The yolk sac is formed when the hypoblast cells multiply and spread to enclose the blastocele. The lining cells get flattened to form Heuser's membrane, which is formed on the 9th day after fertilization. The yolk sac is then called primary yolk sac. After formation of the extra-embryonic coelom, it is known as the secondary yolk sac. Human ova are said to be microlecithal as they contain very little "yolk".
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