CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 177 
approximately from six to ten weeks. The preparations of tail 
epithelium loaned by Professor Sigerfoos were taken from a 
collection which he has been accumulating for a number of years. 
It is probable, therefore, that the counts in these preparations 
represent the chromosome number present during a series of 
years and that the number is constant from year to year. 
Table showing for each tissue studied, the number of different individuals repre¬ 
sented and the number of complexes with their distribution into classes as de¬ 
scribed on page 178. The total number of different individuals represented is 
twenty-three 
TISSUE 
NUMBER 
OP INDI¬ 
VIDUALS 
I 
CLASSES 
II 
ill 
TOTAL 
Peritoneum. 
7 
14 
6 
1 
21 
Mesentery.. 
1 
1 
0 
0 
1 
Lung. 
4 
7 
2 
1 
10 
Tail epithelium. 
5 
9 
4 
0 
13 
Gill plates. 
8 
14 
6 
1 
21 
Totals. 
45 
18 
3 
66 ' 
a. Method of determining number. Since one of the chief pur¬ 
poses of this study is to determine accurately whether there is 
any variation in the number of chromosomes, considerable care 
has been taken to eliminate from the evidence every possible 
source of error. An important part of the presentation of this 
evidence is, then, a concise description of the exact procedure 
employed in obtaining it. 
1. Procedure. In order to avoid overlooking any mitotic 
figures, the entire surface of every piece of tissue was completely 
surveyed systematically before beginning to count any of the 
chromosomes in any of the complexes. The survey was accom¬ 
plished with a 4-mm. objective and an 8X ocular supplemented 
by a mechanical stage. 
In determining the number of chromosomes in each complex, 
a camera lucida sketch of it was first made at a magnification of 
2633 diameters. This sketch was carefully compared with the 
cell in order to make certain that no errors had been made in 
