818 



COMPARATIVE ANATOMY 



shaped much like the ovaries and at first they are at about the same 

 level. The outer surface is smooth, a fibrous envelope, the tunica 

 albuginea, having developed around them, which sends trabeculae in- 

 ^ward, dividing the seminiferous tubules into lobules. Except in the 

 monotremes the testes descend farther into the pelvic cavity, remaining 

 permanently in the pelvis in many insectivores, some edentates, 

 elephants, whales and Hyrax. In other groups they pass outside the 

 pelvic cavity to be enclosed in a special sac, the scrotum. The testes are 

 supported by a cord, the gubernaculum, the homologue of both liga- 

 ments of the ovary. 



"The change in position of ovary and testis is accomplished in part 

 by the unequal growth of body wall and the supporting ligaments. In 

 the case of the male this descent of the testes is complicated. (Fig. 460.) 

 In outline it is as follows : By the unequal growth of gubernaculum and 

 body wall the testes are drawn down into the scrotum which is a pro- 

 truding part of the body wall into which a part of the coelom extends. 

 This wall is formed in part from the genital folds which surround the 

 genital eminence. It lies in front of the penis in the marsupials, behind 

 it in all placental mammals. When the canal connecting the cavity of 

 the scrotum (bursa inguinalis) remains open as it does in marsupials, 

 bats, rodents, insectivores, etc., the descent is temporary, the testes being 

 withdrawn into the peritoneal cavity at the close of the breeding season 

 by the cremaster muscle, developed from the transverse abdominal 

 muscle. In other mammals the descent is permanent, though sometimes 

 it does not occur until the time -of sexual maturity." 



In the monotremes the 

 Miillerian duct is divided into 

 a cephalic portion known as the 

 Fallopian tube and a caudal 

 portion, the uterus, although 

 the line separating these two is 

 not very definite. The broad 

 trumpet-shaped end of the 

 Fallopian tube connects with 

 the coelom while the tube itself 

 secretes the albuminous cover- 

 ing of the eggs. The uterus is more muscular than the Fallopian tube 

 and it is here that the horny shell is formed. The uterus then opens 

 directly into the urogenital sinus to connect the cloaca with the exterior. 

 In other forms of mammals the end of the Mullerian duct between 

 the uterus and the urogenital sinus forms a vagina. In marsupials there 

 are two vaginae and sometimes three. 



When the two caudal ends of the Mullerian ducts fuse as in many 



Fig. 460. 



Descent of the testis ac, abdominal cavity ; g, 

 gubernaculum; pv ; processus vaginalis ; t, testis; , 

 scrotum ; tv, tunica vaginalis ; x, rudiment of pro- 

 cessus vaginalis. 



