200 CHARLES L. PARMENTER 



just as difficult to demonstrate the absence of pairs. It seems 

 to me that the evidence in the Dipteran somatic complexes, 

 where the members of a pair lie parallel and adjacent to one 

 another, together with the already large and well-supported 

 evidence of pairs in the various generations of the germ cells 

 throw the balance greatly in favor of the presence of homologues. 



DISCUSSION 

 A. Introductory stateinent 



The foregoing observations upon the constancy of chromosome 

 number and the existence of pairs in the somatic chromosome 

 complexes have their chief importance in their relation to the 

 Roux-Weismann hypothesis that the chromatin is the idioplasm, 

 which is differentially organized and linearly arranged, and 

 that this organization is perpetuated. This hypothesis received 

 important support from the theory of the individuality of the 

 chromosomes as set forth by Van Beneden ('83) and strongly 

 maintained by Rabl ('85), Boveri ('88, '02), and numerous other 

 more recent investigators. The morphological evidence ac- 

 cepted as supporting this proposition is an essential constancy 

 of number, size, form, and behavior. 



Since McClung ('17) has so recently thoroughly considered 

 the theory of individuality, this discussion is confined to the 

 particular phases of the supporting evidence which are directly 

 related to the observations made upon this material. These 

 phases are essential constancy of number, of size, and of form. 



B. Constancy of chromosome number 



Delia Valle has strongly attacked this theory on the basis of 

 inconstancy of chromosome number. He arrives at the con- 

 clusion ('09, p. 120 ff.) that the number of the chromosomes is 

 the quotient of the quantity of the chromatin divided by the 

 average size of the chromosomes; that their size is variable 

 according to the nature of the elements and the conditions in 

 which they are found, and ('11, p. 188) that the size and number 



