PRESIDENTIAL ADDRESS SECTION D. 87 



when migration of the other chromosomes to the poles occurs. In 

 ■consequence the resulting spermatozoa were of two kinds, namely, 

 those possessing the accessory chromosome and those lacking it. As 

 long ago as 1902 McClung, one of the first observers of this 

 phenomenon, suggested that the accessory chromosome was the deter- 

 mining factor in sex. The famous American cytologist, Professor 

 E. B. Wilson, and others followed up this matter, and found that 

 the accessory or X chromosome, as it began to be called, was paired 

 in the female. The two X chromosomes in the female behave in a 

 general or normal manner, so that all eggs after maturation con- 

 tain a single X chromosome. As a consequence, at fertilisation 

 two classes of eggs are produced — (a) eggs fertilised by a spermato- 

 zoon carrying the X chromosome, and (b) eggs fertilised by a 

 spermatozoon carrying no X chromosome. Two kinds of zygotes 

 (as the fertilised eggs are called) will be produced, namely (a) 

 zygotes with what has been called XX constitution, and (b) zygotes 

 with X constitution. Furthermore, the highly interesting result 

 follows that the first type of fertilised eggs or zygotes develop into 

 females and the second type into males. 



During the early period of the investigations other Hemiptera 

 were found by Wilson to exhibit further chromosome peculiarities, 

 wherein the X chromosome of the male was accompanied by 

 another, a mate, called the Y chromosome, usually much smaller 

 than X. It was found that the X and Y chromosomes conjugated 

 during maturation, so that half the resulting sperm contain the X 

 chromosome and half contain the Y chromosome. The X con- 

 taining sperm are female-producing, the Y containing ones .are 

 male-producing. Morgan's work on the fruit fly, Drosophila 

 ampelophila, revealed some most interesting characters illustrating 

 this principle. In this fly a number of new characters have 

 appeared suddenly, by mutation, and their inheritance can be 

 studied, since the fly breeds rapidly. Morgan's researches on eye 

 colour in this fly may be described best in his own words, thus: 

 "Certain factors follow the distribution of the X chromosome and 

 are therefore supposed to be contained in them. These factors are 

 said to be sex-linked. The inheritance of white eyes may serve as 

 an illustration for the entire group of sex-linked characters. If a 

 white-eyed male is bred to a red-eyed female (wild type), the sons 

 and daughters (F 2 ) have red eyes. If these are inbred the 

 offspring (F 2 ) are three reds to one white, but the white- 

 eyed flies are all males. If we trace the history of the sex 

 chromosomes we can see how this happens In the red- 

 eyed - mother each egg contains an X chromosome bearing 

 a factor for red eyes. In the white-eyed father half of 

 the spermatozoa contain an X chromosome which carries a factor 

 for white eyes, while the other half contain a Y chromosome which 

 carries no factors. Any egg fertilised by an X-bearing spermato- 

 zoon of the white-eyed father will produce a female that has one 

 red-producing X chromosome and one white-producing X chromo- 

 some. Her eyes are red, because red dominates white. Any egg 

 fertilised by a Y-bearing spermatozoon of the white-eyed father 

 will produce a son that has red eyes, because his X chromosome 



