Mammalia: Visceral Systems 



643 



nialian kidneys are not "kidney-shaped." The ureter, the renal blood- 

 vessels, and the nerves (of the autonomic system) join the kidney at 

 the indentation or hilum, which is always on the medial side of the 

 organ. 



All mammals have a urinary bladder. It develops as a midventral 

 outgrowth from the endodermal wall of the cloaca and in close relation 

 to the allantois, whose proximal region may to some extent become 

 incorporated into the wall of the bladder. The ureters at first open into 

 the cloaca. Later, as the bladder becomes expanded and the cloaca is 



Fig. 188. Lobulated kid- 

 ney (metanephros) of otter, 

 Lutra canadensis (Princeton, 

 2234). (a) Aorta; (u) ureter; 

 (v) postcava. (Courtesy, 

 Kingsley: "Comparative 

 Anatomy of Vertebrates," 

 Philadelphia, The Blakiston 

 Company.) 



reduced, the posterior connections of the ureters are shifted to the 

 dorsal surface of the bladder. Exceptional in this respect are the 

 Australian duckbill and spiny anteater which, in the adult stage, 

 retain the embryonic cloaca and the cloacal termination of the ureters. 



Ovaries and Mullerian Ducts 



The ovaries in the mammalian embryo are at first anterior to the 

 metanephric kidneys but, in consequence of later unequal longitudinal 

 growth of the body-wall and the supporting membranes and ligaments 

 of the viscera, the adult position of the ovaries comes to be usually 

 more or less posterior to the kidneys. The right and left ovaries, not 

 always symmetrically placed, are of approximately the same size except 

 in monotremes, whose left ovary is much larger than the right. 



In placental reproduction, characteristic of most mammals, the 

 developing embryo obtains its necessary food by way of the placenta. 

 The egg contains a minimum of stored food and accordingly is of 

 microscopic size. In contrast to anamniotes, in which an ovary may 



