210 



KiyOSHl MASCI. 



tlie increas of theire mitiiber. Owing to this fact, for the determination of 

 the nuraber of chromosomes ouly those cells were selected which were 

 clearly recognized as being nncut, and in which the chromosomes were so 

 distinctly separated from each other that the count coidd be made easily 

 and accurately (Figs. 4, 5). 



Nearly all the cells studied contain forty chromosomes. Dataüs as to 

 the number of chromosomes are given in the following table. 



Table I. Showing number of chromosomes. 



In determiuing the number, every chromosome was cirefally drawn with the aid o£ a 

 ciimera-lucida at the magnification 2000 diameters. In order to avoid miscounting the drawing 

 was carefully c)m::ari;d with the chromosomes of the section under the microscope. 



As shown in tha above table, among the 22 thick sections only one cell 

 contains thirty nine chromosomes. This difference in number, however, is 

 probably due to miscoimting. 



From the above, it is evident that the chromosomes of the male germ 

 cell of the mousa are constantly forty in number. Tlie chromosomes in every 

 spex-matogouial cell vary considerably in size and form, and usually assume a some- 

 what curved rod- or straight rod-shape. When the chromoso'xes are arranged 

 according to size and form, it is found that pairs exist among them, although 

 they form a series wliich falls gi'adually from the largest to the smallest 

 (Figs. 9-12). The following table and the test figure show this relation clearly. 



Table II. Representing area of spermatogonia! chi-omosomes 

 at a magnification of 32000 diametere. 



Method of meastircment : — A definite nuraber of homogeneotts papers (ten pieces of paper 

 to each chromosome) whii-h have the Fame size n:ul form ns each magnified chromosome auil a 

 standard area of the same paper were accurately weighed with a b.ilance. From a comparison 

 between the weight of the f jrmer and that of the latler. the area of each chromosome was 

 calculated. 



