ELECTRICAL AND MECHANICAL APPARATUS FOR USE IN LABORATORIES, AND PARTICULARLY FOR USE IN THE FIELD OF MICROBIOLOGY AND BIOLOGICAL TECHNOLOGY NAMELY, BUNSEN BURNERS, METERING HOSE PUMPS FOR LABORATORY USE, MACHINES FOR AUTOMATICALLY INSERTING STOPPERS IN PIPETTES, MACHINES FOR PREPARING NUTRIENT MEDIA, PIPETTE SIZE SELECTION UNIT, APPARATUS FOR MASS PRODUCING CELL CULTURES, PETRI DISH FILLING ASSEMBLIES, PIPETTE HANDLING APPARATUS, AND APPARATUS FOR PROMOTING CONTROL TISSUE CULTURE GROWTH