GERM-CELL HISTORY IN THE BROOK LAMPREY 85 



two kinds of sexual individuals, each theory to be replaced by 

 others, which further research found equally untenable. In- 

 vestigators now began to search for this odd element in the sex 

 cells of various species of animals. More and more forms, espe- 

 cially among insects were found in which the odd element was 

 present in the germ cells and in which it became distributed to 

 half of the mature cells. 



It is natural that this discovery should have led to a qualita- 

 tive explanation of sex. There apparently was something present 

 in half of the male germ cells which, after fertihzation, was 

 responsible for the development of a male. This was McClung's 

 interpretation and this explanation was accepted by the majority 

 of his immediate followers. 



The early work was done on the accessory chromosome of the 

 male germ cells alone. When cytologists began investigations 

 upon the chromosomal structure of the female germ cells (Wil- 

 son, '05; Stevens, '05, and others) it was found that this odd ele- 

 ment was present there also, not singly but in duplicate. These 

 two accessories were so distributed during maturation that every 

 egg retained one, and consequently all the eggs were ahke in 

 their chromosomal structure. Theoretically, therefore, an egg 

 which happened to be fertilized by a spermatozoon containing 

 the accessory chromosome would give rise to a female, and not 

 to a male as had been supposed to be the case. It became clear 

 that the accessory chromosome could not be sex determining by 

 virtue of any qualities it might possess, but rather that sex was 

 due to a quantitative difference in the amount of the odd 

 chromosomal material present in the fertilized egg. 



Certain studies in heredity have shown that some characters 

 are sex-linked. The interpretation of this fact is that the factors 

 for such characters are carried by the sex chromosome. It was 

 discovered that the inheritance of sex-linked characters in forms 

 like moths, butterflies, and birds was such as to necessitate the 

 assumption that the ova in these forms rather than the sperma- 

 tozoa were dimorphic in regard to the sex chromosome. Later it 

 was discovered by Seller ('14) that there are actually two kinds 

 of eggs in the moth Phragmatobia fuhginosa. In the case of 



