260 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



organ consists of a number of delicate and convoluted 

 tubes, which, when mapped out in diagram, have 



much the appearance shown 

 in the figure (Fig. 109; A). 



Just as in nearly all Ich- 

 thyopsida the pronephros is 

 seen to be a purely larval 

 organ (Balfour), so in the Am- 

 niota the mesonephros makes 

 way for the ittetanephros, which 

 is here preceded for a very 

 short time by the pronephros. 

 The metanephros appears to 

 be a further development of 

 the hinder part of the mesone- 

 phros, and, like it, it retains 

 throughout life evidence of 

 being composed of a system of 

 tubules, which advance in com- 

 plexity as we rise in the series. 

 The truth of this will be 

 shown by a study of the mi- 

 nute anatomy of the kidney 

 of a tortoise (Fig. 109; B), and 

 a comparison of it with that 

 of a pigeon (Fig. 109; c), and 

 of a man (D). 



So, again, with 

 complexity of internal struc- 

 ture, we observe, as we pass 

 from Reptiles to Birds or 

 Mammals, that the length of 

 the kidney diminishes, and 

 that it becomes limited to the lumbar region of the 

 body, while the ducts that open into the pelvis of the 

 kidney are reduced in number and increased in size. 

 The more important macroscopic differences in the 



advancing 



Fig. 107. Diagram of the 

 Mesonephros of an Uro- 

 dele. HO, testis ; N, 

 kidney. ( Modified, from 

 Sprengel.) 



