536 



THE AMEBIC AX NATURALIST [Vol. XLVII 



Analysis of Predeterminative versus Epigenetic 

 Factors in Development 



According to the proponents of the pure line hypoth- 

 eses the genotypic constitution of an individual is fixed 

 at the time of fertilization of the ovum. On this assump- 

 tion the fertilized egg of the armadillo has a fixed and 

 definite hereditary potentiality and, unless inequalities of 

 some sort are introduced during development, i. e., epi- 

 genetically, the four fetuses should be identical. The 

 degree of difference then that actually exists among the 

 individuals of a given set of quadruplets should be a 

 measure of the potency of the epigenetic factors of all 

 kinds, while the degree of correlation among the indi- 

 viduals of a set should serve as a criterion of the relative 

 strength of the predetermining factors. It has been cus- 

 tomary to employ the data derived from comparisons of 

 human duplicate or identical twins as a measure of the 

 extent of predetermination, but such data are unreliable 

 for two reasons. It is impossible, on the one hand, to be 

 sure whether or not such twins are the product of one 

 egg, and in practically all cases the measurements and 

 comparisons arc made comparatively late, so that the two 

 individuals may have had a divergent environmental 

 experience. In both of these respects the armadillo quad- 

 ruplets offer superior advantages and should in the 

 future take the place of human twins as material illus- 

 trating the potency of predeterminative factors in devel- 

 opment, for not only do we know for certain that each set 

 of quadruplets is the product of a single egg, but the 

 amount of material is thoroughly adequate for statistical 

 treatment, and the individuals are compared before birth, 

 so that their pairing and placental relations are known. 

 Coefficients of polyembryonic correlation have been deter- 

 mined for a very large number of characters, such as the 



