250 



GASTRULA-ARRESTED EMBRYOS 



conclusion supported also by the absence from both types of em- 

 bryo of significant amounts of free carbohydrate at any stage in 

 embryogenesis. Glycogen, indeed, seems to be the chief energy 

 source drawn upon by young frog embryos, for we know (Barth 

 and Barth, 1954) that relatively little protein or fat is burned by 

 normal embryos until late in development. Protein and fat me- 

 tabolism of hybrid embryos has not yet been studied. 



It is not known whether the parts of hybrid gastrulae utilize 

 glycogen at different rates. The work of Jaeger (1945) suggests 

 that they do not, for she found that dorsal tissues of Stage HIO 

 and Stage H12 hybrid embryos did not differ in their glycogen 

 contents, and neither did analogous ventral tissues. It seems prob- 

 able that there is a general depression of glycolysis common to all 

 hybrid cells, corresponding to the general lowering of the respir- 

 atory rate discovered by Sze. 



Lactic Acid Production 



In his paper on the respii-atory metabolism of hybrid embryos, 

 Barth (1946) also reported some measurements of their aerobic 

 and anaerobic lactic acid production. Under strictly aerobic con- 

 ditions, neither R. pipiens nor hybrid embryos produce appre- 

 ciable amounts of lactic acid; although embryos allowed to sit 



60 • 





o 

 20 " 



25 



50 



75 



HOURS DEVELOPMENT l8»C 



Fig. 12. Anaerobic lactic acid production of Rana pipiens and hybrid 

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



