The Collaborative International Dictionary of English v.0.48:

Micella \Mi*cel"la\, n.; pl. Micellae. [NL., dim. of L. mica a
   morsel, grain.] (Biol.)
   A theoretical aggregation of molecules constituting a
   structural particle of protoplasm, capable of increase or
   diminution without change in chemical nature.
   [1913 Webster] Mich