CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 171 
cells of the same and other tissues of this closely related species 
and thus to determine whether such a variation as he claims is 
present in the somatic tissues of other Amphibians. Also the 
chromosomes of some cells in my material are sufficiently 
favorable for measurements to permit a reconsideration of their 
length relationships. 
Since this paper is regrettably controversial, it is necessary to 
give careful attention to all the methods and conditions under 
which the preparations and the observations were made. Della 
Valle also lays much emphasis upon this point, and therefore 
considerable space is devoted to this problem. 
For’facilities in collecting and preparing this material I am 
indebted to the courtesy of the Department of Zoology of the 
University of Minnesota, and to Prof. C. P. Sigerfoos I owe the 
loan of several very excellent preparations. The work was done 
under the direction of Prof. C. E. McClung, of the University 
of Pennsylvania, toward whom I feel especially grateful for 
constant encouragement and valuable criticism, and for his 
characteristically generous and kindly interest at all times. I 
am also greatly indebted to other members of the department, 
especially to Dr. Eleanor Carothers and Dr. D. H. Wenrich, for 
helpful suggestions and very painstaking criticisms. 
TECHNIQUE 
The material used was obtained during the spring of 1916 
from larvae of Ambystoma tigrinum, which were abundant in 
the ponds and lagoons near the College of Agriculture of the 
University of Minnesota. Mitotic figures in epithelial cells of 
the tail, gill plates, and lung, and of the endothelium from 
peritoneum and mesentery were studied. 
Tail epithelium 
Very excellent preparations of this tissue were kindly loaned 
me by Prof. Charles P. Sigerfoos, of the University of Minnesota. 
These were made from the tails of larvae f to 1| inches in length 
obtained during the last of May and the first of June during 
several years. 
