THE KIDNEYS SECRETION OF URINE. 



637 



appearance of anything resembling a Urinary apparatus in the Chick, is seen on the second 

 half of the third day. The form at the time presented by it is that of a long canal, extend- 

 ing on each side of the Spinal Column, from the region of the heart, towards the Allantois ; 

 and the sides of this present a series of elevations and depressions, indicative of the com- 

 mencing development of caeca. On the fourth day, the Corpora Wofffiana, as they then are 

 termed, are distinctly recognized, as composed of a series of ccecal appendages, which are 

 attached along the whole course of the first-mentioned canal, opening into its outer side. On 

 the fifth day these appendages are convoluted ; and the body which they form acquires 

 increased breadth and thickness. They evidently then possess a secreting function ; and the 

 fluid which they separate is poured by the long straight canal into the cloaca. Between 

 their component shut sacs, numbers of small points appear, which consist of little clusters 

 of convoluted vessels, exactly analogous to the Corpora Malpighiana of the kidney. The 



Fig. 247. 



Fig. 248. 



Fig. 249. 



Uriniferous Tube, Malpighian 

 Tuft, and Capsule, from Kidney 

 of Frog: a, cavity of the tube; 

 fc, epithelium of the tube; &', 

 ciliated epithelium of the neck 

 of the capsule; b", detached 

 epithelium scale ;' c, basement 

 membrane of tube ; c', basement 

 membrane of capsule. Magnified 

 about 320 diam. 



Distribution of the Renal ves- 

 sels ; from Kidney of Horse ; a, 

 branch of Renal artery ; of, 

 afferent vessel ; m, m, Malpig- 

 hian tufts ; ef, ef, efferent ves- 

 sels ; p, vascular plexus sur- 

 rounding the tubes ; st, straight 

 tube ; ct, convoluted tube. Mag- 

 nified about 30 diam. 



Corpora Wolffiana, with kid- 

 ney and lestes, from embryo of 

 Bird; 1, kidney; 2, 2, ureters; 3, 

 corpus Wolffianum ; 4, its ex- 

 cretory duct; 5, 5, testicles; at 

 the summit are seen the supra- 

 renal capsules. 



Corpora Wolmana, however, have only a temporary existence in the higher Vertebrata ; 

 although it seems that, in Fishes, they constitute the permanent kidney.* The development 

 of the true Kidneys commences in the Chick about the fifth day. They are seen on the 

 sixth, as lobulated grayish masses, which sprout from the outer edges of the Wolffian bodies ; 

 and they gradually increase, the temporary organs diminishing in the same proportion. The 

 sexual organs, as will be hereafter explained ( 866, 6), also originate in the Wolffian bodies; 

 and at the end of fetal life, the only vestige of the latter is to be found as a shrunk rudi- 

 ment situated near the testes of the male. The progress of development in the Human 

 embryo seems closely conformable to the foregoing account. The Wolffian bodies begin to 

 appear towards the end of the first month ; and it is in the course of the seventh week, 

 that the true Kidneys first present themselves. From the beginning of the third month the 

 diminution in the sizeof the Wolffian bodies goes onparipassu with the increase of the Kidneys; 

 and at the time of birth scarcely any traces of them can be found. At the end of the third 



54 



* See Principles of General and Comparative Physiology, 659. 



