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Care is required in filling the "white" pipet of a hemocytometer because of its relatively large bore. Boggs suggested cutting the rubber tube and inserting a Wright "throttle." While this device permits one to draw in blood more slowly and thus more accurately, it is easily stopped and is breakable. Some skill is required in its manufacture.
For the past two years I have been getting satisfactory results by removing the mouthpiece of the pipet and substituting a 25 gage rustless-steel hypodermic needle. I also use one on my "red" pipet. The hub is all that is necessary, but I have never broken the tips off my needles. Any obstruction can be quickly removed by the use of the ordinary cleaning-wire. This method gives perfect control of the flow of liquid into the pipet.
Amstutz OC. MOUTHPIECE PROMOTING ACCURACY IN USE OF HEMOCYTOMETER PIPET. JAMA. 1927;89(25):2113. doi:10.1001/jama.1927.92690250001010