80 DEVELOPMENT OF EXTERNAL GENITALIA IN THE HUMAN EMBRYO. 



DEVELOPMENT OF FEMALE GENITALIA. 



The entire process of development of the external genitalia in the female is 

 accompanied by fewer pronounced morphological changes than occur in the 

 male. It is noteworthy, however, that (in spite of this greater simplicity in struc- 

 ture) completion of development is more protracted, so that the final differentiation 

 of the female genitalia, although brought about by comparatively minor changes, 

 does not synchronize with the more complete transformation of the male (45 mm. 

 CR length), but instead, beginning with a slight change at a length of about 50 mm., 

 extends as a gradual process throughout a considerable period of early fetal life. 



From its beginning until the stage of 21 to 25 mm. the genital tubercle of the 

 female closely resembles that of the male except for the shorter urethral groove. 

 About this period the female shows the beginnings of the caudal decurvation, which 

 is apparently brought about by an excess in the growth of the cavernous over the 

 urethral regions of the phallus. At the same time the urethral folds have become 

 compressed into plate-like caudal projections supporting the slightly overhanging 

 glans, which in this way is more clearly defined than in the male. As has already 

 been pointed out, the coronary sulcus is not formed in either sex until the embryos 

 reach a length of about 45 mm. From 25 mm. to 45 mm. this caudal decurvation 

 becomes a more and more pronounced characteristic of the female phallus, and for 

 this reason is a diagnostic feature of increasing importance as development proceeds. 



In the female important changes occur at about the stage of 45 to 50 mm. CR 

 length, and these likewise mark the termination of the phallus period. While much 

 less extensive than the correlated changes in the male, they are nevertheless char- 

 acteristic and indicate the approximate assumption of the final form. The most 

 pronounced modification occurring at this time is that the caudal ends of the 

 labio-scrotal swellings grow towards each other and finally join in the midventral 

 line to form the posterior commissure (50 mm. CR length). In this manner these 

 originally paired swellings are transformed into a cranially open, horseshoe- shaped 

 rim, inclosing the rest of the external genitalia and separating them from the anus. 



The formation of the posterior commissure in the female thus synchronizes 

 with the formation of the raphe in the male and may be considered as representing 

 the advent of the final differentiation, and from now on we may refer to the genitalia 

 by their adult terms. The labio-scrotal swellings form the labia majora. The 

 glans and cavernous portion of the phallus may be considered as the clitoris, and 

 the urethral folds as the labia minora. The inclosed primary urogenital opening 

 may now be called the urethro-vaginal orifice. It must be pointed out, however, 

 that the application of these terms at this time is an arbitrary one, because the actual 

 separation of the female phallus into these more definitely adult structures does not 

 take place until some time after the close of the period included in the present study 

 (100 mm. CR). Strict accuracy would demand that, until such division had been 

 completed by the formation of the frenula clitoridis, the inclusive term phallus be 

 retained. 



Because of the persistence of the urethral folds (labia minora) in the female and 

 their failure to fuse together as they do in the male, the female phallus retains a more 

 conical outline than does the penis of the male. 



