclei come together and fuse, as already stated, 

 and the tail of the spermatozoon, which consists of cytoplasm, 

 breaks up and becomes absorbed in the cytoplasm of the ovum. 

 The oosperm or zygote formed in this way is the starting-point 

 of a long series of cell divisions which culminate in the formation 

 of a new completely formed individual resembling the parent 

 individuals from which the ovum and spermatozoon were derived. 



Artificial Fertilisation. — Loeb and others have shown that 

 with the sea-urchin and many species of animals the fertilising 

 action of the spermatozoon can be imitated by physico-chemical 

 methods and artificial fertilisation of the ovum may take place, 

 new individuals being produced in this way without the inter- 

 vention of a spermatozoon. Thus Loeb found that by adding a 

 little of a fatty acid to the sea-water which contained the sea- 

 urchin eggs and then transferring the eggs to another sample of 

 sea- water to which sodium chloride had been added, it is possible 

 to cause the eggs to undergo segmentation or cell division, result- 

 ing in the development of new individuals. A number of other 

 methods which are equally effective have been devised, and upon 

 these theories have been based as to the nature of the reacting 

 substances which are carried into the ovum by the sperm in normal 

 fertilisation. There are many animals, however, in which the 

 ovum alone is capable normally of initiating its own development. 

 This condition, which was discovered by Bonnet in 1795 in the 

 aphid or plant louse, is known as Parthenogenesis, and similarly 

 artificial fertilisation has sometimes been called artificial partheno- 

 genesis. Natural parthenogenesis is often a cyclical or seasonal 

 phenomenon, as in the plant louse, with which the female repro- 

 duces without the intervention of the male throughout the 

 summer, after which males appear and generation takes place 

 sexually. This is one example of an " Alternation of Generations," 

 a succession of asexual generations being followed in the autumn 

 by a sexual one. In such forms as the jelly-fish and the polyp 

 the alternation may be between one sexual and one asexual 



