62 



ALICE M. BORING AND RAYMOND PEARL 



In the last paragraph it was assumed that X would not escape 

 notice in many of the cells of a form where present, that is, that 

 the element would be visible if present, in approximately 50 per 

 cent of all dividing spermatocyte cells, 100 per cent of first and 

 0.00 per cent of second spermatocyte cells, or vice versa, 100 per 

 cent of second and 0.00 per cent of first. But any one who has 

 worked much in the cytology of germ cells knows that there are 

 good reasons why X sometimes escapes notice when present. 

 It does not by any means always take up an eccentric position 

 in the equatorial plate; it does not always show itself in the side 

 view of the metaphase spindle by projecting at one side or by 

 appearing visibly univalent instead of bivalent; neither does it 



TABLE 2 



always lag behind the dividing chromosomes during the anaphase 

 of the mitosis. In order, then, that the conclusions of the pre- 

 vious paragraph might be based on facts and not on an assump- 

 tion, a similar statistical study was made of 1000 dividing cells 

 of a bug, Philaenus spumarius, the spermatogenesis of which has 

 been previously worked out and found to exhibit a typical un- 

 paired sex chromosome (Boring '13). The cells were picked out 

 in the same way with the mechanical stage, systematically at 

 random, all the clear ones in a certain area being catalogued. 

 The results are seen in table 2, which shows that the previous 

 assumptions were indeed very close to the facts. Out of 1000 

 cells, 49.4 per cent show X, none of the second spermatocytes 

 show it as a chromosome behaving differently from the others, 

 while in 86.21 per cent of the first spermatocytes, it can be differ- 

 entiated. In the equatorial plate, there were only 8 out of 22 



