174 A. R - Moore 



that of franciscanus than that of the purpuratus (Fig. 7) , and a 

 .marked increase in spininess of the skeleton. 



The whole case may be summed up by saying that the francis- 

 canus characters are retarded in their development in the hybrid, 

 just as is the development of red pigment in LANG'S heterozygote 

 snails. As we have stated, such a result is susceptible of explan- 

 ation if we consider development of the characters of an organism 

 as due to chemical reactions. Since the substance which produces 

 the characters of franciscanus is present in the hybrid in one half 

 the normal amount, the reactions involved in producing these charac- 

 ters will go on at a velocity less than that in the pure strain. To 

 refer to a well known chemical reaction, peptic digestion proceeds 

 at a rate which is proportinal to the square root of the concentration 

 of the ferment present in the system. If this substance which under- 

 lies the formation of dominant characters in the hybrid larvae be 

 governed by such a law as is peptic digestion, then the velocity of 

 development of the franciscanus characters in the hybrid should be 

 seven-tenths that in the pure franciscanus, since the ratio is VI : V J /2* 

 Therefore, the rate of the reaction in the hybrid is only slightly 

 less than in the pure breed. This corresponds well with the small 

 amount of difference in the rate of development of the dominant 

 characters which we have observed in the plutei of sea urchins. 



Summary. 



1) It has been found that the rate of development of the domi- 

 nant characters of body shape and skeleton formation in sea urchin 

 larvae, is less in the heterozygote than in the pure dominant. 



2) Attention is called to the fact that in the case of snails in- 

 vestigated by LANG the rate of development of the dominant color 

 is less in the heterozygote than in the pure dominant. 



3) According to our hypothesis that the substances underlying 

 the formation of dominant characters obey the laws governing enzyme 

 reactions, we should expect these reactions to go forward at a slower 

 rate in the heterozygote than in the pure dominant because the former 

 contains but one half the amount of enzyme to be found in the latter. 



In conclusion I wish most heartily to thank Prof. S. S. MAXWELL 

 and Prof. T. B. ROBERTSON for the helpful interest they have shown 

 during the preparation of this work. 



