76 DEVELOPMENT OF EXTERNAL GENITALIA IN THE HUMAN EMBRYO. 



Stage 4, 14 to 15 mm. (figs. 5, 7, male; figs. 6, 11, female). In this stage the 

 sex difference in the length of the urethral groove becomes clearly evident. But 

 slight changes have taken place in the general shape of the genital tubercle. In the 

 male (figs. 5, 7) the glans area is more clearly indicated than in the female (figs. 6, 11). 

 In both embryos the urethral groove is sharply outlined, making it possible to con- 

 trast its difference in length in the two specimens. In the female the epithelial tag 

 is more pronounced than in the male and the entire tubercle is slightly more swollen. 



PHALLUS PERIOD. 



Stage 5, 16 to 17 mm. (fig. 8, male; fig. 12, female). By the time the embryo 

 has attained a length of 16 to 17 mm. the genital tubercle has elongated into a 

 narrow, conical organ which, because of its modified shape and its separation from 

 the surrounding body areas by the newly formed labio-scrotal swellings, will now 

 be called the phallus. The male embryo representing this stage (fig. 8) shows the 

 phallus as more nearly cylindrical than in any of the younger embryos, the lateral 

 buttresses having, to a marked extent, merged into its body. The groove limiting 

 the glans is present, although not clearly shown in the photograph. The urogenital 

 opening is a narrow orifice extending almost the full length of the phallus, limited 

 distally by a pronounced epithelial tag. The urethral folds are considerably 

 broader than in any of the younger stages, their outer margins being sharply 

 separated from the remains of the lateral buttresses by a concave depression. The 

 most pronounced change, however, is the presence of the labio-scrotal swellings as a 

 pair of distinct, rounded ridges, one on each side of the base of the phallus and 

 separated from it by a broad lateral phallic groove. As regards this feature, it is 

 quite likely that this embryo represents a slightly older stage, as the male specimen 

 of the next stage shows more clearly the probable beginnings of these swellings, and 

 the female selected as the counterpart for this stage does not show them to any 

 marked degree, if they are present at all. It is, of course, very possible that there is 

 normally more or less variation in the time of development of these structures, 

 and that this embryo (fig. 8) is merely slightly precocious in this respect. 



The female of this stage is represented by the dissected-out phallus region of an 

 embryo of the same length. The phallus has the same general configuration as that 

 of the male just described, with the exception of a shorter urogenital opening ter- 

 minated by a larger epithelial tag, a greater fullness in the genital folds, the presence 

 of a straight postanal bar which is grooved by a slight median longitudinal depres- 

 sion flanked on each side by anal tubercles, and the apparent absence of the labio- 

 scrotal swellings. The presence or absence of these swellings in this specimen must, 

 however, remain an open question, although the indications point strongly to their 

 absence. In this respect the embryo seems to be more nearly normal (so far as 

 sequence in the appearance of the modified parts of the external genitalia is con- 

 cerned) than its coordinate male. 



Stage 6, 19 mm. CR (fig. 9, male; fig. 13, female). In the male of this stage the 

 phallus is still of narrow, conical form, with the glans area rather more sharply 

 indicated by a broad, band-like depression, not as clearly shown in the photograph 



