Energy Exchange and Enzyme Development During Embryogenesis 527 



a particular stage of development is related 

 to the amount of embryonic material.* Such 

 a view is consonant with results of respira- 

 tory measurements on gastrula explants 

 (Barth, '42; Boell, '42) in which it appears 

 that oxygen uptake is inversely related to 

 yolk content. It is well known that yolk, 

 separated from the protoplasmic matrix in 

 which it is held, has a negligible oxygen 

 consumption. Synthesis of more embryonic 

 material at the expense of the raw materials 

 stored in the egg would thus be expected to 

 result in increased oxygen consumption. 



Unfortunately, practically nothing is 

 known about the transformation of yolk 

 into active protoplasm or the rate at which 

 the process occurs, because so far it has 

 proven impossible to separate effectively "ac- 

 tive" and "inactive" materials and to meas- 

 ure them with any degree of precision. The 

 term "yolk" has various meanings to differ- 

 ent investigators. To some it refers to dis- 

 tinct cytological entities — the yolk platelets 

 or lipochondria. Yolk may also have a 

 broader connotation and may be used to 

 refer to any stored substances in the egg 

 which are used by the embryo for energy 

 and for growth. Yolk platelets clearly do not 

 contain all of the reserve materials for de- 

 velopment. This is particularly true of lip- 

 oidal substances. When homogenates of am- 

 phibian eggs are strongly centrifuged, the 

 lipoidal material is driven to the centripetal 

 pole and can be seen to consist of a whitish 

 layer, probably lipochondria and a droplet 

 of clear oil (Boell, '42). In the intact egg 

 the oil is most likely dispersed as an emul- 

 sion in the cytoplasm. This interpretation 

 agi'ees with L0vtrvip's ('53a) calculation that 

 30 to 50 per cent of the fat in the amphibian 

 egg is outside the yolk platelets. The yolk 

 platelets also do not contain reducing car- 

 bohydrate (Gregg and L0vtrup, '50). 



If evidence of visible change in the yolk 

 platelets or lipochondria is taken as an index 

 of yolk utilization, one would be forced to 

 believe that the embryo does not call upon 

 its yolk supply for growth or as a source 

 of energy vmtil late in development (Bragg, 

 '39; Daniel and Yarwood, '39; Holtfreter, 

 '46). But visible change in size or pattern 

 of yolk platelets is not a very precise indi- 

 cator of the beginning or course of yolk 

 consumption. Kutsky ('50) has shown, 

 through the use of P^s, that turnover of 

 phosphoprotein in yolk occurs from the very 



* It should be unnecessary to mention that this 

 does not mean that rate of development is controlled 

 by respiratory rate. 



beginning of development in R. pipiens and 

 continues at a low rate throughout. Barth 

 and Jaeger ('47b) found that the enzymes 

 associated with phosphate transfer from yolk 

 are active before gastrulation. Additional 

 evidence for the involvement of yolk in early 

 development has been provided by Friedberg, 

 and Eakin ('49). These workers exposed 

 eggs of Hyla regilla to glycine, containing 

 C^*, and found that the amino acid is taken 

 up by yolk in the early embryo. 



Although these studies show that yolk 

 enters the "metabolic pool" much earlier 

 than has generally been supposed, they give 

 no indication of the rate of yolk utilization 

 for growth. But, if it may be concluded that 

 increase in rate of oxygen consumption is 

 related to increase in magnitude of meta- 

 bolically active components, it may be pos- 

 sible to use respiratory activity, in the ab- 

 sence of better and more orthodox measures, 

 as an index of the transformation of yolk 

 into protoplasm. 



It is clear from Figure 201 and from the 

 work of others (Atlas, Barth, Boell, Fischer 

 and Hartwig, Hopkins and Handford, Spie- 

 gelman and Steinbach, and Wills) that 

 abrupt changes or cyclic variations in respi- 

 ration do not occur during amphibian de- 

 velopment. Nor is there evidence of increased 

 energy expenditure for such processes as 

 gastrulation, differentiation of the primary 

 germ layers, or their further elaboration into 

 the definitive tissues and organs of the em- 

 bryo. Any energy expenditure specifically 

 required for these processes, since they occur 

 gradually, is merely represented as part of 

 the general rise in oxygen consumption that 

 occurs during development. 



In an earlier study of amphibian respira- 

 tion, Parnas and Krasinka ('21) claimed that 

 sudden increases in respiratory rate occurred 

 at distinct stages of development, with pla- 

 teaus of constant oxygen uptake between. 

 The first of these abrupt changes was pre- 

 sumed to occur at the time of gastrulation; 

 another, during closure of the neural folds; 

 and a third, at the time the external gills 

 made their appearance. From these observa- 

 tions, Parnas and Krasinka were led to con- 

 clude that major morphogenetic events were 

 associated with increased energy expendi- 

 ture. There can be no objection to such a 

 conclusion, but it is apparently not validated 

 by the experimental findings of the majority 

 of more recent workers. 



Tuft ('53) has reported, however, that the 

 course of respiration in Xenopus laevis shows 

 a period, interpreted as coinciding with gas- 



