THE UROGENITAL SYSTEM 3OI 



but this affects only the anterior part of the cloaca, which remains 

 undivided posteriorly. The excretory and reproductive ducts, which in 

 amphibians open on the dorsal side of the cloaca, shift their connexions 

 ventrally in reptiles so as to open into the new urogenital passage. 



A phallus comparable with that of mammals also appears first in 

 reptiles, notably in chelonians and crocodiles, in the form of a shallow 

 "seminal groove" in the floor of the cloaca. This groove is bordered by 

 paired "seminal ridges", each of which contains erectile tissue. Pos- 

 teriorly the groove terminates in a free swelling or glans which also con- 

 tains erectile tissue. When the erectile tissue of the seminal ridges is 

 distended, the groove between them is converted into a tubular passage 

 which serves to convey the semen through the intromittent organ to the 

 cloacal cavity of the female. 



Mammals. The genital system of monotremes differs little from that 

 of reptiles and is readily derived from it. In the male, the genital portion 

 of the mesonephros is converted into an epididymis, while the mesonephric 

 duct becomes a ductus deferens. The remainder of the mesonephros 

 atrophies, although remnants persist as a paradidymis. In higher 

 mammals, the testes migrate into a scrotal sac. 



In the female monotreme, the gonad produces large yolk-laden ova 

 similar to those of reptiles. In the placental mammals, however, the 

 ova are reduced in size, and the embryo depends for its nourishment 

 upon the mother. The proliferation of oocytes ceases early in the life 

 of the mammal, and the number of mature eggs produced by the ovary is 

 greatly reduced as compared with the number in Anamnia. Like the 

 testis, the ovary acquires connexions with the mesonephros; but the 

 connexion never becomes functional, and the mesonephros persists only 

 in the form of a rudimentary epoophoron and paroophoron. The meso- 

 nephric duct in the female becomes the functionless Gartner's duct. 

 The descensus of the ovaries is slight compared with that of the testes. 



With the development of a horizontal longitudinal septum, the 

 cloaca disappears in placental mammals, and two cavities, the urogenital 

 and rectal, take its place. As this change occurs the ureters shift their 

 connexions from the cloaca to the bladder while the ductus deferentes 

 open into the urogenital cavity or urethra. 



In female mammals a tendency of the Miillerian ducts — which 

 parallel the Wolffian ducts and lie medial to them — to fuse in the median 

 line is evident. Beginning with the monotremes the posterior part of 

 the Miillerian ducts fuse into a single uterus, while their anterior portions 

 remain separate as the paired uterine or Fallopian tubes. The monotreme 

 uterus opens into the urogenital sinus. In the higher mammals a vagina 

 for the reception of the penis is differentiated between the uterus and 

 the urogenital sinus. The vagina is therefore seen to be a portion of the 



