THE EXTERNAL ORGANS; ANUS 203 



depression (Otis). In front the thickening so produced joins the mesoderm forming 

 the perineal bridge, and forms with it the definitive perineal body separating the 

 anus from the urogenital opening. 



In the female the adult arrangement of parts is readily derived from this neutral 

 condition described above ; the inner genital folds become the Idbia minora, and 

 the outer the labia majora, while the genital papilla forms the clitoris. The groove 

 on the base of the papilla remains open and forms the entrance to the vestibule, 

 which, as already mentioned, is derived from the much shortened urogenital sinus. 

 Further, it is owing to the shortening of the sinus that the urethra comes to open 

 separately on the surface. 



In the male the inner genital folds meet to form the bulbous urethra, which is 

 carried forwards on the cloacal aspect of the genital papilla, first as the solid 

 epithelial ridge already described, then as a groove produced by the shedding of 

 the central cells of the solid cord. This groove is closed from behind forwards, 

 becoming the spongy portion of the urethra. The enlarged end of the papilla becomes 

 the glans penis. In this the urethra is closed independently, so that the last part 

 of the tube to be completed is at the junction of glans and body. The external 

 genital folds meet in the mid- ventral line to form the scrotum. The prepuce in both 

 sexes is developed by the 

 ingrowth of a solid ridge of 



ectoderm (Berry Hart), which, / ' i . i PJp rri *"'*\.' 9lans 



by separating into two 



lamellae, sets free a cutaneous urethra. ./- 



fold as a cap to the glans. 



The anal opening* is 

 formed later than the uro- 



genital; while the urogenital 



opening is effected in a 



15-8 mm. embryo, the anal 



opening is still closed in 



one of 29 mm. (Keibel). 



From the proctodaeal pit the 



ectoderm grows inwards for a FIG. 255. MALE FCETUS 4 CM. LONG (TENTH WEEK). 



short distance and forms the (From Kollmann O 



short ectodermic portion of 



the anal canal. The remainder of that passage is derived from the terminal, last 



closed part, of the cloacal chamber, above which the primitive entodermal tube 



of the rectum ends in an ampullated dilatation (fig. 236, p. 187). 



It appears from the observations of Tourneux, Retterer, Keibel, Fleischmann and his pupils, 

 Disse, and others, that the development of the cloacal region is essentially the same in all mammals. 

 Even in Echidna (Keibel) an entodermic cloaca is completely divided, and the rectum and 

 urogenital sinus open independently into a secondary ectodermic invagination or ectodermic 

 cloaca, just as they do on the surface in placental mammals in which there is no external cloaca. 



Fleischmann ' believes that the cloaca entodermica, or Urodaum, as he terms it after Gadow, 

 is not divided by two lateral folds, as first suggested by Rathke. It is, perhaps, after the first 

 frontal fold separates the rectal opening from the part of the chamber receiving the Wolffian 

 ducts, not further divided at all, but the stage represented in fig. 235, p. 187, may be reached by 

 differential growth, the urogenital section being pushed in a ventral and cranial direction, while 

 the rectum retains its primitive position, and comes to open into a small dorsal chamber 

 of the urodiium, the pars analis. This becomes separated from the ventral chamber to form 

 the entodermic portion of the anal canal. Wood Jones * and Keith 3 deny that the cloaca has 

 any share in the formation of the rectum. The idea involved in Wood- Jones's interpretation, 



1 Morphol. Jahrbuch, 1904. The reader will find here (p. 58) a lengthy historical resume of the 

 literature on this subject. 



2 Brit. Med. Jour. 1904, p. 1630. 3 Human Embryology and Morphology (2nd ed.), London, 1904. 



