530 Energy Exchange and Enzyme Development During Embryogenesis 



a peak of heat production during the meta- 

 phase of mitosis. Similar results were ob- 

 tained in suspensions of sea urchin eggs. 

 Stefanelli ('37) determined respiration of 

 single frog eggs in a capillary respirometer, 

 and, although oxygen consumption showed 

 some erratic fluctuations with time, there 

 was some indication of a peak in respiration 

 just before the appearance of the cleavage 

 furrow. A second minor peak is also evident 

 in his curve, and this occurs a few minutes 

 after the cells have separated. In Rana fusca 

 eggs, respiratory peaks were noted at the 

 time of cleavage furrow formation by Bra- 

 chet ('50), and secondary peaks were also 

 seen between the major oscillations. 



The most systematic and thorough study 

 of respiratory rhythms during cleavage has 

 been made by Zeuthen ('46, '50a,b, '51, '53; 

 see also the theoretical paper dealing with 

 Zeuthen's results by Linderstrctm-Lang, '46). 

 In the frog egg, only the difference between 

 oxygen taken up and carbon dioxide liber- 

 ated was measured; the results obtained 

 indicated that divisions one to foiu- were 

 accompanied by rhythmic waves in respir- 

 atory rate, the amplitude of the wave being 

 4 to 5 per cent of the basal rate. Similar phe- 

 nomena were found in the eggs of Urechis 

 and four species of sea urchin, but in these 

 experiments the total oxygen consumption 

 was measured, not the difference between ox- 

 ygen and carbon dioxide. Zeuthen noted some 

 variation in the time relations of respiratory 

 rhythms and various phases of the mitotic 

 cycle, but in most cases it appeared that the 

 cytoplasm divided dviring the period of 

 decreasing respiratory rate, and Zeuthen 

 believes, therefore, that the respiratory 

 rhythms are more intimately related to 

 nuclear events than to cytoplasmic cleavage. 

 It would appear from the foregoing ac- 

 count that the association of rhythmic varia- 

 tions in respiration with cleavage had been 

 well established, but recently Scholander 

 et al. ('52) have expressed doubt on the 

 occurrence of fluctuations in oxygen con- 

 sumption during cell division. Working with 

 single sea urchin eggs in a diver of ingenious 

 design, Scholander et al. observed cycles of 

 respiration in some cases, but they failed to 

 find them in others although nuclear and 

 cytoplasmic division were proceeding. When 

 cycling was found, the period of enhanced 

 oxygen utilization seemed to coincide with 

 cytoplasmic cleavage rather than with nu- 

 clear division as reported by Zeuthen, and 

 they reported in addition that cycling tended 

 to become strongly damped after the fjrst 



cleavage and to disappear altogether after 

 two or three divisions. 



Zeuthen ('53) has calculated that the 

 standard error of his measurements on the 

 Urechis egg is only 0.66 per cent, much 

 smaller than the error in Scholander's work, 

 and he has suggested that the spread of 

 experimental points in the observations by 

 Scholander et al. wovild make it impossible 

 to detect respiratory rhythms consistently 

 in single eggs. 



Metabolism During Developmental Block. 

 A number of cases are known in which a 

 period of development is followed by one 

 of complete developmental block. The exam- 

 ples to be discussed are of interest because 

 they represent situations in which complete 

 disengagement of growth and differentiation 

 from maintenance is brought about without 

 experimental interference. In amphibian hy- 

 brids {R. pipiens eggs X R. sylvatica sperm) 

 development proceeds normally through the 

 beginning of gastrulation, but then, although 

 the hybrid embryos remain healthy and vi- 

 able, further development ceases (Moore, '41, 

 '46). Barth ('46) has been able to show that 

 both hybrid and normal embryos consume 

 oxygen at the same rate during cleavage 

 and blastula stages. At gastrulation, when 

 developmental block sets in, respiration fails 

 to increase in the hybrids as it normally does. 

 Lactic acid production is also decreased in 

 hybrid embryos, but not quite so drastically 

 as respiration. Apparently, however, metabo- 

 lism is qualitatively identical in the two 

 types of embryos, for the respiratory quo- 

 tients were found to be the same. 



A somewhat analogous situation exists in 

 Drosophila melanogaster eggs with chromo- 

 somal abnormalities. Absence of the X- 

 chromosome leads to developmental failure at 

 about the time of gastrulation. Just as in the 

 case of the amphibian hybrids, "no-X" eggs 

 respire normally so long as development con- 

 tinues, but respiration ceases to increase when 

 development fails (Boell and Poulson, '39). 

 The embryos are viable and continue to 

 respire for many hours, but the rate of res- 

 piration, in the absence of development, re- 

 mains unaltered at a very low level. By con- 

 trast, "attached-X" eggs (those with an extra 

 chromosome) respire and develop normally. 

 The diapause in insects represents a situa- 

 tion where a reversible block to development 

 is interposed between two periods of active 

 growth and differentiation. During diapause 

 in the grasshopper Melanoplus differ entialis, 

 cell division, growth, and differentiation oc- 

 cur sp slowly, if they occur at all, that one 



