Voi,. 7, 1921 
GENETICS: C. B. BRIDGES 
131 
materials. When orthopteran chromosomes are in the extended phase 
preceding their condensation as tetrads they may be seen to consist of 
granules of unequal size distributed at unequal distances along the linin 
thread, both granule size and spacing being characteristic and relatively 
constant for a given chromosome. Another explanation of the uneven 
spacing seems more probable: The amount of crossing over between 
given loci, e. g., black and purple in the second chromosome, is known to 
vary in a definite manner with the age of the mother, with temperature, 
and with genetic modifiers of crossing over. Since a unit of map-distance 
is thus known to represent different lengths of chromosome thread, the 
regions in which loci appear relatively closely spaced may be regions 
in which one unit of map distance represents a longer section of chromosome 
than in those regions that appear relatively unoccupied. It may be sup- 
posed that each region of chromosome has its characteristic relation be- 
tween unit of map-distance and corresponding length of chromosome. 
It is probably significant that in the second and third chromosomes the 
distribution of mutant loci is approximately symmetrical about their mid- 
points, for these chromosomes are V-shaped with median attachment 
of the spindle fibre, while the first chromosome, which has the massing 
at one end, has terminal attachment. The variations, from region to region, 
in the proportion of double crossing over, referred to above, show a dis- 
tribution that is symmetrical in the second and third chromosomes. From 
this and other evidence it is probable that there is a definite relation be- 
tween the region of chromosome, considered morphologically, and the 
variations in the ratio of map-unit to length of chromosome thread. 
When the different chromosomes are compared the average ratios are 
found to be practically the same for all. Thus, the lengths of chromosomes 
as measured directly in metaphase plates are in the ratio of 100 : 159 : 159 : 
12, while the known lengths of the chromosomes as mapped are in the 
ratio of 100 : 165 : 155 : 2. 3 This is a surprisingly close agreement 
between the observed lengths of the condensed chromosomes and their 
lengths in terms of map-units. 
1 Contribution from the Carnegie Institution of Washington. 
2 The maps in the frontispiece of the Mechanism of Mendelian Heredity were made 
in 1914, and are both incomplete and out of proportion. A new frontispiece, practically 
the same as the figure accompanying this paper, has been supplied for a revision of 
the Mechanism and for a French edition. A similar figure is to appear in a book by 
Dr. Sharp of Cornell. The best of the earlier maps of the first chromosome is that 
given in Carnegie Institution Publication No. 237, p. 22. Partial maps of the first 
chromosome that are more modern have been given in connection with special papers 
{Genetics, 1, 1916 (8); Amer. Nat., 1916 (421); Gen. Physiol., 1919 (646). A map of 
the second chromosome, complete to 1916, is given in Carnegie Institution Publication 
No. 278, p. 303. No satisfactory map of the third chromosome has previously been 
published, though partial maps based on special work have been given (Amer. Nat., 1916 
(217); Genetics, 1919 (208 ). 
