EXTERNAL SEXUAL ORGANS. 421 



the groin-canal coalesce. When, however, this remains 

 open, the testes are able to descend periodically (in the rutting 

 season) into the testis-sac, returning again into the abdo- 

 minal cavity (e.g., in Pouched Animals or Marsupialia, 

 Gnawing Animals, Bats, etc.). 



Another peculiarity of Mammals is the formation of the 

 external sexual organs which, as copulative organs, serve 

 to cany the fertilizing sperm from the male into the 

 female organism in the act of copulation. Organs of this 

 sort are altogether wanting in most lower Vertebrates. In 

 those which are aquatic (e.g., Acrania, Cyclostoma, and most 

 Fishes) the eggs and sperm are simply discharged into the 

 water, and their coming together is the result of some lucky 

 accident which in this way brings about impregnation. On 

 the other hand, in many Fishes and Amphibia which bring 

 forth their young alive, there is a direct transfer of the 

 sperm from the male to the female organism ; and this is 

 the case in all Amniota (Reptiles, Birds, and Mammals). In 

 these animals the urinary and genital organs always open 

 originally into the lower part of the rectum, which thus 

 forms a " cloaca " (p. 345) ; but among Mammals the cloaca 

 is permanent only in the Beaked Animals (Ornithostoma), 

 which have, on this account, been called Cloacal Animals 

 (Monotrema, Fig. 327, cl). In all other Mammals a lateral 

 partition wall develops in the cloaca (in the human embryo 

 about the middle of the third month), by which the latter 

 is separated into two cavities. The urinary sexual canal 

 passes into the anterior cavity (sinus urogenitalis), and it 

 .is through this cavity alone that the urinary and sexual 

 products are discharged, while the " anal cavity," which lies 

 behind it, serves merely to eject the excrement through the 



