A- Test of the Metabolic Theory of Sex. 



89 



of oxygen result in the death of some embryos — the male embryos 

 should be somewhat better able than female embryos to withstand 

 an increased concentration of oxygen. Again, if males have a 

 higher metabolism than females the reduced metabolism induced 

 by cooling should perhaps prove more harmful to the male embryos. 



Embryos aged 3 minutes to 12 days have been used; but most 

 frequently the age was between 1 hour and 4 days. Increased 

 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., were used. The time during which embryos 

 were subjected to the altered pressures of oxygen varied from one 

 to five days. For 0.15 per cent. O2 the time was 15 minutes to 

 8 hours. Embryos were normally incubated under birds except 

 during the period of treatment when moisture and temperature 

 (39. 4 0 ) conditions were carefully controlled in the experimental 

 chamber. In survival tests under ''cooling" the eggs were 

 either left at cold room temperature (50°-65° Fahr.) from 10-13 

 hours daily during several days or were placed at ice-box tem- 

 peratures (8°-i5° C.) during 18-24 hours. Sixty-five groups were 

 tested in oxygen, 12 in cooling. 



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 oxygen ; 

 younger embryos by increased pressures of oxygen. It is prob- 

 able that the adequacy or thickness of the shell is also a factor in 

 such survival. This adequacy of the shell has been painstakingly 

 measured in all of the treated embryos. 



The sex data from the reduced oxygen series have one notable 

 defect. Eight embryos in which color was sex-linked were 

 killed at an age when they should have shown eye-pigment or the 

 lack of it which would declare their sex. Unfortunately, greatly 

 reduced pressures of oxygen have been shown 1 wholly to prevent 

 the post-mortem formation of this pigment and we have evidence 

 that it at least interferes with its formation in these embryos. 

 These eight embryos therefore could not be definitely classified 

 and are omitted from the table. Embryos of known sex were 

 not killed in about one half of the experiments. The lower half 



1 Riddle and La Mer, Amer. Jour. Physiol, 19 18, xlvii, p. 103. 



