ZOO501 Final Term Merged PPT

Shortly after the blastula hatches from its fertilization envelope, the vegetal side of the spherical blastula begins to thicken and flatten. At the centre of this flat vegetal plate, a cluster of small cells begins to change. These cells begin extending and contracting long, thin (30 × 5 μm) processes called filopodia from their inner surfaces. The vegetal plate bends inward and invaginates about one-fourth to one half the way into the blastocoel. Then invagination suddenly ceases. The invaginated region is called the archenteron (primitive gut), and the opening of the archenteron at the vegetal region is called the blastopore.