524 Energy Exchange and Enzyme Development During Embryogenesis 



end of development or of a particular phase 

 of development, in which respiration in- 

 creases only slightly, if at all. 



In the sea urchin, the course of respiratory 

 increase during development can best be rep- 

 resented, as shown in Figure 198, by two 



Z 50 

 2 



4 8 12 16 20 24 26 



HOURS AFTER FERTILIZATION 



Fig. 198. Course of oxygen consumption during 

 development of the sea urchin. Curves A and B 

 were obtained from two different lots of eggs. (After 

 Lindahl, '39.) 



connecting curves (Lindahl, '39; Borei, '48). 

 The sigmoid part of the curve represents 

 respiration of the embryo from fertilization 

 to the hatching of the blastula; the ascending- 

 limb of the second curve coincides with gas- 

 trulation, and the terminal portion covers 

 the period in which differentiation is the 

 predominant developmental event. 



Metabolic Rate. It may be asked whether 

 the curves shown in Figures 197 and 198 

 indicate that the rate of metabolism per 

 unit of embryonic material is constantly in- 

 creasing during development. This is not 

 an easy question to answer owing to the dif- 

 ficulties involved in determining accurately 

 the actual amount of metabolically active 

 embryonic material as distinct from the non- 

 metabolizing components in the developing 

 system (see footnote J on p. 520). However, 

 in some cases, it is possible to separate the 

 embryo from its yolk supply pretty com- 

 pletely, and the study of respiratory rate in 

 these embryos provides a negative answer 

 to the question raised above. This is illus- 

 trated in Figure 199, representing a com- 

 posite graph of data from various sources 

 on the metabolic rate (oxygen consumed per 

 milligram dry weight per hour) of the chick 

 embryo. The points for the first six days of 

 development represent in vitro measurements 

 on the isolated embryo; the terminal points 



were taken from Romanoff's ('41) study of 

 the respiration of the intact hen's egg dur- 

 ing incubation and have been corrected for 

 the respiratory activity of the extraembry- 

 onic membranes through the use of percent- 

 age figures from Needham ('32a). 



The graph shows that metabolic rate is 

 essentially constant, at an average of 10.1 

 (range 7.9 to 12.3) between 16 and 288 hours 

 of development. It should not be concluded 

 that Qoo remains at this level during the 

 remainder of the period to hatching, for 

 Romanoff has shown that it declines to 8 

 on the thirteenth day of development, and 

 by the nineteenth day it is only slightly 

 more than 5. The decline in respiratory rate 

 undoubtedly reflects the fact that such meta- 

 bolically sluggish components as feathers, 

 cartilage, and connective tissue now make 

 up an appreciable portion of the total em- 

 bryonic mass. But from the twelfth day of 

 development, the decline is also brought 

 about in part by a decrease in the respira- 

 tory activity of a number of tissues, particu- 

 larly muscle and liver (Romanoff, '43). Slow 

 decline of basal metabolism during the life 

 span of the individual seems to be the gen- 

 eral rule (Needham, '31, '42), but the proc- 

 ess may not begin as early in embryonic 

 life as was previously thought. 



The chief factor responsible for the in- 

 crease in rate of oxygen utilization during 

 the first 16 hours may be regarded as the 

 opposite of that which later contributes to 

 its decline. Philips ('42), who has made the 

 most complete study of the metabolism of 



Fig. 199. Qo2 (m1. oxygen per mg. dry weight per 

 hour) of chick embryo. Values for first 144 hours 

 represent measurements of respiration of chick em- 

 bryos in vitro by Philips ('41, '42), Romanoff ('41, 

 '43), Needham and Nowinski ('37), Dickens and 

 Greville ('33b), Dickens and Simer ('30, '31), and 

 Warburg, Posener, and Negelein ('24). Final three 

 points represent respiratory rate of chick embryos 

 in vivo from Romanoff ('41). 



the very young chick embryo, has suggested 

 that the early increase in rate of respiration 

 is correlated with the conversion of intra- 

 cellular yolk materials into active cellular 

 constituents. 



Essential constancy of respiratory rate. 



