

HUMAN ANATOMY. 



Fie. 1725. 



loacal membrane 



Surface marking* of cloacal region of human embryo 

 of seventeen days (Fig. 1644). X ia. (Ktibel.) 



FIG. 1736. 



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

 Lit. -r 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 gmws slowly and is converted into the glans and body of the clitoris, while 

 the inner g. -nit.il folds become the nymphae and the outer ones the labia majora. 

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

 The : tisMie between the posterior margin of the latter and the anus becomes 



the perineal body. 



ascription ot the development of the glands of Bartholm is given in connec- 

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



In the male the modifications lead- 

 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, 1 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 gro\vs farther forward towards 

 the glans, approximation and fusion of 

 its edges occur behind, whereby the groove 

 is gradually converted into the urethral 

 anal. In this manner the distal opening 

 <1 until its definite position on the glans is reached 

 -d development ..r fusion ,.f the edges of the urethral groove results in defer- 



Genital tubercle 

 Cloacal membrane 

 Lower limb 



Caudal process 



External genitals of human embryo of about twenty- 

 even day*. (Kotlman*.) 



1727 



<;'.,: 



-*G<rnital folds 

 ff Labio-scrotal 



!l III 



urv;rniul sinus 

 x Anal groove 



' Coccygral eminence 

 \lernal grtiiuUof human embryo 







ve resu 



"' . a condition known as hypospadias (page 1927). 

 



urethral groove results in defec- 

 spadias (page 1927). 



"" I'";!'- - begins as a thkkenSg and ingrowth of the surface 

 '" "" "|.-'" anm.lar groove that separates the glans from the 

 n.m tins th.ckenmg the epithelium' gnnvs backward, invading 



, 



U a n.ur.nv ,,.1,,-shaped mass that encircles the glans 

 "Plele and the frmum later appears. In this manner 

 ^ v < - - ;-- 'n,, ^ base <rf fnWl that later, just 



'Ar.luvf. mikn.s. Anatom.. 



ixjjj., 



