52 MICHAEL F. GUl^R 



second spermatocytes, inasmuch as they also would likely each 

 contain chromosomes of different parentage. 



By far the greatest number of division stages to be seen in the 

 hybrid testis were those of the first spermatocytes. These were 

 comparatively plentiful and to my surprise were much more 

 normal in appearance than corresponding division stages in 

 pigeon hybrids from more closely related species. The multi- 

 polar spindles so characteristic of the first spermatocytes in hybrid 

 pigeons were very seldom encountered in the guinea-chicken fowls. 



Perhaps the most interesting feature of this first maiotic divi- 

 sion was the appearance of the accessory chromosome or X-ele- 

 ment. This chromosome, whenever favorably located for obser- 

 vation, was found invariably to be of the guinea and not of the 

 chicken type. 



It will be recalled that in various species of invertebrates the 

 X-element of the male is now known to be represented in the female 

 by two such elements in all cells with the diploid number of 

 chromosomes. In such females, after the reduction divisions 

 each egg thus comes to have a single X-element whereas, since 

 there was only one such element in the somatic and early germ- 

 cells of the male, and inasmuch as this body does not divide in 

 one of the maturation divisions of the cell but goes entire to one 

 of the daughter cells, the X-element is lacking in half of the sper- 

 matozoa. 



In all known cases where an X-element exists in the male, 

 the eggs fertilized by a spermatozoon without the X-element are 

 the ones that give rise to the new males, hence the subsequent 

 X-element of one of these new individuals can only be one which 

 was originally in the egg. The fact that the male zygote must 

 always receive the X-element from the mother was pointed out 

 by Wilson^ in 1906. In the present instance, since it was the 

 mother of the hybrid that was the guinea, the X-element of 

 the hybrid should be of the guinea type, and such, in fact, was 

 found to be the case. 



The chicken and the guinea types of X-element are readily 

 distinguishable, that of the chicken being typically of larger 



^Wilson, E. B.: Studies on chromosomes, III. Jour. Exp. Zool., vi, 1906. 



