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 relationhips. 



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. Delia 

 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. 



