MODIFICATIONS OF THE EEPEODTJCTIVE ORGANS. 109 



as completely devoid of any excretory canal as the ovary ; 

 but, in the higher vertebrates, this want is speedily supplied 

 by the Wolffian body, certain of the tubuli of which become 

 continuous with the tubuli seminiferi, and constitute the vasa 

 recta, while the rest abort. The Wolffian duct thus becomes 

 the vas deferens, or excretory duct of the testis; and its 

 anterior end, coiling on itself, gives rise to the ejnclidymis. 

 A vesicula seminalis is a diverticulum of the vas deferens, 

 near its posterior end, which serves as a receptacle for the 

 semen. 



If the Wolffian bodies, the genitalia, and the alimentary 

 canal of a vertebrate embryo, commitnicated with the exterior 

 by apertures having the same relative position as the organs 

 themselves, the anus would be in front and lowest, the WoK- 

 fian apertures behind and highest, and the genital apertures 

 would lie between the two. But the anal, genital, and uii- 

 nary apertures are found thus related only among certain 

 groups of fishes, such as the Teleostei. In all other Verte- 

 bi-ata, there is either a cloaca, or common chamber, into 

 which the rectum, genital, and urinary organs open ; or, the 

 anus is a distinct posterior and superior aperture, and the 

 opening of a geuito- urinary sinus, common to the imnary 

 and reproductive organs, lies in front of it, separated by a 

 more or less considerate perinceum. 



These conditions of adult Vertebrata repeat the states 

 through which the embryo of the highest vertebrates pass. 

 At a very early stage, an involution of the external integu- 

 ment gives rise to a cloaca, which receives the allantois, the 

 ureters, the Wolffian and Miillerian ducts, in front, and the 

 rectum behind. But, as development advances, the rectal 

 division of the cloaca becomes shut off from the other, and 

 opens by a separate aperture — the definitive anus, which thus 

 appears to be distinct, morphologically, from the anus of an 

 osseous fish. For a time, the anterior, or genito-iirinary part 

 of the cloaca, is, to a certain extent, distinct from the rectal 

 division, though the two have a common termination ; and 

 this condition is I'epeated in Aves, and in ornithodelphous 



