DEVELOPMENT OF EXTERNAL GENITALIA IN 1HE HUMAN EMBRYO. 85 



however, the chief diagnostic feature in the two sexes. In the male it consists of a 

 long slit extending from the base to the apex of the phallus, whereas in the female 

 it extends only to the base of the glans. In the male it is more open distally, the 

 apposition of the urethral folds reducing it proximally to a mere slit. This is in 

 marked contrast to the condition in the female, where the opening (restricted to the 

 shaft) has exactly the opposite shape, being broadest basally, while the terminal 

 portion is narrowed into a slit. 



No further pronounced morphological changes take place in the male genitalia 

 until after the embryo has reached a CR length of 38 mm. At this time a series of 

 changes begins which result in the transformation of the male genitalia into approxi- 

 mately their final form (about 45 mm.) and in the complete separation of the uro- 

 genital opening from the anus. The gutter-like urogenital sinus is transformed 

 into the tubular urethra by the merging of the basal portions of the urethral folds 

 into the raphe", reducing the primitive urogenital orifice to an irregularly shaped 

 opening on the distal portion of the shaft. At the same time the glans becomes 

 sharply defined by the formation of a wide coronary sulcus and the labio-scrotal 

 swellings assume their final position caudal to the penis to form the scrotum, the 

 halves of which become more or less closely approximated to the raphe in the mid- 

 ventral line, although they never entirely lose their bilateral character. 



This period (45 mm. CR length) marks the completion of the definite sex 

 differentiation, characterized by the development of definite structural features in 

 the male in sharp contrast to the lack of these characters in the female. 



As growth continues, the gradual constriction of the urethral opening syn- 

 chronous with the formation of the prepuce, to be described later, is accompanied 

 by an outgrowth of its rim which, in embryos 60 to 85 mm. CR length, results in 

 the formation of a decided cup in the bottom of which the opening is located. With 

 the later stages of prepuce formation the urethral opening is shifted to a more 

 terminal position in the frenular notch of the glans. It then becomes entirely closed 

 and eventually there is formed the new, permanent terminal urethral opening on the 

 apex of the glans. 



The first evidence of the prepuce is found in embryos of about 65 mm. CR 

 length, at which time it can be recognized as a pair of swellings on each side of the 

 urethral opening. Gradually these swellings fuse together over the dorsum of the 

 shaft to form a flattened ridge of skin whose distal margin has enveloped the 

 proximal margin (corona glandis) of the glans (embryos 75 mm. CR length). 

 Subsequent growth results in the progressive inclosure of the glans by this distally 

 migrating fold of skin until at about 100 mm. CR length the originally naked glans 

 is entirely covered. It must be emphasized that these observations upon the forma- 

 tion of the prepuce are based only upon the external examination of the genitalia 

 of these older embryos and have not been confirmed by histological study. For 

 this reason they must be considered merely as suggestions of what apparently takes 

 place and may later be corroborated or contradicted when it becomes possible to 

 make a study of sectioned embryos showing this development. 



