CHEMODIFFERENTIATION 63 



time, both physico-chemically and as regards their potencies. 

 One half of an egg, or two fused eggs, can develop into a single 

 harmoniously built embryo in this case. The one condition for 

 this to occur is, of course, that after the disturbance the co- 

 ordinate system of the egg can readjust itself by the formation 

 of a new equilibrium, essentially similar to the original one, 

 between the axial and cortical gradient-fields. Even in later 

 stages the addition or removal of material may still be regulated 

 in these cases. Such eggs have the character of "regulation 

 eggs". Therefore, the difference between mosaic eggs and 

 regulation eggs is due mainly to an earlier or later occurrence 

 of chemodifferentiation. 



The accumulation, in definite places in the egg cytoplasm, 

 of preformed or newly built determining substances constitutes 

 the first beginning of spatial multiplicity in the egg. At first, 

 this spatial structure is still very simple, but we can imagine 

 how this situation will be the starting point of increasing com- 

 plexity. The local concentration of certain substances will start 

 chemical reactions which previously were unable, or almost 

 unable, to take place because of the dilution of the reagents, 

 or because of the presence of inhibiting substances. New sub- 

 stances will thereby be produced in the egg, which again may 

 be locally concentrated. These local accumulations will affect 

 the physical conditions (pH, rH, interfacial tension, electrical 

 relationships), and the intensity of metabolism in the areas 

 they occupy. They will also interact, e.g. by mutual attraction 

 or repulsion. All these factors give increasingly complex pro- 

 perties to this physico-chemical system, whose spatial multi- 

 plicity originally was characterised by polarity and symmetry 

 only. Once started, chemodifferentiation will steadily proceed, 

 and the egg's intensive multiplicity will be more and more 

 transformed into an extensive multiplicity. 



So far we have considered the egg cytoplasm only. We shall 

 now discuss a new complication in the system of actions and 

 interactions in the egg, namely the activity of the complex of 

 factors localised in the nucleus. 



