416 OUTLINES OF CHORDATE DEVELOPMENT 



remainder of the cloacal tubercle, at the base of the phallus and 

 mostly anterior and lateral to it, is now known as the genital 

 tubercle. The urinogenital aperture is continued forward upon 

 the posterior (ventral) surface of the phallus as a narrow groove, 

 the lateral margins of which are somewhat elevated as the 

 genital folds, which gradually enlarge and so reduce the urino- 

 genital aperture to a narrow elongated slit. 



By the beginning of the third month (Fig. 173, C) the phallus 

 has enlarged considerably, and its extremity has dilated as the 

 rudiment of the glans. Lateral to each genital fold a second, 

 larger ridge, the genital swelling, has appeared. This marks the 

 end of the so-called indifferent period, during which there is 

 but very slight external differentiation between the sexes. As 

 a matter of fact, the sex of the individual is determined at the 

 time of fertilization, and even during the latter part of this 

 "indifferent period" the female embryo can be distinguished 

 by the presence of a groove around the base of the phallus, 

 which is lacking in the male. 



The later development may be sketched very briefly. In 

 the female, where the modifications are less extensive, the 

 glans and the anterior (oral) portion of the phallus are trans- 

 formed into the clitoris (Fig. 173, D), while the posterior 

 (anal) portion of the phallus together with the lateral margins 

 of the urinogenital aperture, become the labia minora. The 

 labia majora are formed from the genital swelling and the 

 genital tubercle (basal portion of the cloacal tubercle). 



In the male (Fig. 173, E), the entire phallus is transformed 

 into the penis, composed of the glans plus the shaft, the pos- 

 terior (anal) portion of which is therefore equivalent to the labia 

 minora. The anterior extension of the urinogenital aperture 

 upon the male phallus is enclosed by the fusion of the genital 

 folds and so added to the lower part of the urethra. The 

 genital swellings in part fuse and are transformed into the 

 scrotal sac, and in part disappear, to be replaced by other 

 scrotal swellings which form the remainder of the scrotal sac. 

 The essentials in the history of the external genitalia may be 

 summarized as follows 



