DEVELOPMENT OF EXTERNAL GENITALIA IN THE HUMAN EMBRYO. 75 



appearance in embryos 17 to 19 mm. long. As yet, however, a sufficient number of 

 these younger embryos has not been examined to permit definite conclusions on this 

 point. 



Stage 3, 8 to 12 mm. (figs. 3 and 4). As growth proceeds, the genital tubercle 

 is transformed into a compressed, conical protuberance. This is brought about 

 by the deepening of the umbilico-phallic groove, so that the cranial slope of the 

 tubercle is nearly straight (i. e., approximately at right angles with the body axis), 

 while the caudal slope remains decidedly convex. At the same time the caudal 

 outline has become markedly triangular by the broadening of the base until it 

 occupies practically the entire area between the bases of the legs. The conspicuous 

 lateral slopes form the "lateral buttresses" which, arising from the cranial border 

 of the tubercle just proximal to its apical area, have a decidedly caudal trend, so 

 that they finally disappear basally opposite the caudal border of the tubercle. The 

 apex of the tubercle is now clearly marked off from the more proximal portion by a 

 shallow circula.1- depression, indicating it as the future glans and separating it from 

 the basal shaft. The urethral groove is a long, lancet-shaped depression, broadest 

 and deepest basally, narrowing distally into a shallow slit limited by a very small 

 "epithelial tag" just proximal to the primitive glans area. The urethral folds 

 (margins of the groove) are elevated as slight rolls of tissue which distally merge 

 into the glans region, while basally they become more tumid and broaden out to 

 surround the anal pit as anal tubercles. Lateral to these urethral folds, the caudal 

 surface of the tubercle is somewhat swollen in the younger specimen illustrating this 

 stage (fig. 3), in marked contrast to the decided concavity of these regions in the 

 older embryo (fig. 4) . In the former (fig. 3) the urethral and anal membranes have 

 not ruptured. The older embryo (fig. 4) agrees with the finding that (in the major- 

 ity of cases) these membranes rupture at about the stage of 12 to 13 mm., although 

 several specimens were found, among both the sectioned and unsectioned material 

 in which the membranes were still imperforate at 17 mm. The perforation of this 

 membrane transforms the shallow urethral groove into a gutter-like, primitive 

 urogenital opening as a direct communication between the phallic portion of the 

 urogenital sinus and the exterior. This is accompanied by an increase in the 

 definiteness of its outlines, as the result of which its sex difference in length is cor- 

 respondingly emphasized. Because of the variation in the time of rupturing of this 

 membrane, as well as on account of normal differences in the breadth of the open- 

 ing thus formed, it seems advisable to continue to use the term urethral groove, 

 except when referring to embryos that clearly show this feature as an opening. 



While there is an apparent sex difference in the lateral outline of the tubercle 

 in embryos 10 to 16 mm. in length, in that in some its tip is placed slightly more 

 cranially than in others, thus producing a wider separation between the tubercle 

 and the tail in the first group than in the second, a sufficient number of embryos has 

 not been examined to permit of definite conclusions on this point, because of the 

 possibility thafc some distortion of the embryos may have been brought about by 

 manipulations during fixation. 



