REMOVAL OF PRONEPHROS OF AMBLYSTOMA 357 
of both pronephroi. The uniform occurrence of a pronounced 
edema after bilateral extirpation, similar to the condition which 
follows certain pathologic conditions in the permanent kidneys 
of the higher animals, suggests a further parallel between the 
larval kidneys on the one hand and the permanent kidneys on 
the other with respect to their function. Extirpation of the 
coiled portion of one or both pronephroi has also afforded the 
opportunity of investigating the question of correlation in develop- 
ment through a study of the effect of its removal on the other 
components of the excretory system. The response of one kidney 
in cases where it has been left functioning alone has further led 
to the consideration of the factors involved in the restoration 
of the normal secreting area through the process of compensatory 
hypertrophy. 
Although no invariable rule can be formulated as to the tj^pe 
of regulation which may be anticipated as a consequence of the 
abnormal conditions imposed by extirpation of an embryonic 
region or organ, a survey of the results obtained in the many 
instances already investigated shows that, in a large proportion 
of cases, there occurs a more or less complete regeneration of the 
excised part. Byrnes ('98 b) and Harrison ('18) have shown, 
for instance, that the limbs of the amphibian embryo, if removed 
at an early age, will soon be replaced through the regenerative 
capacity of the surrounding tissue. This is also true of the audi- 
tory and nasal placodes, the lens, and the gills. The possession 
by the amphibia, of the regenerative power to such a high degree 
would naturally lead to the presumption that removal of the 
pronephric rudiment might result in a similar replacement of 
this organ. This, however, as will be shown later, is not the 
case in the Amblystoma larva, for the adjustment consequent 
on removal of the pronephros is not in the nature of a restitution, 
but is a compensating hypertrophy^ of the remaining head kid- 
2 A peculiar instance of compensatory hypertrophy in another organ is cited 
by Kochs ('97) in his work on Triton, where the amputation of the fore leg often 
resulted in a marked hypertrophy of the tail. Retardation or acceleration in 
the growth of a fore leg in the larvae of Rana esculenta and Bufo viridis may be 
induced by removal of a hind leg, according to Kammerer ('05), the hastening or 
arrest of growth depending on the rapidity of wound healing. 
