526 Energy Exchange and Enzyme Development During Embryogenesis 



It will be noted that the points for the 

 second, third, and fourth days deviate from 

 the straight line drawn in Figure 200. These 

 may represent biologically significant varia- 

 tions since they are all in the same direc- 

 tion, but it may be remarked that Romanoff 

 has shown that the deviations coincide with 

 periods of high variability among embryos 



HOURS 

 Fig. 201. Oxygen consumption during develop- 

 ment of Rana pipiens. Small points represent experi- 

 mental determinations. Large circles represent cal- 

 culated values from equations x = 0.095«?*>o^'^' and 

 X = 0.45e0-03i'. (From Moog, '44a.) 



due to differences in morphological age or 

 mortality during early development. 



Respiration of Amphibian Embryos. Typi- 

 cal data for the respiratory exchange of the 

 developing frog embryo are shown in Fig- 

 ure 201 (Moog, '44a). Similar curves have 

 been obtained for a large number of am- 

 phibian species, both urodeles and anurans, 

 and in all cases where careful attention has 

 been paid to assure normal development and 

 to avoid injury to the embryos during respir- 

 atory measurements, the curves are qualita- 

 tively identical to that shown in Figure 201. 

 It has been claimed that respiratory increase 

 does not occru- during cleavage in the am- 

 phibian embryo, but this may be due to the 

 use of insufficiently sensitive techniques for 

 measuring the very low rates of gaseous 

 exchange in early stages. 



As first indicated by Atlas ('38) and sub- 

 sequently confirmed by Moog ('44a) and a 

 number of others (Barnes, '44; Spiegelman 



and Steinbach, '45; Boell, '45; Earth, '46; 

 Ten Gate, '53), respiratory data for the am- 

 phibian embryo can be closely approximated 

 by the equation z = a.e^\ providing the 

 value of k is changed at an appropriate 

 point in development. On a semilogarithmic 

 plot a break in the curve thus appears similar 

 to that in Figru-e 200. In addition to the pa- 

 pers already cited, semilogarithmic plots of 

 the results obtained by Bialascewicz and 

 Bledovski ('15), Wills ('36), Fischer and 

 Hartwig ('38), and Hopkins and Handford 

 ('43) reveal that the break in the course of 

 respiratory increase is a regularly occurring 

 phenomenon in amphibian development. 

 There are a few exceptions, but in most 

 anurans the break is found approximately 

 at gastrulation; in most urodeles it seems to 

 occur during the tail-bud stages — in Am- 

 blystoma punctatum at Harrison's stage 32 

 to 34. The cause of the break is unknown 

 and it is difficult to assess its developmental 

 significance, especially since it does not ap- 

 pear at the same developmental stage in the 

 embryos of different species. There is some 

 indication that it may be correlated with 

 the development of "total respiratory sur- 

 face" (Moog, '44a; Boell, '45, '48). If this be 

 the case one might expect that the break 

 would occur somewhat later in embryos 

 reared at low temperatures because of the 

 increased solubility of oxygen and decreased 

 respiration of the embryo. A comparison of 

 the work of Atlas, Barth, and Moog, on 

 Rana pipiens, shows that this may be the 

 case.* However, Ten Gate ('53) has fol- 

 lowed the respiration of a number of dif- 

 ferent amphibians over a temperature range 

 from 8 to 26°, and there seems to be no 

 systematic variation in the position of the 

 break with temperature. Unfortunately, 

 some of Ten Gate's experimental points show 

 such wide scatter that it is difficult to dis- 

 tinguish so-called breaks from aberrations 

 due probably to technical errors or abnormal 

 development. 



It seems reasonable to suggest that the 

 chief reason for the increase in respiratory 

 rate in the amphibian embryo is the same 

 as in the fish, chick or mammal, and this 

 appears to be that the oxygen consumed at 



* In experiments conducted at the Carlsberg Lab- 

 oratory, I have found that a break in the respiratory 

 curve occurs at stage 16 in Xenopus laevis. Before 

 and after the break the respiratory curves are per- 

 fectly linear when plotted semilogarithmically 

 against time. These experiments were done at 15° 

 on individual embryos, and the same embryos were 

 used throughout the entire period of development. 



