EXCRETING ORGANS. 635 



extending along each side -of the vertebral column, precede 

 much the development of the kidneys, and disappear before 

 the bird escapes from the ovum : they are more connected 

 with the evolution of the genital glands, the testes and 

 ovaria, than of the urinary organs, in the classes where they 

 are observed. As in other glands, the tubuli of these deci- 

 duous bodies appear at first as pedunculated peripheral 

 vesicles, which become gradually elongated and constricted 

 to form straight narrow tubes, and, at length, long narrow 

 tortuous and interwoven tubuli extending to the interior 

 edge of the organ from the exterior marginal duct. Their 

 structure resembles that of the kidneys of amphibia, but 

 they are not organically connected with the urinary tubuli, 

 and appear to assist in the development of the genital 

 glands. 



The urinary organs of mammalia generally present a more 

 compact and simple external form, and a more extensive 

 secreting surface by the minute divisions and the compact 

 arrangements of their tubuli, than in lower vertebrata ; they 

 eliminate a larger proportion of the aqueous constituents of 

 the blood, and they are always provided with a distinct 

 urinary bladder. The lobed condition of the kidneys, so 

 constant in lower classes, is still, however, observed as a 

 normal adult character in many of the inferior mammiferous 

 tribes, as in the cetacea, many ruminating and pachyderma- 

 tous herbivora, the slow-moving plantigrade carnivorous 

 quadrupeds, and in the amphibious mammalia and the otter. 

 In the higher tribes, the kidneys pass early from the primi- 

 tive lobulated condition to a more concentrated form, by 

 the union of the lobes into a single compact organ, which 

 generally presents internally a distinct cortical and medullary 

 portion, resulting from the straight and parallel course of the 

 minute tubuli in the central part, and their tortuous inter- 

 woven course in the exterior portion. These two portions 

 are alike perceived in the separate renal lobes of the human 

 foetus, and in the component lobes of the adult lobulated 

 kidneys in lower mammalia. The right kidney is generally 

 more advanced in the trunk than the left, and impresses the 

 liver ; they are covered only on the ventral surface with peri- 

 toneum ; they present a depressed and rounded form more 

 or less elongated in different species, and they are largest 



