^^ PRINCIPLES OF EMBRYOLOGY 



that the early development of the molluscan egg is mainly dependent on 

 ooplasmic segregations, that does not tell us anything of the causes which 

 bring about the segregation, or of how the various ooplasms cause the 

 appearance of the specific characteristics of the organs to which they give 

 rise; what we have done is to describe a type of system, but we are still 

 unable to point to particular causes and their particular effects. The same 

 IS true when we attribute the development of the echinoderm egg 

 to a system of gradients or fields. It is only in comiection with evocation 

 phenomena that we begin to attain any real experimental control over 

 important developmental events, smce where evocation comes into 

 play, we can switch the development of a piece of tissue one way or an- 

 other by placing it either near to or far from the source of the evocating 

 stimulus. In this connection, then, we are already in contact with a basic 

 causal system and can hope to go beyond finding out the general nature 

 o the system to the crucial step of discovering what it actually is in detail 

 Unfortunately, as we shall see in Chapter X, it has turned out to be 

 easier to see this bird than to put salt on its tail. 



From the genetical side, also, we are as yet only just approaching the 

 actual causal systems. We know something about the kinds of things 

 genes, or groups of genes, do; but we still want to know exactly what 

 some one defmite gene does and how it produces its effects. It is only in a 

 few cases that we can control the activities of genes, and it is not until 

 we can do so, that we can hope to discover much about them. Again 

 evocator reactions provide one example; by the presence or absence of 

 the stimulus we can bring into play one or another set of gene-controlled 

 processes ; but the genetic variants are not available in any of our laboratory 

 ^ocks which could make it possible to analyse this situation geneticaUy. 

 The other mstances in which it is possible to determine experimentaUy 

 whether a gene shall be active or not occur in lower and more or less 

 undifferentiated organisms (for instance in the control of immunoloo-ical 

 properties m the protozoan Paramecium, or the formation of adaptive 

 enzymes m bacteria and yeasts). From such cases we cannot learn 

 much about the precise mechanisms of differentiation, but we can find 

 some interesting general guidance. 



It is in the analysis, by developmental genetical methods, of the forma- 

 tion of certam particular chemical substances, that we have so far come 

 nearest to a fuU understanding of any developmental process. We know a 

 great deal, for mstance, about the development of the pigments in eyes 

 of the RjDrosophila; not only the genes that affect it, but also the chemical 

 nature of some of the most important changes which those genes produce. 

 Unfortunately, the substances about which we have such detailed know- ■ 



