THE URO-GENITAL SYSTEM 401 



usually wholly subservient to the urinary function, become 

 for the time being wholly reproductive. % 



The marsupials show an intermediate condition by which 

 the transition to the placental mammals can be explained. The 

 cloaca has been divided by a perinaeum and the alimentary and 

 urogenital outlets have become entirely separated. The testes 

 show a marked descensus and usually come to lie in a scrotal 

 sac, which is prepenial in position. The penis is posterior to 

 the testes and is still directed backwards as in monotremes and 

 sauropsids, but becomes attached at its proximal end to the 

 posterior margin of the os pubis. 



The true urethra is very short, as the ductus deferentes enter 

 the tube soon after its origin, but the urogenital tube thus 

 formed is permanently continuous with the lumen of the penis, 

 forming a long urogenital canal. This condition is essentially 

 that found in placental mammals except for the relative posi- 

 tion of the penis, which in the latter animals, retaining its 

 proximal attachment to the lower margin of the os pubis, turns 

 about and becomes directed anteriorly, thus changing its ap- 

 parent relations with the testes, which are now post-penial. 



Connected with the penis are various sorts of glands, em- 

 ployed mainly for the purpose of furnishing a liquid vehicle 

 for the spermatozoa. They are thus the most widely developed 

 in mammals of marked fertility, like rodents and insectivores, 

 and may be arranged in five groups, each associated with a 

 definitive part of the spermatic tract (Fig. 113). The 

 glandules ductus deferentis are thickenings of the wall of the 

 ductus deferens, and are situated near its entrance into the 

 urogenital canal. The glandule? vesicates are large and evi- 

 dent glands, which open near the latter. These have often 

 been considered as receptacles for the spermatic fluid, and are 

 hence usually called seminal vesicles, but they are clearly glan- 

 dular in their nature, and their cavities contain spermatozoa 

 only by accident. Of the remaining three, which open into 

 the urogenital canal, the primitive condition is seen in the 

 urethral glands, tubular glands occurring in the walls of 

 the above canal, especially along its proximal portion. From 



