Energy Exchange and Enzyme Development During Embryogenesis 525 



during a substantial part of embryonic life, 

 may be seen in a number of other cases. 

 This was clearly shown by Gray ('26) for 

 the ti'out embryo, and more recently Hayes, 

 Wilmot, and Livingstone ('51) reported that 

 the respiratory rate of the Atlantic salmon 

 embryo remains unchanged throughout the 

 entire period during which weight data 

 could be obtained. The actively developing 

 grasshopper embryo, which at certain stages 

 can be completely separated from yolk, also 

 has a constant respiratory rate over a con- 

 siderable period of development (Bodine and 

 Boell, '36a, '37). Of course, during diapause 

 respiratory rate falls, but this special situa- 

 tion will be discussed later. In the rat em- 

 bryo, the Q02 is approximately 30 during 

 the cleavage stages, but it soon falls to a 

 value of about 12 (Boell and Nicholas, '48) 

 which is maintained until around the fif- 

 teenth day of development (Dickens and 

 Greville, '33a; Negelein, '25). 



Respiratory Increase and Growth. It seems 

 in those cases in which the yolk content of 

 the embryo is relatively small, or where the 

 yolky parts of the egg can be successfully 

 separated from the embryonic materials, the 

 respiratory rate of the embryo is uniform 

 throughout a major part of the total period 

 of development. In other words, as pointed 

 out by Gray ('27), the total respiratory ex- 

 change at a given stage of development is 

 proportional to the amount of metabolicaliy 

 active embryonic material. It is, therefore, 

 perhaps more than mere coincidence that 

 respiratory data during development should 

 follow a sigmoid curve (Gray, '29a, b; Brody, 

 '45; Thompson, '42). 



During its early phases, growth appears 

 to be an exponential process — that is, ap- 

 proximately the same percentage increase 

 occurs during successive equal intervals of 

 time. The equation x = a.e^'^ has been found 

 empirically to fit growth data in a great 

 many cases, and a plot of the logarithm of 

 the magnitude of the growing entity against 

 time thus yields a straight line.* When 



* It is not intended to attach any strict biological 

 significance to the values of a or A: in the equa- 

 tion X = a.e^'. The semilogarithmic plots of growth 

 and respiratory data are intended as purely descrip- 

 tive; the chief justification for their use lies in the 

 fact that a convenient method is thereby provided 

 for comparing and contrasting curves. In arithmetic 

 plots, the similarities and differences between such 

 curves are not always readily apparent. The reader 

 is referred to Shell's ('54) paper in which is con- 

 tained a thoughtful analysis of the utility and lim- 

 itations of empirical curve fitting (see also Levy, 

 '52). 



growth rate changes abruptly during devel- 

 opment, a series of intersecting straight lines 

 will result. If respiratory increase parallels 

 growth of metabolicaliy active embryonic 

 mass, and the evidence reviewed above 

 strongly suggests such a relationship, the 

 data shown in Figure 197 should yield a 

 linear curve when log respiration is plotted 

 against time (Fig. 200). The points do not 



Days 



Fig. 200. Semilogarithmic plot of oxygen con- 

 sumption dunng development of the chick (data 

 from Table 1 of Romanoff, '41). 



fall on a single straight line, however, for 

 between the seventh and eighth days of de- 

 velopment an inflection appears, and from 

 then on respiratory increase proceeds at a 

 lower rate than earlier. A similar inflection 

 in the semilogarithmic plots of dry weight 

 or wet weight also occurs on the eighth 

 day. What is responsible for the inflection 

 in the respiratory curve is not known. It 

 is interesting to point out that it occurs at 

 about the time that the embryo shifts from 

 predominantly carbohydrate catabolism to 

 protein (see p. 533), and that the rate of 

 protein absorption for growth is minimal 

 on the eighth day (Needham, '31, Table 111, 

 column 13). Novikoff and Potter ('48) have 

 obtained data on increase in PNA and DNA 

 in the chick embryo, and similar results have 

 been described by Reddy, Lombardo, and 

 Cerecedo ('52). A semilogarithmic plot of 

 Novikoff and Potter's PNA figures yields a 

 pair of intersecting curves almost identical 

 in slope with those for respiration, and with 

 a break on the eighth day. Thus a number 

 of lines of evidence suggest that the period 

 around the eighth day of development is 

 one of transition in the chemical growth of 

 the chick embryo. 



