DEVELOPMENT OF THE FAT-BODIES IN RANA TEMPORARIA. 125 



time certain changes are observed, notably the gradual atrophy and 

 subsequent disappearance of the external gills, while the recently 

 acquired internal gills take on more work. 



If at this stage the pronephros be examined, it will be seen that 

 the tubules have a narrower diameter than those of the younger 

 tadpole, while the cells are not so clearly defined. By this time the 

 number of funnel-like openings into the body cavity has increased 

 from three to five, and a new and important structure has made its 

 appearance, the mesonephros, developed, as Sedgwick has shown,* 

 in the mesoblast independently of the peritoneal epithelium. 



The meso- and meta-nephros are not distinct from one another in 

 the tadpole ; they together form the kidney as found in the adult, 

 and it is in this sense that the word kidney will be used. 



The mesonephric tubules extend gradually from behind forwards 

 till they come in contact with the pronephros. The whole nephros 

 then acquires a distinct capsule, becomes separated from the muscular 

 substance of the lateral mass, and lies freely in the abdominal cavity 

 on the ventral aspect of the vertebral column, the peritoneum passing 

 over it. Between the two kidneys is the aorta (Fig, 5). The genital 

 organs, which arise as two hollow ridges, also gradually separate 

 from the body wall, lying internal and ventral to the kidneys 

 (Fig. 5), and are still perfectly well defined anteriorly, the proper 

 genital substance extending quite to the anterior end. 



Concurrently with these changes of conformation, the structure of 

 the pronephros has been undergoing modification of the nature of a 

 fatty degeneration. At the time that the hind limbs are just making 

 their appearance, the degeneration has gone on to the extent 

 represented in Fig. 4. 



The way in which this conversion of kidney parenchyma into 

 fat takes place is a true fatty degeneration, and not simply a fatty 

 infiltration, though the latter occurs in the first stage. The change 

 is seen best in the cells lining the glomeruli and renal tubules. The 

 clearly defined margins of the cells become hazy, and the nuclei less 

 distinct ; fatty droplets appear at various parts of the cell and run 

 together. The cells do not, however, swell up as the fatty matter 



* "On the Early Development of the Anterior Part of the Wolffian Duct and 

 Body in the Chick, together with Some Remarks on the Excretory System of the 

 brata," " Quart. Journ. Micr. yd.," vol. szi., N n S., lbtsl, p, 449. 



