dorsal aorta 



hind limb 



B 



Figure 10-33. Three stages in fhe descent of the testes in the pig. (After Nelsen, 1953) 



body-wall musculature before it, the cremaster. The testis 

 now lies between the skin sac of the scrotum and the nearly 

 enclosing processus vaginalis of the coelom. It is anchored to 

 the scrotal wall by a ligament, the gubernaculum testis, 

 and supported from the body cavity by the spermatic cord. 

 This cord is a bundle of connective tissue anchored to the 

 kidney region, and it encloses the ureter and the nerves 

 and blood vessels passing from the body cavity into the scro- 

 tum. The processus vaginalis is closed off late in development 

 from the body cavity at the inguinal canal. 



Although the scrotum is characteristic of the mammal, it 

 does not occur in all nor is the relationship between penis 

 and scrotum the same. In the placental the scrotum lies be- 

 hind the anteriorly directed and exposed penis, while in the 

 marsupial the posteriorly directed, sheathed penis is behind 

 the scrotum. In some mammals (many rodents), the testes 

 descend into the scrotum during the reproductive period, 

 while in others the testes are retained in the body cavity. 

 This last is described by the term testiconda. Some of the 

 placentals appear to have always been testicondate, whereas 

 others may have secondarily returned to this condition. 



The extreme of mammals is observed in the monotremes 

 (Figure 10-34). In these the testes are retained in the body 

 cavity in the primitive position, ventral to the kidney (Fig- 

 ure 10-37 A). The Miillerian ducts are complete although 

 small in size, and the ureters and vas deferens open at the 

 same level dorsal to the opening of the bladder. These open- 

 ings may be on separate tubercles (Echidna) or on the same 

 one (Platypus). The penis is an erectile tissue tube sheathed 

 by the epithelium of the floor of the cloaca. The urinary sinus 

 opens both through the penis and into the cloaca anterior to 

 the penis. The glans is bifurcate at the tip, and the central 

 canal opens through many small tubules on either division of 



this tip. The prongs of the glans are covered with small horny 

 spines. 



The marsupial agrees with the monotreme in lacking 

 seminal vesicles. They are intermediate in that the penis 

 may lie in a pouch in the cloaca (Perameles) or just below 

 the nearly or quite separate anal opening (most marsupials) 

 — the two openings are, however, enclosed by a common 

 sphincter muscle. The penis may be tubular (Perameles and 

 many marsupials) or may have a dorsal groove for passage 

 of the sperm [Didelphu, the opossum). In the opossum the 

 excretory duct is no longer connected with the cloaca; it 

 opens at the base of the grooved penis. In the kangaroo the 

 penis is like that of the placental, tubular and serving both 

 excretory and reproductive products. The glans of the mar- 

 supial penis is usually bifid, but not in Dasycercus. 



The female The female system of the mammal consists of 

 the ovary, the oviducts, the uterus, and the vagina (Figure 

 10-35). The vagina opens directly to the exterior in most 

 mammals between the urethral opening and the anus. In 

 front of the urethral opening is a clitoris, which represents a 

 rudiment of the penis, and to either side are labia represent- 

 ing the folds between the urogenital and anal divisions of 

 the cloaca and the outer margin of the cloacal aperture. 



The ovary in its development differs from the testis in 

 that the medullary cords are poorly developed, while the 

 cortex becomes the dominant tissue in which the germ cells 

 develop. The medullary cords are of mesonephric blastema 

 origin, while the cortex is produced by thickening of the 

 epithelium overlying the genital ridge. The cortex retains a 

 thin overlying germinal epithelium. The large germ cells 

 lie at first in the epithelium but later penetrate the gonad 

 as it differentiates and separates from the mesonephros. 



320 • THE UROGENITAL SYSTEM 



