213 



The kidneys are developed in three portions. The first 

 formed portion is the anterior pronephros ; it is the functional 

 kidney of the larva, and it degenerates, being succeeded by 

 the mid-kidney or mesonephros (Wolffian body), which is 

 the functional organ of later embryonic life in the Amniota. 

 The mesonephros and the hind-kidney or metanephros 

 together constitute the opisthonephros, which is the func- 

 tional organ of adult Anamnia. In Amniota the meso- 

 nephros atrophies, and the metanephros becomes the 

 permanent kidney of the adult. 



The archinephric ducts, which are at first pronephric 

 ducts, become the segmental or mesonephric (Wolffian) 

 ducts ; and in general these function in the female as 

 ureters only, and in the male as vasa deferentia also. In 

 the female the oviducts are Miillerian ducts, which in 

 Elasmobranchs are split off longitudinally from the meso- 

 nephric ducts. The coelomic funnels of the Miillerian 

 ducts are remnants of the pronephric nephrostomes. 



The metanephric ducts or ureters arise as outgrowths 

 from the "hind ends of the mesonephric ducts. 



In some Craniates the archinephric ducts unite ter- 

 minally, forming a vesicle. This, in Elasmobranchs, be- 

 comes the urino- genital sinus (with an extension from each 

 side, the sperm-sacs) ; and in Teleosteans it forms a urinary 

 bladder. The allantois functions as a urinary bladder in 

 the frog ; and in Mammals that portion of it which remains 

 in the body of the embryo becomes the bladder. 



More or less closely associated with the kidneys are the 

 suprarenal or adrenal bodies (see page 208). Two notable 

 features (indicated above) of the Craniate excretory system 

 are : 



1. The nephridia do not open directly to the exterior, 

 as in Annelids, but indirectly through a duct. 



2. A close connection with the reproductive system. 

 (See also answer to next question.) 



Give a General Account of the Reproductive System of 

 Craniates. 



The germ cells, which form the gonads (testis of male, 

 ovary of female), arise from the germinal epithelium (meso- 

 derm) of the wall of the coelome. (See Part I., page 8.) 



The gonads develop (within the peritoneal cavity) as 



