204 PHYSIOLOGICAL GENETICS 



direction, which will be followed by morphogenetic differences. 

 The evocator then would determine the center of a metabolic 



gradient, and the different levels of the gradient would determine 

 the different parts of a pattern. 



Another viewpoint of a more general type is Lillie's theory of 

 segregation (1929). According to Lillie, embryonic segregation is 

 the process of origin of the diverse specific potencies that appear 

 in the organism in the course of the life history, which express 

 themselves later in tissues of specific structure and function. 

 The criterion of segregation is self-differentiation. Segregation 

 proceeds from the more general to the more special until 

 further segregation is closed. Thus far, this conception is 

 practically identical with all other similar conceptions. Lillie, 

 however, postulates that this segregation is dichotomous. It is 

 not clear whether he means only a dichotomy into parts with 

 restricted and nonrestricted determination or thinks that simul- 

 taneously only two areas may be formed, not a complete pattern 

 of areas with different restricted potencies. This dichotomy may 

 coincide with cell divisions in the case of cell lineage, and it may 

 also cover multicellular areas. Only a comparatively small 

 number of such segregations occur in ontogeny. Inductive 

 processes are those wdiich cause segregation to take place. 

 Segregation, however, does not separate organ-forming stuffs 

 but possibly only different proteins (which is the same, Author). 

 This theory, though couched in somewhat different terms, is 

 after all not very different from others here reported. 



Another more generalized point of view is the theory of 

 embryonic fields in the slightly differing types worked out by 

 Gurwitsch, Weiss, and Guyenot (see Weiss's review, 1935). 

 They compare the area that is to be subdivided to a field of 

 force (without making special physical assumptions) that is 

 being subdivided by the evocation of new conditions of equi- 

 librium. (This is only a very generalized statement of these 

 views.) Another view has been developed by Goldschmidt 

 (1927c). He uses as an inorganic model the formation of 

 Liesegang rings in a colloidal solution, evoked by the infusion of 

 a definite substance. With a generalized expression he calls such 

 a diversification within a unit, produced by a chemical or physical 

 evocation, a process of stratification; this might work via all 

 known chemical or physical processes such as change of charge, 



