a blood sampling device comprises a pointed cannula at each end , a pre - evacuated sample container with a self - sealing stopper which can be perforated , a self - sealing membrane which can be perforated and a blood - viewing chamber the walls of which are surrounded by transparent material arranged in front of the stopper in the direction of perforation . the chamber is arranged on the stopper and is sealed off essentially from the sample container by said stopper and from the patient by the membrane .