176 CHARLES L. PARMENTER 



Dehydration was accomplished by passing the material through 

 the above grades. The fluids were removed from, and added to, 

 the containers without handling the material. Alcohols were 

 followed by half xylol and half absolute alcohol, and finally by 

 xylol. 



The pieces of peritoneum were transferred from xylol to a 

 slide where the above-mentioned pigmented area, with the at- 

 tached muscle fibers, was removed quickly with a sharp scalpel 

 just before mounting. After mounting in damar under a cover- 

 glass, they were put under a light pressure for twenty-four 

 hours or more while drying to insure as flat a preparation as 

 possible. 



OBSERVATIONS 



It should be emphasized that the preparations upon which 

 these observations were made are unsectioned surface mem- 

 branes. This makes it possible to study the mitotic figures with 

 the confidence that all of the chromosomes are present and that 

 none have been cut and are being counted more than once. 

 This is an important consideration in determining whether the 

 number of chromosomes is constant. 



A. The number of chromosomes 



There are twenty-eight chromosomes in the somatic complexes 

 of Ambystoma tigrinum. In forty-five unquestionable enumera- 

 tions and in eighteen which contained either one or two chro- 

 mosomes that might possibly be considered subject to interpre- 

 tation, there are none which vary from twenty-eight. In three 

 complexes, because of the alternative interpretations possible at 

 one or more points, the number cannot be definitely determined 

 and is interpreted to be either twenty-seven or twenty-eight. 

 The fact that these numbers are so close to twenty-eight is 

 strong evidence that these cells contain the usual number of 

 chromosomes. 



The counts as indicated in the accompanying table have been 

 obtained from twenty-three different individuals varying in age 



