THE METABOLISM OF EGGS, II 77 



in non-protein amino acids after ageing in sea water. These changes 

 are said to reflect the fact that unfertilized eggs synthesize their 

 own yolk proteins, free amino acids and peptides being supplied to 

 them from other cells in the ovary; though this is not in accord 

 with the fairly prevalent view that large molecules such as proteins 

 are supplied to the growing oocyte in finished form. Unfertilized 

 eggs from freshly collected Strongylocentrotus purpuratiis had a high 

 free amino acid content which dropped after fertilization, but this 

 was not so in unfertilized eggs obtained from urchins kept under 

 conditions inhibiting spawning. In the latter, the free amino acid 

 content increased after fertilization. Kavanau states that a high 

 free amino acid content is characteristic of eggs in which yolk 

 synthesis is proceeding rapidly, which suggests under-ripeness; 

 a low free amino acid content means that yolk synthesis has ceased, 

 free amino acids and small peptides are no longer being absorbed, 

 and the eggs are ripe (or perhaps even over-ripe), and ready to 

 be spawned. A different interpretation of the facts is, however, 

 possible. The low free amino acid content of unfertilized eggs 

 obtained from animals kept under conditions which inhibited 

 spawning and feeding may be correlated with the nutritional state 

 of the animal, rather than with changes in the synthetic activities of 

 the egg. Freshly collected and, presumably, well fed animals might 

 be expected to be relatively rich in free amino acids, while stored 

 and probably semi-starved animals might have a rather low content 

 of them, the difference being reflected in all the body tissues, in- 

 cluding the ovaries and eggs. According to Kavanau (1954a, p. 

 566), 'the early post-fertilization changes in non-protein amino 

 acids (and probably other metabolites) must apparently be viewed 

 as adjustments which bring an initially flexible system to a more 

 rigidly defined state from which normal development can proceed.' 

 These views recall those of Whitaker (19336) on the 'regulation' of 

 respiration by fertilization. The idea that metabolic changes at 

 fertilization may be positive or negative according to the past 

 history of the unfertilized egg may seem startling, but there is 

 evidence to support this idea. The fact that the increase in O, up- 

 take after fertilization of Psammechinus eggs depends on the delay 

 between spawning or removal from the ovary (Fig. 11) is one 

 example, as is the paradoxical behaviour of Urechis eggs after 

 fertilization (Tyler & Humason, 1937). Another concerns the 

 contradictory results of Infantellina & La Grutta (1948) and 



