316 UNITY AND DIVERSITY IN BIOCHEMISTRY 



A remarkable example of the influence of the milieu on the biochemical form 

 of the phenotype is provided by the influence of the partial pressure of oxy- 

 gen in the milieu on the biosynthesis of haem derivatives. It has been shown 

 in yeasts that the presence of air stimulates the synthesis of cytochromes a, b 

 and c and that the absence of air inhibits this synthesis. On the other hand, 

 it is vjtW known that a fall in oxygen pressure increases the blood haemo- 

 globin in man at high altitudes and in fishes living in waters poor in oxygen. 

 In mammals, a fall in the external oxygen pressure also increases the cyto- 

 chrome-c and myoglobin concentrations in muscles. Among the Cladocera, 

 a subdivision of the Crustacea, a fall in the oxygen content of the medium 

 increases the haemoglobin concentration in muscles, ganglia and blood. In 

 Daphniapulex, D. magna and D. obtusa, an increase in blood haemoglobin in 

 poorly aerated water has been demonstrated by H. M. Fox. Fox and Phear 

 also observed that the animals lost their blood pigment in well aerated waters. 



Whatever the type of differences in the biochemical order which can be 

 brought about by the influence of the medium on the biochemical phenotype, 

 it is possible to determine the spectrum of possible phenotypes and so to find 

 the limits set to the organism by its own genetic potential in the realization 

 of the phenotype by the interaction of the environment and the genotype. 



REFERENCES 



Baker, J. R. (1948). The cell-theory : a restatement, history and critique. Part I. 



Quart, y. Micr. Sc, 89, 103-125. 

 Jones, T. Chester (1957). The Adrenal Cortex. Univ. Press, Cambridge. 

 Knox, W. E., Auerbach, V. H. and Lin, E. C. C. (1956). Enzymatic and metabolic 



adaptations in animals, Physiol. Rev. 36, 164-254. 

 Prosser, C. L. (1955). Physiological variation in animals. Biol. Rev., 30, 229-262. 

 Wagner, R. P. and Mitchell, H. K. (1955). Genetics and Metabolism. Wiley, 



New York. 



