346 A. W. BELLAMY. 



(Fig. 22, A) indicated by the marked dorsal convexity with the 

 tail extending upward, in some cases at right angles to the body. 

 The medullary folds may fail to close in the brain region, in 

 the brain region and at the posterior end, or they may not close 

 at all throughout their entire length. In some cases anencephalic 

 forms appear. Spina bifida of all degrees is of course common 

 under conditions that inhibit development, and result primarily 

 from inhibition of the dorsal lip region. 



V. DISCUSSION. 



It has been shown that gradients in susceptibility to several 

 external agents do exist in the egg and embryo of frog. Concern- 

 ing the fact there can be no question. It was pointed out that 

 the experimentally produced modifications described in this paper 

 are readily and logically accounted for on the basis of differential 

 susceptibility. Now, since no evidence was found of: (a) "speci- 

 ficity" in the sense in which the term has been used to explain 

 terata produced by various chemical agents; (b) of " blastolysis " 

 or (r) of any "disorganization" effect of the chemicals used to 

 modify development as effective factors in the production of 

 terata, the interpretation of the data is now largely a matter of 

 pointing out the basis and significance of differential suscepti- 

 bility. This is the chief task of the discussion. 



i. The Nature of Physiological Axes. These differences in 

 susceptibility to external agents exhibited by different parts of the 

 living egg and embryo (see pp. 322-344) must represent differences 

 of some sort in protoplasmic conditions. Living protoplasm is 

 a complex system of correlated, interdependent reactions and 

 conditions, and it is unquestionably true that different agents 

 affect it differently or attack it at different points. But, what- 

 ever the particular factor or factors chiefly concerned in deter- 

 mining these differences in susceptibility to particular agents 

 may be, whether it be rate of fundamental oxidation-reduction 

 processes, state of colloidal aggregation, permeability of limiting 

 membranes, or something else, the general uniformity and lack 

 of specificity in the susceptibility relations to different agents 

 and conditions, not only as regards the egg and embryo of the 

 frog but as regards animals and plants generally, force us to 



