THE INHERITANCE OF BIOCHEMICAL CHARACTERISTICS 315 



It is generally accepted that the active cells in the adrenal cortex are those 

 of the zona fasciculata, which secrete corticosteroids under the influence of 

 ACTH. The secretion of the various corticosteroid hormones implies that 

 a complex system of enzymes is involved which characterizes the biochemi- 

 cal differentiation of the cells of the zona fasciculata. The reader will find 

 an excellent account of the biology of the adrenal cortex in the recent work 

 of Chester Jones (1957). 



When we come to consider the biosynthesis of the steroid hormones by 

 the gonads, or by the placenta, here also we have enzyme systems special 

 to these cells. 



The preceding pages give only a slight idea of the biochemical diversity 

 accompanying cellular differentiation in the organism of a mammal such as 

 man. Also, the description of these differentiations has been made in an 

 altogether too schematic manner without taking into account the variations 

 existing from cell to cell in the framework of the same cellular 

 differentiation. 



Among the same class of differentiated cells we may also observe an 

 individual biochemical variation depending on time, age and the reproduc- 

 tive cycle, etc. In the water flea Daphnia, for example, the haemoglobin 

 content of the blood varies with the instar. The smallest concentration 

 corresponds to the time when the eggs have reached the development of 

 late-stage embr}'os ready to be released. After this the blood haemoglobin 

 rapidly diminishes, passing from the blood into the ovaries before the 

 eggs are laid. After the laying of the eggs the blood of the female 

 gradually recovers its haemoglobin content (Fox, Hardcastle and Dresel, 

 1949). 



III. PHENOTYPE AND ''MILIEU" 



Under given conditions of the milieu, cellular differentiation controlled 

 by a given genotype can produce an extreme diversity of results in the way 

 of biochemical systems. When to this is added the influence of the milieu 

 even further differences in a given biochemical system may be produced. 



Grasshoppers which have been raised at an elevated temperature are 

 pale whilst those which have grown at a lower temperature are dark. This is 

 because at temperatures higher than 40° the biosyntheses of m.elanin and of 

 insectorubin are inhibited. Here we have a direct action on an enzymatic 

 component of the existing phenot}^pe, and we may call this a phenotropic 

 action of the milieu. 



A further example, the green pigment present in the tegument of solitary 

 locusts is not present in the insects of the gregarious type. The haemolymph 

 of the solitar}^ form is a brilliant green whilst that of the gregarious form is 

 a golden-yellow. The characteristic difference here is the production of 

 mesobiliverdin in the solitar\' locust. 



