410 



PRINCIPLES OF EMBRYOLOGY 



required for a system which will tend towards a limited number of 

 alternative end states. 



It is not so easy to give an actual example of a system in which the 

 alternative end states depend on specific inhibitions, of the kind postulated 

 by Delbriick. However, Horowitz (195 1) has mentioned a case, which, 

 although it is still somewhat hypothetical, may serve as an example of the 

 kind of situation which it would be most desirable to analyse. When 

 the mould Neurospora is grown in sulphur-deficient media it develops 



Simultaneous adaptation 



maltose and galaaoK 



D 



240 360 



Minutes 



Figure 19.2 



Simultaneous adaptation of yeast to maltose and galactose. The curve B 

 shows the activity in fermenting galactose, when that is the only sugar 

 present; D is the corresponding curve for maltose. When both sugars are 

 present, the splitting of galactose is not much affected {A) but the growth in 

 maltose-splitting activity is competitively inhibited (C). (After Speigelman 



1948.) 



the enzyme tyrosinase, and tends to become blackened with melanin 

 pigment if any suitable substrate is available. It seems probable that in fact 

 this enzyme is also produced in normal media containing sulphur, but 

 that in them some sulphur-containing compound which inliibits tyro- 

 sinase activity is also formed. Horowitz asks what would happen if one of 

 the products of tyrosinase activity could combine with the inhibitor and 

 destroy it, or in some other way prevent its appearance. Then, he suggests, 

 if a mould were grown for some time in a sulphur-deficient medium, it 

 would acquire a high content of tyrosinase, and on transference to a 

 normal medium this enzyme would continue to be active unless or until 

 the production of inhibitor could overtake its inactivation by the tyro- 

 sinase-system. We might then have a very simple example of a system 



