142 ESSENTIALS OP ZOOLOGY 



carries excess nitrogenous materials and must be eliminated. These 

 wastes are principally in the form of urea, uric acid, and creatinin 

 in water solution. The passage of urine from the blood to the 

 uriniferous tubules is a complex process combining secretory activ- 

 ity and osmosis. 



The kidneys of the rat, all other mammals, birds, and reptiles be- 

 long to the type known as metanephros or true kidney. Fishes and 

 amphibians in adult form have a different kidney structure, called 

 mesonephros. The higher vertebrates possess this as an embryonic 

 structure also. 



Reproductive System 



In the early stages of the development of the individual the organs 

 of sex show no indication as to its future sex. The gonads and ex- 

 ternal genitalia of both sexes are similar up to a certain point, and 

 both sets of accessory tubules develop in each individual. At a 

 certain stage in the development of the individual the sex becomes 

 established. The gonads take on characteristics of either ovaries or 

 testes, one set of accessory ducts is accelerated in development while 

 the other is inhibited, and there is a modification in the developing 

 external genitalia in keeping with the sex to be. The manner in 

 which this differentiation occurs is shown in the following table : 



INDIFFERENT EMBRYONIC 

 CONDITION 



MATURE MALE MATURE FEMALE 



Primitive genital gland Testis Ovary 



Mesonephros Epididymis Epoophoron 



Paradidymis Paroophoron 



Mesonephric duct Vas Deferens Disappears 



Miillerian duct Vestigial appendix of Oviduct, uterus, and 



testis vagina 



The principal function of this system is the production of germ 

 cells (sex cells or gametes). Female germ cells are produced in 

 the ovaries and are called ova. The male germ cells are produced 

 in the testes and are called spermatozoa. 



The Female Reproductive Organs. — Included under this title are 

 external genitalia, two ovaries, pair of oviducts (Fallopian tubes), 

 uterus, and vagina. The external structures are in the perineal 

 region just ventral to the anus. The diminutive clitoris is a fold of 

 tissue here which is terminated by the glans clitoris. The clitoris 



