Sex Dclcrmi)ialion 187 



occurrence of occasional individuals of these exceptional tyix's led 

 Bridges to suspect that non-tlisjunction had taken j)lace. Later 

 he effected a striking confirmation by cytological demonstration 

 that the exceptional vermilion females possessed a Y chromosome, 

 and that the exceptional red males had no Y chromosome. This 

 work provides the linal convincing demonstration that the chromo- 

 somes are the bearers of hereditary characters, since abnormalities 

 in the distribution of a certain chromosome set arc accomjjanied 

 by corresponding abnormalities in the distribution of those genes 

 which were assumed to be located on that chromosome set. 



In the chapter on bud variation the phenomenon of ''chromo- 

 some elimination" was discussed (p. 121). Morgan* and Bridges 

 (17) have discovered this sort of thing in connection with the sex 

 chromosomes in the fruit fly. An individual which starts its 

 development as a normal female, A'A', has one of the A' chromo- 

 somes eliminated from one of the daughter-cells at an early 

 embryonic division. Tissues arising from this daughter-cell 

 have only one A" chromosome and show the characteristics of the 

 male sex, while the rest of the tissues are female. Individuals of 

 this part female — part male type are known as gynamiromorphs. 



Physiological theories.— In 1906 Hertweg (14) 

 performed some sex determination experiments with 

 frogs. The eggs are laid free in the water before fer- 

 tilization, so that they furnish unusually good material 

 for such experiments. Normally the eggs are fertilized 

 very soon after they are laid, with the result that the 

 progeny consists of api^roximately 50 j)er cent males and 

 50 per cent females. Hertweg took some of these eggs 

 and allowed them to overripen before fertilization took 

 place, that is, he put aside some eggs as soon as they 

 were laid and allowed them to remain unfertilized iov an 

 unusually long period. While these eggs were standing 

 in the water he found that they absorbed an unusual 

 amount of water, and the obvious conclusion was that 

 overripe eggs show high water content. He then allowed 



