  Although the concept of defect of an analytic disc attached to a generic
manifold of $\C^{n}$ seems to play a merely technical role, it turns out to be
a rather deep and fruitful notion for the extendability of CR functions defined
on the manifold.
  In this paper we give a new geometric description of defect, drawing
attention to the behaviour of the interior points of the disc by infinitesimal
perturbations. For hypersurfaces a stronger result holds because these
perturbations describe a complex vector space of $\C^{n}$.
  For a big analytic disc the defect does not need to be smaller than the
codimension of the manifold. Indeed we show by an example that it can be
arbitrarily large independently of the codimension of the manifold.
Nevertheless we also prove that the defect is always finite. In the case of a
hypersurface we give a geometric upper bound for the defect.
