74 DEVELOPMENT OF EXTERNAL GENITALIA IN THE HUMAN EMBRYO. 



separated from the rest of the tubercle by a pair of converging depressions which 

 unite in the midventral line to form a longitudinal groove extending to the caudal 

 border, giving to the entire groove a Y-shape. Caudal to the arms of the Y and 

 lateral to its stem is a pair of broad, swollen areas. This condition is slightly 

 different from the stage described by Keibel (1896) of the beginning development of 

 the genitalia in a 3-mm. embryo. In his specimen he described the cloacal tubercle 

 as consisting of a pair of eminences separated by the cloacal membrane. The 

 difference between the stage described by Keibel and my first stage may be accounted 

 for entirely by the difference in the size of the two specimens. 



The term genital tubercle, which I use to describe the entire genital eminence, 

 has a slightly different meaning from the one given to it by Felix (1912, p. 948). 

 That author speaks of the genital eminence at first as the cloacal tubercle, which is 

 later divided into a basal genital tubercle and a terminal phallus. According to 

 his usage, the former develops into the genital (labio-scrotal) swellings, while' the 

 latter forms the phallus proper. My observations indicate that the entire primor- 

 dial genital eminence should be considered as the precursor of the phallus and as 

 such should be termed the genital tubercle. As will be shown later, my observations 

 also indicate that the labio-scrotal swellings apparently originate from the outlying 

 tissue and not from the basal portion of the primary tubercle. But even without 

 this modified interpretation of their origin, the term genital tubercle seems most inap- 

 plicable to these swellings (which are decidedly secondary, both in genital structure 

 and in function) and is therefore much more a propos of the genital eminence as the 

 precursor of the phallus. 



Stage 2, 8 mm. (fig. 2, plate 1). The second stage, represented by the recon- 

 struction model of an 8-mm. embryo, shows considerable advance in the develop- 

 ment of the genital tubercle. This now forms a somewhat rounded mass which 

 occupies almost the entire area between the umbilical cord and the base of the tail. 

 Its cranial margin is but barely indicated by a slight groove between its apex and the 

 umbilical cord. The apex is broadly rounded and from it the slightly convex caudal 

 surface slopes toward the base of the tail. The caudal slope is almost bisected by 

 the urethral groove, a shallow depression extending from the base to the tip of the 

 tubercle. The basal end of the groove is separated from the anal pit by a narrow 

 transverse bar. It is significant that, while there is this external separation of the 

 primitive cloacal groove into the urethral groove and anal pit, the internal division 

 of the cloacal cavity into urogenital sinus and rectum is not yet complete, at least in 

 this embryo. The margins of the urethral groove are but slightly elevated into the 

 urethral folds, although the margins of the anal pit are more pronounced. Laterally 

 the caudal surface of the tubercle is rounded out into pronounced swellings. 

 Almost in front of the tubercle the ventral body-wall bears a pair of swellings lying 

 in the umbilico-phallic angles, the significance of which has not yet been definitely 

 ascertained. These masses show a gradual increase in size until the embryo 

 reaches a length of 16 mm. They then apparently disappear in some embryos, 

 while in others they undergo a caudal shifting; this would seem to indicate that 

 they are the primordia of the labio-scrotal swellings, which definitely make their 



