Energy Exchange and Enzyme Development During Embryogenesis 541 



ence of aldolase in the eggs of Paracentrotus 

 lividus, and Cleland and Rothschild ('52a) 

 showed that lactic and pyruvic acids could 

 be produced from the following intermedi- 

 ates in the Embden-Meyerhof scheme: gly- 

 cogen, gkicose, glucose- 1 -phosphate, glucose- 

 6-phosphate, fructose-6-phosphate, hexosedi- 

 phosphate, and phosphoglycerate. Further- 

 more, they found that glycolysis was greatly 

 depressed by phenylmercviric nitrate, phlor- 

 izin, fluoride, and iodoacetate. 



Respiratory Mechanisms in Embryos Other 

 than Echinoderms. Respiratory mechanisms 

 in the embryos of other species have not 

 been so thoroughly investigated as in the 

 sea urchin. However, the functional signifi- 

 cance of cytochrome oxidase is indicated 

 for most embryos by their sensitivity to 

 cyanide. In a number of cases, cytochrome 

 oxidase has been assayed histochemically 

 (Moog,'43) or manometrically, and the re- 

 sults of the quantitative measurements show 

 that the enzyme is present in sufficient 

 amovmts to account for the total respiratory 

 activity of the intact embryo. This is true 

 for the grasshopper (Bodine and Boell, '36a; 

 Allen, '40), various amphibians (Brachet, 

 '34; Spiegelman and Steinbach, '45; Boell, 

 '45), the chick (Albaum and Worley, '42; 

 Albaum, Novikoff, and Ogur, '46; Levy and 

 Young, '48), and the mammalian cerebral 

 cortex (Flexner, Flexner, and Straus, '41). 



Attempts to demonstrate the presence of 

 cytochrome c in early embryos have been 

 no more successful than in the sea lurchin 

 already mentioned. In Amphibia, the pres- 

 ence of pigment, of course, makes direct 

 spectroscopic examination impossible. Yaoi 

 ('28-'29) was unable to find cytochrome c 

 in the chick embryo, and he stated that if 

 it was present at all between the fourth and 

 eleventh day its concentration was so low 

 as to be negligible. Likewise, Potter and 

 Dubois ('42) were unable to find cytochrome 

 c in the chick embryo until the tenth day. 

 In the cerebral cortex of the pig fetus, the 

 bands of reduced cytochrome c were only 

 faintly visible after the middle of gestation 

 (Flexner, Flexner, and Straus, '41). Stotz 

 ('39) stated that cytochrome c is present 

 in the early rat embryo (age not specified) 

 in a concentration of 0.3 mg. per gram of 

 tissue (dry weight); in the late rat embryo, 

 he found 0.18 mg. per gram. 



It has been claimed that succinic dehydro- 

 genase and other dehydrogenases in the 

 citric acid cycle are absent from the early 

 chick embryo (Banga, '37; Elliott and Greig, 

 '38; Greig, Munro, and Elliott, '39). But 



Booth ('35) showed that succinic, lactic, 

 hexosediphosphate, a-glycerophosphate, and 

 glucose dehydrogenases were present on the 

 eighth day. More recent work has shown the 

 presence of an active succinoxidase system 

 as early as the twenty-fifth hour of incuba- 

 tion (Albaum, Novikoff, and Ogur, '46), 

 and Spratt ('52) has found succinic dehydro- 

 genase in the early blastoderm. Potter, 

 Schneider, and Liebl ('45) made measure- 

 ments of the succinoxidase system in rat 

 liver and brain from the seventeenth day 

 of gestation through the thirtieth day after 

 birth and showed that the activity increased 



Table 21. Effect of Citric Acid Cycle Inter- 

 mediates on Oxygen Consumption 

 of Sea Urchin Egg Homogenates* 



* Data from Cleland and Rothschild ('52b). 



during this period. Flexner and Flexner 

 ('46) believe that the succinoxidase system 

 of the cerebral cortex of the fetal pig is 

 inactive until after the middle of gestation 

 owing to insufficiency of cytochrome c. Noth- 

 ing is known about the presence of succinoxi- 

 dase in the early stages of amphibian de- 

 velopment, but in Amblystoma punctatum 

 the enzyme has considerable activity at the 

 first stage tested (Harrison stage 20), and 

 it may reasonably be assumed that it is 

 present even earlier (Boell, '48). 



As will be shown below, there is good evi- 

 dence that the Embden-Meyerhof system of 

 phosphorylating glycolysis exists in the chick 

 embryo (Novikoff, Potter, and Le Page, 

 '48), and Barth and Jaeger's series of studies 

 ('47a,b, '50a,b) give strong evidence of the 

 presence of a mechanism in the amphibian 

 embryo, from early cleavage on, for oxida- 

 tive production of high-energy phosphate 

 bonds. 



Alternate Pathways of Energy Release. Cya- 

 nide-insensitive Respiration. Cyanide does not 

 inhibit completely the oxygen uptake of 

 most embryos, and in some few cases res- 

 piration is almost completely resistant to 

 this reagent. These facts have been mainly 



