194 CHARLES L. PARMENTER 



However, although the measurements may not meet the above 

 criteria in all chromosomes, there are certain cases which do meet 

 them definitely, and strongly evidence the existence of pairs. 

 This fact, together with the above consideration, makes it 

 possible that all the chromosomes are in pairs. 



Furthermore, it may be mentioned here that conditions which 

 do not meet the above criteria fall far short of proving that 

 pairs do not exist. The possibility still remains that two or 

 more pairs may be of equal or nearly equal length. Such a 

 condition is known to exist in certain Orthopteran chromosome 

 pairs (Carothers, '17, pi. 1, tetrads 7 and 8) where the chromosomes 

 are unquestionably known to be paired. 



2. Evidence for the existence of pairs. On plate 9, figures 33 

 to 37, are five rows of vertical lines representing the relative 

 lengths of the chromosomes of as many cells. The differences 

 in the lengths of these lines and also the space between adjacent 

 lines represent relative differences in chromosome length. For 

 convenience the lines are made twice the length of the chromo- 

 somes as drawn and the width of the spaces are made eight times 

 the difference in length. The lengths of the chromosomes, the 

 amounts included for foreshortening, and the form of chromo- 

 somes for each of these cells are also shown, respectively, in 

 figures 27 to 31 and 33 to 37. 



A part of the evidence which these graphs present for the 

 existence of pairs is three outstanding characteristics common 

 to all of them. First, there is a graded series of chromosome 

 lengths from the shortest to the longest; second, there is a 

 marked sameness in the relative chromosome lengths of these 

 cells which appears in the approximately constant presence of 

 groups containing the same chromosome pairs, and, third, a 

 similarity of form between homologues. 



The pairs, in accordance with the above criteria, were de- 

 termined primarily on the basis of chromosome length, supported 

 by a comparison of form. The graphs and figures of each com- 

 plex measured show that certain chromosomes are very probably 

 homologues. In other cases a number of chromosomes are so 

 nearly of the same length that, according to the criteria, the 



