484. Charles Zeleny 
3 Differences in Laboratory History. All the egg-masses, with 
the exception of egg-mass J, had the same laboratory history. 
They were collected at one time from the same pond and had the 
same laboratory environment. 
4. Changes in Rate During the Regeneration Period. Compari- 
sons are made only during identical parts of the regeneration pro- 
cess. 
5 Differences in the Level of the Cut. All. cut surfaces from 
which successive regenerations were compared were made as nearly 
at the same level as possible. Differences between individuals 
could not, however, be avoided, (a) because of the activity of the 
larve, and (b) because the line between the old and new tissue 
sometimes becomes obscured. In connection with the former 
difficulty all animals with second or later cuts evidently away from 
the line of the first cuts were discarded. The latter difficulty is a 
more serious one, since a slight change in level involves not only a 
difference in itself, but also because it involves a difference between 
old and new tissue. Fortunately this source of error was possible 
in only a few individuals. 
6 Additional Injury. All salamanders with additional injury 
were discarded. The precaution of putting the larve into sepa- 
rate dishes is necessary, not only because of the desirability of 
keeping separate records, but also because of the cannibalistic 
tendencies of the larva. When several larve are kept together 
most of the tails, legs, and gills are kept closely cropped. 
7 Temperature. he larvae were not kept at a constant tem- 
perature, but were all kept close together at one end of the room 
in order that the temperature of all dishes might be nearly equal. 
Furthermore, larve to be compared with each other occupied 
alternate positions on the table. 
& Food. All larvae were fed the same number of Tubifex 
worms of a uniform size each day. 
g Individual Variation. his was kept down by the above 
major precautions in addition to the following minor ones. ‘The 
dishes were of equal size and were all filled to the same level with 
water. To further insure the good health of the larva, dust was 
excluded by glass covers slightly raised at one edge and the water 
was changed every day. 
