CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 181 
considered and would otherwise constitute a part of class I. In 
fact, an experienced cytologist of this laboratory, in examining 
these, without a knowledge of the number of chromosomes 
present, could see no reason for considering them as subjects 
for interpretation, and it seems almost absurd to place them in 
a separate class. 
Complexes of the third class. There was a very large number 
of cells which were beautifully clear everywhere except in regard 
to one or two chromosomes. However, only three of these were 
sketched, because the number of clear counts was so large that an 
increased number of these uncertain counts is of little value. 
Each of the three cells drawn contains two points of uncer¬ 
tainty as to whether there are one or two chromosomes present. 
The number is interpreted as either twenty-seven or twenty-eight. 
The minimum number of chromosomes possible of interpreta¬ 
tion in one cell is twenty-six, the maximum is twenty-eight; in 
the other two cells the minimum is twenty-seven, the maximum 
is twenty-nine. 
These three cases were interpreted while the sketch was being 
made and before it was known how many chromosomes were 
present. It is not true, therefore, that the interpretations were 
prejudiced nor that any cases which did not agree with the 
expected numbers were cast aside and consequently ignored. 
On the contrary, they are here included as part of the evidence in 
forming the conclusions drawn from this study. 
Rationally considered, then, of the cells sketched there are 
sixty-three in which the enumeration of chromosomes is accurate 
and dependable and three in which there are unavoidable 
interpretations necessary. These sixty-six complexes constitute 
very strong evidence that the number of chromosomes in Am- 
bystoma tigrinum is constant. 
b. Possible variation in number in uncounted complexes. As to 
whether or not there was any variation in chromosome number 
in this species can be judged from the results obtained from the 
sixty-six cells which were studied. If as few as 2 per cent of the 
total complexes studied varied from the usual number, at least 
one of these should have made its appearance. Furthermore, 
