THE EVOLUTION OF SEX 39 



taneous or successive. There remain rare 

 cases, such as some of the parasitic flukes and 

 tapeworms, in which there is self-fertilisation 

 (autogamy), the ova being fertilised by sperma- 

 tozoa from the same individual. The sperms 

 may reach the ova after these have been laid, 

 as in the case of salmon ; or just as they are 

 laid, as in the case of frogs; or within the 

 body of the female, as in all reptiles, birds, 

 and mammals. It is evident, therefore, that 

 the sexual union which leads to " insemina- 

 tion " and which from observation of the 

 higher animals long seemed to be the very 

 essence of the process, is really of later acquire- 

 ment. The gist of fertilisation throughout 

 life remains as the union of two sex-cells, 

 however that may be accomplished. It may 

 be noted that artificial insemination has been 

 successfully effected in some of the higher 

 animals, such as sheep, cows, mares, white 

 mice, and hens. The spermatozoa can be 

 kept for some time in weak salt solutions. 



What does fertilisation imply ? (1) It 

 implies an intimate and orderly combination 

 of the paternal and maternal inheritances, 

 which are probably borne in greater part 

 by the nuclear elements or chromosomes of 

 the spermatozoon and ovum respectively, 

 and in some measure also by the extra- 

 nuclear protoplasm of the two germ-cells. 

 (2) During the maturation of the spermatozoon 

 and ovum, the number of chromosomes (which 

 is always definite for each kind of organism) 

 is reduced to half the normal, so that fertilisa- 

 tion restores the normal number. (3) Besides 

 bearing the paternal contribution to the 



