580 CLARENCE E. McCLUNG 



suffer division of their substance and they are obhged to re- 

 store this loss through metabohc changes. Since these changes 

 of substance take place at surface contacts there is an obvious 

 advantage in increased superficies and, in common with other, 

 larger structural elements, the chromosomes become extended 

 and their substances are diffused. In this state their bound- 

 aries may not be well defined and this circumstance has been 

 seized upon as a disproof of their continuity. 



For myself, even were the visible limits of the chromosome 

 completely lost, this would not appear as a convincing disproof 

 of persistent individuality in the face of the large number of 

 facts pointing in the opposite direction. Of such facts those 

 relating to the history of the multiple chromosomes, outlined in 

 this paper, are of great importance. Their bearing upon the 

 various aspects of numbers, sizes, forms and behavior, has been 

 mentioned under these different headings. At this point how- 

 ever I wish to summarize this evidence upon the general topic of 

 individuality. But before doing so I should like to repeat state- 

 ments previously made that the accessory chromosome in the 

 germ cells of the male presents us with the history of a particu- 

 lar chromosome, at all times distinct and well delimited, whose 

 physical identity suffers no eclipse during the various metabolic 

 changes of growth and division. It is readily distinguished 

 where it occurs, not alone in a given cell, but in all the germ 

 cells of the species, genus, family and order. The same element, 

 always recognizable by reason of its structural character, is 

 identified in different phyla and is shown to have a definite rela- 

 tion to the development of sex characters. It would seem im- 

 possible to conceive any more definite marks of individuahty 

 than is possessed by this chromosome, in whose history there is 

 no confusion of character or break of continuity, and yet it 

 does not receive even mention in some attacks upon the theory of 

 indi\dduaHty. 



Again, its position may be granted, but with the reservation 

 that it is unique and no criterion for other chromosomes. 

 Upon this point it is now merely necessary to say that the 

 accessory chromosome differs only in degree from the euchro- 



