280 



ZOOLOGY. 



Its exact origin, and that of its duct (the ureter) have 

 never been fully traced out, but they appear to be derived 

 from the posterior part of the mesonephros and its duct. 

 As the metanephros develops, the mesonephros aborts in 

 the female, but in the male it becomes connected with the 

 testis and forms the epididymis. 



It will be useful at this point to tabulate the facts about 

 the excretory system of our Vertebrate types (see p. 271). V j 



In the rabbit only, an important change takes place in 

 the position of the apertures of the ducts (fig. 147). Primi- 

 tively they open into the cloaca on its dorsal side, as they 



rksr/s 



! 



Fig. 147. UROGENITAL ORGANS OF MALE BABBIT. 

 A, embryo. B, adult, showing shifting of parts. 



do permanently in lower types. Afterwards they shift their 

 positions round the sides of the cloaca, and the metanephric 

 duct shifts further still and opens into the base of the 

 allantois (or urinary bladder). 



The ventral portion of the cloaca, into which the 

 allantois and (at this stage) the Wolffian and Miillerian 

 ducts open, becomes separated off as the urethra. A further 

 complication is caused by the shifting of the gonads from 

 being more median than the kidneys to being more lateral. 

 This shifting involves the Miillerian duct (oviduct) in the 

 female, and the Wolffian duct (vas deferens) in the male : 

 the shifting is much greater in the male, where the testes 

 pass into the scrotal sacs. 



