528 Energy Exchange and Enzyme Development During Embryogenesis 



trulation, during which respiratory increase 

 does not occur, and he has cited the work of 

 Barnes ('44) as showing a similar phenome- 

 non. L0vtrup ('53a, b) and Ten Gate ('53) 

 have also emphasized that periods of rela- 

 tively constant oxygen consumption have 

 been noted in their experiments. It should 

 be mentioned that, in a number of instances, 

 the period immediately following the break 

 in the rate of respiratory increase seems to 

 be one in which respiration remains con- 

 stant, but such plateaus, if they may be so 

 designated, are generally of short duration. 

 Extremely long periods of essentially con- 

 stant respiration, similar to those shown by 

 Tuft or Barnes, are not the rule. It is likely 

 that these plateaus have little fundamental 

 significance, for they are not found con- 

 sistently, even by the same investigator. 

 Moreover, when they are found, they do not 

 occur uniformly at gastrulation, as stated 

 by Tuft. In this connection, it may be noted 

 that the amphibian embryo is particularly 

 sensitive to injiiry during gastrvdation, and 

 Smith ('46) has found the same to be true 

 for the trout. It seems possible, therefore, 

 that long respiratory plateaus during gastru- 

 lation may be an indication of damage to 

 the embryo. The possibility of injury cannot 

 be eliminated from Barnes' experiments, for 

 the eggs used were shaken at the rate of 

 120 cycles per minute over an amplitude 

 of 8 cm. for 6 to 8 hours during a mano- 

 metric run, and she has reported that a care- 

 ful census of the eggs was made routinely 

 after each experiment "to check on the nvmi- 

 ber living." It is also possible that plateaus 

 of respiration represent intervals of develop- 

 mental retardation due to handling of the 

 eggs or other unfavorable conditions. Under 

 such circumstances chronological time and 

 developmental time would not be equivalent. 

 Time is an adequate base line for de- 

 velopment only when it is certain that devel- 

 opment is normal. That acceleration of de- 

 velopmental rate occurs after embryos have 

 been depressed by adverse conditions is clear 

 from the work of Buchanan ('38, '40). The 

 heightened respiratory activity after a pla- 

 teau may reflect a regulatory phenomenon 

 of this kind. 



Energy for Maintenance vs. Energy for De- 

 velopment. Respiratory Metabolism and Cell 

 Division. Calculations of the amount of en- 

 ergy necessary for cytoplasmic cleavage or 

 for the production of "new surfaces" during 

 mitosis have generally yielded figures so 

 small as to suggest that the energy cost of 

 cell division is negligible. The problem can- 



not be dismissed in this way, however, for 

 the cell may not be completely efficient as 

 an energy transformer diiring division, just 

 as muscle is unable to channel all of the 

 energy released during contraction into 

 mechanical work. 



Apparently, respiratory increase does not 

 necessarily follow the increase in number 

 of cells. This was clearly shown in the work 

 of Gray ('27), and more recently Tuft ('53) 

 and Zeuthen ('53) have indicated that in- 

 crease in number of nuclei bears only a 

 casual relationship to increase in respiration. 



A number of indirect approaches to the 

 question of the energetics of cell division 

 have been made through measurements of 

 respiration when cell division is depressed. 

 The difficulties of interpreting the results of 

 such experiments are illustrated by the fact 

 that hypertonic sea water and phenyl ure- 

 thane inhibit cleavage without appreciably 

 affecting oxygen consumption (Warburg, 

 '08, '10), cyanide depresses respiration with- 

 out affecting cleavage (see p. 523), and the 

 svibstituted phenols block cleavage reversibly 

 and at the same time bring about a tremend- 

 ous increase in oxygen utilization (Glowes 

 and Krahl, '34, '36; Tyler and Horowitz, 

 '38a). 



Brachet ('38) studied the respiration of 

 Ghaetopterus eggs that had been artificially 

 activated by potassium chloride. Such eggs 

 will develop into ciliated objects resembling 

 trochophores through a process of "differen- 

 tiation without cleavage." Brachet observed 

 that the oxygen consumption of KGl-activated 

 eggs increased less than in normal eggs. One 

 might be tempted to consider such a differ- 

 ence in respiration as representing the energy 

 cost of cleavage, but Brachet went on to show 

 that the rate of development of the KGl- 

 activated eggs was much slower than normal. 

 This was indicated by the fact that their 

 DNA content was only 30 per cent of that 

 in normally activated eggs. Similarly, Tyler 

 and Horowitz ('38b) investigated respiration 

 of cleaving and non-cleaving parthenoge- 

 netic Urechis eggs and found that the res- 

 piration of cleaving eggs increased at a 

 faster rate than it did in non-cleaving ac- 

 tivated eggs or in fertilized eggs whose 

 cleavage had been blocked by phenyl ure- 

 thane. Just as in Brachet's experiments, the 

 absence of cleavage was not the only differ- 

 ence between the lots of eggs. To conclude 

 from such experiments that the difference 

 in respiration between normal and non- 

 dividing eggs represents the energy cost of 

 cleavage would, as Holter ('49) has pointed 



