196 COMPARATIVE PHYSIOLOGY 



the facts elicited. Next consider the resuhs of back-crossing 

 these XXY abnormal F.i white females to a normal X'Y male. 

 According to whether the X elements segregate with respect 

 to one another or the Y chromosome, the F.i white females 

 will lay four types of eggs : XX, Y, XY, X. If these are 

 fertilised by a Y sperm (which cannot bring in the red factor), 

 we get four types : {a) XXY white females which will obviously 

 behave in the same way, thus agreeing with breeding 

 experience ; (b) YY — individuals with such constitution 

 cannot exist ; (c) XYY — white males which should produce 

 XY sperms so that in crossing with normal white females 

 daughters of the XXY type producing exceptional progeny 

 would result ; (d) XY — normal white males. When, on the 

 other hand, the same four classes of eggs are fertihsed by an 

 X' sperm carrying the red factor, four red types of offspring 

 would result, as follows : (a) X'XX — a triploid female which 

 usually dies ; (b) X'Y — normal red males ; (c) X'YX red 

 females with abnormal offspring ; (d) X'X normal red females. 

 Thus the non- disjunction of the X chromosome in the forma- 

 ion of the eggs of some of the females of the parental white- 

 eyed stock accounts for the entire series of exceptional genetic 

 phenomena which occur in these strains. 



Recently Bridges has shed further light on the genetical 

 aspect of sex- determination by the discovery of non- disjunction 

 in chromosomes other than the sex-chromosomes, sometimes 

 referred to in contrast to the latter as autosomes. In an 

 experiment in which a brown mutant of Drosophila was 

 crossed back to a parental stock, a culture was obtained in 

 which the individuals were almost exclusively females or sex 

 intermediates. These " intersexes " displayed intermediate 

 sex-characters throughout, notably in the abdomen and in 

 the sex-combs of the tarsal joint of the forelegs, and also 

 genitalia. On the whole they fell into two groups, one tending 

 more to the female, the other to the male condition. Genetical 

 evidence led Bridges to conclude that for one group of genes 

 at least the female individuals of these cultures were triploid, 

 i.e. inherited a double instead of a single set of genes from 

 their fathers. Microscopic examination of the germ cells 



