The Collaborative International Dictionary of English v.0.48:

Myeloidin \My`e*loid"in\, n. [Myelin + -oid + -in.] (Physiol.
   Chem.)
   A substance, present in the protoplasm of the retinal
   epithelium cells, and resembling, if not identical with, the
   substance (myelin) forming the medullary sheaths of nerve
   fibers.
   [1913 Webster]