Mechanism of Mendelian Heredity 247 



franciscanus and purpuratus,^ are crossed, certain 

 features of the skeleton of the embryo, e. g., the so-called 

 cross-bars, are a dominant, inasmuch as they are found 

 in purpuratus and both the crosses, while they are 

 absent in franciscanus. The development prior to 

 the formation of the skeleton is purely maternal. 

 These observations again lend support to the idea that 

 the Mendelian factors of heredity must have the 

 embryo to work on and that the organism is not to be 

 considered a mere mosaic of Mendelian factors. This is 

 further supported by the idea that the species specificity 

 resides in the proteins of the unfertilized egg (see Chap- 

 ter III), and it is quite likely that this species specificity 

 decides which type of animal should arise from an egg. 

 The idea had been suggested that the factors which 

 determine the future character might be ferments or 

 enzymes, or substances from which such ferments de- 

 velop. A. R. Moore ^ pointed out that the cross-bars in 

 the skeleton of the hybrid between S. purpuratus and 

 franciscanus develop more slowly than in the pure breed 

 and that this should be expected if the determiners were 

 enzymes. Since the pure purpuratus has two deter- 

 miners for the development of the cross-bars (from 

 both egg and sperm), the hybrids only one (from either 



* Loeb, J., King, W. O. R., and Moore, A. R., Arch.f. Entwcklngsmech.f 

 19 10, xxix., 354. These experiments have been repeated at different 

 seasons of the year and in different years and have been found to be 

 constant. 



'Moore, A. R., Arch.f. Eniwcklngsmech., 1912, xxxiv., 168. 



