558 Theophilus s. Painter, 



and significant fact that tlie accesoiy chromosome shonld show the 

 same behavior and the same form in thirteen families of spiders, 

 families which must be separated from one another, phylogenetically, 

 by very long periods of time. One conclusion may be drawn from 

 this fact with areasonable degree of certainty, namely, that the accessory 

 chromosome must have had a very important and constant function 

 in the life of these spiders. This conclusion is the more striking 

 when we remember the changes which have taken place in the 

 autosomes, particulary with regard to the reduction in the number 

 present in the cells of the différent species. It is difficult to conceive 

 how the accessory chromosome could have persisted with the same 

 form and the same behavior unchanged through the great lengths 

 of time which these spiders have lived, unless its function has been 

 the same throughout the group. 



3. Determination of sex. 



Through the works of Wilson, it has become a thoroughly 

 established fact, that the accessory chromosome has to do with the 

 determination of sex ands sex-linked inheritance. Wilson (1911) 

 has given such a comprehensive review of this whole subject that 

 we shall do no more than call attention to the fact that the behavior 

 of the accessory chromosome elements in spiders is in entire accord 

 with the results which AVilson and others have obtained through 

 a study of insects. 



In spiders half of the sperm carry the accessory chromosome 

 elements and half lack these bodies. The female carries two measures 

 of the accessory chromosomes and while the oogenesis has not been 

 worked out, it seems certain that each egg will be found to carry one 

 measure of these bodies. Sex will be determined then according to 

 the following scheme. 



ZYGOTES 



Female 



Diagram IV. 



