CHROMOSOMES IN CAVIA COBAYA. 75 



three to seven micra proved best for study. It was difficult to 

 isolate individual cells in thicker sections. Sections three micra 

 did not seem to be particularly advantageous over those five or 

 six micra. The most favorable stain was Heidenhain's iron- 

 alum hematoxylin. 



DESCRIPTION OF MATERIAL. 



It was found that the actively dividing cells are not distributed 

 throughout the whole lining of the seminiferous tubules but are 

 in very restricted areas. These areas are larger and more 

 numerous in the superactive male. They are elliptical in shape. 

 The greatest diameter is always lengthwise of the tubule. This 

 dimension never exceeds two thirds of the circumference of the 

 tubule and may be as small as only a few cells. 



I. Spermatogonial Divisions. 



Many and repeated counts have given us thirty-eight spermato- 

 gonial chromosomes. Eight of these are distinctly U-shaped 

 which may be arranged in four pairs. Twenty-eight chromo- 

 somes are slightly bent rods of various sizes which, according to 

 size and shape, may be arranged in pairs. The remaining two 

 chromosomes are unequal in size and irregular in shape. We have 

 designated these as the XY-pair. Fig. 2 is a polar view of a 

 metaphase plate of a spermatogonial division. The chromo- 

 somes which form the members of a given pair have been desig- 

 nated by the same number. Reference to this figure shows that 

 the chromosomes are not arranged merely around the periphery 

 of the spindle as is so often the case in insects (Harman, 1915 and 

 1920), but fill in the entire diameter of the spindle. No one kind 

 of chromosome occupies one area of the spindle exclusively. The 

 large, slightly bent rods are deep in the spindle as well as around 

 the periphery. The same may be said of the U-shaped chromo- 

 somes. However, the relative arrangement of these various 

 chromosomes is approximately the same. 



2. The Growth Period. 



Evidently a longer period of time is spent in the stages between 

 the last spermatogonial division and the first spermatocyte 

 division than in any other stage, for there are always many times 



