The results of Gayer and Hamburger (1943) that homozygous Creeper eye anlagen develop normally in orthotopic transplantation to normal while the reciprocal does not (1 case), do not necessarily indicate an indirect effect of the Creeper factor on eye development. Their findings may also be explained by assuming all Creeper tissues are equally deficient in a substance necessary for normal differentiation. This substance can be supplied to Creeper anlagen when these are transplanted orthotopically to normal. On the further assumption that adequate oxygen supply is necessary for the elaboration of this substance, the Creeper phenocopies produced by transplanting normal anlagen to normal flank also provide support for this explanation.