2044 



HUMAN ANATOMY. 



Cloacal membrane 



Surface markings of cloacal region of human embryo 

 of seventeen days (Fig. 1644). X 12. (Keibel.) 



Genital tubercle 

 Cloacal membrane 

 \ Lower limb 



Caudal process 



the urogenital sinus that appears when the cloacal membrane ruptures. Somewhat 

 later, about the ninth week, a pair of thick crescentic swellings, the outer genital, or 

 labio-scrotal folds, make their appearance on either side of the genital tubercle. 



In the female, in which the original relations are largely retained, the genital 

 tubercle grows slowly and is converted into the glans and body of the clitoris, while 

 the inner genital folds become the nymphae and the outer ones the labia majora. 

 The urogenital sinus remains as the vestibule and its opening as the vulvar cleft. 

 The wedge of tissue between the posterior margin of the latter and the anus becomes 

 the perineal body. 



A description of the development of the glands of Bartholin is given in connec- 

 tion with the consideration of these organs (page 2026). 



In the male the modifications lead- 

 FiG. 1725. ing to the fully differentiated external 



organs are more pronounced in conse- 

 quence of the formation of the urethra. 



The genital tubercle rapidly increases 

 in size, becomes somewhat conical and 

 differentiated into the glans and shaft of 

 the penis. The parts of the outer genital 

 folds behind the penis soon become en- 

 larged, rounded, approach each other, 

 and, finally, unite along a line afterward 

 indicated by the median raphe, so that in 

 embryos of 45 mm. length the scrotum is 

 already well defined. According to Her- 

 zog,^ the development of the urethra pro- 

 ceeds from an epithelial ridge that appears 

 on the cloacal membrane and extends for- 

 ward along the under surface of the geni- 

 tal tubercle towards its distal end. This 

 ridge sinks into the mesoblastic tissue of 

 the elongating genital tubercle as a nar- 

 row longitudinal strand (urethral septum), 

 and later becomes partially divided by a 

 superficial furrow, the urethral groove, the 

 lips of which correspond to the inner geni- 

 tal folds. In consequence of the cleavage 

 of the posterior third of the epithelial 

 ridge, the cloacal membrane is ruptured 

 and communication established with the 

 urogenital sinus by means of a small canal 

 that opens into the urethral groove. As 

 the latter grows farther forward towards 

 the glans, approximation and fusion of 

 its edges occur behind, whereby the groove 

 is gradually converted into the urethral 

 canal. In this manner the distal opening 

 of the urethra is carried forward until its definite position on the glans is reached. 

 Arrested development or fusion of the edges of the urethral groove results in defec- 

 tive closure of the canal, a condition known as hvpospadias (page 1927). 



The formation of the prepuce begins as a thickening and ingrowth of the surface 

 epithelium at the bottom of an annular groove that separates the glans from the 

 body of the penis. From this thickening the epithelium grows backward, invading 

 the young connective tissue as a narrow wedge-shaped mass that encircles the glans; 

 except below, where it is incomplete and the frenum later appears. In this manner 

 an annular fold, the prepuce, is defined around the base of the glans that later, just 

 before or shortly after birth, becomes free by the partial solution of the intervening 

 solid epithelial stratum and its conversion into the preputial sac. 

 ^ ^Archivf. mikros. Anatom.. Bd. Ixiii., 1904. 



External genitals of human embryo of about twenty- 

 seven days. [KoHmann.) 



Fig. 1727 



rGlans 

 — 4-GenitaI folds 



y-~ Labio-scrotal 

 folds 



Opening of 

 urogenital sinus 



Anal groove 



Coccygeal eminence 

 Indifferent stage of external genitals of human embrvo 

 of thirty-three days (Fig. 1647). X 8. {Keibel.) 



