1328 



THE UKO-GENITAL SYSTEM. 



WD 



the pronephros is a vestigial organ, and its development in all higher vertebrates is 

 very incomplete. It disappears almost as soon as it is formed, and it is replaced by the 

 far more important mesonephros. With the development of the permanent kidney the 

 mesonephros atrophies, yet some of its tubules persist in the adult. The ductuli 



efferentes, the ductuli aberrantes, and the rudimentary 

 paradidymis (organ of Giraldes) in the male, and the 

 rudimentary tubules of the ep-oophoron and of the 

 par-ob'phoron in the female, are structures which owe 

 their origin to the tubules of the mesonephros. 



Soon after the formation of the Wolffian ducts two 

 other longitudinally disposed canals, called the 

 Mullerian ducts, are developed. These open at their 

 cephalic ends into the body cavity, and at their caudal 

 ends, unlike the Wolffian ducts, they unite with one 

 another in the median plane. From them are formed, 

 in the female the uterine tubes, the uterus, and the 

 vagina ; and in the male the appendices of the testis 

 and the utriculus prostaticus. 



The Wolffian and Mullerian ducts open at their 

 caudal ends into the ventral or urogenital part of 

 the cloaca, which in the course of development becomes 

 transformed into the bladder and the uro-genital 

 canal of the embryo. The developing ureter at first 

 arises as a diverticulum from the Wolffian duct, at a 

 short distance from the point where the latter joins 

 the cloaca. Soon, however, the ureters acquire inde- 

 pendent openings into the cloaca, which become gradu- 

 ally shifted further from one another and from those 

 of the Wolffian ducts. The ureters are now found to 

 open into the anterior portion of the cloaca which lies 

 nearer to the head of the embryo than the part with 

 which the Wolffian ducts are connected. This cephalic 

 portion of the anterior subdivision of the cloaca which 

 receives the ureters becomes the bladder and the 

 upper part of the urethra. The caudal part, lying 

 below the level of the entrance of the Wolffian ducts, 

 is called the uro-genital canal, and is represented in 

 the adult male by the lower part of the prostatic and 

 by the membranous portions of the urethra ; in the 

 female by the lower part of the urethra and the part of 

 FIG. 1037,-DiAGRAM TO ILLUSTRATE THE the uro-genital fissure which immediately surrounds 

 MANNER IN WHICH THE URETER, THE the openings of the urethra and vagina (Figs. 1 

 DUCTUS DEPERENS, AND THE URINARY 1045). The united Mullerian ducts open into the lower 

 BLADDER ARISE IN THE EMBRYO. part of the cloaca or ur0 -genital canal between the 



The structures developed from the cloaca Wolffian ducts of opposite sides. In the male the 

 are indicated in blue, those from the position of this opening, which is represented in the 

 Wolffian duct in red, and the ectoderm ^^ , the Qrifice Qf the utriculug prostaticuS| rem ains 



111 DlctCK. *' . i i i i J 



, almost unchanged ; in the female, on the other hand, 



^adTTecle^ird rlTcqute a downgrowthVm the fused Mullerian ducts giv- 

 openings into the ectodermal cloacal origin to a new passage, the vagina, which establis 

 fossa is shown in II. and III. (A. H. an opening behind that of the urethra in the uro- 

 genital fissure of the adult. 



After the complete separation of the cloaca into 

 anterior or uro-genital and posterior or rectal sub- 

 divisions, the rectum establishes a communication with 

 the exterior in the floor of the shallow depression 

 known as the ectodermal cloacal fossa. At a little later 

 time the uro-genital canal also joins this fossa at a point in front of the opening of the 

 rectum. The ectodermal cloacal fossa lies in front of the vestigial tail, and extends 

 forwards as far as a tubercle known as the cloacal tubercle, which later gives rise to the 

 genital eminence and a pair of elevations called the labio-scrotal folds. The genital 

 eminence becomes converted into the clitoris or penis according to the sex. The labio- 

 scrotal folds extending backwards on each side form the labia majora of the female, and, 



III 



CM 



Young and A. Kobinson). 



A. Allantois. R. Rectum. 



B. Bladder. Ur. Uro-genital canal. 



C. Cloaca. U. Ureter. 



CM. Ectoderm of VD. Ductus deferens. 

 cloacal fossa. VS. Seminal vesicle. 

 K. Pelvis of kidney. WD. Wolffian duct. 



