GENITAL ORGANS 



483 



tremes, most Edentates, Hyrax, Elephant), the testes are retained 

 within the abdomen. 



Originally, the descent of the testes did not occur until sexual 

 maturity in all cases, but in many Mammals (e.g. Marsupials, 

 Ungulates, Carnivora, Primates) the process has gradually become 

 shifted backwards ontogenetically to earlier periods, so that the 

 formation of the scrotum takes place independently in the embryo 

 in the form of the external genital folds, from which also, in the 

 human female, arise the " labia majora " of the vulva. 1 



The testes are smooth, and somewhat oval in form ; their 

 relative size varies in different Mammals, and they mav become 

 periodically enlarged at the breeding season (e.g. Rocfents and 

 Insectivores) ; they are covered by a fibrous investment (Fig. 363), 



FIG. 363. DIAGRAMMATIC SECTION OF THE TESTIS OF A MAMMAL. 



A, tunica albuginea of the testis, which gives rise to the trabeculse (t, t) and the 

 corpus Highmori (t) ; Cv, coni vasculosi, which are connected together by 

 the collecting duct, Vep ; Ho, testis ; L, L, coils of the seminal tubules ; NH, 

 epididymis ; Va, vas aberrans ; Vd, vas deferens ; Ve, vasa efferentia (rete 

 testis). 



from which processes (trabeculse) usually extend inwards. Thus 

 the seminal tubules become separated into definite, lobed masses, 

 and a sort of lattice-work is also formed (corpus Highmori) by 

 means of which the elements of the vasa efferentia pass to the 

 epididymis. In the latter the seminal tubules become rounded off 

 to form the so-called coni vasculosi, and these are connected together 

 by a collecting duct, the vas epididymitis. The vas deferens arises 

 from the last conus vasculosus, and gives rise towards its distal end, 

 shortly before it opens into the urinogenital sinus close to an 

 elevation, the colliculus seminalis, to glandular outgrowths (vesiculce 

 seminales), which may attain a relatively enormous size (Fig. 364). 



1 " Labia majora" also occur in certain other Primates, but in most Monkeys 

 " labia minora" are alone present bounding the vulva, and these belong morpho- 

 logically to the clitoris and not to the scrota! folds. 



I i 2 



