24 LIBBIE H. HYMAN. 



all of the abnormalities arose from embryos "aged less than 

 two days at the time treatment was begun." Byerly ('26) 

 allowed eggs to incubate for 24 hours and then coated them 

 with water-glass after removing the entire shell and outer shell 

 membrane from the air chamber. Such embryos were then 

 incubated for three days. Some of the embryos thus obtained 

 were very abnormal, the posterior end being most affected. 

 This agrees with my finding that at 24 hours the posterior end 

 of the embryo is more susceptible than any other part. 



Although the modifications induced by these various pro- 

 cedures agree with the expectations on the basis of differential 

 susceptibility, nevertheless the applicability of this interpretation 

 in these cases may be open to question : for there is no certainty 

 that all parts of the embryos were equally exposed to these 

 agents. Variations in different regions of an egg as regards 

 thickness and porosity of the shell, viscosity and thickness of 

 the albumen layer, permeability of the shell and egg membranes, 

 and similar factors render it highly probable that external agents 

 and conditions will not reach all parts of the embryo simul- 

 taneously. To mention only one condition, an altered gaseous 

 environment obviously requires some time to affect the embryo. 

 Obviously the principle of differential susceptibility will apply to 

 the induced abnormalities with certainty only when the conditions 

 or agents applied have reached all parts of the embryo equally 

 and simultaneously. Apparently the only conditions under 

 which these prerequisites are fulfilled are ageing of the eggs and 

 alterations of the temperature of incubation. 



Dareste ('82, '830) found that if eggs are kept for some time 

 before being submitted to incubation, development is abnormal. 

 Typical abnormalities were: blastoderms without embryos, 

 embryos with absent or reduced posterior ends, reduced and 

 abnormal heads and brains, and celosomia (protrusion of the 

 viscera due to a failure of the lateral limiting sulci to close below). 

 It is possible of course that a beginning slow development at 

 room temperature as well as senescence of the embryo is a factor 

 in these cases. 



A number of experimenters have tested the effect of subnormal 

 or supernormal incubation temperatures on the development of 



