Hypercyclic subspaces on Fr\'echet spaces without continuous norm

Known results about hypercyclic subspaces concern either Fr'echet spaces with a continuous norm or the space \omega. We fill the gap between these spaces by investigating Fr'echet spaces without continuous norm. To this end, we divide hypercyclic subspaces into two types: the hypercyclic subspaces M for which there exists a continuous seminorm p such that M\cap \ker p={0} and the others. For each of these types of hypercyclic subspaces, we establish some criteria. This investigation permits us to generalize several results about hypercyclic subspaces on Fr'echet spaces with a continuous norm and about hypercyclic subspaces on \omega. In particular, we show that each infinite-dimensional separable Fr'echet space supports a mixing operator with a hypercyclic subspace.