86 



systems, which might otherwise get out of 

 hand. 



Hypothetically, at least, the scheme of 

 competitive interaction is applicable to the 

 problem of embryonic differentiation. Since 

 no egg is completely homogeneous in the 

 distribution of its chemical constituents, a 

 way is immediately open for the differential 



Cellular Structure and Activity 



bryos is alterable by addition of succinate 

 (Boell, '49), but in this case normal develop- 

 ment is possible over only a narrow range of 

 enzyme enhancement. Probably a more fav- 

 orable type of material for experiments on 

 adaptation is made available in the im- 

 portant experiments of Spratt ('49), who 

 has shown that explanted chick blastoderms 



3 



< 

 U 



z 



I 



OR 



r 



Fig. 9. Diagram to illustrate some of the principles pertinent when two or more enzymes, subject to deg- 

 radation, are competing for a common stock of enzyme precursor. Raw materials, presumably from endog- 

 enous synthetic mechanisms, supply precursor. Enzymes, once formed, are stabilized by (A and 5) com- 

 bination with substrate or (fi') combination with inhibitor or structural analog of normal substrate. If pre- 

 cursor is limiting, A or B enzyme systems might dominate depending on (1) the effectiveness of the syn- 

 thetic system for A or B and (2) the relative stabilization by substrate. Thus starvation might have highly 

 differential effects. 



production of various enzymes in different 

 regions. As the enzymes so produced must 

 themselves affect their surroundings, con- 

 tinued differentiation becomes self-perpetu- 

 ating. How amenable this scheme is to ex- 

 perimental verification is, however, another 

 matter. Work with great nvimbers of inhibi- 

 tory agencies has taught us little except how 

 easy it is to upset fatally the delicate bal- 

 ance of developmental events. Will excess 

 substrates merely add themselves to the list 

 of inhibitors, if they have any effect at all? 

 One preliminary report does indicate that 

 succinic dehydrogenase in amphibian em- 



can use a variety of six-carbon sugars as 

 exogenous energy sources. 



The Influence of Structural Orientation. Em- 

 bryology provides us with many instances 

 of systems held in an inhibited state. The 

 dormant seed of a green plant, the orthop- 

 teran egg in diapause, the winter egg of 

 rotifers, the unfertilized egg of marine in- 

 vertebrates, the prolonged blastocyst of the 

 deer, the unincubated cicatrix of the hen's 

 egg, are examples of such restraint. In many 

 cases the system can be released suddenly, 

 indicating that structural factors were re- 

 sponsible for the blockage. Experimentally 



