SECONDARY SEXUAL CHARACTERS 125 



special genital ducts or gonoducts (g.d.) are developed which 

 pierce the body wall and serve for the passage of the germ cells 

 to the exterior. The sexes are usually distinct in the higher 

 forms (Vertebrata), but many of the invertebrate coelomates 

 (e.g., the earthworm) are hermaphrodite, the same individual 

 bearing both male and female gonads (testes and ovaries) with 

 the corresponding gonoducts (vasa deferentia and oviducts). 



Fertilization of the ova by the spermatozoa, or in other words 

 conjugation of the gametes, may take place either within the 

 body of the parent, as in most terrestrial forms, or externally, as 

 in a very large proportion of aquatic animals. In the former case 

 special organs are developed for the transference of the sperma- 

 tozoa from one individual to another, and such transference 

 usually occurs even in hermaphrodite forms, which, as a rule, are 

 incapable of self-fertilization. Further modifications may arise 

 in connection with the nutrition of the embryo, which may 

 remain within the body of the parent in an enlarged portion of 

 the oviduct known as the uterus until it has reached an 

 advanced stage of development. This takes place more par- 

 ticularly in the females of the higher vertebrates. 



In connection with the sexual differentiation, more 

 especially in the higher animals', numerous secondary sexual \ 

 characters may arise which are not directly connected with the ^ 

 organs of reproduction. Such are the various ornamental out- 

 growths of hair, feathers and so forth, which distinguish the males 

 of many vertebrates and are supposed to appeal to the aesthetic 

 sense and thus to contribute towards the mutual attraction 

 between male and female, and the special weapons, such as 

 antlers and spurs, which male animals frequently develop and. 

 which are used in combat for the possession of the females. 



Although not directly connected with the gonads these 

 secondary sexual characters seem to depend for their development 

 in some curious way upon the presence of these organs. Thus 

 it is well known that if the testes be removed by castration the 

 secondary sexual characters will not, in most cases at any rate, 

 develop properly. We see an excellent illustration of this in the 

 case of the antlers of the stag, which are confined to the male and 

 do not develop at all if the animal be castrated in early youth, 

 while if the operation be performed after the antlers are fully 

 developed these are prematurely cast off and replaced by 

 imperfect ones. 



