CHROMOSOMES OF INDIAN RUNNER DUCK. 345 



that I was examining entire cells, not sections of cells.) These 

 rows may extend in any direction across the nucleus, but they 

 always lie within the circle of the larger chromosomes. 



DISCUSSION. 



The chromosome number in the somatic cells and early germ 

 cells is traceable half of it to the paternal parent and half of it to 

 the maternal parent. There is, then, in the typical cell, an even 

 number of chromosomes. If in any form an odd number is 

 found, the question is immediately presented as to the disposition 

 of the odd chromosome in maturation and what part it might 

 play in the transmission ot hereditary factors. 



An attempt to solve the problem of the odd chromosome in the 

 case of the Indian runner duck, as well as in any other form, 

 necessitates not only a tracing of its disposition or probable dis- 

 position in maturation and zygote formation but also a dis- 

 covery of a parallelism with the vast amount of results obtained 

 from experimental breeding. 



In the Indian runner duck the male has an even number, 

 seventy-six chromosomes, but the female has an odd number, 

 seventy-seven, and strange to say, one more rather than one 

 less than is present in the male. Of course where the odd num- 

 ber occurs in the female, one would immediately look to the 

 chromosomes in polar body divisions for an explanation. The 

 difficulty of getting eggs in the right stage of maturation is at 

 once apparent. It seems best, therefore, at this time to attempt 

 an explanation of the phenomenon from indirect evidence and in 

 the light of the results of the work on Lepidoptera, in which the 

 female is known to be digametic for sex as is probably the case in 

 the aves. 



In the duck the additional unpaired chromosome present in the 

 cells of the female is without a homologue in the cells of the male 

 and of course could not then be a sex-linkage chromosome. If it 

 is not concerned with sex-linkage it must probably belong to the 

 class of sex-determining bodies known as the W-chromosomes, 

 occurring only in females. 



With this chromosome (W) ruled out of consideration it then 

 becomes the problem to find which one of the remaining chromo- 



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