540 



THE AMERICAN NATURALIST [Vol. XLVIII 



It will be noted that several of the gametic ratios in- 

 volved here closely approach 2 : 1. YV, YM, WV and WM 

 are the most conspicuous examples. It may seem that 

 such a simple ratio is due to a very simple reduplication 

 series, but I do not think such an assumption can be suc- 

 cessfully maintained. The tables given above show that 

 YM and WM have approximately this same value when 

 they appear as secondary series, and the data for the 

 combination YWVM show the same thing for YV (see 

 Sturtevant, '14). 



If, as I have maintained above, the same series of redu- 

 plications must occur in all flies, whether we can follow it 

 or not, then it follows that in these three cases the 2 : 1 

 ratio is never due to a simple series, but always to a long 

 and complicated one, since in all three one of the primary 

 series is of high intensity. 



It was pointed out by Trow that the intensities of the 

 reduplication series afford a method of calculating the 

 number of cell divisions necessary to complete the series. 

 If we assume that approximately the same series is 

 occurring both in homozygous and in heterozygous flies, 

 we have the following series in Drosophila as a basis for 

 such calculations. 



Sex-linked Group 



YW= 90.1 



WV= 2.1 



VM = 31.8 



MR = 5.0 



RBr= 21.7 



BVg = 3.6 

 VgCv= 10.4 

 CvSp = 2.8 

 SpBa= 10 + 



Third Group 



PEb = 100 ± 



