GENES AND DEVELOPMENT 155 



tuents which have definite properties which allow only a certain Hmited 

 number of reactions to occur; in so far as this is true, one may say that 

 development proceeds on a basis of the "preformed" qualities of the 

 fertihzed egg. But equally it is clear that the interaction of these con- 

 stituents gives rise to new types of tissue and organ which were not 

 present originally, and in so far development must be considered as 

 "epigenetic." In particular, the discovery of the organizer, and of the 

 way in which the primary organizer induces tissues or organs which 

 later act as secondary organizers, has led to great emphasis being placed 

 in recent years on the epigenetic character of developmental processes. 



These considerations have a bearing on the common genetical terms 

 genotype and phenotype. Neither of these is very easy to define pre- 

 cisely, partly because their usage is even now not strictly standardized. 

 The genotype was originally defined^ as the sum total of the genes 

 contained in the fertilized egg, but the word is usually used to refer 

 comprehensively to the whole genetic system of the zygote considered 

 both as a set of potentialities for developmental reactions and as a set 

 of hereditable units ; that is to say, it includes not only the mere sum 

 of the genes, but also their arrangement, as expressed in position effects, 

 translocations, inversions, etc. The question arises as to whether the 

 cytoplasmic characteristics of the zygote are to be included in the 

 genotype, but although they are obviously a very important part of the 

 developmental potentialities of the zygote, it seems advisable not to 

 include them in the genotype: probably it is better to consider them 

 as part of the phenotype determined by the genes of the mother. 



The main difficulty in defining the concept of phenotype is caused by 

 the fact that animals change in time. In its original sense, the word 

 referred to the characters, both anatomical and physiological, of the 

 adult. But clearly if we have an animal whose eye colour darkens with 

 age there is no essential diflference between the fight eye of the young 

 animal and the dark eye of the older one ; both must be included in the 

 phenotype. But if this is allowed, there is no reason to exclude from the 

 phenotype the processes (about which we usually know very fittle) by 

 which the eye pigments are synthesized during development. The 

 phenotype in fact must be used as a name for the whole set of charac- 

 ters of an organism, considered as a developing entity. Phenotypic 

 differences between two organisms may be caused by genotypic differ- 

 ences or may be produced by different environments acting on the 

 same genotype. 



1 Johannsen 191 1. 



