522 Energy Exchange and Enzyme Development During Embryogenesis 



structures with a fairly modest expenditure 

 of food." 



Activity metabolism refers to the energy 

 required to sustain the specific activities of 

 embryonic structures whose functional ca- 

 pacities have been realized. Most organs do 

 not become functional as soon as they appear 

 in the embryo. Nevertheless, when function 

 does commence, the energy expenditure of 

 the organ increases. As an example, the heart 

 may be mentioned. The respiratory rate 

 (m/ttl. oxygen consumed per Mg. per hour) 

 of the beating rat heart, at the earliest stage 

 that it can be removed from the embryo, 

 is 40; it is only 6 for the heart at rest (Boell 

 and Nicholas, unpublished). The difference 

 in metabolic rate in this instance clearly 

 represents the energy used for muscular con- 

 traction. It is of interest to note that total 

 oxygen consvimption of an embryo from 

 which the heart has been removed is indis- 

 tinguishable from that of an embryo with its 

 heart in situ. This means simply that the 

 heart is so small in relation to the total em- 

 bryo that its absence produces no noticeable 

 effect on total metabolism. 



Another example of activity metabolism is 

 seen in the development of the grasshopper. 

 The smooth course of respiratory increase, 

 which characterizes postdiapause develop- 

 ment, is punctuated, between the third and 

 fifth days, by a significant decline in respira- 

 tory rate. "At precisely this time the lateral 

 walls of the embryo beat more slowly than 

 they do immediately before or afterward; 

 moreover, blastokinesis (revolution of the 

 embryo around the yolk) has just been com- 

 pleted on the third day. Since Slifer ('32) 

 showed that blastokinesis 'is accomplished by 

 vigorous movements on the part of the em- 

 bryo itself the decreased respiration may be 

 attributed to lessened embryonic activity" 

 (Boell, '35).* 



It may be concluded that the processes as- 

 sociated with development — maintenance, 

 growth, differentiation, and functional activ- 

 ity — require energy. In the long run, these 

 energy requirements are met through oxi- 

 dative processes. The embryo is not required 

 to pay a premium for the large synthetic job 



* Tuft ('53) apparently has misunderstood this 

 description, for, in referring to the experimental ob- 

 servation in support of his contention that "phases 

 (of development) during which the cells of the 

 embryo spread over the yolky parts of the egg are 

 accompanied by a decrease in O2 uptake," he states 

 that "a similar phenomenon seems to occur at the 

 same stage in the eggs of Melanoplus differentialis 

 (Boell, '35), but this author attributes it to a tem- 

 porary decrease in the frequency of the heart beat." 



it has to do other than that necessitated by 

 its own inefficiency as an energy trans- 

 former, but this is a characteristic shared by 

 all living organisms. Neither is the embryo 

 exempt from the construction cost of protein 

 synthesis. "Developmental phenomena," as 

 Weiss ('53) has recently pointed out, "do 

 not violate the laws of thermodynamics . . . 

 the old problem of 'energy of shape' is still 

 with us, presumably because of the fact 

 that the energy requirements in growth and 

 differentiation may be greatly overshadowed 

 by the energy requirements for the continu- 

 ous anabolic renewal of the protoplasmic 

 system. . . ." 



ENERGY RELEASE DURING PERIODS OF 



REDUCED OXYGEN SUPPLY OR 



ANAEROBIOSIS 



The eggs and embryos of most species will 

 develop continuously and normally only in 

 the presence of oxygen, but no very close 

 correlation exists between the oxygen ten- 

 sion of the environment and that required to 

 svistain normal development. The sea urchin 

 egg will respire and develop normally under 

 oxygen tensions as low as 40 mm. Hg (Am- 

 berson, '28) ; normal development and res- 

 piration in the grasshopper egg are possible 

 in a gas mixture containing 10 per cent of 

 oxygen (Bodine and Boell, '34a), and even 

 an egg as large as that of the frog can with- 

 stand some variation in oxygen tension with- 

 out developmental retardation (Parnas and 

 Krasinka, '21). On the other hand, numerous 

 investigators have shown that respiration of 

 the mammalian embryo in vitro requires an 

 atmosphere of pure oxygen, and Philips ('41) 

 has suggested that the normal oxygen ten- 

 sion in air may not be sufficient to sustain 

 an optimal level of respiration in the chick 

 embryo during the period before circulation 

 commences. 



Oxygen supply seems to be indispensable 

 for continued development, but most em- 

 bryos can safely withstand the effects of 

 oxygen lack or reduced respiration for some 

 time. But the embryos of different species 

 vary considerably with respect to resistance 

 to anoxia, and, by implication, with respect 

 to their ability to derive energy from an- 

 aerobic reactions or to accumulate an oxygen 

 debt. In the sea urchin, Arbacia punctulata, 

 the activation of the egg and the initial cor- 

 tical changes associated with the process can 

 occur in the complete absence of oxygen 

 (Kitching and Moser, '40), but cell division 

 is immediately blocked as soon as oxygen is 



