THE EMBRYO. 



1213 



a stage in which there is no distinction of sex. It is therefore necessary to 

 describe this stage, and then follow the development of the female and -male 

 organs respectively. 



As stated above, the anal depression, or proctodseum, at an early period is 

 formed by an involution of the epiblast, and the intestine is still closed at its lower 

 end. When the septum between the two opens, which is about the fourth week, 

 the urachus in front and the intestine behind both communicate with the anal 



^J_Posterior 

 jBr* extremity. 



Genital eminence. 

 ^..Genital ridge. 

 Cenital fold. 

 3J" Genital groove. 



I) 



Glanft penis. 

 GfantttU ridge. 



( leu i tid groove. 

 Genital fold. 

 Perinseum. 

 Anus. 



Clitoris. 

 Genital ridge. 

 Genital fold. 

 Entrance to vagina. 

 Perinseum. 



FIG. 780. Stages in the development of the external sexual organs in the male and female. 

 Ecker-Ziegler wax models.) (From Hertwig's Entwickclungsgeschichte.) 



(After the 



depression. This, which is now called the cloaca, is afterward divided by a trans- 

 verse septum, the perinceum, which appears about' the second month. Two tubes 

 are thus formed : the posterior becomes the lower part of the rectum, the anterior 

 is the urogenital sinus. In the sixth week a tubercle, the genital eminence, is 

 formed in front of the cloaca, and this is soon surrounded by two folds of skin, the 

 genital ridges. Toward the end of the second month the genital tubercle 

 presents, on its lower aspect, a groove, the genital groove, which extends down- 

 ward toward the cloaca. This groove becomes deeper, and is bounded laterally 

 by projecting folds of skin, the genital folds. All these parts are well developed 

 by the second month, yet no distinction of sex is possible. 



Female Organs. The female organs are developed by an easy transition from 

 the above. The urogenital sinus persists as the vestibule of the vagina and the 

 urethra. The genital eminence forms the clitoris, the genital ridges the labia 

 majora, and the lips of the genital groove the labia minora, which remain open. 

 An innovation of the epithelium takes place on either side close to the root of the 

 genital tubercle, which becomes the glands of Bartholin. 



Male Organs. In the male the changes are greater. The genital eminence is 

 developed into the penis, the gla-ns appearing in the third month, the prepuce and 

 corpora cavernosa in the fourth. The genital groove closes and thus forms a canal, 

 the spongy portion of the urethra. The urogenital sinus becomes elongated and 

 forms the membranous urethra. The genital ridges unite in the middle line to 

 form the scrotum, at about the same time as the genital groove closes, viz., 



