gastrodisc

from The Century Dictionary.

• noun The endoderm of the vertebrate embryo in the discogastrula stage, that is, when both the ectoderm and the underlying endoderm are spread out on the yolk in the form of a disk.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Biol.) That part of blastoderm where the hypoblast appears like a small disk on the inner face of the epibladst.