Comparative ultrastructure of the yolk material in preimplantation stages of the hamster, mouse, and rat embryos

Yolk material of preimplanation stages of embryos of the hamster, mouse, and rat were examined by a standardized electron microscopical procedure. The material was encountered as fibrils, scattered more or less densely in the cytoplasm. In the hamster, the material was present in large masses and the fibrils had a chain‐like appearance when cut longitudinally. The ultrastructure of the fibrils was compatible with a helical pattern. The fibrils had a width of about 40 nm and the pitch (the axial distance of the repeating unit) was about 30 nm. In the mouse, the yolk material was dispersed in the cytoplasm forming small plaque‐like groups. Also, in this species the fibrils were chain‐like but smaller than in the hamster. The fibrils were often closely situated, resulting in images with varying crystalline appearances. In the rat, the yolk appeared as light areas occupying a substantial part of the cytoplasm. The fibrils in the yolk plaques were sparse and diffusely outlined. They were thinner than the fibrils of the mouse‐yolk material, did not display any helical pattern at the resolution used, but showed a periodicity.