Abstract: A device comprises an elongate, tubular sheath (4) having an anterior and a posterior end (6; 8), a pointed needle (18) arranged in the sheath (4) for displacement between a first and a second end position, a catheter (26) initially supported by the needle (18), and an attachment for displacing the needle (18) between the first and the second end position. The novelty of the invention resides in that the sealing means for sealing the anterior, initially open end (6) of the sheath (4) is detachably connected to the attachment, that the sealing means further is detachably connected to the catheter (26), that the sealing means, during the displacement of the needle (18) from the first to the second end position, is moved towards the anterior end (6) of the sheath (4) to a sealing position, and that the sealing means is fixed in the sealing position for substantial sealing of the anterior end (6) of the sheath (4).