mesomere

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A blastomere of intermediate size, larger than a micromere but smaller than a macromere.
• noun The middle zone of the mesoderm of a chordate vertebrate embryo, from which excretory tissue develops.

from The Century Dictionary.

• noun In embryology:
• noun In the segmenting egg, a blastomere, or cell which gives rise to the mesoblast.
• noun In the vertebrate embryo, a mesoblastic somite, or protovertebra.