206 FOUNDATIONS OF BIOLOGY 



When fertilization occurs within the body, the egg may 

 soon pass to the exterior, usually after being wrapped up 

 in nutritive and protective coats secreted about it during its 

 passage down the oviduct. Or, as is the case sporadically 

 among lower forms and the rule among the highest Verte- 

 brates, the Mammals, the egg on reaching the lower part 

 of the oviduct may become attached to the wall of an en- 

 largement of the oviduct, or of a chamber formed by the 

 union of the two oviducts, called the UTERUS. Here the 

 embryo derives nourishment from the maternal blood sup- 

 ply, and proceeds far along in development before it is ex- 

 pelled to the exterior, or born. (Fig. 113.) 



Thus, except in the simplest animals, there is a special 

 REPRODUCTIVE SYSTEM; a series of organs connected with 

 the reproductive function. But it must be emphasized 

 that the essential organs are the gonads themselves and all 

 the rest are accessory. Furthermore, in relation to the sexual 

 differentiation of male and female individuals, many so-called 

 SECONDARY SEXUAL CHARACTERS arise which are not directly 

 connected with the reproductive organs, but nevertheless 

 depend very largely for their development upon hormones 

 liberated by the gonads. For .example, early castration of 

 the Stag inhibits the growth of a distinctive male secondary 

 sexual character, the antlers; while if performed later when 

 the antlers are full grown, they are shed and abnormal ones 

 take their place. Similarly, the development and functioning 

 of the mammary glands during pregnancy in the human fe- 

 male is induced by hormones produced, not by the ovary it- 

 self, but by its product, the developing embryo within the 

 uterus. Here at least two hormones are involved; one 

 directly stimulates the development of the glands, while 

 another inhibits their active functioning until it is removed 

 by the birth of the offspring. (See p. 181.) 



