4SO THE DOGFISH chap. 



of as the internal ear. In the frog there is also an accessory apparatus — 

 the tympanic cavity and membrane, together with the columella — which 

 is called the middle ear (compare p. 49 and 189). 



TJrinogenital organs. — In order to understand the mor- 

 phology of the kidneys, and the close relations existing 

 in most Vertebrates between them and the generative 

 organs, it is necessary to know something of the develop- 

 ment of these parts. In the embryo, the kidneys appear 

 in the form of separate, segmentally arranged tubes having 

 the general character of nephridia, opening on the^ one 

 hand by nephrostomes into the ccelome, and on the other 

 into a longitudinal duct which discharges into the cloaca. 

 Thus the primitive structure of the vertebrate kidney 

 furnishes another example of metamerism, which can no 

 longer be distinctly recognised in the adult kidney. (Figs. 

 46 and 47.) 



At a later stage of development in most vertebrate orders 

 two longitudinal ducts can be recognised on either side, 

 which in some cases (e.g. Dogfish) are formed by the 

 subdivision of the single primary duct. These are known 

 respectively as the Wolffian and the Miillerian ducts : the 

 former takes on the function of a spermiduct in the male, 

 although it may {e.g., in the frog, p. 193) retain also its 

 function as a ureter ; the latter gives rise to the oviduct 

 in the female. 



In the dogfish the kidneys (Fig. 118, ep, k) are long, 

 narrow, lobulated organs, lying close to the vertebral 

 column on either side, covered ventrally by the thick 

 peritoneum, and extending primarily along almost the 

 whole length of the coelome. But in the course of de- 

 velopment, certain important modifications take place in 

 them and in their ducts (WolfiSan ducts). In the 

 male, about the anterior half of the kidney takes on a 



