MOKPHOLOGY OF THE HUMAN URINOGENITAL TRACT. 337 



From a mesial skin-fold the genital eminence develops, giving 

 rise to clitoris and penis. From two lateral folds below it, we 

 get the labia with the genital furrow between, the cloacal pit 

 lying posteriorly. Two eminences develop at the side of the 

 cloaca, and by their junction in the middle line, and union with 

 the septum dividing bladder from rectum, above, we get the 

 perineum formed, and the cloaca with its hoivcl opening obliterated. 

 In the cases narrated by Eeichel and von Eosthorn the cloaca 

 has remained in part, but the bowel opening has become closed 

 at the apex of the cloacal pit. Marital relations had caused a. 

 breaking down of the tissue between bowel and the rest of the 

 cloaca, and thus an incontinence of fseces was estabhshed. 



The permanent anus is not developed from the bowel opening 

 into the cloaca. This closes, as has been described, and the 

 permanent anus develops as follows : — 



The development of the amis is well seen at the stages between 

 the second and third and a half month, and we also get a very 

 clear view of how the epidermis, by passing in deeply, forms a 

 lumen. The sagittal sections show the epidermis passing in as 

 a series of tubular ingrowths (fig. 19). When at the top of the 

 anal orifice, the superficial cells cease, and the deep cells pass 

 up in two layers to meet the epithelium of the Lieberklihnian 

 glands. The anus is thus formed like the prepuce— viz., the 

 epidermis, deep layer and svperficial layers, passes in ; the super- 

 ficial cells {now central) desquamate, and thus the Imnen forms. 



I may finally, under this division, briefly consider the ques- 

 tion of the production of the cavities or spaces already alluded 

 to. I have mentioned the formation of the primitive gut, the 

 division of the entodermal cloaca into bladder and rectum, and, 

 finally, the formation of the ectodermal cloaca at the 6th 

 week. Each of these shews a different mechanism. 



1. The p)rimitive gut is formed by the ventral infolding of the 

 early embryonic area— i.e., by head, tail, and lateral folds con- 

 verging centrally to the site of the future navel. 



2. The bladder with the urinogenital sinus and the rectum are 

 formed by mesoblastic folds tvhich pass partly from above and 

 partly from the sides. They are thus partition folds, and there 

 is no central breaking down. 



3. The cloaca — ectodermal cloaca, it may be termed — is formed 



