60 ALICE M. BORING AND RAYMOND PEARL 



That this is not the explanation in the Barred Plymouth Rock 

 will be shown by the statistical tables given later in this paper 

 (pp. 61-62). 



It is interesting to notice that it is easy to pick out from either 

 the photomicrographs or the drawings a series of pictures which, 

 taken by themselves, would show an X-chromosome to be present 

 and to find it dividing in the first maturation division. Figures 

 26 a, 30, 34, 36 c, 35 a, 35 b are such a series of photographs, 

 and figures 45, 62, 66, 70, 78, 84 such a one of drawings. Further, 

 figures 3 a, 17 a, 20, 22 a, 23 b, and the series 39, 54, 63, 77, 82, 

 83, taken by themselves, show just as conclusively that an X- 

 chromosome is present and fails to divide in the second sperma- 

 tocyte division. Still further, a series of figures made by a com- 

 bination of the figures of the previous two series, for example, 

 figures 3 a, 17 a, 20, 36 c, 35 a, 35 b, or 39, 54, 63, 70, 78, 84 

 shows that there is no X-chromosome present at all. 



The above reductio ad absurdum, proving three conflicting 

 hypotheses out of the same material, is the crux of the argument 

 of this paper. In difficult cytological material, such as this, one 

 can prove anything one wants, by picking out certain cells and 

 ignoring all others. The only way to find out the real facts is 

 to study a large number of cells and find out what happens in 

 the majority. So this investigation comes to a focus around the 

 question : Is this so-called X-chromosome in the Barred Plymouth 

 Rock present in enough cells to be justly regarded asthehomologue 

 of the X-chromosome in insect spermatogenesis? 



The following statistics were worked out to answer this ques- 

 tion. Slides were systematically examined by means of the me- 

 chanical stage, and every clear dividing cell within a certain 

 area was recorded under one of the twelve headings in table 1. 

 Several nuclei are often grouped in one individual mass of 

 cytoplasm, as described by Guyer for the Langshan birds ('09). 

 It i^ easy to distinguish the secoild spermatocytes from the first 

 by the amount of chromatin and the size of the spindle. The 

 criteria used for the presence of X were those used by Guyer 

 for the Langshan material, either a separate mass of chromatin 

 as in figures 10 and 14, or a very conspicuous arm protruding 



