Dictionary.net

• Isolated reddish patches in the mesoderm in which primitive erythroblasts are found enclosed in a mesodermal syncytium.

1920 - A dictionary of scientific terms.
By Henderson, I. F.; Henderson, W. D.

• Collections of greatly enlarged hollow cells in which red b. corpuscles are being developed; these cells also unite to form b. vessels.

1916 - Appleton's medical dictionary
By Smith Ely Jelliffe