CHROMOSOMES IN CAVIA COBAYA. 77 



pleted its division and each component is half way to the pol.- 

 Fig. 10 is an anaphase of a primary spermatocyte division. Thi> 

 figure shows a lack of synchronism in the division of the auto- 

 somes. 



Figure 9 is an anaphase of a secondary spermatocyte division. 

 It will be seen that some of the chromosomes divide much in 

 advance of the others. Figs. 11 and 12 are polar views of meta- 

 phase plates of the secondary spermatocyte divisions. In each of 

 these figures are seen nineteen chromosomes. Fig. 12 is not a 

 direct polar view but is a view of the metaphase plate at a 

 tangent. 



DISCUSSION. 



The large number of chromosomes, the comparative small size 

 and the tendency of the chromatin material to mass together and 

 form into clumps in which the individual parts are not distinguish- 

 able have made the study of mammalian chromosomes a difficult 

 task. This may, in part, account for the differences in the 

 interpretation of the chromosome complex in observations on the 

 same species as was cited in the beginning of this paper. Other 

 difficulties are the scarcity of cells in the various stages of active 

 division, the lack of synchronism in the division of the chromo- 

 somes and the distribution of the chromosomes throughout the 

 entire diameter of the spindle. 



We are of the opinion that a selection of animals which show 

 sexual activity at the time of securing the material greatly in- 

 creases the size and number of areas of cells in active stages of 

 division. We found this true not only for the animals used in 

 this study but also for a number of other guinea pigs which we 

 have observed. This also agrees with Painter's (Painter, 1924) 

 observations on the monkey and other mammals. 



Since the chromosomes are closely packed together the selection 

 of a fixing agent which does not swell the chromatin material is 

 very desirous. We have found that the modification of Flem- 

 ming's solution with urea used at a temperature slightly ab<>\ 

 freezing gives the most satisfactory results. We have used 

 Allen's modification of Bouin (Allen, 1919) at about bo<l\ 

 temperature with less satisfactory results. This is the technique 

 which Painter has used so successfully. 



