Section 19. Figure 3, Sheet 17, Is a Generalized Diagram of the

uro-genital organs in the vertebrata; M.L. is the middle line of the body, G. is the genital organ, Pr. is the pronephros, or fore kidney, a structure which is never developed in the dog-fish, but which has functional importance in the tadpole and cod, and appears as a transitory rudiment in the chick. A duct, which is often spoken of as the pronephric duct (p.d.), and which we have figured under that name, is always developed. Anteriorly it opens into the body cavity. It is also called the Mullerian duct, and in the great majority of vertebrata it becomes the oviduct, uniting with its fellow, in the case of the dog-fish, ventral to the oesophagus. In the male it usually disappears; the uterus masculinus of the rabbit is still very generally regarded as a vestige of it. Kolliker has shown, however, that this interpretation is improbable. Ms. is the mesonephros, some or all of which becomes the epididymis in the male of types possessing that organ, and is connected with G. by the vasa efferentia. Mt., the metanephros, is, in -actual fact- [the frog], indistinguishably continuous with Ms., and is the functional kidney, its duct (metanephric duct) being either undifferentiated from the mesonephric (as is the case with the frog) or largely split off from it, as in the dog-fish, to form the ureter.