THE EEPRODUCTIVE ORGANS. 107 



the modificatiou of certain embryonic structures called the 

 Miillerian ducts. 



The Miillerian ducts are canals which make their appear- 

 ance alongside the ducts of the Wolffian bodies, but, through- 

 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. Beyond these ostia they 

 generally remain narrow for a space, but towards their 

 hinder openings into the genito-urinary part of the cloaca, 

 they commonly dilate again. In all animals but the didel- 

 phous and monodelphous Mammalia, the Miillerian ducts 

 undergo no further modification of any great morphological 

 importance ; but, in the monodelphous Mamynalia, they be- 

 come united, at a short distance in front of their posterior 

 ends ; and then, the segments between the latter and the 

 point of imion, or still farther forward, coalesce into one. 

 By this process of confluence the Miillerian 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 coalesce, until both 

 Miillerian ducts are represented by a single vagina, a 

 single uterus, and two Fallopian tubes. The didelphous 

 Mammalia have two vaginae which may, or may not, coalesce 

 anteriorly for a short extent ; but the two uteri remain 

 perfectly distinct. So that what takes place in them is, pro- 

 bably, a differentiation of each Miillerian dvict 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 

 earlier stages of development in Monodelphia. The "Wolffian 

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

 canals of Gaertner, which open into the vagina, or dis- 

 appear altogether. Remains of the Wolffian bodies consti- 

 tute the parovaria, observable in certain female mammals. 



In the male vertebrate embryo, the testis, or essential 

 reproductive 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 



