yolk plug

The exposed surface of the yolk cell now comprises a large yolk plug.

Yolk plug is the remaining patch of endodermal cells that is created during the formation of the dorsal lip of the blastopore.

Cell marking experiments actually show that to be the case (Long, unpublished), thus we use the terms dorsal lip, blastopore, and yolk plug as names for the appropriate embryonic structures in this report.