J. R. GREGG 245 



attempts to analyze hybrid morphogenesis from a physiological 

 or biochemical point of view. 



Metabolis7n of Hybrid Embryos 

 Respiration 



Oxygen Uptake. Barth (1946) has made a special study to 

 compare the respiratory rates of hybrid and control embryos. His 

 results are summarized in Fig. 7. The rate at which R. pipiens 



15 



30 46 60 



HOURS 



76 



90 



106 



120 



Fig. 7. Respiration (cu mm Oj/hour embryo ) of developing B.ana 

 pipiens and hybrid embryos. (From Barth, 1946.) 



embryos utilize environmental oxygen increases with time, 

 roughly in exponential fashion. Hybrid embryos at first respire 

 oxygen at the same rate as R. pipiens control embryos, but when 

 they are late blastulae their respiratory rate becomes constant 

 and remains so until they are in Stage H18, when a small, abrupt 

 increase in respiratory rate (not clearly associated with any mor- 

 phological change, external cilia, for example, begin to function 

 at about Stage 17, according to Moore) initiates a very slow ex- 



