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 



Peritoneum 



Mesentery 



Lung 



Tail epithelium 

 Gill plates 



Totals 



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 



