THE ACCESSORY CHROMOSOME SEX DETERMI- 

 NANT? 



C. E. McCLUNG. 



PART I. OBSERVATIONS AND COMPARISONS. 



The peculiar chromatic element discussed under this name in 

 several recent papers is one that gives promise of throwing con- 

 siderable light upon the nature of the chromosomes. So long 

 as all chromosomes of the nucleus were observed to pass 

 through a cycle of changes apparently identical in each case, 

 there was little chance to gain an insight into their interrelations. 

 With the discovery of the accessory chromosome and the recog- 

 nition of its true chromosomic character, however, there has 

 been offered an opportunity to draw comparisons and so to formu- 

 late conclusions which, in time, are certain to materially increase 

 our knowledge of these most important nuclear structures. 



In recognition of this fact, and with the hope of hastening such 

 a desirable end, I have devoted some time to the study of the 

 accessory chromosome and have also encouraged students in my 

 laboratory to direct their attention to it. Much material has 

 been collected, and is still being accumulated, in order that as 

 broad a view of the subject as possible could be obtained. 



Since, however, the difficulties involved in securing and prepar- 

 ing material from widely different forms would unduly delay the 

 attainment of any comparative results, I have confined my stud- 

 ies largely to the Orthoptera. This has been done in the belief 

 that more substantial good can be derived from a thorough knowl- 

 edge of a limited group than from a superficial acquaintance 

 with a wider field. Once the basic principles underlying the cel- 

 lular phenomena of one group are discovered, their recognition 

 in other forms will be rendered much easier. 



As a result of the studies so far pursued, it has been found 

 that individual forms rarely present all the details of a problem 

 equally well. Different species excel in the clearness with 

 which certain points are brought out. A feature obscure in one 

 species will appear distinct enough in another while for the elu- 



