228 OSCAR RIDDLE 



lower metabolism). I have now obtained two cases of female 

 identical twins, and am able to know that the ova (yolks) which 

 produced both of them were extraordinarily and abnormally 

 large. 



We may first note that these two instances supply a strong 

 confirmation of my earlier conclusion concerning the correlation 

 of high-storage yolk values and femaleness. Next we may 

 examine the data which demonstrate that the twin-producing 

 yolks were of exceptionally large size. 



SIZE OF EGGS AND YOLKS IN RELATION TO THE TWO CASES OF TWINS 



The yolk of a dove's egg cannot, of course, be directly weighed 

 on a balance and then be incubated with the hope of producing 

 young. But, as we shall see, the two procedures on the same egg 

 are unnecessary to the demonstraiton that the twin-producing 

 yolks were of extraordinary size. Again, the weights of two en- 

 tire eggs (yolk + albumin + shell) from the same bird, and the 

 same clutch, may be different and yet this difference not indicate 

 which egg contains the larger yolk (ovum). But the amount 

 which two such eggs may thus differ, without showing the direc- 

 tion of difference in the yolks, is limited. And, for the species 

 under consideration, as well as for several others, the limits of 

 such difference are now approximately known. Accurate direct 

 weighings of nearly 15,000 yolks of various pigeon species and 

 hybrids have been made. Probably nearly 4,000 of these are 

 eggs of the species which produced these two instances of twins. 

 Certain aspects of these data will be utilized here for the purpose 

 just indicated. 



One of the twin-producing eggs was the seventy-sixth egg laid 

 by that particular female. The other twin arose from the 

 fiftieth egg of another female. In our main study the complete 

 egg-laying history is kept of some hundreds of doves and pigeons. 

 We are here able, therefore, to give the exact weight of every 

 egg produced by each of these two twin-producing females prior 

 to, as well as after, the appearance of their twin-producing eggs. 

 For the blond ring female (No. A248) this record is given com- 



