REMOVAL OF PRONEPHROS OF AMBLYSTOMA 371 
first loop being removed, while in the second group (B), the 
funnel rudiment was cut off just anterior to the first bend. Of 
the twenty specimens used, all exhibited symptoms identical 
with those induced by bilateral excision. 
The extent of regeneration which would occur in a defective 
tubule if the opposite kidney were allowed to remain intact is 
still a matter for investigation, but it is not improbable that 
a given portion of a tubule may possess a prospective potency 
which would insure the restoration of an excised section. How- 
ever, in dealing with an organ where the demand for functional 
activity follows so closely upon this disturbance of normal con- 
dition, the requisite time for readjustment may be the factor 
lacking. Accumulation of excretory fluid may inhibit the regen- 
eration which might be the normal consequence, if excretory 
activity were maintained by an undisturbed kidney. A funda- 
mental difference thus places excision of the kidney in a category 
apart from the majority of regeneration or transplantation 
experiments upon the amphibian embryo which have been 
reported up to the present, for the effects consequent on extirpa- 
tion and transfer of limb rudiments, optic vesicles, or nasal pits, 
though abnormal, are not of a nature to interfere with any of the 
vital functions of the embryo. 
UNILATERAL EXCISION OF PRONEPHRIC RUDIMENTS 
Conclusive evidence having been obtained as to the essential 
nature of the pronephros in the life of the embryo, a further 
study of the correlation of the development of this organ with 
that of the other components of the excretory system was then 
undertaken.'' Unilateral excision of the pronephric rudiment 
served. as a practical means to this end. 
The technique of operating has already been discussed in the 
previous section, but a word of explanation is necessary regard- 
ing the controls used in this series. Since a more or less pro- 
nounced retardation in growth was the unavoidable consequence 
' As has been previously stated, the glomerulus was found to develop normally 
even in the absence of the pronephric coil. 
