DIFFERENTIAL DEVELOPMENTAL MODIFICATION. Ill 265 



appear, with a few experiments, to be regionally specific effects, may 

 prove, on more extensive investigation, to be nothing of the sort. Leh- 

 mann's conclusions may be entirely correct, but the experimental evi- 

 dence does not, at present, exclude other possibilities, and the develop- 

 mental modifications concerned do not appear to be essentially different 

 from others, apparently not specific. Further experiment with different 

 developmental stages, different agents, and a wide range of concentra- 

 tions and exposure periods is still highly desirable. 



HIGHER VERTEBRATES 



Experimental modification of development in the chick by subjection 

 of the embryo to particular external agents and altered environmental 

 conditions presents certain difiiculties. When a chemical agent is applied 

 inside the shell, there is no certainty that all embryonic regions are sub- 

 jected to the same concentration at the same time ; and when it is allowed 

 to diffuse through the shell, the possibihty of control of distribution is 

 even less. Uniform decrease of oxygen supply to different parts of the 

 embryo cannot be certainly attained by decreasing the surface through 

 which oxygen can enter or by decreasing oxygen in the surrounding at- 

 mosphere, because differences in oxygen consumption of different embry- 

 onic parts may decrease the supply at very different rates about different 

 embryonic regions. The only external factors which meet the require- 

 ments seem to be change of temperature and perhaps X-rays and radium. 

 In view of this situation and of the possibility of differential tolerance, 

 conditioning, and recovery it is often impossible to determine the signifi- 

 cance of a particular teratological embryo. Nevertheless, the general sim- 

 ilarity of many modifications of chick development to those of fishes and 

 amphibia, which apparently depend on differential susceptibihty, leaves 

 little doubt that essentially similar physiological factors are involved in 

 all three groups. More than forty years ago Dareste (1891) concluded 

 that similar modifications are produced by different external conditions, 

 that the same external factors do not always produce the same modifica- 

 tions, and that the teratological forms depend on time, intensity, and 

 duration of action rather than on the nature of the external agent. Later 

 work, in large part, confirms these conclusions. Hyman (1927a, b), dis- 

 cussing differential susceptibility in the chick, points out that the regions 

 most modified are those most susceptible to lethal agents. 



Inhibition of head development, approximation of eyes and other lateral 

 cephalic organs to the median plane, their development as single median 



