mesomere

A blastomere of intermediate size, larger than a micromere but smaller than a macromere.

The middle zone of the mesoderm of a chordate vertebrate embryo, from which excretory tissue develops.

In embryology:

In the segmenting egg, a blastomere, or cell which gives rise to the mesoblast.

In the vertebrate embryo, a mesoblastic somite, or protovertebra.