322 CHORDATE ANATOMY 



Sterility since spermatozoa are unable to survive the normal temperature 

 of the body. The scrotal sac appears to act as a thermoregulator. 



The factors involved in the descensus of the testes are complex. 

 Chief among them appears to be the contraction of the connective-tissue 

 gubemaculum testis which extends from the testis to the posterior wall 

 of the scrotum. The gubernaculum contracts to one quarter of its 

 original length and, after the descensus, almost completely atrophies. 

 During the course of its descent, each testis rotates through an arc of 

 1 80° so that its anterior and posterior ends are reversed. 



Rarely, the ovaries undergo a similar descent into the labia majora. 

 Normally, however, the enlargement and relations of the uterus prevent 

 this migration. 



Reproductive Ducts. The mesonephric or Wolffian ducts are utihzed 

 by the male as reproductive ducts, the ductus deferentes. Mliller's 

 ducts are also developed in both sexes. Each Miiller's duct arises from 

 a longitudinal groove on the lateral side of the mesonephros. The 

 peritoneal epitheUum sinks into the underlying mesenchyma (Fig. 67). 

 Except at its anterior end, where it remains open as the ostium tubae, the 

 groove closes over and grows posteriorly as far as the cloaca into which it 

 acquires an opening. 



Male Reproductive Ducts. In a three-month embryo, the anterior 

 and posterior regions of the mesonephros differ. In the anterior region, 

 which consists of five to twelve renal tubules, the collecting portions 

 of the tubules separate from the excretory portions and acquire connexion 

 with the tubules of the rete testis. In this manner, anterior mesonephric 

 tubules form the ductuli deferentes and serve as outlets for the external 

 products of the testis. The remaining, posterior, portion of the meso- 

 nephros mostly degenerates, remnants persisting as the paradidymis and 

 ductuli aberrantes. The cranial portion of the mesonephric duct increases 

 greatly in length to form the ductus epididymidis. The posterior portion 

 becomes the ductus deferens. 



In the male, Miiller's ducts begin to atrophy in the third month. 

 In the adult, remnants of the anterior and posterior extremities may 

 persist as functionless rudiments. The former is the appendix testis 

 and the latter the uterus masculinus. 



Remnants of the mesonephros persist also in the female. Some 

 anterior mesonephric tubules unite with the rete ovarii to form the 

 epoophoron. The posterior part of the mesonephros becomes the rudi- 

 mentary paroophoron. The functionless remnant of the Wolffian duct 

 in the adult is known as Gartner's duct. 



Female Reproductive Ducts. Miiller's ducts parallel the meso- 

 nephric ducts and open into the urogenital sinus median to them. The 

 position of the primary opening marks the place where later the hymen is 

 located. During the fourth month, the posterior portions of the paired 



