Identical Twins in Pigeons. 



13 



and 6, are known to be rather unreliable indices of the weights of 

 the enclosed yolks because the parent birds (K469, P843) suffered 

 from special reproductive disorders which involved the production 

 of irregular and inadequate quantities of shell and albumen, un- 

 paired eggs and embryos often incapable of hatching. The early 

 death of these three pairs of twins is probably to be associated with 

 this circumstance. The significance of the egg weights of the four 

 cases listed at the top of the table is wholly clear since differences of 

 1 5 per cent, in egg size (between the two eggs of a clutch) have been 

 found in normal birds to reliably indicate that differences of yolk 

 size lie in the same direction. The two cases listed at the top of 

 the table have been earlier fully described 1 and the data given 

 there will likewise demonstrate the abnormally large yolk size 

 which must have been present in cases 3 and 4 of the present 

 tabulation. 



Apparently the known facts concerning these cases of twins do 

 not well accord with a strict application of Stockard's 2 conclusions 

 as to the cause of twinning and "double monsters," particularly 

 as he has described it in relation to birds, since in the present cases 

 we learn that the twin-producing ovum of the pigeon is "marked" 

 for twinning even before it leaves the ovary. However, it seems 

 possible that even these instances may fall within the range of 

 his general explanation. We have learned that extraordinary 

 yolk-size means a low oxidizing level of the ovum. Since this 

 level is lowest in the largest ovum of the given bird this excep- 

 tionally low level may account for the first 4 cases of the list. In 

 the last three cases the size of the contained ova is questionable 

 but in these cases a disorder of the reproductive organs — already 

 known to involve the abnormal functioning of some of the en- 

 docrine glands — may conceivably effect a retardation of develop- 

 ment previous to gastrulation as Stockard's theory demands. The 

 early death of many of these particular embryos, as well as an 

 apparent excess of twins derived from the meager amount of this 

 material, may afford evidence for such retardation. It should 

 also be stated that in case 3 the embryo was subjected to ice-box 

 temperatures (i3°-i6° C.) during the first 23 hours after laying; 

 and that the parent in case 4 was a generic hybrid. 



1 Riddle, O., Jour. Exp. Zodl., 1918, xxvi, 227. 



2 Stockard, C. R., Amer. Jour. Anat., 1921, xxviii, 115. 



