372 KUTH B. ROWLAND 
of such operations, the controls were always more advanced 
than the operated embryos, so that, for the comparison of the 
excretory organs, others had to be selected as described below 
(p. 373). 
Postoperative effect on the embryo as a whole 
Every outward evidence of successful readjustment to the 
new conditions imposed was shown by the operated larvae. 
Adverse symptoms, such as edema and general sluggishness, were 
absent, and, barring the slight retardation already mentioned, 
normal progress continued, except in those cases in which the 
limb bud was disturbed or entirely removed. The ectodermal 
surface which showed a slight concentration of pigment in the 
initial stages of wound healing gradually became indistinguish- 
able from the surrounding regions, and differed from the opposite 
side only in the absence of the distention caused by the under- 
lying pronephric coils. 
Effect of unilateral excision on the remaining pronephros 
The pronephros remaining after removal of one head kidney 
obviously takes over the function of excretion usually per- 
formed by the two organs. Beginning with the fourth 
day after excision, operated embryos were killed for obser- 
vation each day for a period of two weeks. Sections showed 
distinct changes in the several remaining components of the 
excretory system, particularly in the head kidney functioning 
alone, the size of which was indicative of a marked compensa- 
tory hypertrophy. Since the controls taken frcm the same 
egg mass and carried along under the same conditions as those 
to which the operated forms were subjected invariably showed on 
sectioning a more advanced stage of development, the first step 
in determining the nature and extent of the change in the oper- 
ated kidney was the establishment of a criterion for comparison 
of an operated with a normal embryo. An operated individual 
(PN 7) was chosen as a typical case and a large number of nor- 
mal larvae of apparently the same age was examined to obtain 
