SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 



721 



and, if the egg is infertile, no further change takes place, even though 

 it be kept for a month or more. These alterations are shown in 

 Fig. 158. If the egg is fertile, the carbon dioxide content then rises 

 during the ist day to 1-89 per cent., a phenomenon probably due 

 to the effect of heat on the egg-contents. The subsequent course of 

 the curve is shown in Fig. 159. A slight diminution on the 2nd or 

 3rd day brings the value to a level at which it remains until roughly 

 the nth day (i-o6 to 0-33 per cent.), but after that time it pro- 

 gressively augments until hatching. Clearly it is at about the nth 

 day that the carbon dioxide produced by the metabolism of the 

 embryo becomes greater per unit time than that which can get away 



Unfertile eggs 

 2-0 '" 



Airspace composlbion 



- 1-0 - 



5 10 20 



Days 

 Fig. 158. 



through the egg-shell per unit time, hence an increased concentration 

 in the air-space. At the end of development the percentage has risen 

 to between 4-50 and 5-43. During the first week the oxygen content 

 remains unchanged, and a little below that obtained in unincubated 

 eggs, but after that time it begins to fall, presumably because it is 

 being used up by the embryo rather faster than it can diffuse in 

 through the egg-shell. At the end of development it only reaches 

 the figure of 13*65 per cent. If now the curve in Fig. 159 be com- 

 pared with that found by various workers for the output of carbon 

 dioxide from the egg (Fig. 151), it can easily be seen that, although 

 the latter increases during incubation more than fifty times, the 

 former hardly increases five times; the obvious inference is that 

 the shell becomes more permeable to gases as development proceeds. 

 It is unfortunate that no direct measurements with a diffusiometer 



