236 OSCAR RIDDLE 



incubated) to test its fertility on the third day of incubation, two 

 embrj^os were plainly seen. It was then noted that the two were 

 close together; that in moving, turning, or shaking the egg, they 

 invariably turned together, and that their position with reference 

 to each other could not be altered. Clearly they were contained 

 within the same ovum. It was thus known in advance that this 

 was a twin-bearing single yolk. On the fourteenth day of incu- 

 bation — just before the young were due to hatch — the egg was 

 opened so as better to learn the conditions presented by the twins. 

 Two young were found, one dead, the other alive. The dead 

 young was practically a full-term embryo, perhaps slightly larger 

 than the live one which seemed nearly ready to hatch. There 

 remained here also a considerable amount of unabsorbed yolk; 

 and, as in the previous case, the umbilici had a practically common 

 point of union on the yolk-sac. Both birds were plainly females^ 

 and both birds possessed right ovaries which were one-half as 

 large as the left ovaries. 



Five ' doubled-yolked' eggs have appeared among the approxi- 

 mately 20,000 doves' eggs that have been examined. The size 

 of these eggs compared with the other egg of the clutch, and 

 with the size of the eggs of the immediately preceding and 

 succeeding clutches, is given in table 13. Four of the five cases 

 occurred among the eggs of hybrids. The one case of a female of 

 pure species (Stigmatopeha senegalensis) was supplied by a 

 female which otherwise showed the following reproductive 

 abnormalities: Two clutches immediately preceding the double- 

 yolked egg were clutches of one egg each; previous to these she 

 had laid fifteen clutches, fourteen of which consisted of two eggs. 

 The double-yolked egg was the last egg produced during the year 

 (November 25), and the last in life for this bird, except that an 

 egg was present in her oviduct when she died three and one-half 

 months (March 5) later. 



The second of the double-yolks was produced by a female hybrid 

 (alba X risoria) from her third egg in life. The two yolks of this 

 egg were of most strikingly abnormal size — both together being 



' The sex can usually be definitely learned in nine- to ten-day embryos of those 

 species whose incubation period is from fourteen to fifteen days. 



