436 THE SOLIDS. 



that the afferent and the efferent vessels of each tuft are one 

 and the same by direct continuity. In these classes, the ef- 

 ferent vessel readily admits of being injected from the artery, 

 and this in consequence of the simplicity of the plexus sur- 

 rounding the Malpighian enlargement. 



The size of the Malpighian bodies varies, not merely in the 

 same kidney, but also to a still greater extent in the kidneys 

 of different animals: they are largest in the elephant and 

 horse, and smallest in birds. 



Development of the Kidney. 



According to Dr. Carpenter, " The first appearance of any thing re- 

 sembling a urinary apparatus in the chick, is seen in the second half of 

 the third day. The form at the time presented by it is that of a long 

 canal, extending on each side of the spinal column, from the region of the 

 heart towards the allantois ; and the sides of these present elevations and 

 depressions, indicative of the commencing development of coeca. 



" On the fourth day, the Corpora Wolffiana, as they are termed, are dis- 

 tinctly recognised, as composed of a series of caeca! 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 Corpora Wolffiana, how- 

 ever, 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 greyish 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, also originate in the 

 Wolffian bodies ; and at the end of foetal life, the only vestige of the 

 latter is to be found as a shrunk rudiment 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 size of the Wolffian 

 bodies goes on pari passu with the increase of the kidneys ; and at the 



