Energy Exchange and Enzyme Development During Embryogenesis 531 



may consider the diapause period as one in 

 which development is completely suspended.* 

 Respiration of the egg, or of the embryo iso- 

 lated from it, is much lower during diapause 

 than when development is going on even 

 though blocked and developing embryos are 

 of identical size and morphologically indis- 

 tinguishable (Bodine, '29; Bodine and Boell, 

 '34a,b, '36b, '37; Boell, '35). Quantitative 

 respiratory data for the two types of embryos 

 are svimmarized in Table 17. During dia- 

 pause, the embryo, as far as can be ascer- 

 tained, is simply maintaining itself, and it is 

 reasonable to suppose that the total energy 

 expenditure, as indicated by the oxygen con- 

 sumed, is channeled into maintenance proc- 

 esses. The same can be said for the amphib- 

 ian hybrid and the "no-X" Drosophila em- 

 bryo. 



Are Different Respiratory Mechanisms In- 

 volved in Maintenance and Development? 

 Fisher and his co-workers have attempted 

 to determine the relative proportions of the 

 total respiratory activity concerned respec- 

 tively with maintenance and with activity 

 (Fisher, '41; Fisher and Henry, '40, '44; 

 Fisher, Henry and Low, '44; Fisher and 

 Stern, '42; Henry and Henry, '45). Working 

 with yeast, luminous bacteria, and sea virchin 

 eggs, they have produced evidence that a 

 fraction of respiration that is relatively sensi- 

 tive to urethane or chloral hydrate is con- 

 cerned with such cell activities as division or 

 luminescence, and that another fraction, 

 much more resistant to these narcotics, is 

 concerned with maintenance. The mass law 

 relation has been applied to their data 

 through the equation 



U 



— X [/V]« = K 



U is the fraction of respiration not inhibited 

 by narcotic and is proportional to the free 

 or active enzyme in the cell; / represents the 

 fraction of respiration that can be inhibited 

 by narcotic and is proportional to the amount 

 of inactivated enzyme, and [A^]", for prac- 

 tical purposes, may be regarded as concentra- 

 tion of narcotic. Such a relation assumes, and 

 reasonably so in the case of most narcotics, 

 a reversible combination of narcotic and en- 



U 

 zyme. When Fisher plotted log — against 



the log of narcotic concentration, not one 

 straight line resulted, btzt two, and these in- 



* Fitzgerald ('49) has shown, however, that al- 

 kaline phosphatase increases late in the diapause 

 period. 



tersected at a narcotic concentration about 

 the same as that required to block cleavage 

 completely. At this concentration respiration 

 was inhibited about 70 per cent. Fisher and 

 his co-workers have thus been led to con- 

 clude that two respiratory mechanisms oper- 

 ate in the cell: one, accounting for about 70 

 per cent of the total oxygen consumption, is 

 concerned with cellular activities; the other, 

 corresponding to approximately 30 per cent 

 of the total respiration and more difficult 

 to inhibit with narcotic, is regarded as in- 

 volved in maintenance metabolism. 



Table 17. Respiratory Rates of Diapause and 

 Developing Eggs and Embryos 



* The embryos were dissected free of yolk. 



A similar attack on the problem of energy 

 for maintenance and energy for development 

 has been made by Moog ('44a) through the 

 inhibition of respiration and development of 

 the frog egg with Chloretone. Moog suggests 

 that "from the neurula on, normal develop- 

 ment requires almost all of the normal oxy- 

 gen consumption, and that development can- 

 not proceed at all unless at least 50% of the 

 normal oxygen consumption is operating." 

 In Chloretone solution capable of suppress- 

 ing development completely without killing 

 the embryo, she finds that respiration persists 

 at a value between 40 and 50 per cent of 

 normal, and she interprets these results as 

 suggesting the presence of two systems of 

 respiration concerned respectively with main- 

 tenance and growth-differentiation. These 

 conclusions are svipported in part by applica- 

 tion of the mass law in the same manner as 

 done by Fisher. 



The action of lithium on respiration and 

 development in the sea urchin egg may be 

 appropriately mentioned at this time. Lith- 

 ium, it will be recalled, exerts a vegetalizing 

 influence on the sea urchin embryo. In an 

 attempt to discover biochemical correlates 



