1. The quantitatively outstanding effects of beryllium on embryonic development may be correlated with the rarity of this element in nature.
2. Qualitatively the effects are such as may be produced by other chemical agents in appropriate concentration and by other experimental means. The most notable are: exogastrulation, spina-bifida, axial defects in general, hemicephaly and microcephaly.
3. Situs inversus of heart and viscera has not previously been produced in Amphibia by chemical means, but no doubt is part of the general complex of effects.
4. The common action of all reagents would seem to be essentially one of inhibition, acting differentially on different parts of gradients of activity in the embryo.
5. There is evidence for the existence of three main gradients in the embryo, in the three dimensions of space. Two gradients alone cannot explain the organization of the Vertebrate embryo.
6. The form and orientation of the gradients changes during normal development. This is reflected in the complexity of the experimental effects produced.
7. The question of the evolutionary significance of the “mosaic” stage of development is raised.
8. Rhythms in susceptibility to external agents during development are revealed.
9. Examples of dependent and of independent differentiation are noted.