TWO PAIRS OF FEMALE TWINS 235 



Unquestionably, in the parents which supplied a test of the 

 matter, the twin-bearing egg was immediately succeeded by the 

 production of a high proportion of female-producing eggs. 



The question of variations in the relation of yolk weight to 

 egg weight under the conditions of reproductive overwork and of 

 'crowded reproduction' is not answered by the data of the tables 

 given here and lies outside the scope of this paper. For the 

 present purpose, and by way of summary, it may be observed 

 that both cases of twin-producing eggs occurred, a) in reproduc- 

 tively overworked females; b) in periods of 'continuous activity;' 

 c) in very short intervals — six and seven days — since the pre- 

 ceding clutch, and, finally, that such crowded reproduction tends 

 to produce an excess of females. 



DOUBLE-YOLKED EGGS IN DOVES AND PIGEONS 



There remains for consideration the possibility of the origin 

 of the two cases of twins from 'double-yolked' eggs. Two sets 

 of facts show that this did not occur. We may first note the con- 

 clusive data obtained from the twins themselves, and from the 

 eggs that produced them, and later produce the record of the 

 few cases of double-yolked eggs that have appeared in our studies 

 with doves and pigeons. 



The pertinent facts concerning the first pair of twins (from 



9 60) are as follows : This egg was laid on March 7, and failing 

 to hatch on March 22, was opened for examination. Two 

 nearly full-term dead embryos were found; both birds were 

 plainly smaller than normal birds ready to hatch, but both seemed 

 practically completely formed and ready to hatch. A very 

 considerable amount of yolk, however, remained unabsorbed, 

 and both umbilici were plainly united at a nearly common point 

 on the single yolk-sac. Both young were plainly females; in 

 one of the two there was a distinct right ovary as well as the usual 

 left ovary, but I was unable to make sure that a similar right 

 ovary was also present in the other. 



The facts obtained on the second twin-bearing egg (from 



9 A248) were as follows : When this egg was candled (held toward 

 the light, as is done on the second to the fifth day for all eggs 



