84 DEVELOPMENT OF EXTERNAL GENITALIA IN THE HUMAN EMBRYO. 



SUMMARY. 



A more comprehensive idea of the sequence of events in the development of 

 the external genitalia may be gained if we briefly review their development in each 

 sex. 



DEVELOPMENT OF THE MALE GENITALIA. 



The genital tubercle, from which is derived the phallus and which includes the 

 urethral and anal openings, arises as a broad conical eminence barely separated 

 cranially from the umbilical cord, but with a well-defined caudal slope. Its 

 rounded free end, as the primordium of the glans, is faintly separated from the basal 

 portion. The lateral slopes, below the glans area, extend into the outlying basal 

 tissue as the "lateral buttresses, " which I interpret as enlargements for the develop- 

 ing corpora cavernosa. The caudal slope is bisected by the shallow urethral groove 

 whose margins are slightly elevated into the urethral folds. Distally, these margins 

 merge into the glans area, while proximally they continue into the basal enlarge- 

 ments surrounding the anal pit. In the majority of embryos which have attained 

 a length of 12 mm. the urethral membrane is ruptured to form the primitive uro- 

 genital opening, although a few cases have been found in which this perforation 

 had not taken place at the stage of 16 to 17 mm. Practically from its first appear- 

 ance the urethral groove shows a sex difference in its length, this difference being 

 further accentuated upon the formation of the urogenital opening. 



As development proceeds, the elongation of the tubercle and the medial migra- 

 tion of its lateral buttresses transform it into the somewhat cylindrical phallus, 

 whose base is separated from the surrounding body areas by the newly formed 

 labio-scrotal swellings (embryos 17 to 19 mm.). The latter, appearing first as 

 broad, elevated areas extending laterally from the base of the phallus, are soon 

 transformed into swollen ridges separated, in both sexes, from the base of the 

 phallus by the lateral phallic grooves. It may be emphasized that in the majority 

 of specimens the labio-scrotal swellings seem to be from the first separated from the 

 phallus. In fact, only one embryo was found which showed these areas as definitely 

 merging into the phallus, while in all of the others of about the same age the swell- 

 ings were already separated from the phallus by pronounced grooves, the lateral 

 phallic grooves. 



Accompanying the elongation of the tubercle into the phallus, the uro- 

 genital opening becomes gradually more sharply outlined and the sex differ- 

 ence in its length correspondingly emphasized. The urethral folds which form 

 the margins of the opening also increase in definiteness, partly through their 

 own elevation and partly through their lateral demarcation from the cavernous 

 portion of the shaft; the result of these combined factors being that the folds 

 extend as plate-like caudal projections from the more cylindrical shaft. At the 

 same time the terminal glans has increased in density so that it now forms an opaque 

 white area in contrast to the translucent shaft, although the limiting coronary 

 sulcus is not formed until the close of the phallus period. 



As a result of these combined changes the male phallus at this time is markedly 

 different from that of the female. The length of the urogenital opening is still, 



