EGG (AS. crg, Icel. ego, OHG. ci, Cer. Ei; con nected with ()Church Slay. eye, yelp'. Lat. orate, Gk. 416e. /ion, egg). The present article proposes to consider the external characteristics, qualities, and utilities of the eggs of animals. and more especially of birds, leaving to the articles EM BRYOLOGY, REPRODUCTION, etc.. their biological relations and development. All animals except Protozoa produce eggs, which are essentially single cells of protoplasm. containing a nucleus in which rests the potentiality of life. plus, in most cases. food-material and an integument. Eggs may be produced singly or in very small numbers at long intervals, or frequently and in great numbers: they may be so minute as to be invisible to the unahled human eye, or may exceed a foot in diameter: and in relative size, as compared with the parent, they may be nearly a quarter as large or less than a millionth; they may never leave the parent except as waste matter, or may be extruded, either before or after fertilization: finally. those which are 'laid' may he east out wholly naked to take an infinitesimal chance of survival, or may be more or less well secured against harm by protective coverings, or even become objects of care and protection on the part of the Eia.s of Ittans. The eggs. of birds will be con sidered first, they are typical, familiar, and of most interest to the general reader. They are the largest. relatively, to the size of the parent. of any in the animal kingdom, due to the fact that the embryos must reach a higher degree of maturity (and bulk) before hatching than among inferior ela.ses of animals. The egg originates in the ovary, where, as the breed ing season approaches. a number of them become greatly enlarged. and one by one break loose from the ovarian niembranes and begin to descend the oviduct. They are at this time simply spheres of yolk surrounding a small quantity of germinal protoplasm (the nucleus of the original cell), which appears on the surface of one side, as the 'tread' of popular speech (see ALLA:viols; EMBRYOLOGY ) • and covered with an almost im perceptible membrance. the vitelline membrane, formed from the protoplasm of the egg itself. Yolk or deutoplasin is a granular. fatty, nutritive material or highly compact bmil, stored in the egg for the nourishment of the young. it is
of two kinds—white and yellow. The white yolk forms only a thin film over the surface of the yellow yolk. and pa sseS under the germinative vesicle where it forms a sort of bed. and front this point extends downward to the centre of the yellow yolk. The yolk is completed in the sac like mouth of the oviduct. and then descends into the oviduct itself, where it is. or may be, fer tilized, and along which it descends in a slow spiral course.
Here the yellow ball begins to receive its outer coverings, the first of which is tile 'white.' or albumen, which is poured out from glands. and consists of about 86 per cent. of water. 12 per cent, of proteids, 1.5 per cent. of fat, and a little salt. This 'white of the egg' is formed in con centric layers (as may be seen in a hard-boiled egg), the innermost of which is somewhat tena cious, and is drawn out by the spiral rotation of the egg in its progress into threads at the opposite poles. "These threads," writes Cones, "which become twisted in opposite directions, . . are called chelavr; they are the 'strings' rather unpleasantly evident in a soft-boiled egg, lint serve the important office of mooring and steadying the yolk in the sea of white by adhe sions eventually contracted with the membrane which immediately lines the shell. They are also intrusted with the duty of ballasting, or keeping the yolk right side up. For there is a 'right' side to the yolk-hall. being that on which floats the •icatriele or 'tread.' This side is also the lightest. the white yolk being less dense than the yellow; and the ehala.:a. are attached a little below the central axis. The result is that if a fresh egg be slowly rotated on its central axis the tread will rise by turning of the yolk ball in the opposite direction, till, held by the twisting of the elude:cc, it can go no further: when. the rotation being continued, the tread is carried under and up again on the other side. resuming its superior position." In view of this it is no longer necessary to believe that brood ing birds carefully turn their eggs each day in order to equalize the heat imparted to them. It is not essential that any part should be kept warm except the germinal vesicle. and this natu rally remains uppermiist.