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 Httle 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 chrojnosomes in Am- 

 bystoma tigrinum is constant. 



h. 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, 



