4 Edmund B. Wilson 



best have been selected for illustration) and to satisfy myself thor- 

 oughly of the constancy of the relations as described. Everyone 

 familiar with such objects will, however, realize that in regard to 

 such matters as the arrangement and size-differences of the 

 chromosomes certain apparent variations appear that are due to 

 slight differences in the form and position of the chromosomes, 

 and to the various degrees of foreshortening thus caused. This 

 introduces a slight error, into both the observations and the draw- 

 ings, that can hardly be avoided. A second source of error lies 

 in the degree of extraction, which produces surprising variations 

 in the apparent size of the chromosomes I have found, for 

 instance, that by successive extraction the chromosomes may be 

 reduced almost to one-half their original apparent size, and the 

 smaller chromosomes may thus be caused almost to disappear from 

 view. Camera drawings at successive stages of the extraction show, 

 however, that the relative sizes of the chromosomes remain sub- 

 stantially unchanged, and the comparison of the same object after 

 a shorter and a longer extraction has thus, in a number of cases, 

 given a more certain result than could otherwise have been 

 obtained. I have, whenever it was possible, figured different 

 stages of the same species from the same slide, so as to avoid the 

 error due to different degrees of extraction; but this is not always 

 possible, since as a rule longer extraction is required to give a 

 perfectly clear view of the spermatogonial groups than is desirable 

 for the spermatocyte-divisions. For the comparison of the two 

 sexes, different slides must of course be used, and to this is due, 

 I am sure, some of the size-differences between the oogonial and 

 spermatogonial groups that appear in the figures. 



Making all due allowance for the sources of error mentioned, 

 it remains perfectly clear that the chromosomes in each species 

 show among themselves constant and characteristic size-differ- 

 ences; and further, that with the special exceptions in the male 

 described beyond, the chromosomes of the unreduced groups 

 (/. e., those of the oogonia and spe'rmatogonia) may be paired off, 

 two by two, to form equal or symmetrical pairs. The pairing of 

 the chromosomes is most evident in the case of especially small 

 chromosomes (such as the m-chromosomes of Anasa, Alydus, 



