ZOOLOGY: A. H. STURTEVANT 
555 
in two days. This number of eggs therefore has not received the ex- 
posure necessary to produce the change in crossing over. The culmina- 
tion of the two days exposure is to be expected in those eggs so 
situated that 125 to 175 eggs will be laid before them. Such an 
interpretation makes it extremely likely that the change in the amount 
of crossing over is finally affected in the earliest oocytes, that is, at 
the beginning of the growth period. The above evidence on the time 
of applying the new temperature and the time when the change in 
crossing over occurs, suggests that the crossing over process takes place 
in the stage when the chromosomes of Drosophila are known to be finely 
drawn out threads. 
The decrease in the strength of linkage caused by temperature in no 
way weakens the chromosome interpretation of linkage. It rather adds 
to it considerable support, for it localizes the process of crossing over at 
a period in oogenesis when twisting between homologous threads seems 
possible. The evidence positively establishes the fact that crossing over 
does not take place during the early oogonial divisions, and makes it 
extremely improbable that it occurs at so late a stage in the growth 
period as the thick thread stage favored by Janssens as the chiasmatype. 
1 Bridges, C. B., /. Exp. Zool, Wistar Inst., Philadelphia, 19, No. 1, July, 1915. 
^Sturtevant, A. H., these Proceedings, 3, 1917, (555-558). 
^Cf. Snyder, C. D., Amer.J. Physiol., 22, 1908, (309). 
GENETIC FACTORS AFFECTING THE STRENGTH OF LINKAGE 
IN DROSOPHILA 
By A. H. Sturtevant 
ZOOLOGICAL LABORATORY. COLUMBIA UNIVERSITY 
Communicated by T. H. Morgan, July 5, 1917 
In September, 1913, a wild female Drosophila of a stock from Liver- 
pool, Nova Scotia, was crossed to a male bearing the second chromo- 
some mutant characters vestigial and speck. A single daughter of this 
mating was tested, and gave no crossovers among 99 offspring, though 
vestigial and speck usually show about 37% crossing over. This 
strain has since been bred in very large numbers, and the experiments 
are being continued ; but it has seemed advisable to report briefly on some 
of the results obtained. ^ 
It has become clear that the original result was due to something in 
the second chromosome derived from the Nova Scotia female'. Two 
of her granddaughters and all of her later descendants that were known, 
from linkage, to have received the second chromosome in question gave 
