440 



SCIENCE PROGRESS 



separate to pass indiscriminately one to each pole of the division 

 spindle. In this way the chromosomes are randomly dis- 

 tributed to the resulting cells. 



Up till quite recently it has always been assumed that the 

 chromosomes of the two sexes of an animal or plant were alike 

 in number, and a connection between sex and chromosomes was 

 never anticipated. In 1902, however, McClung,^ arguing on 

 d priori grounds, suggested that the accessory chromosome 

 was a sex-determiner. This accessory chromosome was dis- 

 covered by several observers, who found that in the males of 

 certain insects of the Hemiptera and Orthoptera groups there 

 was one chromosome which behaved differently from its 

 fellows during the maturation divisions. Thus it took no 

 part in the pairing process just described, but passed undivided 

 to one pole of the division spindle when the other chromosomes 

 did so. In this way the resulting spermatozoa were of two 

 classes, viz. those possessing the accessory chromosome and those 

 lacking it. Wilson ' and others who took up the investigation 

 speedily discovered that this unpaired heterotropic or X 

 chromosome — names by which it is often known — is paired 

 in the female, who thus possesses two such chromosomes. These 

 behave, during maturation, in a normal manner, with the result 

 that all the mature eggs contain a single X chromosome. 



Thus at fertilisation two classes of eggs are produced. 



(i) Eggs fertilised by a sperm carrying the X chromosome 

 giving rise to a zygote with the XX constitution. 



(2) Eggs fertilised by a sperm carrying no X chromosome 

 giving rise to a zygote with the X constitution. 



The first type of zygote will develop into a female, the second 

 type into a male. This is indicated in the first diagram below, 

 the symbol O representing the absence of an X chromosome. 



Diploid Nuclei 



Female. 



XX 



Male. 



xo 



Gametes 



Fertilisation 



Zygotes 



XX X o 



V 



XX 



xo 



XX 



XY 



Fig. I. 



Fig. 2. 



1 McClung, C. E., Biol. Bull., 3, 1902. 



2 Wilson, E. B., Journ. Exp. Zool., 3, 1906, p. i. 



