Em´bry, an immature organism, especially in the earlier stages of development from the fertilized ovum onwards. The embryo of a mammal (except in the egg-laying duck-bill and spiny ant-eater), which develops internally, is known as a fœtus. The dormant plantlet in a seed is also known as an embryo.
Embryol´ogy, the branch of biology concerned with the fertilization of the ovum and its development into the adult. Aristotle and Galen made some observations on the subject, as regards animals, while Harvey and his successors considerably advanced our knowledge, but as a distinct and important subject embryology only dates from the nineteenth century. In the course of its development an organism repeats the evolution of its group in an abbreviated fashion, thus furnishing a clue to its actual affinities. The subject also throws light on the problems of heredity.
A, Antipodals. P, Polar nuclei. E, Egg-cell. S, Synergidæ.
Embryo-sac, the name given to the megaspore of seed-plants. In Angiosperms the ripe embryo-sac is typically a large ovoid sac enclosed in a thin membrane and lined by cytoplasm; it contains seven or sometimes eight nuclei with associated masses of cytoplasm, distributed in a characteristic manner. At the apical end, next the micropyle, is the egg-apparatus, comprising the egg-cell or ovum flanked by the two synergidæ or helping-cells, while the basal or chalazal end is occupied by the three antipodal 'cells'. The cavity of the sac is filled with cell-sap traversed by stout protoplasmic strands, suspended in which, near the centre, are two polar nuclei, or a single fusion-nucleus formed by the union of these. A characteristic feature of Angiosperms is the process of 'double fertilization'. The pollen-tube contains two male gametes, both functional; one fertilizes the egg-cell, the resulting zygote or oospore giving rise by cell-division and growth to the embryo; the other unites with the fusion-nucleus (or the two polar nuclei), and the product is the origin of the special nutritive tissue or endosperm, which is used up by the embryo, either during ripening of the seed (exalbuminous seeds), or at germination (albuminous seeds). See Gymnosperms; Ovule.
Em´den, a town of Prussia, province of Hanover, near the mouth of the Ems, occupying a low flat intersected by numerous canals. It was raised to the rank of a free Imperial city in 1595, was made a free port in 1751, was incorporated with the kingdom of Hanover in 1815, and in 1866 was united to Prussia. The principal building is the great church, built in 1455. The harbours admit large vessels, and several canals run inland. It exports grain, dairy produce, and gin, and has shipbuilding yards, and manufactures hosiery and leather. Pop. 24,000.
Em'erald, a variety of beryl, a well-known gem of pure green colour, somewhat harder than quartz; specific gravity, 2.67 to 2.73. It is a silicate of aluminium and the rare element glucinum or beryllium, which was detected in it by Vauquelin after it had been discovered by the same chemist in the beryl. Its colour is due to the presence of chromium, of which there may be 0.2 to 0.3 per cent present. Its natural form is either rounded or that of a short six-sided prism. It is one of the softest of the precious stones, but is not acted on by acids. Emeralds of large size and at the same time free from flaws are rare and more valuable than diamonds or rubies; the largest on record is said to have been possessed by the inhabitants of the valley of Manta in Peru when the Spaniards first arrived there. It was as big as an ostrich egg, and was worshipped as the mother of emeralds. The ancients, who valued them, especially for engraving, are said to have procured them from Ethiopia and Egypt. The finest are now obtained from Colombia. The Oriental emerald is a variety of the ruby, of a green colour, and is an extremely rare gem.
Emerald Green, a vivid light-green pigment, an aceto-arsenite of copper, used both in oil and water-colour painting. It is extremely poisonous. Hydrated chromium sesquioxide is another mineral emerald green. There is also an aniline dye of this name.
Emerald Lore The emerald as a sacred
stone was anciently believed to blind a serpent which gazed at it, but to strengthen human eyes, and was, according to Theophrastus, worn as a ring-stone on account of this property. It was confused with 'false emerald', a kind of malachite. This fact explains the reference to ancient kings presenting blocks of emerald 1 or 2 cubits long to temples. The Babylonian name was barraktu, the Sanskrit marakata, the Hebrew bareket or barkat, the Greek maragdos or smaragdos.