DEVELOPMENT OF EXTERNAL GENITALIA IN THE HUMAN EMBRYO. 79 



In the female (fig. 20) the glans has become more clearly defined, in a negative 

 way, by changes which have taken place in the character of the urethral folds. 

 As a result of this modification the entire shaft is terminally decurved upon the 

 sharply demarked urethral folds, which now apparently form plate-like supports for 

 the thicker cavernous shaft, the terminal portion of which (the future glans) is 

 slightly more knob-like than that of the male. The inclosed urogenital opening is 

 narrow and lanceolate, clearly restricted to the region of the genital folds and not 

 extending onto the decurved glans. In general, the labio-scrotal swellings are 

 similar to those of the male; their caudal prolongation into a curving, connecting 

 postanal ridge is, however, more pronounced. Here it is without tubercular 

 enlargements, but is almost bisected by a median groove. 



Some of the older embryos in this stage show further changes of a minor nature 

 in the phallic region. Thus the glans may be more strongly indicated by a further 

 increase in the density of its tissue (corpus cavernosum glandis) , so that it appears 

 as an opaque white tip in sharp contrast to the more translucent shaft. The post- 

 anal ridge, joining the tips of the labio-scrotal swellings, becomes more V-shaped, 

 while the inclosed anal opening shifts from a transverse to a longitudinal slit. 



Reconstruction models of three embryos of this stage show quite clearly the 

 correlation between the sex differences of the external and internal organs. In one 

 specimen (No. 584a, 25 mm. CR) showing externally the long male type of uro- 

 genital opening, the tip of which extends onto the glans, the internal organs are 

 male also. The gonad is rather short, barely reaching to the rim of the pelvis, 

 while the tubar portion of the urogenital folds, containing the Wolfnan and Miil- 

 lerian ducts, unite with the dorsal wall of the bladder without the formation of a 

 genital cord (primordium of the uterus). 



In the females modeled (No. 840, 24.8 mm. CR, and No. 782, 28 mm.) the 

 opposite condition is found. Externally the phallus of each shows a short uro- 

 genital opening. Internally the organs are decidedly more of the female type. 

 The gonads are slightly longer, while the tubar portions of the urogenital folds 

 (containing the Mlillerian ducts) unite to form a genital cord before fusing with 

 the dorsal wall of the bladder, thus producing a sinuous ridge which projects from 

 the dorsal wall of the bladder and partially subdivides the cavity of the pelvis. 

 In the older specimen the gonads are relatively larger and more compact. 



Stage 9, 30 to 38, mm CR (figs. 18 to 23, males; figs. 22 and 29, females) .The 

 changes in form between this and the previous stage are slight and consist mainly 

 of a further decurvation of the female phallus and a great condensation of the 

 glans area, although as yet the definite coronary sulcus has not been formed. In 

 the male, the urogenital opening still extends the entire length of the phallus and 

 there is no indication of the raphe". 



DEFINITIVE PERIOD. 



Stage 10, 38 to 45 mm. CR (figs. 24 to 28, 31, 32, males; figs. 30, 35, 36, fe- 

 males). The series of important changes occurring between the lengths of 38 and 

 45 mm. culminates in the final differentiation of the external genitalia. The forma- 



