750 



THE AMERICAN NATURALIST 



[Vol. LI 



In our embryological study there were 33 litters in the 

 yellow X yellow mating. Of these, as previously stated, 

 two consisted entirely of dead embryos A. This means 

 that for 31 litters containing living embryos there were 

 two that did not, or 6.45 per cent. If we assume a like 



percentage of litters of size with our 140 normally born 

 litters from ''yellow X yellow" (Table VIII) the total 

 number of litters would be increased to 149 and their 

 average size would be 5.04 instead of 5.36. Taking 5.04 

 as a basis for comparison with 6.51, the average size of 

 litters from the non-yellow X yellow mating, we find that 

 the percentage relation is 77.42 per cent. This is reason- 

 ably close to 75 per cent., the expected percentage. In 

 this connection it may be well to call attention to a fact, 

 previously stated, that as the average litter size increases 

 the percentage relation tends to approach 75 per cent. 

 This is exactly what one should expect. For with in- 

 creased size of litter there would be fewer litters of size, 

 and the averages as actually found would more nearly 

 represent the true averages. 



Curves (Fig. 1) have been constructed showing the 

 frequency of the litter sizes for certain of the types of 

 matings. The data upon which they are based may be 

 found' in Table IX. A careful survey of these curves, 



s 



35 



