400 EMBRYOLOGY. 



In the following weeks of development the eminence protrudes still 

 more, and thereby becomes converted into the genital member, which 

 is at first possessed by both sexes in the same condition ; meanwhile 

 the groove (gr) on its under surface becomes deeper, and surrounded, 

 at the right and left, by projecting folds of the skin, the genital 

 folds (g f ). (Compare also the diagrams fig. 219 gho, gw, cl' and 

 fig. 229 cp, Is, cl.) 



Alterations follow (fig. 231 M and TF) by which the cloaca is 

 differentiated into two openings, one lying behind the other, the anus 

 (a) and the separate urogenital opening (ug). The deep partition 

 (fig. 229) by which the sinus urogenitalis and the rectum are separated 

 from each other begins to grow outward, and at the same time folds 

 also arise on the lateral walls of the cloaca and unite with it. Thus 

 a membrane (fig. 231 d) is developed which separates a posterior 

 opening (a), the anus, from an anterior opening, the entrance to 

 the sinus urogenitalis (ug). Inasmuch as this partition continues to 

 become thicker up to the end of embryonic life, it finally crowds the 

 two openings far apart and forms between them the perinaeum (fig. 

 231 M* and W* d). In this way the anus (a) moves entirely out of 

 the territory of the previously mentioned genital ridge (fig. 230 gw). 



From the fourth month onward great differences arise in the develop- 

 ment of the external sexual parts in male and female embryos. 



In the female (fig. 231 W and W*) the metamorphoses of the 

 originally common embryonic foundations are on the whole only 

 slight; the genital eminence grows only slowly and becomes the 

 female member, the clitoris (cl). Its anterior end begins to thicken 

 and to be marked off from the remaining part of the body as the glans. 

 By a process of folding in the integument there is developed around 

 it (fig. 231 W* vh) a kind of foreskin (the praeputium clitoridis). 

 The two genital folds (W gf), which have bounded the groove on the 

 under surface of the genital knob, take on a more vigorous develop- 

 ment in the female than in the male, and are converted into the labia 

 minora ( W* ksch). The space between them (W ug), or the sinus 

 urogenitalis, which receives the outlet of the urinary bladder and 

 the vagina developed by the fusion of the MUllerian ducts, is called 

 the vestibulum vaginae (W* vv). In the female the genital ridges 

 ( W gw), owing to the deposition of fatty tissue, become very volu- 

 minous, and are thus converted into the labia majora (W* gsch). 



The corresponding fundaments pass through much more essential 

 metamorphoses in the male (fig. 231 3f and M*). By an extra- 

 ordinarily vigorous growth in length the genital eminence is 



