1. A reference to the middle embryonic germ layer, lying between the ectoderm (outer skin) and the endoderm (inner skin), from which connective tissue, muscle, bone, and the urogenital and circulatory systems develop.
2. Relating to the mesoblast; as, the mesoblastic layer which is the middle germ layer that develops into muscle and bone and cartilage and blood and connective tissue.

The germ layer is a layer of cells produced during the process of gastrulation (series of cellular movements) during the early development of an animal embryo, which is distinct from other such layers of cells, as an early step of cell differentiation (distinguishing one cell from another cell).

The three types of germ layers are the endoderm (innermost layer that develops into the lining of the respiratory and digestive tracts), ectoderm (the outermost of three cell layers of an embryo, from which the epidermis, nervous tissue, and sense organs develop), and mesoderm (the middle of the three cell layers in an embryo, from which connective tissue, muscle, blood, dermis [skin], and bone develop).