256 



GASTRULA-ARRESTED EMBRYOS 



hybrids do not expend esterified phosphorus more than R. pipiens 

 controls. Since they do not expend enough E? anaerobically to 

 account for the increase in I, it appears that they draw upon some 

 as yet unidentified store of esterified phosphorus for the excess I. 

 Whether R. pipiens embryos do also remains to be established. 



The evidence thus warrants the conclusion that hybrids meet 

 relatively short anaerobic demands for energy about as well as 

 R. pipiens controls, whatever the actual mechanism, but whether 



Fig. 14. Liberation of inorganic phosphate by acid breis of Rana 

 pipiens and hybrid gastrula tissues. (Reconstructed from data of Mezger- 

 Freed, 1953.) 



they preserve their entire store of phosphate bond energy as well 

 as controls is open to further inquiry. 



Additional information about the phosphorus metabolism of 

 hybrid embryos has been obtained by Mezger-Freed ( 1953 ) . 

 This author measured the inorganic phosphorus liberated by acid 

 breis of embryos in various stages of development and also by 

 acid breis of explanted gastrula parts. In this respect, she discov- 

 ered little that would distinguish hybrids from controls, except 

 that breis of dorsal tissues from the animal hemispheres of hybrid 

 gastrulae liberate much less inorganic phosphate than those of 

 corresponding parts of controls ( Fig. 14 ) . She regards her results 

 as evidence for the existence of a phosphatase catalyzing the lib- 



