726 



AMNIOTA. 



cord of tissue, known as -the genital cord (fig. 407, gc), within the 

 lower part of which the MUllerian ducts are also enclosed. In 

 the male the MUllerian ducts in this cord atrophy, except at 

 their distal end where they unite to form the uterus masculinus. 

 The Wolffian ducts, after becoming the vasa deferentia, remain 

 for some time enclosed in the common cord, but afterwards 

 separate from each other. The seminal vesicles are outgrowths 

 of the vasa deferentia. 



In the female the Wolffian ducts within the genital cord 

 atrophy, though rudiments of them are for a long time visible or 

 even permanently persistent. The lower parts of the MUllerian 

 ducts unite to form the vagina and body of the uterus. The 

 junction commences in the middle and extends forwards and 

 backwards ; the stage with a median junction being retained 

 permanently in Marsupials. 



The urinogenital sinus and external generative organs. 

 In all the Amniota, there open at first into the common cloaca 

 the alimentary canal dorsally, the allantois ventrally, and the 

 Wolffian and MUllerian ducts and ureters laterally. In Reptilia 

 and Aves the embryonic condition is retained. In both groups 

 the allantois serves as an embryonic urinary bladder, but while 

 it atrophies in Aves, its stalk dilates to form a permanent 

 urinary bladder in Reptilia. In Mammalia the dorsal part of 

 the cloaca with the alimentary tract becomes first of all partially 

 constricted off from the ventral, which then forms a urinogenital 

 sinus (fig. 407, ug). In the course of development the urino- 

 genital sinus becomes, in all Mammalia but the Ornithodelphia, 

 completely separated from the intestinal cloaca, and the two 

 parts obtain separate external openings. The ureters (fig. 407, 

 3) open higher up than the other ducts into the stalk of the 

 allantois which dilates to form the bladder (4). The stalk 

 connecting the bladder with the ventral wall of the body con- 

 stitutes the urachus, and loses its lumen before the close of 

 embryonic life. The part of the stalk of the allantois below the 

 openings of the ureters narrows to form the urethra, which opens 

 together with the Wolffian and MUllerian ducts into the urino- 

 genital cloaca. 



In front of the urinogenital cloaca there is formed a genital 

 prominence (fig. 407, cp), with a groove continued from the 



