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ARTHUR E. GILES. 
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 structure 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 the case of the fat-body goes a step farther, since 
the primary connection with the kidney is lost, while the vasa 
efferentia remain connected with both kidney and testis. 
Again, the question may be asked, “ Why should only a 
part of the kidney structure undergo fatty degeneration — why 
should not the meso- and meta-nephros share in the change?” 
The answer would be even more difficult to find if, on the 
supposition that von Wittich was right, such a question were 
asked concerning the genital organs, for they are of equal value 
in all their parts, and when the metamorphosis occurs no 
portion of them has had any reproductive activity. But in the 
case of the pronephros it is different. It is true that the nature 
and origin of the pronephros are still matters of discussion, but 
it is at least evident that the pronephros is in many respects 
different from the mesonephros ; that the former, in the case 
of the Frog and of all animals with a larval stage, has a period 
of activity before the mesonephros appears at all, and in most 
cases disappears as the latter begins to take on active functions. 
On the other hand, in Vertebrates possessing no larval stage, 
the existence of the pronephros is only dimly shadowed forth 
by rudimentary traces, the meso- and meta-nephros performing 
all the excretory functions from the first. 
The answer then to the question, “ Why this change should 
occur resulting in the formation of the fat-body,” seems to be 
