The vitelline membrane is of double origin. The yolk and blastodisc are bounded by a plasma membrane and outer vitelline membrane, which is a converted form of zona radiata. The central flask-shaped white yolk called latebra runs from the centre to the lower side of the blastodisc and there it spreads to form the nucleus of Pander. The yolk has a central mass of white yolk around which are alternate concentric layers of yellow and white yolk. It is entirely filled up by the yolk and over it, i.e., in animal pole lies a small cytoplasmic disc with a nucleus. The egg is about 3.0 cm in diameter and is polylecithal. The various egg membranes secreted around egg are albumen, shell membranes and shell. The nucleus of one sperm fuses with the female nucleus (amphimixis) and the nuclei of other sperms degenerate.Īs the fertilised egg passes downward inside the oviduct, it rotates, undergoes cleavage and various accessory egg membranes are laid down over the developing egg. The secondary oocyte or ovum after second maturation division is surrounded by several (5 or 6) sperms which enter in it (polyspermy), but only one sperm succeeds in the fertilisation process.
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