466 THE DOGFISH CHAP. 



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 Fig. 108 and pp. 189 and 449). 



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 (see Chapter XII). In the embryo, 

 the kidneys appear in the form of separate, segmen tally 

 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 verte- 

 brate kidney furnishes another example of metamerism, 

 which can no longer be distinctly recognised in the adult 

 kidney (compare 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 Mullerian ducts : the 

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

 although it may to a greater or less extent (compare p. 193) 

 retain also its function as a ureter ; the latter gives rise to 

 the oviduct in the female, and vestiges of it may be re- 

 cognised in the male (Figs. 117 and 127). 



In the dogfish the kidneys (Fig. 127, ef, K) are, long, 

 narrow, lobulated organs, lying close to the vertebral 

 column on either side, covered ventrally by the thick 

 peritonium, and extending primarily along almost the 

 whole length of the ccelome. But in the course of 

 development, certain important modifications occur in 



