Answer:
blastopore
Step-by-step explanation:
Involution is the second stage after invagination during frog gastrulation. Involution occurs at the blastopore when the marginal zone cells get to the tip of the blastopore, make an inward turn and then move along the inner exterior of the outer cells sheets.
This means that the cells found at the lip of the blastopore and always undergoing change and the first of these cells to develop into the dorsal lip are the endodermal cells. Next are the mesoderm cells that form the Notochord.