;;r,6 



THE AMERICAN NATURALIST [ Vol. L 



single or double), and 101 in which chromosome II crossed 

 over. If it was a matter of pure chance whether or not 

 crossings-over occurred in I and II at the same time, 

 coincident crossing-over should have happened in X 

 101 = 49 + cases. The actual number of cases in which 

 crossing-over occurred in both chromosomes at once was 

 52. Thus there is neither interference nor synchronism of 

 these crossings-over, and this result too is strikingly dis- 

 similar to the relations found between two crossings-over 

 in the same chromosome. 



Since the results in the two chromosomes were found to 

 be independent in all respects, it is deemed unnecessary to 

 list here all the different combinations which were found 

 of factors in group I with factors in group II, and their 

 observed frequencies. The results for the two chromo- 

 somes may more advantageously be separated and added 

 to the other results, obtained when groups I and II were 

 followed in different crosses. 



The data for the first chromosome are given in the tables 

 which follow. In all, 712 offspring of females heterozy- 

 gous for the 12 mutant factors in group I have been tested. 



The above results give a direct demonstration of. the 

 fact that the factors behave as though they are joined in 

 a chain; when interchange takes place, the factors stick 

 together in sections according to their place in line and are 

 not interchanged singly. The fact is shown so patently 

 as to require no further comment. 



Non-crossing-over occurs in this chromosome in 54.4 per 

 cent, of cases, single crossing-over in 41.7 per cent., double 

 crossing-over in 4.2 per cent. No triple crossing-over was 

 obtained in this count, although one, which will be de- 

 scribed later, was obtained in the next generation, in one 

 of the " testing out" bottles. 



(To be concluded) 



