A description of the fine structure of the yolk of the unincubated hen's egg has been provided, which will serve as a basis for further studies on yolk digestion. The gross components of the yolk (that is, free-floating lipid drops, yellow and white yolk spheres together with their enclosed lipid subdroplets, and aqueous protein fluid) could be recognized by phase contrast and low power electron microscopy. The majority of the lipid drops, whether free floating or enclosed within yolk spheres, were composed of particles about 30 to 60 A in diameter. The protein component of the yolk was found to consist of round profiles about 250 A in diameter. The surfaces of the yolk spheres were of three types, and it is difficult to decide which represents the true structure although reasons are given for believing that yolk spheres are not normally enclosed by membranes identical with cell membranes.

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