THE URO-GENITAL SYSTEM 393 



tached to the epididymis; the posterior by a median vesicle 

 which leads from the dorsal wall of the urethra and lies im- 

 bedded in the prostate gland, the prostatic vesicle, sometimes 

 referred to as the uterus masculinus. 



The history of the urogenital organs, as thus far consid- 

 ered, with their correspondence in the two sexes, may be con- 

 veniently shown in a table, in which the first column gives the 

 part in its primary morphological significance, while the second 

 and third state their ultimate fate in the male and female 

 Amniota respectively. Vestigeal parts are given in italics. 

 This table may be studied in connection with the one at the 

 end of the chapter, in which the external parts are considered 

 in the same way. 



In the monotremes the ductus deferentes [vasa deferentia] 

 open into a urogenital sinus, the ventral recess of a common 

 cloaca, in common with the urethra or excurrent duct of the 

 urinary bladder; in all higher mammals, however, with the 

 formation of a perinaeum or division between this uro-genital 

 sinus and the rectum, the ductus deferentes are received by 

 the much prolonged urethra so that the distal portion of this 

 is a common duct for both urinary and reproductive products, 

 a resumption of early conditions under another form. 



While in the monotremes and in certain placental mammals 

 the testes remain throughout life in or near the original posi- 

 tion, in others they experience a more or less marked change 

 of location. This is termed the descensus testiculorum, and 

 is more or less comparable to a similar descent on the part of 

 the ovaries, although the procedure involves different parts, 

 and is quite likely of a different historical significance. Aside 

 from the monotremes, no appreciable descent takes place in 

 elephants and in certain insectivores, while in sloths and ant- 

 eaters the testes descend considerably and take up a final posi- 

 tion in peritoneal folds between bladder and rectum, but still 

 within the pelvic cavity. The only other placental mammals 

 in which there is no external manifestation of this process are 

 the armadillos, related to these last, and the two aquatic or- 

 ders of Cetacea and Sirenia, in which the condition is plainly a 



