250 000 distant galaxies from $z = 8$ to $1.5$ are documented, where more than half of the star-forming galaxies are clumpy [@2014ApJ...780...57B and references therein]. The main origin of bound clumps is expected to be a result of in situ gravitational disc fragmentation, as demonstrated by a large number of numerical simulations of self-gravitating gas-rich discs, either in a cosmological context [@Agertz:2009wd; @Dekel:2009bn; @Ceverino:2010eh; @2012MNRAS.420.3490C] or in an isolated box . In general, local gravity has to overcome the stabilizing effects of thermal or turbulent pressure and differential rotation. This is quantified by the parameter $Q_0$ of @1964ApJ...139.1217T. Below a critical value ($Q_0<1$), axisymmetric instabilities (rings in an axisymmetric disc) can form, contract and finally break up into several clumps [@Dekel:2009bn], faster than spiral patterns (non-axisymmetric modes) can form. Furthermore, to finally form strongly bound clumps that later on collapse to stars, sufficient cooling is required to ensure contraction to higher densities within short enough time-scales [@Gammie:2001hv; @Dekel:2009bn].\
A more detailed insight can be achieved from linear perturbation theory [@Lin:1964cx]. It can describe the properties of structure formation, with respect to their sizes, masses and time-scales. Understanding these processes in detail is important in order to get a better 