428 TEXT-BOOK OF EMBRYOLOGY. 



side of the ridge becomes raised still farther above the surface, forming a dis- 

 tinct protrusion, the genital tubercle. The tubercle continues to increase in 

 size, and the distal end forms a knob-like enlargement. Along the ventral (or 

 rather caudal) side a groove appears, which extends distally as far as the base 

 of the enlarged end. The ridges along the sides of the groove increase in 

 size and form the genital folds. In the meantime a second pair of elevations 

 appears lateral to the genital folds to form the genital swellings (Fig. 382). 



After the cloacal membrane ruptures, a single opening is produced which 

 leads from the exterior into the cloaca. This opening is then separated by the 

 further growth of the urorectal fold (p. 403) into the opening of the urogenital 

 tract and the anal opening. The caudal part of the fold then enlarges to form 

 the perineal body, which serves to push the anus farther away from the genital 

 ridges. The latter, together with the genital tubercle and swellings, all of which 

 lie in the immediate vicinity of the urogenital opening, constitute the anlagen 

 of the external genital organs (Fig. 383). These at this time are in the 

 "indifferent" stage, from which development proceeds in one of two directions, 

 accordingly as the embryo is a male or a female. Up to the fourth month 

 there is little difference between the structures in the two sexes. After this the 

 differences become more and more obvious. 



In the female the changes in the originally "indifferent" structures are 

 comparatively slight. The genital tubercle grows slowly and becomes the 

 clitoris. The enlarged extremity becomes more clearly marked off from the 

 other part to form the glans clitoridis. The skin covering the glans is converted 

 by a process of folding into a sort of prepuce. The genital folds, which 

 bound the opening of the urogenital tract, become elongated and form the 

 labia minora. The opening of the urogenital tract is the vestibulum vagina. 

 The genital swellings enlarge still more than the genital folds, by a deposition 

 of a considerable mass of fat in the mesenchyme, and become the labia majora. 

 The latter are the structures (mentioned on p. 424) which mark the points 

 at which the inguinal ligaments of the mesonephroi pierce the body wall, and 

 are homologous with the scrotum in the male (Figs. 384 and 385). 



In the male the "indifferent" anlagen undergo more extensive changes 

 than in the female. The genital tubercle continues to grow more rapidly and 

 forms the penis, which is homologous with the clitoris. The enlarged extremity 

 becomes the glans penis, and an extensive folding of the skin over the glans 

 forms the prepuce. The groove on the caudal or lower side of the tubercle 

 elongates as the latter elongates and becomes deeper. Finally the ridge (or 

 genital fold) on each side of the groove meets and fuses with its fellow of the 

 opposite side, thus enclosing within the penis a canal — the penile portion of 

 the urethra. The groove is primarily continuous with the opening of the uro- 

 genital tract, and as the fusion takes place the penile portion forms a direct 



