Biology of Finfish and Shellfish (2+1)

4.11.8. 6^th Stage

The rudiments of the head and arms have now assumed the typical position to form the "head". The embryo is now altogether distinct from the yolk, to which it merely hangs instead of, as before, lying upon it. The blastoderm finally grows round the yolk and forms a yolk sac. At first this sac is four or five times the size of the embryo, but in proportion as the latter grows at the expense of the yolk and develops further, the sac becomes smaller, so that when the embryo is hatched out, the size of the yolk-sac is only one-third of the young animal.

It must further be mentioned with regard to the yolk sac that it is at no time in communication with the intestine. As the embryo becomes constricted from the yolk, the latter divides into two parts-an inner part, lying inside the embryo and an outer part, filling the sac. These two parts are connected by means of the stalk of the yolk sac, which projects downward from the "head". The yolk within the embryo is divided into three unequal parts, the largest of which fills the visceral dome; another mass of considerable size fills the "head", and these two masses are connected with a smaller portion lying in the nuchal region.