Utilization of yolk platelets during early embryonic development of Rana temporaria and Bufo bufo.

Utilization of yolk platelets in cleaving embryos of Rana temporaria and Bufo bufo was studied by different methods. Morphological observations of yolk platelets of R. temporaria embryos at tail bud stage by transmission electron microscopy indicated four initial phases of platelet degradation. The pattern of these events is similar to that found in embryos of B. bufo. The morphological observations were confirmed by energy-dispersive X-ray microanalysis of the elemental content of platelets and by selected-area electron diffraction of platelet cores. Covalently bound sulphur content decreased during cleavage and the content of different inorganic ions changed, whereas the structure of crystalline core remained constant. Morphological changes found in the amorphous cortex of yolk platelets were due to their utilization. Stereological measurements indicated that utilization during cleavage increased, but only the initial phases of yolk platelet degradation were seen. The volume of the cortex did not decrease and the crystalline core did not fragment.