64 LIBBIE H. HYMAN. 



in embryos otherwise nearly normal, if the inhibiting agent is a 

 rather weak one. This is not possible in other forms ; I venture 

 to predict that such a result could not be obtained in Tautogo- 

 labrus but that defective eyes in this species would always be 

 associated with marked defects of the brain and other parts of 

 the head. Such is the case in the frog, where cyclopic eyes occur 

 only in markedly microphthalmic embryos. Owing also to the 

 high metabolic rate of the Fundulns eye it is possible in this form 

 for the eye to recover and survive when nearly all other parts of 

 the embryo are killed. I also venture to predict that the occur- 

 rence of such isolated and solitary eyes in the absence of other 

 parts of the embryo will be found to be rather rare and unusual 

 in other species. 1 



In conclusion I may reiterate that the study of the metabolic 

 gradients such as has been made in this paper furnishes a rational 

 basis for the understanding and interpretation of normal and 

 teratological development. While the particular organism which 

 is to develop from a given egg is determined by the hereditary 

 constitution of that egg, the orderly sequence of development, the 

 spatial relations and proportions of parts, and the general axial 

 organization are controlled by physiological, metabolic differ- 

 ences between different parts of the developing egg. Such physio- 

 logical differences arise in the final analysis through the action of 

 external conditions on protoplasm. By modifying in a purely 

 non-specific, quantitative manner the metabolic differences at dif- 

 ferent levels, orderly predictable departures from the normal 

 course of development are obtainable. 



VI. SUMMARY. 



1. The susceptibility of developing eggs of Fundulns, the din- 

 ner and the cod to toxic solutions at various stages was studied. 



2. In early blastoderms the central region is more susceptible 

 in Fundulus and the dinner, the peripheral region in, the cod. 



1 Monopthalmia, often observed in Fundulns embryos, is simply due to a 

 greater susceptibility of one side than the other; the eye on the more sus- 

 ceptible side is inhibited. In the course of my studies on vertebrate embryos, 

 this asymmetrical susceptibility has frequently been noted although the figures 

 are drawn as if the susceptibility were always bilaterally symmetrical. 



