Yolk Utilization in the Notochord of Newt as Studied by Electron Microscopy.

W H E N 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, 19636) 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. In some cases a yolk platelet at such a stage is found to be bounded by an envelope consisting of a few layers of thin lamellae. The present paper demonstrates that in the notochord tissue of newt at stages during the utilization of the yolk, the platelets are surrounded by an exceptionally thick and well developed lamellar zone. The present observations were made in the course of experimental work by Selman & Jurand (1964) on the effect of ultrasonic vibrations upon the endoplasmic reticulum of notochord cells. The appearance of the yolk to be described here was identical in both control and experimental material, and it was clear that none of the ultrasonic treatments had any effect upon the yolk structures.