^^8 H. H. ISJ ewjnan 



correlated with the dominance of certain good combinations of 

 vital characters. For example it appears that intensity of pigmen- 

 tation is, in the H hybrid strain, associated with unusual rapidity of 

 heart-beat. 



Might we suggest the possibility that cases of dominance in 

 other experimental fields might be explained physiologically in 

 some such way as has been suggested in the foregoing discussion ? 



High Degree of Variability in Hybrid Strains the Result of Vary- 

 ing Degrees of Compatibility in the Germ Cells of the Two 

 Species 



That hybrids are more variable than pure breeds is a fact long 

 established and is only emphasized by the data brought out in 

 this paper. The range of variability is, however, far wider than 

 might be supposed were only the successful types of hybrids con- 

 sidered. The following series of types showing the various degrees 

 of success in development o{ H hybrids may prove instructive: 



1 Many eggs never cleave although fertilized by foreign sperm. 



2 A few cleave but break down in the two, four, eight, or six- 

 teen-cell stages, through some disharmony in the germinal materials 

 involved. 'This disharmony is usually noticed in one cell, from 

 whence it spreads through all the cells of the blastodisc. 



3 Many embryos die and disintegrate in advanced blasto- 

 disc stages. 



4 A small percentage of embryos are weaklings, due to some 

 incompleteness in organization. Many of them fail to establish a 

 circulation and die at an early age. Others reach a stage similar 

 to that of an H pure at 48 hours and then die. 



5 A large percentage of embryos develop into incomplete and 

 monstrous forms, such as those shown in Figs. 3 to 8. These show 

 all grades of incompleteness and discord of parts. 



6 A few embryos develop to full size but never hatch unless 

 released artificially from the egg membrane. Even then they are 

 sluggish and soon die. 



7 A large percentage of embryos are rather pale in color, hatch 

 rather late, and live only a few days, but do not grow. 



