All eggs, like any other cells, are of course covered by the cell membrane or plasmalemma. Under the electron microscope the plasmalemma sometimes can be seen to be double, with two layers of electron-dense material about 50 Å thick separated by a less dense interval of about 60 Å.
In addition to the plasmalemma, eggs of all animals, with the exception of sponges and some coelenterates, are surrounded by special egg envelopes. Depending on their origin these may be subdivided in two groups – the primary envelopes and the secondary envelopes (In older literature, the egg envelopes were referred to as egg membranes).
The primary egg envelopes are those which develop in the ovary between the oocytes and the follicle cells in the space occupied by the inter-digitating microvilli. In the initial stages of its formation the envelope shows positive reactions for mucopolysaccharides, but in later stages it is fortified by addition of further substances and may consist of fibrous proteins, or in insects, it may be sclerotized and may become highly impermeable.
The envelope thus formed bears different names in different animals – it is known as the vitaelline envelope or vitelline membrane in insects, molluscs, amphibians, and birds; in tunicates and fishes it is usually called the chorion. In mammals the envelope of an exactly similar nature is called the zona pellucida. The zona pellucida thus takes the place of the zona radiata. The jelly coat surrounding the eggs of sea urchins belongs in the same group.
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The origin of the materials forming these envelopes is not quite obvious; usually the envelope is believed to be secreted by the follicle cells, but the oocyte may perhaps contribute to the formation of the envelope in some cases.
In insects a second, thicker coat is secreted by the follicle cells on top of the vitelline envelope. This second envelope is called the chorion. In many insects its surface shows a complicated sculpture, which is typical for each species. The primary egg envelopes usually closely adhere to the surface of the oocyte, but at a later stage a space filled with fluid may appear between the cytoplasm and the envelope; this space is called the perivitelline space.
In mammals an egg escaping from the ovary carries with it on the surface of the zona pellucida a layer of follicle cells known as the corona radiata. The cells of the corona radiata are peeled off later, as the egg descends the oviduct.
The second group of egg envelopes is those which are secreted by oviducts and other accessory parts of the genital organs while the egg is passing from the ovary to the exterior.
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The eggs of amphibians are surrounded by a layer of jelly, which protects the egg and sometimes serves to make the eggs adhere to one another and to submerged objects such as water plants. This jelly is secreted as the eggs pass through the oviducts.
When the amphibian egg is deposited in water, the jelly absorbs water and swells. In the oviparous sharks and rays the egg is surrounded in the oviducts (in the special parts called the shell glands) by a hard shell of a complicated shape. The shell is drawn out into long twisted horns which serve to entangle the eggs among seaweed.
The most complicated egg envelopes, however, are found in the eggs of birds, where no less than five envelopes can be distinguished, the innermost being the vitelline envelope—a very thin envelope covering the surface of the yellow of the egg (which is the true egg cell). This envelope actually has a double origin.
An inner layer of the envelope is produced in the ovary, in the space between the oocyte and the follicle cells. This layer is composed of very rough fibers. An outer, more finely fibrous layer is then formed on top of the first layer when the egg enters the upper portion of the fallopian tube.
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The next envelope is the white of the egg. Eighty-five per cent of the egg white is water; the rest is a mixture of several proteins, mostly albumins, which make up 94 per cent of the dry weight. A denser part of the egg white forms strands, known as the chalazae, which help to keep the egg cell in the center of the egg white. Next to the egg white are two layers of shell envelopes which consist of fibers of keratin matted together.
Over most of the surface of the egg the shell envelopes are in contact with one another, but at the blunt end of the egg they are separated; the inner envelope adheres to the egg white, and the outer envelope adheres to the shell, leaving a space in between filled with air. The outermost envelope is the shell, which consists chiefly of calcium carbonate (CaCO3), about 5 gm. in a hen’s egg.
The shell is pierced by many fine pores which are filled by an organic (protein) substance related to collagen. In an average hen’s egg the pores have a diameter of 0.04 to 0.05 mm., and the total number of pores is estimated at about 7000. The envelopes of a bird’s egg are secreted one after another as the egg proceeds down the oviduct. The whole process takes slightly longer than 24 hours.
After the egg has been released from the ovary, it quickly passes into the oviduct and descends through it for about three hours, during which most of the egg white is secreted and envelops the egg cell. The lowest portion of the oviduct is widened and is termed the uterus. Here the egg remains for 20 to 24 hours, while the remainder of the egg white and eventually the shell envelopes and the shell itself are secreted.
Not only do the envelopes of a bird’s egg protect the egg cell, but the egg white serves also as an additional source of nourishment and is gradually used up in the course of the development of the embryo.