EXPERIMENTAL EVOLUTION. 



123 



concentrations of oxygen varying from 26.8 per cent to 96.6 per cent 

 and decreased concentrations varying from 18.3 per cent to 0.15 per 

 cent have been used. The time during which embryos were subjected 

 to the altered pressures of oxygen has varied from 1 to 5 days. For 

 0.15 per cent O2 the time was 15 minutes to 8 hours. 



During treatment the embryos were kept in a sealed chamber (a 

 modified desiccator) into which prepared washed gas of known con- 

 centration was continually fed and then led away. Two to five analy- 

 ses were made daily of gas samples drawn from the chamber. Before 

 and after treatment in the chamber (kept at 103.0° F. in a Freas oven) 

 the embryos were incubated by doves, either by the parents or by 

 generic hybrid doves maintained for this purpose. 



The age of the embryo has been found the most important factor in 

 survival under alteration of the gaseous environment. Older embryos 

 are most affected by reduced pressures of O2; younger embryos most 

 affected by increased pressures of O2. It is probable, but not now cer- 

 tain, that the adequacy of thickness of the shell is also a factor in such 

 survival. Probably the egg-shell normally acts as a buffer against the 

 oxygen of the air. This adequacy of the shell has been painstakingly 

 measured by Dr. Riddle in all of the treated embryos. A small num- 

 ber of embryos has been simultaneously treated with increased pres- 

 sures of oxygen and carbon dioxide. From 8.0 per cent to 46.0 per 

 cent of CO2 have been employed. In the embryo these two substances 

 doubtless have in part antagonistic effects. Table 4 presents the chief 

 data obtained concerning sex. It will be understood that the sex of 

 many embryos which were killed, and even of some which sundved 

 treatment but died later, could not be ascertained. The results thus 

 far obtained, with the year three-fourths completed, indicate that sex 

 is also a factor in survival. 



Table 6.- 



-Comparison of effect of variations of percentage of oxygen in the atmosphere on 

 male and female embyro pigeons. 



The tabulated data show that fewer males than females were killed 

 by increased pressures of O2, and that more males survived this treat- 

 ment. More males were killed by decreased pressures of Oo, and 



