METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 63 



clearly saw that " the principles enunciated by Child serve to 

 rationalize the results of heterogenic hybridization." He gave 

 what is for the most part the correct explanation of the terata 

 originating in his hybridization experiments but fell into a num- 

 ber of errors because little was then known about the metabolic 

 gradients in these fish embryos, and he assumed them to be like 

 those of the flatworms. 1 The most complete analysis of verte- 

 brate teratology which has been made is that of Bellamy ('19) on 

 the frog, since in this case both the metabolic gradients and the 

 terata resulting from the differential action of external agents on 

 the eggs are known. 



I have now shown in a general way how the terata produced 

 experimentally in teleost embryos can be explained on the basis 

 of the metabolic gradients. Such terata are of two general types, 

 those due to differential susceptibility, in which the parts of 

 highest activity are inhibited and defective, and those due to dif- 

 ferential recovery or acclimation, in which the parts of highest 

 activity alone survive. A more detailed discussion seems to be 

 unnecessary in view of the extensive treatment of the matter in 

 the papers of Child, Newman, and Bellamy already cited. I may, 

 however, as an illustration of the application of the susceptibility 

 results to a specific organ take the case of the Fundulus eye. It 

 happens that in Fundulus, as I have shown, the eyes are very 

 susceptible with reference to other parts of the body, more so 

 than in other species of fish. This indicates that the region from 

 which the eyes arise must be one of very high activity, and as the 

 data of Stockard show this region must be affected before the 

 eyes appear in order that defective eyes result. Consequently 

 inhibition of this region results first in approximated and later 

 in defective eyes. Now since it happens that in Fundulus this 

 region is so much more susceptible than in other forms, the oc- 

 currence of eye defects in Fundulus will also be more common 

 than in other forms and further it is possible to obtain eye defects 



i In particular the statements made by Newman about the gradient of the 

 heart and circulatory system are quite erroneous. Further the posterior end 

 of fish embryos is a region of high metabolic rate and embryos with defective 

 posterior ends are probably due to direct inhibition and are not recovery types 

 as supposed by Newman. 



