REMOVAL OF PRONEPHROS OF AMBLYSTOMA 373 
one in which the stage of development was identical. In many 
embryos where superficial features, such as length, breadth, and 
condition of limb and gill rudiments, were the same as those of 
PN 7, it was found on sectioning that the internal organs varied 
widely in degree of development. The normal larva finally 
selected (PN 7 d) tallied^ not onl}^ in external measurements, 
but showed the several internal organs (retina, lens of eye, 
digestive tract, etc.) to be in a stage corresponding to those of 
PN 7. 
As a further check against the possibility of error in the choice 
of a normal duplicate, a second duplicate was chosen, and the 
respective volumes of the kidneys of the two roughly compared 
by the following method:^ On drawing-paper of uniform thick- 
ness the serial sections of the entire kidney of PN 7 d and of the 
second duplicate (PN 7 d, no. 2) were projected and the lijmen of 
the tubule outlined. The drawings of each kidnej^ were then 
carefully cut out. No attempt was made to assemble them in 
the form of a model, but the weight of the paper used for each 
was taken as a standard for comparison. The weight of PN 7 d 
was 2.35 grams and that of PN 7 d, no. 2, 2.26 grams, giving a 
difference of only 0.09 gram, or about 4 per cent — a variation so 
small as to be considered negligible. The larger normal kidney 
(PN 7 d) was used for comparison with the hypertrophied one 
in order to lessen the possibility of exaggerating the difference 
between the two. 
After the normal duplicate (PN 7 d) had been selected, several 
methods were open for the determination of the nature and 
degree of the hypertrophy of the remaining pronephros in the 
embryo from which the organ on one side had been removed. 
^ The slight variation would tend rather to minimize the contrast than to 
accentuate it, since, if either, PN 7 d is the more advanced. 
" In connection with the review of Kittleson's paper (see previous reference), 
I find that somewhat the same method was employed by him in his estimation 
of the relative surface areas and weights of the kidneys of rats. The weight in 
grams was reduced to square centimeters by estimating the average area in 
square centimeters of one gram of paper, and from this the total volume of the 
kidney was estimated. 
