Energy Exchange and Enzyme Development During Embryogenesis 523 



withdrawn. Indeed, Amberson ('28) has 

 shown that cleavage is retarded when the 

 oxygen tension falls to 11 mm. Hg, and be- 

 low 4 mm. mitotic activity is completely 

 blocked. Hultin ('53) has concluded that 

 oxygen is necessary from the beginning to 

 assure development in the sea urchin. 



Ascaris eggs are likewise very dependent 

 upon oxygen supply in order to cleave 

 (Brachet, '50, p. 165). It is of interest, there- 

 fore, that mitosis continues in Ascaris eggs 

 after centrifugation at 400,000 Xg (Beams 

 and King, '36) although this treatment re- 

 duces the rate of oxygen consumption to 25 

 per cent of normal (Huff and Boell, '36). 



The eggs of Fundulus, on the other hand, 

 are remarkably resistant to complete removal 

 of oxygen. Loeb (1895) maintained Fundu- 

 lus eggs under vacuum for four days and 

 noted that cleavage could occur. In the pres- 

 ence of cyanide, which exerts a strong de- 

 pressing effect on respiration, development 

 appears to proceed qualitatively normally 

 but at a slower rate than in controls. By 

 contrast, the mitotic process is blocked by 

 cyanide in the eggs of the mackerel, cunner, 

 and scup (Philips, '40). 



Frog eggs have been shown to develop 

 through the cleavage stages in the absence 

 of oxygen and in the presence of cyanide 

 when respiration is only 10 per cent of 

 normal (Brachet, '34; Barnes, '44; Spiegel- 

 man and Moog, '45). Brachet believes that 

 energy is supplied, during short periods of 

 anaerobiosis, through the utilization of an 

 oxidative reserve, typified by but not neces- 

 sarily identical with glutathione, rather than 

 through processes leading to the formation 

 of lactic acid. During recovery from anaero- 

 biosis, the respiratory quotient of frog eggs 

 is not imity, as one would expect if accumu- 

 lated lactic acid were being burned, but 

 ranges between 0.27 and 0.52. During longer 

 periods of anaerobiosis lactic acid is pro- 

 duced, however (Lennerstrand, '33; Brachet, 

 '34; Barth, '46). 



That avian and mammalian embryos can 

 derive energy through anaerobic processes is 

 a well established fact (Needham, '31, '42). 

 However, most of the studies on these em- 

 bryos have been concerned with the analysis 

 of glycolytic mechanisms (see p. 541 ff.) 

 rather than with the ability of the embryo to 

 survive under conditions of reduced oxygen 

 supply. A comparative study of the rate of 

 anaerobic glycolysis and respiration in pre- 

 somite and early somite rat embryos has re- 

 vealed the interesting fact that the embryo 

 can derive more energy from anaerobic 



breakdown of glucose than from oxidation 

 (Boell and Nicholas, unpublished). This 

 may have considerable significance for the 

 embryo, for a condition of "Everest in utero" 

 probably exists during the period before 

 placentation as well as at birth. 



Examples could be multiplied, but those 

 enumerated show that many embryos have 

 the ability to derive energy, and in some 

 cases to develop, under conditions where 

 oxygen is lacking. Survival, diiring the exi- 

 gencies of anaerobiosis, seems to be associated 

 with the ability of the embryo to contract 

 an oxygen debt. Sooner or later, however, 

 this must be redeemed aerobically. For the 

 vast majority of embryos, development de- 

 pends upon an adequate oxygen supply and 

 release of energy through respiratory proc- 

 esses. 



RESPIRATION 



Oxygen Consumption During Development. 



With minor variations, the curve shown in 



2 4 6 8 10 12 14 16 18 20 22 

 INCUBATION AGE IN DAYS 



Fig. 197. Oxygen consumption during development 

 of the chick (from Romanoff, '41). 



Figure 197 describes the course of respira- 

 tion during development of a wide variety 

 of animals, both invertebrate and vertebrate. 

 The chief characteristics of the curve are 

 an initial period in which there is a con- 

 stant percentage increase in oxygen con- 

 sumption during equal intervals of time, 

 then a period in which the rate of increase 

 gradually lessens, and finally one, near the 



