CHROMOSOMES OF INDIAN RUNNER DUCK. 351 



SUMMARY. 



1. This study was undertaken with two points in view: First, 

 to determine the number of chromosomes and second, to discover 

 the mechanism for sex-determination and sex-linkage in the 

 duck as a representative of the aves. 



2. Material from thirteen individuals was used, seven males 

 and six females. 



3. Chromosome counts were made on entire cells, both somatic 

 and spermatocytic. 



4. In the somatic cells there appear to be seventy-six chromo- 

 somes for the male and seventy-seven chromosomes for the 

 female. There is present in the cells of the female a long un- 

 paired chromosome which is not found in the cells of the male. 

 There is reason to suppose that there are probably among the re- 

 maining six largest chromosomes two more unpaired chromo- 

 somes, one of which, the largest, is probably homologous to the 

 largest pair (sex-linkage) of chromosomes in the male complex, 

 while the other, it is thought may be some one of the five remain- 

 ing long chromosomes. 



5. There appear to be 38 bivalents in the primary spermato- 

 cytes of the male. These agree with the pairs of somatic chromo- 

 somes in size gradations. Most of the chromosomes in the 

 spermatocytes appear to be bivalent or tetrad in form. 



6. Sutures accompanied by constrictions seem constant in 

 position in at least two points in the largest rod-shaped chromo- 

 somes. These sutures seem to lend additional pliancy to these 

 regions in the chromosomes. 



7. Gonomeric grouping of chromosomes occurs in the amnion 

 cells of the duck. It is also thought to be present in the chromo- 

 some plates of the gonial cells in the embryo of the chick and the 

 lizard. 



8. Filamentous linkage between certain of the smaller chromo- 

 somes appears to be present in certain stages of the prophase. 



9. There is reason to believe that the sex-mechanism is of the 

 WwZ-ZZ type similar to that found in the moth Phfagmatobia. 



10. This would give the female more chromatin than the male 

 and yet preserve female digamety, which would bring this type 

 into harmony with the usual conditions found in the XX-Xy 

 type. 



