140 DEVELOPMENT. 



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

 organs respectively. 



As stated above, the anal depression at an early period is formed by an invo- 

 lution of the external epithelium, and the intestine is still closed at its lower end. 

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

 allantois in front and the intestine behind both communicate Avith the anal depres- 

 sion. This, which is now called the cloaca, is afterward divided by a vertical 

 septum, whose lower edge thickens to form the perineum, and which appears about 

 the second month. Two tubes are thus formed ; the posterior becomes the lower 

 part of the rectum, the anterior has uniting with it the urogenital sinus. In the 

 sixth week a tubercle, the genital tubercle, is formed in front of the cloaca, and 

 this is soon surrounded by two folds of skin, the genital folds. Toward the end 

 of the second month the tubercle presents, on its lower aspect, a groove, the 

 genital furrow, turned toward the cloaca. All these parts are well developed by 

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



Female Organs (Fig. 108, A, B, c). The female organs are developed by an 

 easy transition from the above. The portion of the cloaca in front of the septum 

 persists as the vestibule of the vagina, and forms a single tube with the upper part 

 of the vagina, which, as we have already seen, is developed from the united 

 Miillerian ducts. The genital tubercle forms the clitoris, the genital folds the 

 labia majora, and the lips of the genital furrow the labia minora, which remain 

 open. 



Male Organs (Fig. 108, A', B', c'). In the male the changes are greater. The 

 genital tubercle is developed into the penis, the glans appearing in the third month, 

 the prepuce and corpora cavernosa in the fourth. The genital furrow closes and 

 thus forms a canal, the spongy portion of the urethra. The urogenital sinus 

 becomes elongated and forms the prostatic and membranous urethra. The genital 

 folds unite in the middle line to form the scrotum, at about the same time as the 

 genital furrow closes viz. between the third and fourth months. 



The following table is translated from the work of Beaunis and Bouchard, with 

 some alterations, especially in the earlier weeks. It will serve to present a resumS 

 of the above facts in an easily accessible form. 1 



1 It will be noticed that the time assigned in this table for the appearance of the first rudiment 

 of some of the bones varies in some cases from that assigned in the description of the various bones 

 in the sequel. This is a point on which anatomists differ, and which probably varies in different 

 cases. 



