"Mesenchyme" is a term introduced by Oscar Hertwig in 1881.In order to differentiate the use of the word mesenchyme in invertebrate zoology (an ecto- or entomesodermal middle layer of some invertebrates) and the use in vertebrate embryology (that is, undifferentiated tissue found in embryonic true mesoderm - entomesoderm - from which all connective tissues like blood vessels, blood cells, the lymphatic system, and the heart are derived.