532 Energy Exchange and Enzyme Development During Embryogenesis 



with this phenomenon, Lindahl ('39) was 

 led to study the effects of lithium on respira- 

 tion. In brief, he found that exposure of sea 

 urchin eggs to lithium soon after fertilization 

 largely prevents the rapid increase which 

 occurs normally between fertilization and 

 hatching of the blastula (see Fig. 198). He 

 thus concluded that respiration in the sea 

 urchin egg consisted of two fractions. One 

 remained constant, or essentially so, through- 

 out development; the second, the lithium- 

 sensitive fraction, rose continuously from 

 fertilization and was largely responsible for 

 the exponential increase in respiration. Pre- 

 sumably normal development depends upon 

 the proper operation of both fractions; if the 

 lithium-sensitive portion of respiration is de- 

 pressed, vegetalization of the embryo results. 



Studies of the effects of carbon monoxide 

 and cyanide on the respiration of yeast, sea 

 urchin eggs, and grasshopper embryos pre- 

 sent analogous situations. Here too, carbon 

 monoxide- and cyanide-sensitive and -insen- 

 sitive fractions have been demonstrated. But 

 some question seems now to exist as to 

 whether there is complete cyanide-insensitiv- 

 ity in any case. Robbie, Boell, and Bodine 

 ('38) showed that approximately 20 per cent 

 of the respiration of the diapause egg, formerly 

 regarded as completely insensitive to cya- 

 nide, could be depressed by 0.001 M potas- 

 sium cyanide when care was taken to avoid 

 loss of cyanide from the medium surround- 

 ing the eggs. And Robbie ('46) went on to 

 show that the unfertilized sea urchin egg, 

 long regarded as the classic example of cy- 

 anide insensitivity, also had reduced respira- 

 tory activity in the presence of this reagent. 



Critique and Evaluation. From the ac- 

 count given above, it is apparent that it is 

 difficult, if not impossible, to assess the en- 

 ergy requirements of particular events in the 

 developmental process by a stvidy of the over- 

 all gaseous metabolism of the embryo. One 

 can never be certain that minor ripples on 

 the total curve of respiratory increase are 

 significantly correlated with the develop- 

 mental phenomena with which they appear 

 to coincide temporally. They may represent 

 nothing more than random and fortuitous 

 fluctuations brought about by developmental 

 conditions. Furthermore, it should be empha- 

 sized that a period of increased oxygen utili- 

 zation is not necessarily contemporaneous 

 with an energy-requiring developmental 

 event. The energy needed at the time the 

 event is occiirring might be supplied by 

 breakdown of high-energy phosphate com- 

 pounds such as adenosinetriphosphate (ATP). 



Thus a period of enhanced oxygen consump- 

 tion would occur after the event and would 

 be concerned only with regeneration of the 

 energy stores. This would be identical to the 

 situation in muscle, where oxidative recov- 

 ery follows contraction. 



Nevertheless, when all the observations 

 reviewed above are taken together, they 

 strongly suggest that a certain fraction of 

 the total respiratory exchange is used by the 

 embryo for purposes of maintenance while 

 another fraction is used to support develop- 

 mental processes. 



The question may now be asked whether 

 the energy reqviirements for these processes 

 can be fixed quantitatively or even relative- 

 ly. (For various points of view on this ques- 

 tion, the reader may consult Needham, '42; 

 Tyler, '42; Brachet, '50; Tuft, '53.) One may 

 be tempted to conclude that the amount of 

 energy needed to maintain a developing em- 

 bryo is the same as the total energy released 

 by a blocked embryo, so that the difference 

 in metabolic level between embryos in the 

 two states might be regarded as representing 

 the cost of developmental work. Similarly, 

 it might be thought that the energy require- 

 ments for development could be derived by 

 subtracting the total metabolism of the un- 

 fertilized egg from that of the fertilized egg. 

 Such considerations lead into difficulties, 

 however, for in some cases fertilization is not 

 associated with change in respiratory rate, 

 and in others it actually decreases (Whitaker, 

 '33). Before firm conclusions could be drawn 

 from data on blocked and developing em- 

 bryos, or fertilized and unfertilized eggs, it 

 would have to be demonstrated that a unit 

 of oxygen consumed had the same calorific 

 value for the two kinds of eggs or embryos. 

 To obtain information on this would require 

 very precise knowledge not only of the 

 amount but also of the kind of foodstuffs be- 

 ing oxidized as energy sources. Although it 

 might be possible to demonstrate by chemi- 

 cal analysis that a decrease in a potential 

 energy source occurs during development, it 

 may be pointed out that it does not follow 

 that such disappearance is brought about ex- 

 clusively by oxidative or other energy-yield- 

 ing processes. The substance in question may 

 be synthesized into some other compound, 

 or it may be lost from the egg without par- 

 ticipating in energy-yielding processes. There 

 seems to be evidence that considerable quan- 

 tities of metabolites may disappear from the 

 egg by processes which do not involve the 

 simultaneous utilization of oxygen (Ldvtrup, 

 '53b). 



