embryo-sac

The endosperm-nucleus divides rapidly to produce a cellular tissue which fills up the interior of the rapidly-growing embryo-sac, and forms a tissue, known as endosperm, in which is stored a supply of nourishment for the use later on of the embryo.

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The nucellus is a cellular tissue enveloping one large cell, the embryo-sac or macrospore.

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Isolated cases show that any of the cells within the embryo-sac may exceptionally form an embryo, _e. g._ the synergidae in species of

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This view is still maintained by those who differentiate two acts of fertilization within the embryo-sac, and regard that of the egg by the first male-cell, as the true or generative fertilization, and that of the polar nuclei by the second male gamete as a vegetative fertilization which gives a stimulus to development in correlation with the other.

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The number of chromosomes (see PLANTS: _Cytology_) in the nucleus of the two spores, pollen-grain and embryo-sac, is only half the number found in an ordinary vegetative nucleus; and this reduced number persists in the cells derived from them.

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The gametophyte or prothallial generation is thus extremely reduced, consisting of but little more than the male and female sexual cells -- the two sperm-cells in the pollen-tube and the egg-cell (with the synergidae) in the embryo-sac.

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If in its extension to contain the new formations within it the embryo-sac remains narrow, endosperm formation proceeds upon the lines of a cell-division, but in wide embryo-sacs the endosperm is first of all formed as a layer of naked cells around the wall of the sac, and only gradually acquires a pluricellular character, forming

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The germination of the macrospore consists in the repeated division of its nucleus to form two groups of four, one group at each end of the embryo-sac.

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At the period of fertilization the embryo-sac lies in close proximity to the opening of the micropyle, into which the pollen-tube has penetrated, the separating cell-wall becomes absorbed, and the male or sperm-cells are ejected into the embryo-sac.

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(Liliaceae) polyembryony results from an adventitious production of embryos from the cells of the nucellus around the top of the embryo-sac.

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