THE EPIGENETICS OF THE EMBRYONIC AXIS 



201 



which would not be expected to consume oxygen, a better comparison 

 would be obtained on a basis of yolk-free cytoplasm rather than of the 

 nitrogen content of the whole cell. An approximation to this can be 

 reached by crushing and centrifuging the various regions of the gastrula 

 and estimating the percentage of volume occupied by yolk. Applying 

 this correction, the Q'o^ of the active cytoplasm for the four regions is 

 7.3, 4.8, 5.2 and 3.8 (Boell 1948) (Figs. 10.13, 10.14). BoeU suggested that 

 one should probably also make a further correction for non-yolky but 

 non-respiring cytoplasm; and if this is done, the gradients vanish. Sze 



o 



Figure 10.13 



On the left, a section of an axolotl gastrula, divided into regions, whose 



relative respiratory rates in four different experiments are shown at the right. 



(From Boell 1948.) 



(1953^) also fmds that in the frog egg, although there are animal-vegetative 

 and dorso-ventral gradients of respiratory activity reckoned on a dry- 

 weight basis, all regions respire at the same rate when compared in terms 

 of their content of extractable (= active?) protoplasm. Fhckinger (1954) 

 fmds that the rates of incorporation of radioactive CO2 into the different 

 regions are related in a similar way. It seems then that there is nothing 

 special about the rate of oxygen uptake of the blastopore region; it falls 

 simply into its place in a gradient between the animal and vegetative 

 poles. 



The rate of oxygen uptake is, however, by no means the only factor 

 involved in respiratory metabolism. Brachet (1936) found that there is a 

 higher output of CO2 from the blastopore region than from comparable 



