636 EXCRETING ORGANS. 



and most divided into lobes in the aquatic and the larger 

 terrestrial forms of this class. In several of the cetacea, 

 there are more than two hundred deeply isolated lobes in a 

 single kidney, but in the monotrema, they are united into a 

 simple compact organ ; in the lobulated kidneys, the number 

 of papillae and infundibula corresponds with the number of 

 lobes, but in the compact forms of the organ, the number of 

 these conical tufts of tubuli is often reduced to a single 

 papilla, and the entire pelvis to a single calyx. The relative 

 development of the cortical and medullary portions, varies as 

 much as the outward form of the organ in different mam- 

 malia. The structure and relations of the straight and 

 tortuous tubuli uriniferi of the simple and lobulated kidneys 

 of mammalia and lower vertebrata, and the connections of 

 the small round vascular corpuscules with the arterial 

 branches, were already investigated and described by Mal- 

 pighi. 



The kidneys of mammalia, as of lower vertebrata, are 

 preceded in the embryo by the corpora Wolffiana, composed 

 each of elegant series of simple transverse tubuli opening 

 into a common longitudinal duct, which extends along the 

 outer margin of these deciduous glands and terminates in 

 the cloacal end of the intestine ; these bodies are interposed 

 between the situations of the renal and genital glands, they 

 are most developed long before the middle of foetal life, and 

 they have entirely disappeared before birth. The kidneys 

 appear at first as consisting each of a congeries of minute 

 tortuous follicles radiating to the periphery of a small round 

 soft gelatinous blastema, and terminating around the ex- 

 terior surface of this primitive mass in minute closed pyri- 

 form sacs, like the vesicular terminations of the bronchial 

 tubes of the lungs or of the early tubuli of most other glands. 

 The peripheral terminal vesicles diminish in size and disap- 

 pear, as the tubuli become lengthened, tortuous, and inter- 

 woven, and there is no trace of distinction between cortical 

 and medullary portions in the interior, nor of lobes on the 

 surface. The tubuli in the central part of the kidney become, 

 at length, more straight and parallel, and grouped into conical 

 fasciculi, which compose the medullary portion, while the 

 tortuous interwoven peripheral parts of the tubuli form the 

 cortical portion of the organ. These conical groups of 



