MAMMALS. 371 



the lungs, and a peritonial cavity containing the other viscera 

 (see p. 106). This diaphragm, while foreshadowed in some 

 sauropsida, is only developed in the mammals. 



The functional kidneys (metanephridia, p. 122) are small 

 compact organs, only occasionally, as in some seals and whales, 

 showing lobulations. The ureters leading from them empty 

 into the dorsal part of the urinary bladder. The urethra lead- 

 ing from the bladder either empties into the cloaca (mono- 

 tremes) or into the urogenital sinus. No renal portal system 

 occurs. 



In the monotremes and whales the testes remain in their 

 primitive position near the kidneys, but in all other mammals 

 they sink into the pelvis. In the elephants they do not proceed 

 farther ; in the rodents, bats, and some insectivora, they emerge 

 during the breeding-season into a temporary sac or scrotum, 

 and after this time is passed are retracted again by a cremaster 

 muscle. In the other mammals the testes remain permanently 

 in the scrotum after their descent, and the opening through 

 which they descended closes. The cremaster muscle persists, 

 but with more limited functions. Closely connected with the 

 male genitalia are the prostate and Cowper's glands, the ducts 

 of which empty into the genital duct (vas deferens), the secre- 

 tion being added to the spermatozoa, rendering the whole more 

 fluid. 



The ovaries are relatively small, and are always abdominal 

 in position. The oviducts have their inner ends wide, the inter- 

 nal apertures being usually fimbriate. In each duct three re- 

 gions occur, (i) a somewhat narrow Fallopian tube leading to 

 (2) a uterus with muscular walls, and (3) an external canal or 

 vagina. In the lower mammals the ducts of the two sides may 

 remain distinct, but in the higher fusion begins at the lower 

 end, resulting in a single vagina and a uterus, which shows 

 more or less clearly traces of its double origin. 



In the monotremes the large eggs, covered by a flexible 

 calcareous shell, pass to the exterior, but in all other mammals 

 the embryo passes through a considerable portion of its devel- 

 opment in the uterus, and is brought into the world in a more 

 or less perfect condition. 



