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THE AMERICAN NATURALIST [Vol. LI 



tached, or the perishing of some may make the chances 

 greater that the rest will successfully complete their de- 

 velopment. Or secondly, the production of a relatively 

 small number of young at one birth may lead indirectly 

 to more free ovulation subsequently, and so to the pro- 

 duction of a large litter at a second birth." 



It will be seen in the first place that the above theories 

 are based on false premises. It was not known at the time 

 these theories were proposed that the homozygous yel- 

 low zygote does not perish in the sense of disintegrating 

 and finally disappearing. It merely ceases to develop 

 after a certain stage has been reached, and then remains 

 more or less stationary till parturition. It might still 

 be maintained that since these undeveloped zygotes take 

 up very little room there would still be the possibility for 

 the "other" zygotes to develop. In that case the average 

 number of total embryos (including dead embryos A and 

 B) should be greater in the yellow X yellow mating than 

 in the non-yellow X yellow mating. Our data, presented 

 in Table VI, indicate that this is not the fact. The other 

 theory that ' ' the production of a relatively small number 

 of young at one birth may lead indirectly to more free 

 ovulation subsequently, and so to the production of a 

 larger litter at a second birth" still remains a possibility 

 so far as our data are concerned. 



There are several other possibilities which seem worthy 

 of consideration. Instead of looking for causes that tend 

 to increase the size of litters from yellows X yellows it 

 may be profitable to determine if possible causes that 

 may decrease the size of litters from the non-yellow 

 X yellow mating. The first and most obvious possibility 

 is that overcrowding in the uterus may have this effect 

 by causing the death of some of the embryos. However, 

 none of our results bear this out. In the first place there 

 are proportionately just as many dead embryos B (whose 

 death is probably caused by overcrowding) in the one type 

 of mating as in the other, and in the second place thiis 

 would mean that for our race of mice, having a high 

 average litter size, there should be a proportionately 



