The small-angle neutron scattering (SANS) was measured from networks containing deuterated un-cross-linked small chains (of molecular weights M(WD) = 14 370 and 73 000) under uniaxial extension of ratio II up to the maximum value before breaking (lambda similar to 2). In the isotropic state, the scattering increases with cross-linking, in a stronger way for large M(WD). In the deformed state, the scattering, in particular its zero-q limit, varies with the direction with respect to the stretching axis. It increases in all directions, corresponding to a dilation of the sample (i.e., parallel). It decreases slightly, back to the random mixing scattering, along contracted directions (i.e., perpendicular). The isointensity contours on the two-dimensional detector have a double-winged shape, often called butterfly patterns. The most striking effect is the appearance and the enlargement under strain of an "intermediate regime" of scattering, characterized by a limit curve with a power law dependence of I(q) with q. The zero-q Limit, I-0, and the correlation length, xi, vary as an apparent power law of lambda. Such an effect is not predicted by the classical models of elasticity combined with random mixing, but by other models developed recently. Quantitative comparisons lead to discarding the amplification under strain of spontaneous fluctuations (Y. Rabin and R. Bruinsma), at least in its version for small deformation. We discard also a perturbation model of uncorrelated frozen fluctuations (A. Onuki) which predicts a too strong variation of I(q). Considering correlated frozen heterogeneities and unscreening effects associated with the spatial separation under cross-linking and deformation of percolation-like clusters leads to a qualitative prediction of the limit curve and the power laws. The percolation exponents are larger than observed. This discussion can be extended to similar butterfly patterns observed in other permanent or temporary (entangled) networks permeated by a mobile species.