338 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



fold continues down the genital ducts forming the ligament of the 

 ovary or testis. In the male broad ligament and ligamentum testis 

 together form the gubernaculum. Unequal growth of body and 

 these ligaments draws the gonads (except in the monotremes) farther 

 back into the pelvic region. 



There is some variation in the ovaries. In the monotremes the left is larger 

 (cf. birds) and it is interesting to note that eggs have been found only in the left 

 oviduct. There is also some variation in shape in the marsupials. Elsewhere 

 the ovaries are relatively small (sometimes increasing in size at the breeding season), 

 rounded or oval and with the surface smooth or furrowed. 



In male whales, elephants, some edentates, etc., the testes remain 

 permanently in the abdominal cavity. In all others a descent of the 

 testes occurs. By the same relative difference of growth of body 

 and gubernaculum the testes are drawn out of the abdomen into a 

 pouch (scrotum) really a part of the body wall into which a part 

 of the coelom (bursa inguinalis) extends. The wall of this is formed 

 in part from the genital folds (see copulatory organs) which surround 

 the genital prominence. This scrotum is in front of the penis in the 

 marsupials, behind it in all placentals. When the canal connecting 

 the cavity of the bursa with the rest of the ccelom remains open (mar- 

 supials, insectivores, rodents, bats, etc.) the descent is temporary, the 

 testes being withdrawn into the ccelom at the close of the breeding 

 season by a 'cremaster muscle.' In other mammals the descent 

 is permanent, though in some species it does not occur until the time 

 of sexual maturity. 



In the oviducts (Miillerian ducts) two regions can be recognized 

 in monotremes (figs. 338, 339, A), three in all other forms. The two 

 are the Fallopian tube, which opens into the body cavity by a broad, 

 fringed ostium tubae, and second the uterus, in which the egg is retained 

 for a part of its development. In the other mammals Fallopian tube 

 and uterus are retained, the latter being specialized for the longer 

 development of the young, and the third region is added the vagina, 

 which receives the copulatory organ of the male. The vagina opens 

 into the urogenital sinus (fig. 339, B), but in the monotremes the 

 vagina is lacking and the uterus and the sinus are directly connected. 

 In the marsupials a vagina is developed for each Miillerian duct, and 

 in some there is a peculiar fusion of the ducts distal to the vaginae so 

 that a caecal pocket results, and in a few this pocket also connects with 

 the urogenital sinus, thus forming a third vagina (fig. 339, B). 



