SPERMATOGENESIS OF THE CHICKEN 59 



of the chromatin on the spindle, and that this, if considered by 

 itself apart from all other elements, resembles an X-chromosome. 

 For instance figures 11 a and 51, 22 a and 73, show equatorial 

 plates with a separate chromosome at one side; figures 14 and 61, 

 23 b and 82 show metaphase spindles with a chromosome toward 

 one end of the spindle, apparently passing to one end without 

 dividing. Figures 34, 65 and 83 show anaphases with project- 

 ing chromatin arms at only one pole. These three appearances 

 are usuallj^ taken as signs that an X-chromosome is present. 

 The more conclusive and unequivocal evidence based on the 

 number of chromosomes in the spermatogonia, primary, and 

 secondary spermatocytes respectively is not here available, be- 

 cause it is impossible to count chromosomes accurately in this 

 material. 



However, to follow out the only kind of evidence available, it 

 is clear that half the above sample figures represent primary 

 spermatocytes, and half secondary. Now the tj^pical X-chro- 

 mosome fails to divide in only one spermatocyte division : either 

 the first, as in Coleoptera (Stevens '06) and Hemiptera homop- 

 tera (Boring '07) or in the second, as in some Hemiptera heterop- 

 tera (Wilson '05), and Orthoptera (McClung '00). It behaves 

 like any other chromosome in the equatorial maturation division, 

 whichever that may be. But the apparent X-chromosome in 

 the Barred Plymouth Rock can be seen passing undivided to one 

 pole in both primary (fig. 65) and secondary (fig. 83) spermato- 

 cytes. This would result in one-quarter of the spermatozoa with 

 X and three-quarters without it. Following this further; if the 

 chicken were like the majority of forms with sex chromosomes so 

 far studied, and had one less chromosome in the male than in 

 the female, there would be on the basis of the above observa- 

 tions a sex ratio among chickens of one female to three males. 

 Davenport's ('06) work on poultry, as well as very extensive 

 unpublished statistics on the pbint in the records of this labora- 

 tory, show that such a ratio does not exist in this form. 



Another possibility is that the X-chromosome may divide in 

 either the first or second spermatocyte division. Edwards de- 

 scribes a case of this in Ascaris ('10), but this is a rare occurrence. 



