Introduction The increase in size of the vertebrate eye as a whole is regulated by a number of factors. Thus, the results of heteroplastic transplantations of amphibian eye priomordia by Harrison1and Twitty and Schwind2have implicated genetic factors. In addition to genetic factors, nutritional factors may influence the enlargement of the amphibian globe.3,4Recent work with the chick embryo has revealed two additional factors. Not only is intraocular (vitreal) pressure a cardinal factor influencing the increase in the size of the eye, as shown by Weiss and Amprino5and me,6,7but the resistance to expansion offered by the eye wall as it elaborates cartilage and collagen appears to be important in progressively decreasing the rate of ocular expansion as development proceeds.7 If we turn our attention from the eye as a whole to its parts, we find much observational and experimental work supporting