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THE AMERICAN NATURALIST [Vol. XLVI 



For example, if r = 1.5 (so that the nuclear volume of any 

 cell increases 50 per cent, when that cell divides), then for the 

 result of four cleavages (producing 16 cells) the formula gives 



x = ^TCT = TT " 1696 P el ' cent 



If Conklin employed the 1-cell stage as his standard of com- 

 parison, the above formula would be 



It will be found that for any increase less than 100 per cent, 

 of what was present before division (that is, r = 2), Conklin 's 

 ratio (from formula 1 or 2) decreases in the later stages of 

 cleavage, even though the law of increase, so far as each cell by 

 itself is concerned, remains absolutely the same. Thus, if at 

 the division of every cell its nuclear volume increases 50 per 

 cent., Conklin 's ratio (formula 1) will give 25 per cent, for the 

 result of the second cleavage, 20.83 for the third, 16.96 for the 

 fourth, 13.54 for the fifth, 10.64 for the sixth, 8.25 per cent, for 

 the seventh, etc. This appears to be the reason why Conklin 

 finds the rate of nuclear increase, as shown by his ratio, to b6 

 less in later stages; it is not due to any change in the relations 

 so far as what happens in each cell is concerned. 



IS THERE ASSOCIATION BETWEEN THE YELLOW 

 AND AGOUTI FACTORS IN MICE? 

 In the generally accepted formulae for the colors of mice, as 

 worked out by Cuenot, Bateson, Durham and others, there is 

 assumed to be a factor, Y, for self yellow color, 1 which is epi- 

 static to T, the ticking or agouti factor (also known as G). The 

 various types have the following constitution : 



Yellow Ytyt, or YtyT. 2 



Agouti (including cinnamon) yTyT, or yT yt. 



Black and chocolate (including dilute forms) . . yt yt? 



1 On the formulae adopted here this factor ¥ is probably to be considered 



2 It has been shown by Cuenot and others (see especially Castle and 

 Little, Science, N. S., 32, 868, 1910) that mice homozygous for ¥ do not 

 exist, the reason probably being that the YY zygotes, though formed in the 

 expected proportions, do not develop. 



3 Blacks differ from chocolates in having a black factor, B. Agoutis 

 also carry this factor, while cinnamons lack it. Yellows may or may not 

 bear it, but the two types are distinguishable by their eye color. 



