96 THE ANATOMY OF VERTEBRATED ANIMALS. 



out their whole extent, remain distinct from them. Their 

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

 dilated to form the so-called ostia. I3ejond 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 Mullerian ducts undergo no 

 further modification of any great morphological importance ; 

 but, in the monodelphous 3Iaininalia^ 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 Mullerian ducts are primarily converted into a single 

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

 Monodelphia^ the two uteri also more or less completely coa- 

 lesce, until both Mullerian ducts are represented by a single 

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

 phous Mammalia have two vaginae which m.ay, 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 Mullerian 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 Aimpjhioxus and in the 3farsip)o- 

 branchii^ this tissue appears to pass directly into spermatozoa ; 

 but, in most Vertehrata^ 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 higher 

 vertebrates, this want is speedily supplied by the Wolffian 

 bodv, certain of the tubuli of which becom.e 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 seininalis is a 



