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CHORDATE ANATOMY 



inal pores, which in structure and relations resemble the paired pores 

 of the gonadic sacs of flatworms. The elongated body-cavity of verte- 

 brates may be considered as formed by the fusion of the cavities of a 

 similar series of paired coelomic sacs. This assumption is further justified 

 by the fact that the relations of the abdominal pores in vertebrates 

 resemble those of the gonadic pores of flatworms. Moreover, their 

 function is similar. In Petromyzon both the primitive pronephric ducts 

 and the abdominal pores open posteriorly into the urogenital sinus and 

 to the outside through a urogenital papilla. 



Elasmobranchs. The gonads of elasmobranchs are paired, and in 

 some species greatly elongated, especially in the male. The testes 

 acquire secondary connexions with the anterior part of the mesonephros 

 and use the mesonephric ducts as an outlet for the sperm. Elasmobranchs 

 retain abdominal pores, but they appear to be functionless. In this 

 group, the primitive pronephric duct sphts longitudinally to form Wolffian 

 and Mullerian ducts. In the female the Wolffian ducts are purely 

 excretory, while the Mullerian ducts form the oviducts. In the male, 

 the Wolffian ducts combine both urinary and reproductive functions, while 

 Miiller's ducts atrophy to form the uterus masculinus. Many elasmo- 

 branchs are viviparous, and the eggs are retained until hatched within 

 uterus-like enlargements of the oviducts. 



Amphibia. The gonads of amphibians resemble those of elasmobranchs. 

 As in the latter, the testis becomes connected by means of vasa efferentia 

 with anterior tubules of the mesonephros, and this part of the mesonephros 

 tends to atrophy and lose its excretory function. The Wolffian duct 

 serves as a urogenital outlet in the male. The Mullerian ducts, which 

 persist as rudiments in the male, become the oviducts of the female. 

 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-Hke 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. 



Reptiles. The differences between the reproductive systems of 

 amphibians 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; 



