Discontinuous at horizontal axis, U and K points, in energy

[Tana-ami]

I have a question about horizontal axis of energy band.
In band structure of FCC lattice, like Si, GaAs, the horizontal axis have discontinuous at U point and K point. But other region have continuous like from Gamma to X or to L.
I cannot hit on an idea about this discontinuous.
Would anyone who understand the reason answer this question, please teach me.

 

[DrDu]

I don't see a discontinuity. At best a discontinuity of the derivatives at X. That's not miraculous as the path in k space has a sharp bent there, see
http://en.wikipedia.org/wiki/Brillouin_zone