62 CHEMODIFFERENTIATION 



amount of these substances will modify the ''histochemical equi- 

 librium" and thereby the direction of development. Peltrera 

 (1940) has given a clear statement of this view on the basis of his 

 work on the distribution of various physico-chemically important 

 substances in the egg of the marine snail Aplysia. What we 

 have so far called "determining substances" are probably only, 

 for some fortuitous reason, the more conspicuous components 

 of this complicated system of factors; they may themselves be 

 mixtures of a great number of substances. 



We have discussed in this chapter how, after the beginning 

 of development, the originally more or less homogeneous egg 

 cytoplasm acquires a complicated spatial structure because of 

 the concentration of various substances in definite parts of the 

 egg. These substances may at first have been evenly distributed 

 throughout the cytoplasm, their accumulation now taking place 

 under the orienting influence of the egg's "coordinate system", 

 especially its cortex (see p. 49). On the other hand it is possible 

 that the substances were not originally present in the egg, but 

 that they arose de novo as a consequence of chemical reactions 

 taking place in the egg, e.g. of such an interaction between its 

 axial and cortical gradient systems, as was assumed by Dalcq 

 for the amphibian egg. At any rate, the various parts of the 

 egg, which were all alike before, now begin to show differences 

 in chemical composition. Following J. S. Huxley, we may call 

 this phenomenon "chemodifferentiation". 



The differences in physico-chemical composition, now existing 

 between the parts of the germ, result in differences in their 

 developmental potencies. In broad outline, the egg at first was 

 an "equipotential system"; now, it is divided up into a mosaic 

 of parts with diverging potencies. 



Chemodifferentiation may set in at earlier or later stages in 

 development. We have seen above (p. 58) that in ascidians the 

 uncleaved egg is already a mosaic of different plasmatic sub- 

 stances. In such a case, the developmental potencies of the 

 different parts of the germ are unequal from a very early 

 stage, and development has the character of a "mosaic devel- 

 opment". In other cases chemodifferentiation begins much later, 

 the various parts of the germ remaining equivalent for a long 



