THE EXTERNAL GENITALIA 



225 



midline of the ventral body wall, between the tail and umbilical cord, the cloacal 

 tubercle. Upon this appears a knob-Hke structure, the phallus, and the two 

 together constitute the genital eminence (Fig. 220). Cranially about the phallus 

 the cloacal tubercle forms a crescent-shaped genital tubercle, which later gives 

 rise to the right and left genital swellings. The phallus grows rapidly carrying 

 with it the phallic portion of the urogenital sinus (Fig. 219). At the end of the 

 phallus the epithehum of the sinus forms a solid urethral plate. Along the anal 

 surface of the phallus, in the midhne, the wall of the urogenital sinus breaks 

 through to the exterior and forms the sht-Hke primitive urogenital opening 

 (Fig. 235). In embryos of 21 to 26 mm., at the end of the phallus, the glans is 

 marked off from the base by a circular groove, the coronary sulcus (Figs. 232 

 and 235 B). 



ABC 

 Fig. 235. — Three stages in the development of the external genitalia in human embryos of 24 to 

 34 mm. (after Tourneux in Heisler). Indifferent stage: 1, Phallus; 2, glans; 3, primitive urogenital 

 opening; 4, genital tubercle or swelling; 5, anus; 6, coccyx. 



Female. — A deep groove appears about the base of the phallus, separating 

 it from the genital tubercle, which becomes a circular swelhng (Fig. 235 C). 

 From the swelhng differentiates: (1) cranially, the mons pubis; (2) laterally, the 

 right and left labia majora; (3) caudally, the posterior commissure of the labia 

 majora (Fig. 236). The glans of the phallus forms the glans clitoridis of the female. 

 On the anal surface of the phallus, beginning at the coronary sulcus, the primitive 

 urogenital opening closes distally, forming the urethral groove. Proximally it 

 remains open, as the definitive urogenital opening near the base of the phallus. 

 The Hps of this groove and opening enlarge and become the labia minora. The 

 cranial surface of the phallus forms a fold, the prepucium, which, however, is 

 not the homologue of the male fore-skin. This in the female is represented by a 

 ring-hke rudiment at the base of the glans chtoridis. 



Male. — The phallus grows rapidly at its base so that the glans and primitive 

 urogenital opening are carried some distance from the anus (Fig. 237). A cyhn- 

 drical collar of the epithelium, incomplete on the anal side, grows down into the 

 IS 



