"Dr. Marrus asked a black Junior Fellow: "You know this is your master, eh? Do you feel the lash?""

And the Junior Fellow (JF) should have retorted that 'well, he is your master too and you are going actually going to feel the lash' then and there instead of all this backbiting by all. Since JF did not throw the ball back, it is incumbent on Dr. M to apologise immediately to JF.

Do not know Dr. M from Adam. But  Dr. M must have clearly felt so collegial with JF to have commented in such a manner as Dr. M cannot be so obtuse to speak to a stranger.