When cells of amphibian embryos differentiate they begin to synthesize the structural proteins characteristic of the cell type, and this is immediately preceded by the beginning of the absorption of the yolk platelets. These events may be studied in cultured cells (Jones & Elsdale, 1963) or in sections of fixed embryos. When amphibian platelets are utilized by the cell, they undergo certain wellmarked changes in ultrastructure that have been studied by electron microscopy, for example by Karasaki (1959, 1963b) and Sung (1962). The details of these changes depend in part upon the species, but also upon the particular tissue concerned. However, it is possible to generalize to the extent of saying that the utilization of a yolk platelet involves a decrease in the volume of the regularlyordered crystalline core, which before differentiation occupied the bulk of it. This is accompanied by a corresponding increase in the volume of its outer shell of irregularly-packed granular matrix.

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