STUDIES ON CHROMOSOMES. 253 



one rather than a pair of elements in the male somatic cells made 

 the cytological evidence, if we were to continue to regard this 

 accessory element as specifically concerned with the sex-linked 

 characters, apparently stand at variance with the facts established 

 by breeders. The evidence as presented in this paper, if I have 

 correctly interpreted it, does away with this difficulty, since the 

 female soma as seen in chick embryos is heterozygous and the 

 male soma is homozygous for a special curved element which 

 to my mind fulfills the requirements foi being regarded as a 



bona fida X-element. 



SUMMARY. 



1. My later studies confirm my earlier ones as regards the 

 finding of a large curved chiomosome in primary spermatocytes, 

 comparable to the so-called sex-chromosome of other forms. 



2. The piesence of this element is recorded in 963 primary 

 spermatocytes which were sufficiently well prepared to show inter- 

 pretable detail. It has been observed in many others. It 

 probably exists in all although often obscured by the other 

 chromosomes which tend to stick together. 



3. It is, for variously fixed material, surprisingly constant 

 in shape and size, in Langshan, Plymouth Rock and Rhode Island 

 Red fowls. 



4. A similarly constant element, differing in form from that 

 of the common fowl, is found in the guinea and in the guinea- 

 chicken hybrid. 



5. At the division of the primary spermatocyte this element 

 passes undivided to one pole thus producing two classes of 

 secondary spermatocytes, one with nine and one with eight 

 chromosomes. 



6. The element in question is probably a bivalent chromosome 

 formed by the union of two characteristic curved chromosomes 

 which occur in spermatogonial and somatic cells. These ele- 

 ments may be seen to best advantage in the testicular or nephri- 

 dial cells of 10 to 14 day chick embryos. The remaining chromo- 

 somes, typically 16 in number, are usually rod or block shaped. 



7. In female chick embryos of 10 to 14 days, a relatively 

 large percentage of dividing cells which were found in the ovarian 

 and nephridial tissues showed a single large curved element. 



