- Mast cells contain a rounded nucleus.  Their cytoplasm is loaded with basophilic, methachromic granules.  Paul Ehrlich (1877) interpreted these as alimentary storage granules (Ehrlich's mast cells).

Reference:
1. Color Atlas of Cytology, Histology, and Microscopic Anatomy 4th Edition. Wolfgang Kuehnel, M.D. Thieme Publications. 2003.