1428 



THE URINOGENITAL ORGANS 



Ureter. 



Wolffian duct. 

 Mullfrian duct. 

 Bladder 



cloaca which ultimately becomes the bladder (Fig. 1126). The manner in which this separation 

 is brought about is not fully known. 1 



The secretory tubules of the kidney become arranged into pyramidal masses or lobules, and 

 the lobulated condition of the kidneys exists for some time after birth, while traces of it may be 

 found even in the adult. The kidney of the ox and many other animals remains lobulated 

 throughout life. 



The Urethra. In the female the urethra is formed from the upper part of the urogemtal 

 sinus viz., that part which lies above the openings of the Wolffian and Miillerian ducts. The 

 portion of the sinus below these openings becomes gradually shortened, and it is ultimately opened 

 out to form the vestibule, and in this manner the urethra and vagina come to open separately on 

 the surface. Wood-Jones regards the female urethra as "the cloacal remnant in its simplest 

 form," and points out that " it does not remain tubular throughout fetal life, but is for a jime 

 obliterated more or less completely by the proliferation of the vaginal bulbs." Developmerifally 

 considered, the male urethra consists of two parts (1) the prostatic and membranous portions, 

 which are derived from the urogenital sinus, and correspond to the whole of the female urethra; 

 (2) the penile portion, which is formed by the fusion of the inner genital folds. 



The prostate gland originally consists of two separate portions, each of which arises as a 

 series of diverticular buds from the epithelial lining of the urogenital sinus, between the third and 

 fourth months. These buds become tubular, and form the glandular substance of the two 



lobes, which ultimately meet and fuse behind 

 the urethra and also extend on to its ventral 

 aspect. The third or middle lobe is formed 

 as an extension of the lateral lobes between 

 the common ejaculatory ducts and the blad- 

 der. Skene's ducts in the female urethra 

 are regarded as the homologues of the pros- 

 tatic glands. 



The glands of Cowper in the male, and 

 of Bartholin in the female, also arise as 

 diverticula from the epithelial lining of the 

 urogenital sinus. 



The Urinary Bladder. The trigone of 

 the bladder is formed from the upper part 

 of the urogenital sinus; the remainder of the 

 viscus is developed from the part of the 

 cloaca which lies above the sinus (Fig. 1187). 

 The bladder is at first tubular in shape, its 

 canal being continuous with that of the allan- 

 tois, but after the second month its cavity 

 expands to form a sac, from the summit of 

 which the tube of the allantois extends to 



the umbilicus; this tube undergoes obliteration to form the fibrous cord of the urachus. In some 

 cases the allantoic canal remains patent, and urine may escape by it at the umbilicus. If the 

 urethra be looked upon as the remnant of the cloaca, then the bladder, with the exception of 

 the trigone, must be regarded as being developed by a dilatation of the proximal part of the 

 allantois. 



The external organs of generation (Fig. 1189), like the internal, pass during development 

 through an indifferent stage in which there is no distinction of sex. It is therefore necessary to 

 describe this stage, and then follow the development of the female and male organs, 

 respectively. 



The cloacal membrane, which is composed of ectoderm and entoderm, originally extends 

 from the umbilicus to the tail. The mesoderm around the cloacal chamber gradually 

 extends between the layers of the membrane, stopping short, however, around the margins of the 

 entodermal cloaca, so that the bilaminar cloacal membrane is limited to this part. About the 

 fifth week a prominence, the genital tubercle, arises in front of the cloacal membrane, while at the 

 sides the edges of the mesoderm are elevated to form the labioscrotal or outer genital folds. 



Along the under surface of the genital tubercle the ectoderm is thickened, and at the apex of 

 the tubercle projects forward as an epithelial horn. In this ectodermal thickening a longitudinal 



1 The separation of the uterus from the Wolffian duct may be brought about by the absorption of the hinder 

 end of the latter into the genitourinary chamber, and by the growth of the wall of this chamber between the 

 openings. Robinson (Proceedings of the Anatomical Society of Great Britain and Ireland, May, 1903, p. 63) 

 states, regarding an embryo of about seven weeks, that "from the posterior or lower opening of the Wolffian 

 duct a grooved ridge, the Wolffian ledge, runs caudally on the wall of the genitourinary chamber and gradually 

 disappears at the junction of the Wolffian angle with the body of the chamber. The lateral margins of the 

 groove are continuous anteriorly with the lateral margins of the Wolffian duct, and apparently fuse together 

 to form the ventral wall of the lower part of the duct. . . . Obviously, if the lateral margins of the groove 

 were to fuse from before backward, the aperture of the Wolffian duct would be carried farther backward in the 

 chamber, and its distance from the opening of the ureter increased." 



Glans penis. 



Urethra. 



Vertebral column. 



FIG. 1188. Tail end of human embryo, from eight and a 

 half to nine weeks old. (From model by Keibel.) 



