1336 



THE UKO-GENITAL SYSTEM. 



from the tail to the umbilical cord. At its cephalic end is a tubercle known as the 

 cloacal tubercle, and behind lies the coccygeal tubercle. Immediately in front of the latter 

 the anus is formed, and between this opening and the cloacal tubercle the uro-genital canal 

 opens on the surface by a median slit-like aperture, the primitive uro-genital opening. 



The cloacal tubercle early becomes subdivided into an apical genital eminence which 

 occupies the middle line and lies at the cephalic end of the slit-like uro-genital opening, 

 and a basal portion which lies nearer to the umbilicus and also curves round the sides of 

 the genital eminence. At a later time the basal part is continued to form a prominent 

 fold on each side of the ectodermal cloacal fossa. These folds are called the labio-scrotal 

 folds and, in' the female, give rise to the labia majora. The lateral margins of the primitive 

 uro-genital opening give origin to the labia minora, and the genital eminence becomes 

 the clitoris. On the clitoris at a very early date a relatively large glans is marked off by 

 a surrounding sulcus. In the male the scrotal folds grow backwards, and meeting behind 

 the primitive uro-genital opening, fuse together. In this way the opening is pushed 

 forwards. The genital eminence elongates rapidly owing to a growth at its basal part, and 



FIG. 1048. EXTERNAL GENITAL 

 ORGANS, HUMAN EMBRYOS. 

 Indifferent stage. A. Embryo 

 of 20 mm. B. Slightly larger. 



The genital eminence and the 

 labio-scrotal folds are well 



FIG. 1049. EXTERNAL GENITAL 



ORGANS, MALE EMBRYO. 

 Formation of scrotum. The labio- 

 scrotal folds, formerly best 

 marked at the sides of the genital 

 eminence, have grown backwards 

 and united behind the primitive 

 uro-genital opening, to form the 

 raphe scroti. The genital folds 

 embrace the base of the genital 

 eminence or penis. The glans is 

 very prominent. 



FIG. 1050. EXTERNAL GENITAL 

 ORGANS, MALE EMBRYO. 



Behind the glans penis the urethra 

 opens in a diamond-shaped fossa 

 in the posterior wall of which 

 the median raphe ends. The pre- 

 puce is formed behind the con- 

 striction which marks off the 

 glans, and as it grows forwards 

 the constriction disappears. 



A little horn-like process of epi- 

 thelium is present on the summit 

 of the genital eminence. 



a sulcus which is formed on its cloacal aspect gradually becomes converted into a canal 

 by the closure of the lateral lips of the primitive uro-genital opening. Soon the uro- 

 genital opening is found to lie nearer the apex than the base of the eminence, which has 

 now given rise to the penis. For some time the opening in the male lies at the base of 

 the glans penis and is somewhat rhomboidal in outline. At a later time, owing to the 

 breaking down of a dense septum of epithelial cells which appears within the glans, a sulcus 

 and finally a canal arises within this part of the penis and thus the terminal part of the 

 urethra is formed. When the opening at the base of the glans is closed the continuous 

 urethral passage is established. The main portion of the urethra is entodermal in origin, 

 but there is some evidence to indicate that the part which traverses the glans has its origin 

 from the ectoderm. 



THE MAMMAEY GLANDS. 



The mammae or breasts are accessory organs connected with the female repro- 

 ductive system. Each gland is situated in the superficial fascia covering the 

 anterior aspect of the thorax, and usually extends from the level of the second or 

 third to that of the sixth rib. The hemispherical projection formed by the gland 



