THE UROGENITAL SYSTEM 



411 



and the abdominal pores open posteriorly into the urogenital sinus and 

 to the outside through a urogenital papilla. 



Elasmobranchs. The gonads of elasmobranchs are paired, and in 

 some species greatly elongated, especially in the male. The testes 

 acquire secondary connexions with the anterior part of the mesonephros, 

 and use the mesonephric ducts as an outlet for the sperm. Elasmobranchs 

 retain abdominal pores, but they appear to be functionless. In this 

 group, the primitive pronephric duct splits longitudinally to form Wolffian 

 and Muellerian ducts. In the female the Wolffian ducts are purely 



OVARIAN OVUM 



DEGENERATING 

 OVUM 



.-fby®5^j^0 



SPERMATOZOA 



INTERSTITIAL TISSUE 



HISTOGENESIS OF TESTIS - RANA. 

 PORTION OF SECTION C-C MUCH ENLARGED. 



Fig. 338. — An enlarged portion of a section of the posterior lobule shown in 

 337, A. The ovarian sacs are being changed into crypts containing spermatozoa, 

 degenerating sac on the left spermatozoa have already made their appearance. 



Fig. 

 In a 



excretory, while the Muellerian ducts form the oviducts. In the male, 

 the Wolffian ducts combine both urinary and reproductive functions, while 

 Mueller's ducts atrophy to form the uterus masculinus. Many elasmo- 

 branchs are viviparous, and the eggs are retained until hatched within 

 uterus-like enlargements of the oviducts. In some species, the embryonic 

 yolk-sac, which is richly supplied with blood-vessels, unites with the wall 

 of the oviduct to form a yolk-sac placenta. 



Amphibia. The gonads of amphibia resemble those of elasmobranchs. 

 As in the latter, the testis becomes connected by means of vasa efferentia 

 with anterior tubules of the mesonephros, and this part of the mesonephros 

 tends to atrophy and lose its excretory function. The Wolffian duct 

 serves as a urogenital outlet in the male. The Muellerian ducts, which 

 persist as rudiments in the male, become the oviducts of the female. 



