246 



GASTRULA-ARRESTED EMBRYOS 



ponential rise (possibly associated with the onset of cytolysis) 

 that continues to Stage H20, beyond which respiratory rates have 

 not been measured. It is easy to see that, from late Stage H9 on, 

 hybrid embryos are characterized by increasingly subnormal re- 

 spiratory rates. 



Respiratory depression seems to be a feature of all parts of 

 hybrid gastrulae. Sze (1953) has compared the respiration of 

 four different regions of the animal hemisphere of hybrid em- 

 bryos in early Stage Hll with that of corresponding regions of 

 control embryos. From the graph of his results (Fig. 8), it can 



^ 



^ 



2 



W-D 



Regions 



Fig. 8. Respiration of parts of Rami pipiens and hyl^rid gastrulae. Hol- 

 low circles, Rana pipiens; solid circles, hybrid. (Adapted from Sze, 1953.) 



be seen that the rate of respiration of hybrid tissues from em- 

 bryos in the stage mentioned is everywhere about half that of the 

 corresponding R. pipiens tissues. The difference is a little greater 

 than might be expected if Earth's results on whole embryos are 

 taken as standard, but, pending evidence to the contrary, Sze's 

 results must be interpreted as warranting the supposition that 

 there is no tissue-localized block to respiration in hybrid em- 

 bryos. 



Respiratory Quotient. In the work cited above, Earth also 



