CHAPTER I 

 THE DIFFERENTIATION OF THE SEXES 



The sexes are distinguished primarily by the presence of ovary 

 or testis, i.e. by the power to produce ova or spermatozoa. This 

 production of germ cells is the essential function of the gonads, 

 and in lower animals, where the reproductive processes are 

 of the simplest, their only function. During the course of evolu- 

 tion, however, the tendency has been to limit the number of ova 

 produced, and to provide each embryo with a greater degree of 

 parental care. This tendency has culminated in the prolonged 

 period of internal gestation found in mammals, and has 

 resulted in the appearance of many organs and characters 

 designed to facilitate reproduction. Coincidently, a second 

 function of the gonad has appeared, the control of the develop- 

 ment of these accessory structures. These organs have reached 

 a higher degree of elaboration in the female, upon whom the 

 care of the young largely falls in most species. 



In addition to the accessory organs, there are found secondary 

 sexual characters, which, though of no direct use in the repro- 

 ductive processes, are nevertheless valuable or necessary in 

 dioecious propagation. 



It is thus possible to distinguish three types of sexual 

 differentiation : 



(a) The gonad — ovary or testis. 



[h) The accessory reproductive organs. 



[c) The secondary characters. 



The accessory reproductive organs in the male are all designed 

 to convey the spermatozoa in a suitable medium to the exterior 

 and thence into the female genitalia. The accessory organs of 

 the female mammal (individually described in Chapter II) are 

 adapted for reception of the spermatozoa, gestation of the 



P.S.O. A 



