hypomere

from The Century Dictionary.

• noun In embryology, the portion of the mesoderm from which the walls of the pleuroperitoneal cavity of vertebrates arise.
• noun That lower part of some sponges which has all three fundamental layers, ectoderm, mesoderm, and endoderm, but develops no flagellated chambers or choanosome: distinguished from spongomere.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology A part that the walls of a pleuroperitoneal cavity grow.