556 CLARENCE E. McCLUNG 



While these unequivocal conditions certainly obtain and jus- 

 tify the effort to homologise the chromosomes on the basis of 

 size, it must be admitted that there appears to be some fluc- 

 tuation in voluiTie, both absolute and relative, between the 

 same members of the series in different individuals. How much 

 this may signify it is difficult to say. There are so many ways in 

 which the size of a chromosome may be affected in the processes 

 of microscopical technique that a just estimate of the significance 

 of size variation is hard to reach. Thus it is kno^vn, for in- 

 stance, that very marked differences in size may result from the 

 method by which the animal is killed. If this be true, there are 

 doubtless other ways in which these delicate structures may be 

 similarly affected. Since however the animals themselves may 

 vary greatly in dimensions without in any other way modifying 

 their character, the same may be true of their chromosomes. 



A very careful study needs to be made of chromosome dimen- 

 sions, based upon material prepared with the utmost refinement 

 of technique and studied with most careful mensuration. Under 

 ordinary conditions there are many circumstances which would 

 serve to obscure differences between nearly related members 

 of a series and make their identification uncertain. While this 

 is true, most of our observations would indicate that agencies 

 affecting the size of chromosomes operate more or less uniformly 

 on the entire complex, so that a proportionate change is found in 

 each elenient. With the subject as difficult as it is there is no 

 wonder that views as widely divergent as those of Meek and 

 Delia Valle exist. The former author thought at one time that 

 he had detected a uniformity so great that only two diameters 

 appeared in all the Metazoa, but a more extended experience did 

 not confirm this opinion. It is true that there is much uniformity 

 in the diameter of the chromosomes of a complex, and Meek's 

 careful measurements disclosed this; but, on the other hand, 

 there are extensive movements in the chromatin substances and 

 the diameter varies inversely with the length. There is always 

 the possibility of such movements and the extent of them at 

 any given time is variable, so that we may expect to find differ- 

 ences in dimension although the volume remains constant. 



