DEPARTMENT OF GENETICS. 



147 



enon, for the embryo is accessible throughout the period of its embry- 

 onic development, and, because of the transparency of the shell, 

 twins can be early and definitely identified. Moreover, when the egg- 

 weights have been carefully kept, the prospective sex of the embryo is 

 often known. It appears that one-yolk twins, of the female sex at 

 least, can develop fully within the eggs. The chief difficulty in the 

 study is the rarity of this form of twinning; only 7 cases have occurred 

 in from 15,000 to 25,000 eggs examined. Of these seven, 2 produced 

 in 1915 and 1916 have been already described. This year 2 new 

 cases have been found; and the occasion is taken to give data con- 

 cerning all 7 cases in table 13. 



Table 13. 



—Summary indicating the abnormal size of 7 eggs from which arose 7 pairs of 

 identical twins and the origin of female twins from ova of largest size. 



The weights of twin-bearing eggs are set in Italic type. 



^The upper number in each pair below is the average of 5 first eggs of the clutch, the lower 

 number of 5 second eggs. 

 ^Common pigeon; all other groups are ring-doves. 



An inspection of table 13 shows that in the case of the first 4, the 

 twin-producing eggs were distinctly larger than any other eggs pro- 

 duced by the prospective parent during the year. In all cases (3) 

 where the twins developed to an age at which sex could be determined 

 the sex was female. Dr. Riddle points out that the large size of these 

 female twin-producing eggs harmonizes, in so far, with his earlier ob- 

 servations that females arise from eggs (yolks) of high storage metab- 

 olism, hence large eggs. It is noteworthy, also, that the small eggs 



