CHAPTER 6 



THE METABOLISM OF EGGS, II 



Carbohydrate metabolism. One of the objects of studying the meta- 

 bohsm of eggs is to translate the observed biological results of 

 fertilization into quantitative chemical language. This will probably 

 not be achieved in the next twenty-five years and, as always 

 happens when subjects are in their infancy, our knowledge of the 

 metabolic changes induced by fertilization is disordered and con- 

 tradictory. We can follow up the investigations described in the 

 previous chapter by asking the following questions: what are the 

 principal sources of energy in the egg and what are the metabolic 

 changes which make this energy available for such purposes as 

 maintenance of structure, pumping sodium out of the egg if that 

 is necessary, rotation of the sperm head, cytoplasmic movements 

 when they occur, aster formation, and movements of the pro- 

 nuclei? The R.Q. experiments already discussed show that it is 

 far from certain that carbohydrate is always the principal endogen- 

 ous substrate of eggs; but because our knowledge of the pathways 

 of carbohydrate catabolism is greater than that of lipid or protein 

 catabolism, more attention has naturally been paid to the former 

 than the latter. Tables 12 and 13 summarise the evidence that sea- 

 urchin eggs have a normal anaerobic carbohydrate metabolism, or 

 at any rate the machinery for it. The reason for the qualification 

 is that, because of the impermeability of eggs (unlike spermatozoa, 

 Mann, 195 1), experiments to investigate metabolic pathways in 

 eggs almost always necessitate the use of homogenates, with or 

 without the addition of various cofactors; in these experimental 

 conditions, the normal egg structure and, therefore, conceivably, 

 the normal egg metabolism, is destroyed. 



Cleland (1950a, b) has obtained evidence which is very similar 

 to that given in Tables 12 and 13 for the existence of a classical 

 glycolytic system in the eggs of the rock-oyster. 



In spite of what has been said above, opinions are not unanimous 

 about the principal pathways of carbohydrate metabolism in the 

 sea-urchin egg. Lindberg & Ernster (1948) were the first to suggest 

 that the mechanism often known as the hexose monophosphate 



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