78 DEVELOPMENT OF EXTERNAL GENITALIA IN THE HUMAN EMBRYO. 



The coordinate female (fig. 13) shows, more clearly than any of the younger 

 specimens, the distinction between the glans and shaft, although the division is 

 indicated more by changes in the surface modeling than by the formation of a defi- 

 nite coronary sulcus. There is also a very slight indication of the caudal decur- 

 vation which was first pointed out by Herzog (1904) as an early sex difference. 

 The urogenital opening is clearly limited to the shaft, being broader basally and 

 narrowing distally into a mere slit. The urethral folds are still quite broad and are 

 separated from the cavernous portion by shallow depressions. The labio-scrotal 

 swellings resemble those already described for the male of the preceding stage. 



Stage 7, 21 to 23 mm. CR (figs. 10, 15, males; figs. 14, 19, females). While there 

 is a slight increase in the length of the phallus at this stage, the most conspicuous 

 changes are the increase in the relative sex differences in the length of the urogenital 

 opening and the greater development of the genital folds and the labio-scrotal 

 swellings. In the males figured, the greater length of the urogenital opening is clearly 

 seen to be in sharp contrast to the much shorter opening of the females . The urethral 

 folds at this stage are more pronounced than in any of the embryos previously 

 described. As a result of the deepening of the lateral depression between these 

 folds and the cavernous portion of the shaft, their median portions now project from 

 the shaft as compressed ridges. Distally they merge into the terminal glans area, 

 while proximally they broaden out into the overhanging pre-anal enlargements 

 (bulbo-urethral swellings?). As a result of this modification, the shaft of the 

 phallus now shows clearly two distinct regions: the heavily thickened, cavernous 

 portion, formed by the medial migration of the lateral buttresses, and the caudal 

 urethral portion, represented by the projecting urethral folds outlining the uro- 

 genital opening, a condition which persists throughout the remainder of the phallus 

 period. The labio-scrotal swellings have now grown into /-shaped enlargements, 

 separated from the phallus by grooves more pronounced in the males than in the 

 females. Cranially, these swellings are separated from each other by a triangular 

 prolongation of the base of the phallus, so that there is no indication of the horse- 

 shoe shape which is so characteristic of most of the older illustrations. The caudal 

 extremities of these swellings are joined together by a low, curving postanal ridge 

 (slightly larger in the female), which is almost bisected by a median sagittal groove. 



Stage 8, 24- to 28 mm. CR (figs. 16, 17, males; figs. 20, 21, females). This stage 

 is represented by an exceedingly interesting pair of twins. In the male (fig. 16) 

 the nearly cylindrical phallus projects at approximately right angles to the body 

 axis, in marked contrast to a pronounced decurvature of that of its mate (fig. 20). 

 The glans is not so sharply demarked as in the female, although there are slight indi- 

 cations of its extent. The urogenital opening extends almost to the tip of the 

 phallus as a shallow, open groove deepened basally by the caudally extended, plate- 

 like urethral folds. As a result of this increase in the folds, the base of the phallus is 

 considerably longer than its apex. The labio-scrotal swellings are separated from 

 the base of the phallus by broad, shallow grooves; their tips curve cranio-laterally 

 away from the phallus, leaving a rather broad, unswollen area between the latter 

 and the umbilical cord. 



