628 EXCRETING ORGANS. 



tions, the tabulated exterior so constant in the oviparous ver- 

 tebrata. The secreting surface of these ducts is thus greatly 

 extended for the reception of a larger distribution of renal 

 capillary arteries and veins over their parietes, and the diffe- 

 rent modes of distribution of the blood-vessels in the interior 

 of these organs in the different classes, contributes to the 

 differences perceptible in the intimate texture of the kidneys, 

 as of other glandular organs. The tubuli uriniferi are almost 

 always long, cylindrical, narrow, and more or less tortuous 

 in the adult lobules of the kidneys of the plagiostomi, and 

 the higher osseous fishes, and more short, straight, parallel, 

 and wide, in the earlier stages of their development, and in 

 the lowest cyclostome species. Each lobule of the adult is 

 composed of the tubuli proceeding from a single branch of 

 the common duct or ureter, and appears, in the embryo, to 

 consist of a single lamina of the formative blastema. The 

 primitive vascular blastema of the embryo divides into 

 laminae, and each lamina developes a cluster of tubuli, which 

 open by a common orifice, or short duct, into the cylindrical 

 narrow ureter extending along the whole kidney. The group 

 of tubuli uriniferi, composing a renal lobe, are held together 

 by the remaining portion of the soft formative blastema. 

 The blastema at first extends undivided on the median plain, 

 along the middle of the back, between the mucous and serous 

 layers of the embryo ; a duct for each kidney is, at length, 

 perceived extending through it longitudinally, and giving off 

 lateral tubuli in its course, which develope through the sub- 

 stance of the lobules. 



In the rays and sharks, the kidneys are composed of long, 

 fine, very tortuous and convoluted tubuli, which open 

 separately along the course of the ureter, and in the early 

 embryo, they appear, as in higher classes, to be accompanied 

 with a corpus Wolffianum, composed of very minute convo- 

 luted tubuli. In the torpedo marmorata, the kidneys form two 

 tabulated organs extending along the outside of the ureters, 

 and consist of large tubuli, about T v of a line in diameter, long, 

 and remarkably convoluted like the tubuli seminiferi of mam- 

 malia. In many of the long, equal, contorted tubuli, forming 

 the entire mass of the kidneys in the cyprinus carpio, Muller 

 observed a distinct dichotomous division, at some distance 

 from their closed extremities, preserving, however, as usual in 



