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 ('16) observes that chromo- 

 somes A and B condense before the other chromosomes in the 

 spermatogonia and tetrads of Phrynotettix, and ('17) 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 ('11, 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 bj' 

 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. 



