It's bone, growing from the back of the head outward.

Originally, this ep was to be broadcast later in the run, because I
didn't want two PsiCorps episodes back-to-back, and for one little detail that
will become clearer after "Babylon Squared" airs; it would've been better to
have followed that episode, but again, these eps are made to be watched in
just about any order, so it's okay in the long run.

jms