190 
CHARLES L. PARMENTER 
only approximate, because it is very difficult to determine 
accurately the amount of vertical deviation, and its longitudinal 
extent, as well as its exact course. In figures 27 to 30 and 33 
to 36 corrected figures are used, and the amount included in each 
measurement for foreshortening is indicated. 
There are two other conditions which do not give rise to actual 
errors in measurement, but do interfere with precision of results 
and may well be considered here. 1) A possible unequal con¬ 
traction of chromosomes. Wenrich (T6) observes that chromo¬ 
somes A and B condense before the other chromosomes in the 
spermatogonia and tetrads of Phrynotettix, and (T7) he shows 
that one homologue of chromosome 4, cell E, plate 2, contracts 
more rapidly than the other. 2) Since so many of the chromo¬ 
somes of Ambystoma are so long and composed of two inter¬ 
twined chromatids, there is considerable possibility of a stretching 
due to bending and other stresses still present in the complexes 
nearing the metaphase. As Meves ('ll, p. 247) points out, 
under these conditions two chromosomes could be of different 
length and of equal volume. Even an imperceptible difference 
in diameter of parts or all of two chromosomes of equal volume 
might cause considerable difference in their lengths. This dif¬ 
ference would of course be proportionally greater in the longer 
chromosomes so that measurements of the shorter chromosomes 
of a cell might strongly indicate the presence of pairs while the 
homologues of the longer pairs would show quite wide differences 
in length. A case of very perceptible stretching is to be seen in 
chomosomes ‘s,’ figures 9 and 12. 
Effects of technique. The differences which may arise in the 
chromosomes of different cells of even the same tissue due to 
different effects of fixatives, and all other effects of technique, 
do not affect relative measurements of chromosomes in the same 
cell. For it is extremely improbable that the lengths of chro¬ 
mosomes of the same cell which are so equally close to the 
surface of these membranes would not be similarly affected by 
the action of these various reagents and processes. It is also 
improbable that inherent differences among the homologues 
would cause a differential change of length under these 
conditions. 
