File:Image from page 337 of "Chordate morphology" (1962) (19989513674).jpg

Title: Chordate morphology Identifier: chordatemorpholo00joll Year: 1962 (1960s) Authors: Jollie, Malcolm Subjects: Morphology (Animals); Chordata Publisher: New York, Reinhold Contributing Library: MBLWHOI Library Digitizing Sponsor: MBLWHOI Library

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Text Appearing Before Image: dorsal aorta

Text Appearing After Image: 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

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