298 COMPARATIVE ANATOMY 



is the development of the cochlea. This structure, which in the 

 frog is represented only by a small dilatation of the sacculus (vol. 

 i. fig. 12, pbc.^, is produced in the mammal into a long tubular 

 offset twisted into a spiral of from i|^ to four turns, and lodged 

 in a corresponding spiral chamber of the periotic bone. The 

 epithehum lining the cochlea is differentiated to form a com- 

 plicated sensory organ called the organ of Corti. 



The urogenital organs of mammaha (excepting the Proto- 

 theria) offer many special features. The complete separation 

 of the urogenital from the the rectal aperture has already 

 been noted. The kidneys are formed by the metanephros, 

 and their ducts, the ureters, open into the bladder. In the 

 male the Wolffian ducts become the sperm ducts or vasa 

 deferentia. The upper end of each is thrown into many 

 convolutions, and forms a structure known as the epididymis, 

 which lies close to the testis and is connected with it by 

 the vasa efferentia. The mesonephros is represented by a 

 small mass of coiled tubules, the caput epididymis, lying 

 at the anterior end of the epididymis. The Wolffian ducts 

 open below into the narrow stalk of the bladder, or urethra. 

 The so-called Miillerian duct (which is formed independently 

 of the Wolffian duct, as in the frog, and is not split off from 

 it as in the dogfish) disappears almost entirely in the adult 

 male, but the lower ends persist as a pouch, the uterus 

 masculinus, which opens into the urethra in close connection 

 with the openings of the Wolffian ducts. 



In the female the Wolffian ducts are completely aborted in 

 most mammals, but traces of them are to be found in the struc- 

 tures known as Gaertner's canals in the pig and a few other 

 forms. The mesonephros is represented only by some function- 

 less coiled tubes which bear the same relation to the ovaries as 

 the epididymes bear to the testes. They are known as the 

 parovarium. The Miillerian ducts form the oviducts. The 

 lower ends of the oviducts always coalesce to form a single 

 tube, the vagina, and the upper ends are differentiated into 

 the uteri and the oviducts or Fallopian tubes, the latter open- 

 ing by funnel-shaped mouths into the abdominal division of 

 the coelom in close proximity to the ovaries. In the rabbit the 

 uteri are separate from one another and open independently 

 into the vagina, but in many mammals the right and left uteri 

 are partially or wholly fused together. 



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