REPRODUCTION AND SEX 483 



Similarly, pricking the ovum of a frog or toad with a platinum needle 

 and the entrance of several blood corpuscles may serve to activate, 

 while the return to the normal medium may serve as the indis- 

 pensable corrective to disintegration. 



One must not conclude that the rdle of the complex living sperma- 

 tozoon is exhaustively replaced by the chemico-physical agencies 

 referred to, for normal fertilisation implies more than activation 

 and a regulation of the subsequent cleavage. It implies a mingling 

 of the heritable qualities of the two parents. What the experiments 

 show is that the ovum is quite complete in itself, that certain 

 factors involved in what the spermatozoon effects may be artificially 

 mimicked, and that perfectly normal larvae may be reared from 

 various unfertilised eggs which are not known ever to develop 

 parthenogenetically in natural conditions. The remarkable facts 

 that have come to light since 1899 show that one cannot set limits 

 to the possibility of the occurrence of parthenogenesis. Some of the 

 experimental conditions which are effective in inducing partheno- 

 genetic development might find a parallel in natural conditions. 

 As yet, no instance of either artificial or natural parthenogenesis 

 has been observed in the animal kingdom above the level of 

 Amphibians. 



SEX-DIMORPHISM 



Since the beginning of the present century the difficult problem of 

 the origin, evolution and development of sex-characters has been 

 illumined by a series of brilliant experimental researches, which 

 have made reconsideration imperative. This revision has been 

 facilitated by the masterly work of Kammerer (Ur sprung der 

 Geschlechtsuntersckiede, 1912), who gathered together the data and 

 submitted them to an analysis, at once fair-minded and critical. 

 We shall state his conclusions and indicate where we differ from 

 them. 



It is usual to classify the differences between the sexes as 

 "primary" and "secondary". The "primary" differences refer to 

 the reproductive organs or gonads, the "secondary" to those that 

 appear in other parts of the body, such as the larynx or the hair. 

 It is clearer to follow Poll, Kammerer, and others in recognising 

 {a) essential or gonadial differences which must be present if there 

 are sexes at all — the differences between ovaries and spermaries; 

 and (6) accessory differences which may or may not be present, 

 some of them subsidiary to the reproductive organs, either internally 

 or externally, and others affecting extra-genital parts of the body. 

 Our scheme of classification, slightly modified from Poll's and 

 Kammerer's, may be thus expressed : 



