OS ALICE M. BORING AND RAYMOND PEARL 



figure 88 eight, and figures 89, 90 and 91 nine. Figures 50 and 

 85 are the only two cells from smear preparations, where count- 

 ing was at all possible, and here it seems rash. Both of these 

 cells seem to show nine chromosomes. In the notes and rough 

 sketches left by Dr. N. M. Stevens on the Barred Plymouth Rock 

 material she has recorded more cells where she could count nine 

 than any other number, but she had counts recorded all the way 

 from 5 to 22. 



Another note of Dr. Stevens indicated that she did not find 

 any sign of the double reduction recorded by Guyer for pigeons, 

 chickens and guineas. Figures 70 to 75 look as though the num- 

 ber of chromosomes present in the secondary spermatocytes must 

 be more than half of nine, and figure 90, an aceto-carmine prep- 

 aration of a secondary spermatocyte, shows nine clearly as the 

 second spermatocyte number. However, these facts as to num- 

 ber are too meager to be offered as anything more than isolated 

 observations. The result of this investigation is to show that 

 there is no good evidence for a sex chromosome in the germ cells 

 of the Barred Plymouth Rock males and that therefore there is 

 no cytological evidence to conflict with the experimental evi- 

 dence that the female fowl is heterozygous and the male homozy- 

 gous for sex. 



SUMMARY 



There is no good observational or statistical evidence of the 

 existence of a sex chromosome in males of domestic chickens of 

 the Barred Plymouth Rock breed. In 11.82 per cent of first 

 spermatocytes and 3.06 per cent of second spermatocytes, there 

 is a piece of chromatin similar to that called an X-chromosome 

 by Guyer in Langshan males. This is not to be regarded as an 

 X-chromosome in Barred Plymouth Rock males, because: 



1. It is present in spermatocytes of both orders. 



2. A statistical study of Barred Plymouth Rock cells in com- 

 parison with those of the hemipteran Philaenus spumarius shows 

 that it is present in too few I spermatocytes and in too many II 

 spermatocytes, or vice versa. 



