V THE DEVELOPMENT OF THE FROG 113 



the mesoderm ; the spaces enlarge, become continuous, and 

 the cells surrounding them take on a definite arrangement 

 and form the walls. The blood corpuscles arise either from 

 cells originally inclosed in the vessels or from cells budded 

 off from the lining membranes. 



In the development of the renal organs there first appears 

 on each half of the body a temporary organ known as the 

 pronephros, which later disappears without contributing to 

 the formation of the permanent kidney. The duct of the 

 pronephros, or segmental duct, arises, according to Field,^ as 

 a thickening of the mesodermic wall of the body cavity. 

 It becomes hollowed out secondarily, and at its anterior end 

 it divides into three tubules which open into the coelom. 

 Posteriorly the duct joins the cloaca. The tubules increase 

 in length and become more convoluted, and the duct itself 

 in the region of its tubules becomes bent and twisted owing 

 to its increase in length, but its hinder portion remains straight. 

 The mouths {nephros tomes) of the tubules become lined with 

 ciHa which carry material into the canals. 



The pronephros, which is the functional kidney of early 

 larval life, is replaced by the mesonephros, or iVolffian body, 

 which is the renal organ of the adult. The mesonephros 

 makes its first appearance as a series of small tubules on 

 either side of the body, between the aorta and the segmental 

 duct. The tubules are at first solid, but they soon acquire a 

 lumen which communicates with that of the segmental duct, 

 with which they fuse. Their distal ends become swollen 

 out into a sort of sac, one wall of which becomes pushed in 

 by a knot of blood vessels or glomerulus derived from the 

 renal arteries, thus forming the Malpighian bodies found in 

 the adult kidney. Owing to their growth in length the 

 tubules become contorted ; branches are given off which 



1 Field, Dull. Mus. Comp. Z.00I. Harvard, Vol. 21, 1891. 



