The membrane shell of an implantable dosage system according to the invention is particularly suitable for subcutaneous applications to release an active agent with steady amounts during a longer period of time. The membrane shell (3) according to the invention comprises a first half (1) and a second half (2), which both halves comprise a continuous closure edge (8, 9), and are adapted to be connected to each other through a closable joint. The closure edges (8, 9) of the halves comprise at least one groove (10) and/or at least one protrusion (11) as continuous or discontinuous, and the membrane shell (3) is adapted to be closed so that at least one protrusion (11) and/or at least one groove (10) of the second half becomes opposed to at least one groove (10) and/or at least one protrusion (11) of the first half through a snap-fit joint.
