DEVELOPMENT OF THE FAT-BODIES IN RAN A TEMPORARIA. 127 



has undergone fatty degeneration, as was thought hy von Wittich, 

 but that its attachment to the genital organ is secondary. 



The fatty degeneration is always complete before the attachment 

 to the genital organ takes place ; almost any tailed frog that has 

 not very long had its four limbs showing the fat-body attached to 

 the anterior end of the kidney. That it is, in reality, fat-body, is 

 shown by its macroscopic and microscopic characters, and by its 

 staining with osmic acid. The part marked (/) in Fig. 7 has 

 exactly the same structure and appearance as that similarly marked 

 in Fig. 8, the two specimens having been stained and cut at the 

 same time. 



Having thus decided that the fat-bodies are derived not from the 

 genital organs but from excretory structures, we have to consider 

 what part of the nephros it is to which they owe their origin. It 

 can only be pro-, meso-, or meta-nephros, or their ducts ; the ducts 

 can be at once put aside, because their destination has been clearly 

 and definitely made out. The meso- and meta-nephros are also known 

 to form together the permanent kidney, as found in the adult. 



There remains, therefore, only the pronephros, which, in the 

 Amphibians at least, has hitherto received but little attention, 

 though Sedgwick* mentions that it undergoes atrophy in the young 

 frog. "Atrophy," however, implies diminution in size, or even 

 total disappearance ; the pronephros of the tadpole, on the contrary, 

 not only persists but actually gets larger (in its modified form) as 

 the frog grows. Now, we saw that at an early stage the pronephros 

 undergoes a fatty degeneration ; that the degenerated part remains 

 for a time continuous with the rest of the kidney (Fig. 6), and then 

 becomes secondarily attached to the genital organ. Hence the fat- 

 bodies represent the persistent pronephros, profoundly modified both in 

 struckvre and in function. 



If it be objected that it is a priori improbable that the fat-bodies 

 should consist of the anterior part of the nephros detached and 

 fastened on secondarily to the genital organ, it will be sufficient to 

 recall the fact that the vasa efferentia are formed by a quite 

 parallel growing out of kidney structure — the mesonephric tubules ; 

 the only difference being that the process in tho case of tho fat- 



* Op. cit., p. 445. 



