J. R. GREGG 247 



compared the respiiatory quotients (R.Q. = CO2 produced per 

 O2 consumed) of hybrid and R. pipiens embryos (Fig. 9). The 

 respiratory quotients of hybrid embryos in all stages between Stage 

 HI and Stage H12 are not significantly different from those of 

 R. pipiens control embryos. Beginning at Stage H12, however, 

 the respiratory quotients of hybrid embryos become increasingly 

 larger than those of controls. Aging hybrid embryos are thus 

 characterized by hypernormal CO2 production, relative to their 

 oxygen consumption. We have no knowledge at present about 

 the R.Q.'s associated with various parts of hybrid gastrulae. Barth 

 notes that his results are consistent with the view that up to Stage 



0.95 



'0.85 



0.75 + 



PIPIENS 



2"+ lf8 



HOURS DEVELOPMENT l8»C 



72 



Fig. 9. Respiratory quotients of developing Rami pipiens and hybrid 

 embryos, (Reconstructed from data of Barth, 1946.) 



H12, hybrid embryos are oxidizing substrates of the same kind as 

 control embryos, and that not until thereafter do they oxidize 

 substrates of different kinds. Therefore, to the extent that R.Q. 

 measurements warrant such statements, we are deprived of sup- 

 port for the hypothesis that hybrids fail to gastrulate because 

 they do not oxidize substrates providing the normal types of nec- 

 essary metabolic intermediates. But because of technical difficul- 

 ties involved in the determination of respiratory quotients, espe- 

 cially on pre-gastrula embryos, Barth urges that the absolute 

 values of the R.Q.'s obtained should not be construed as evidence 

 for any hypothesis stating what substrates embryos are oxidizing. 



