UROGENITAL SYSTEM 



817 



tinues, retractor muscles are developed which draw the organ back into 

 pockets where they are retained at all times except when used for copu- 

 lation. The simplest form of the copulatory organ is produced by a 

 thickening of the ventral wall of the cloaca. There is a longitudinal 

 groove formed in the upper surface of this through which the sperm may 

 pass. It may be divided into right and left halves, the tip of which 

 forms the glans penis. The homologous structure is the clitoris which 

 forms in the female though all parts but the glans are lacking. 



In the mammals, while there are two pronephric tubules outlined in 

 the embryo, they never are functional and the pronephros degenerates. 

 The mesonephros, however, is definitely used during foetal life, and in 

 the marsupials and monotremes it even functions sometimes after birth. 

 However, in all forms of mammals it disappears in time, with the ex- 

 ception of the efferent ductules of the testes and a few remnants in both 

 sexes. The metanephros, which becomes the permanent kidney, has 

 several lobes in the early stages. A definite lobe is formed for each 

 end branch of the ureter so that each lobule has its own duct. This con- 

 dition is retained in "adult elephants, some ungulates, carnivores and 

 primates, and especially in the aquatic species (whales, seals), the lobules 

 being most numerous (200-|-) in some whales. In all other species the 

 ducts fuse and the lobules unite later into a compact mass lying in the 

 lumbar region near the last rib." These lobules are the cause of the 

 cortex and medulla of the kidneys forming two series of interlocking 

 pyramids. (Fig. 459.) 



In the early embryonic stages the 

 gonads lie cephalad to the kidneys. 



The ovaries are usually equally de- 

 veloped in the mammals, except in the 

 monotremes where the left is the larger. "It 

 is of interest that eggs one in the Echidna, 

 two in Ornithorhynchus have been found 

 only in the left oviduct." The ovaries, con- 

 trary to the testes, always remain in the 

 body, and in the monotremes retain their 

 early position. "They are supported by the 

 mesovaria which are attached to the median 

 side of the double fold of the .peritoneum 

 which supports the mesonephros. When the 

 Wolffian body degenerates, the fold becomes 

 the broad ligament while another fold con- 

 tinues down the Mullerian duct as the 

 ligament of the ovary. In some mammals 

 the ovaries have, in addition, a special fold 

 of the peritoneum, which in the rats and 

 mice encloses the ovary and the ostium 

 tubae connected with its opening." 



. "The testes are relatively small and are 



Fig. 459. 



Longitudinal section through 

 kidney. 1, cortex, 1', medullary 

 rays; 1", labyrinth; 2, medulla; 2', 

 papillary portion of medulla ; 2", 

 boundary of medulla ; 3, transverse 

 section of tubules in the boundary 

 layer ; 4, fat of renal sinus ; 5, 

 artery ; transverse medullary rays ; 

 A, branch of renal artery ; C, 

 renal calyx ; U, ureter. The pyra- 

 mids are located between the fat 

 portions and form the papillae. 

 (From Hill after Tyson and 

 Henle. ) 



