from The Century Dictionary and Cyclopedia
- n. That part of the yolk of a meroblastic egg which serves to nourish the embryo, as distinguished from the formative or germinative substance; deutoplasm. Thus, in a hen's egg all of the ball of yellow except the little tread or cicatricula is food-yolk.
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Egg: a simple cell, capable of fertilization, containing the germ, the food-yolk necessary for its nutriment, and a covering membrane: a single ovum or cell from an ovary: the first stage of the insect.
The most important phenomena to be included under the general heading of cenogenesis are, first, the occurrence of food-yolk, and second, those anomalies of development which are classed by Haeckel as heterochronies and heterotopies.
In the higher animals a good deal of food-yolk is stored up in the germ, and so the vase-shaped structure has been flattened and altered.
When this connection (called the placenta) between embryo and mother through the egg-shell became more perfect, not only oxygen but food-material was obtained from the blood-vessels of the mother; and, in consequence, it became unnecessary for the eggs to be provided with a large supply of food-yolk.
Among existing marsupial animals, which, on the whole, represent a lower type of mammalian structure than ordinary mammals, there is more food-yolk than in ordinary mammals, and less food-yolk than in the two egg-laying mammals.
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