544 CLARENCE E. McCLUNG 



case before us the indi\'idual animals in class 1 have twelve 

 chromosomes, those in class 4 have nine while the ones in class 

 5 have ten. The question we have to decide is, whether these 

 conditions represent absolute differences in organization, or 

 w^hether, within the nine and ten groups, the twelve elements 

 of class 1 are morphologically present. On inspecting a com- 

 plex of class 4 we find that the accessory chromosome is miss- 

 ing from its usual position in the series, but upon looking further 

 it is discovered, with all its usual characteristics of structure 

 and behavior, attached to one of the tetrads. As a structural 

 element of the cell it is demonstrably present and must be 

 counted, although it is not a free chromosome. In a complex 

 which is of almost identical composition otherwise, (class 5) the 

 accessory chromosome is a free element and would of course be 

 counted. In both cases the entire history of the accessory 

 chromosome in all cell generations is typical, whether free or 

 attached. The remaining two chromosomes of classes 4 and 5 

 are strikingly different from all the others, and upon careful 

 inspection are found to have morphological subdivisions, which 

 if counted in the usual way, restore the apparently missing two. 

 In other words, if the number of chromatids is counted the 

 total is forty-six, just as it is in class 1. So far as the number 

 of elements is concerned, therefore, we are dealing with the 

 same series in each case. It is obvious that if the differences in 

 number were the result of an unordered variation there would 

 be every reason to expect numbers in excess of twelve, together 

 with modifications of all the elements and not of precise changes 

 in particular ones. 



In a similar way, when the numbers in classes 2, 3 and 6 are 

 considered, the same e\ddences of ordered change are seen. Only 

 eleven separate chromosomes are found in class 2 but the largest 

 of these shows very clearly the accessory chromosome as a con- 

 is that the chromosomes are unstable and indefinite structures of the cell. Back 

 of such a position is the implication that evidence must be taken at its face 

 value without interpretation. This I consider entirely wrong. The essence of 

 observational work is interpretation, and in microscopical investigations it 

 reaches its fullest development. The value of evidence submitted by a micro- 

 scopist is in direct proportion to the quality of his interpretative ability. 



