96 THE ANATOMY OF VERTEBRATED ANIMALS. 



out their whole extent, remain distinct from them. Tlieir 

 proximal ends lie close to the ovary, and become open and 

 dilated to form the so-called ostia. Beyond these ostia they 

 generally remain narrow for a space, but, toward their hinder 

 openings into the genito-urinary part of the cloaca, they com- 

 monly dilate again. In all animals but the didelphous and 

 monodelphous Mammalia, the Miillerian ducts undergo no 

 further modification of any great morphological importance ; 

 but, in the monodelphous Mammalia, they become united, at 

 a short distance in front of their posterior ends ; and then the 

 segments between the latter and the point of union, or still 

 farther forward, coalesce into one. By this process of conflu- 

 ence the Miillerian ducts are primarily converted into a single 

 vagina with two uteri opening into it ; but, in most of the 

 Mo^iodelphia, the two uteri also more or less completely coa- 

 lesce, until both Miillerian ducts are represented by a single 

 vagina, a single uterus, and two Fallopian tubes. The didel- 

 phous 3Iaram,alia have two vaginae which may, or may not, 

 coalesce anteriorly for a short extent ; but the two uteri re- 

 main perfectly distinct. So that what takes place in them is 

 probably, a differentiation of each Miillerian duct into Fallo- 

 pian tube, uterus, and vagina, with or without the union of 

 the two latter, to the extent to which it is effected in the ear- 

 lier stages of development in Monodelphia. The Wolffian 

 ducts of the female either persist as canals, the so-called ca- 

 nals of Gaertner, which open into the vagina, or disappear 

 altogether. Remains of the Wolffian bodies constitute the 

 parovaria, observable in certain female mammals. 



In the male vertebrate embryo, the testis, or essential re- 

 productive organ, occupies the same position, in front of the 

 Wolffian body, as the ovary ; and, like the latter, is composed 

 of indifferent tissue. In Amphioxus and in the 3Iarsipo- 

 branchii, this tissue appears to pass directly into spermatozoa ; 

 but, in most Vertebrata, it acquires a saccular or tubular struct- 

 ure, and from the epithelium of the sacs, or tubuli, the sperma- 

 tozoa are developed. At first, the testis is as completely de^ 

 void of any excretory canal as the ovary ; but, in the higlier 

 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 epididymis. A vesicula seminalis is a 



