| 1. |
Involuting mesodermal and entodermal cells within the blastoporal groove ofHyla regilla were studied with the electron microscope. The mesodermal cells differ from the entodermal cells in having a greater concentration of cytoplasmic particles (presumably glycogen and RNP particles), more highly developed mitochondria, more numerous and smaller cytoplasmic vesicles, fewer yolk platelets and larger lipid droplets.
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| 2. |
Evidence is presented to support the hypothesis that cytoplasmic vesicles and endoplasmic reticulum arise from the outer nuclear membrane, as well as from Golgi bodies andde novo in the cytoplasm in association with yolk platelets and lipid droplets.
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| 3. |
Bodies, presumed to be forming mitochondria, were characterized as follows: double outer membrane, tubules or cristae interspersed with a granular matrix restricted to the periphery of the body, and an apparently structureless interior.
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| 4. |
Yolk platelets presumed to be undergoing dissolution were described.
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Grateful acknowledgement is made to Prof.Richard M. Eakin for his generous and invaluable assistance in preparing this paper for publication, to Prof.William Berg for a critical reading of the manuscript, to MissEmily Reid for the drawing, and to the United States Public Health Service for fellowship support.