412 COMPARATIVE ANATOMY 



As in elasmobranchs, the oviducts open far forward in the body cavity 

 near the liver. The primary gonads show the hermaphroditic potencies 

 manifest in cyclostomes. Ovaries and testes at first resemble one another 

 and have the appearance of ovaries, but either before or after meta- 

 morphosis of the larva, the ovary-like gonad of the male is transformed 

 into a testis. In exceptional instances, this metamorphosis is retarded 

 and may take place in a full-grown individual. Frequently the trans- 

 formation of an ovary into a testis may occur on one side before it has 

 begun on the other. Such an individual may appear superficially as a 

 unilateral gynandromorph. The so-called fat-bodies of the frog are 

 formed from the anterior portion of the genital ridges and appear to serve 

 as a reserve food-supply for the germ-cells. (Figs. 337, 338) 



Reptiles. The differences between the reproductive systems of 

 amphibia and reptiles are relatively slight. In the reptiles the gonads 

 of both sexes have shifted posteriorly. Also the beginnings appear of a 

 division of the cloaca into a dorsal rectal, and a ventral urogenital moiety; 

 but these affect 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 as an intromittent organ to convey the semen 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 



