52 /'// YSIOLOGICAL GENET h '.s 



ment. These might be rates of growth or differentiation, rates of 

 product ton of stuffs accessary for differentiation, rates of read ions 

 leading to definite physical or chemical situations at definitr 

 times of development, rates of those processes which are respon- 

 sible for segregating the embryonic potencies at definite times. 

 Thus far such conclusions were derived only from the facts 

 regarding the phenocopies and similar phenomena. Originally, 

 however, the interpretation of gene action in terms of rates of 

 reaction was derived from a different type of facts, analyzed by 

 Goldschmidt (19176, c, 19206), viz., observed differences in rates 

 of certain gene-controlled processes. For a special case, the 

 pigmentation of rabbits, Wright (1916) had also derived the con- 

 cept of rate of production of an enzyme to explain the action of 

 different alleles for coat color (see page 73). 



A. Rate Genes 



Goldschmidt (1917cZ) first drew attention to rate genes and 

 their importance for an understanding of gene action. He 

 described different genetic races of the gypsy moth, Lymantria 

 dis-par, the caterpillars of which were different in regard to their 

 markings. The main difference was that in some races a pattern 

 of light markings on the back persisted throughout development; 

 in others, it became covered during development by a dark cuticu- 

 lar pigment which was deposited at a certain rate with the 

 progress of development. This rate was different in different 

 races and intermediate in heterozygotes. Thus different genetic 

 constitutions could be linked with different rates of production of 

 pigment during development. Numerous curves for the process 

 are found in Goldschmidt (19246). 



Simultaneously Goldschmidt studied another phenomenon 

 which led to an explanation of gene action via rates of develop- 

 mental processes. The phenomenon of zygotic intersexuality 

 produced by crossing the sex races of Lymantria dispar permitted 

 such an analysis. It was shown that a definite event in the 

 development of intersexes, the turning point at which sex 

 changes, could occur at different times, depending in an orderly 

 way upon the genetic constitution of the individual. It was 

 argued that such a perfectly orderly dependance of the time of 

 such a change from a similarly orderly set of conditions of the sex 

 genes could be understood only if two competing sets of reactions 



