  Filaments are crucial intermediaries in the star formation process. Recent
observations of filaments show that - \textbf{(i)} a number of them are
non-singular entities, and rather a bundle of velocity coherent fibres, and
\textbf{(ii)} while a majority of filaments spawn cores narrower than their
natal filaments, some cores are broader. We explore these issues by developing
hydrodynamic simulations of an initially sub-critical individual filament that
is allowed to accrete gas from its neighbourhood and evolves under
self-gravity. Results obtained here support the idea that fibres form naturally
during the filament formation process. We further argue that the ambient
environment, i.e., the magnitude of external pressure, and not the filament
linemass alone, has bearing upon the morphology of its evolution. We observe
that a filament is susceptible to the \emph{sausage}-type instability
irrespective of the external pressure. The fragments, however, are pinched in a
filament experiencing pressure comparable to that in the Solar neighbourhood
($\sim 10^{4}$ K cm$^{-3}$). By contrast, fragments are broad and spherical -
having density profiles similar to that of a stable Bonnor - Ebert sphere -
when the filament experiences a higher pressure, typically $\ge 10^{5}$ K
cm$^{-3}$, but $\le 10^{6}$ K cm$^{-3}$). The filament tends to rupture at even
higher external pressure ($\ge 10^{7}$ K cm$^{-3}$). These observations
collectively mean that star formation is less efficient with increasing
external pressure.
