12 



Scientific Proceedings (117). 



tance, not merely for the vegetable world but also for the higher 

 animals. Furthermore, the fact that sunlight is efficacious in 

 the rickets of both human beings and rats, serves to show the 

 similarity of this disorder in these two species. These results indi- 

 cate that in the prevention and causation of rickets at least one 

 hygienic factor plays an important role which will have to be 

 carefully considered in future studies of this disorder. 



6 (1753) 



Identical twins in pigeons arise from ova of 

 markedly aberrant size. 



By OSCAR RIDDLE. 



[From the Carnegie Station for Experimental Evolution, Cold Spring 



Harbor, N. 7.] 



During 10 years data have been accumulated for yolk size and 

 total egg size in 15,000 to 18,000 eggs of doves and pigeons. Such 

 measurements of these two associated structures permit us, within 

 certain limits, to know some definite things concerning the size 

 of either structure if the weight of the other is known. Another 

 group of 15,000 to 20,000 eggs have been weighed, incubated, and 

 later observations made upon the embryos and young. Incidental 

 to these latter observations 7 instances of identical twins have been 

 found. Such twins other than the seven listed here have almost 

 certainly not appeared; or, if present, they attained a stage of 

 less than 2-day embryos. 



The figures of Table I make it clear that at least most of the 

 particular eggs which gave rise to twins were of markedly different 

 size from all other eggs then being produced. This is particularly 

 well shown in the first four instances — given in the upper eight 

 rows of figures — since the twin-bearing egg was in these four cases 

 by far the largest egg produced by its parent during one entire 

 year, — and so much larger as to indicate, in all probability, that 

 it contained the largest ovum produced during the year. The 

 seventh case was likewise of aberrant size — being the smallest of a 

 group of undersized eggs. However, the weights of all eggs ob- 

 tained in connection with this seventh case, as also with cases 5 



