254 



EMBRYOLOGY 



Respiration of the embryo 



As more fat and carbohydrate become oxidized, the amount of oxygen 

 which is taken up by the embryo increases. This increase is very rapid as de- 

 velopment proceeds. If we plot the rate of oxygen consumption at different 

 times during development we obtain a curve which rises very sharply, as 

 illustrated in Figure 166. As time goes on, the rate of oxygen consumption 

 increases rapidly, following a logarithmic curve. Similarly, the rate of carbon 

 dioxide production increases; the tissues must get rid of carbon dioxide. 



Oxygen gets into cells by diffusion, and the amount of oxygen taken up is 

 limited by the extent of the cell surface exposed to the gas. As the embryo in- 

 creases in volume it is evident that some mechanism must be provided for 

 increasing the amount of surface exposed to oxygen. Likewise, the carbon 

 dioxide must leave the embryo by means of diffusion. In the frog an increased 

 surface results from the development of fingerlike external gill filaments 



Fig. 166. The rate at which 

 oxygen is consumed by the develop- 

 ing frog egg increases sharply with 

 time. As growth and differentiation 

 proceed, more and more oxidation 

 of the stored foods takes place. 

 This oxidation makes available the 

 energy for development. 



TIME FROM FERTILIZATION 



