The collection of stock sera is not a matter of any difficulty. With strict aseptic precautions 20 cc. of blood are withdrawn in a syringe from persons known to belong to Groups II and III; the bloods are put into a sterile test-tube and allowed to clot. As soon as the serum has separated it is drawn up into sterile glass bulbs of suitable capacity, which are sealed off at each end. The most convenient form of storage for actual use is a capillary glass tube sealed at each end. Each tube may be made to hold a single drop, which is the amount used for a test. There is then no wastage of serum, and no chance of contaminating the remaining stock. When the blood has been withdrawn and has clotted, the complete settling of the corpuscles can be hastened by the use of the centrifuge. If the serum be left in contact with the corpuscles for more than twelve hours, some auto-hæmolysis may take place, so that the serum will become tinged with hæmoglobin. It is exceedingly important that the two stock sera should not become confused, and this may easily happen unless each tube has some distinguishing mark. The methods of testing for blood groups have been simplified by successive observers since the existence of the groups was first demonstrated in 1907. Moss used an elaborate technique such as was essential for putting a new discovery upon a secure scientific basis. In order to obtain a suspension of corpuscles, blood was drawn into a syringe containing a solution of sodium citrate to prevent clotting. The corpuscles were collected by means of the centrifuge, and were thoroughly washed twice in normal saline solution so that they were finally collected free from serum and from citrate. Serum was collected in the manner already described. A series of small tubes was then filled with equal quantities of serum and the suspension of corpuscles, and was incubated for two hours at 37·5° C. At the end of this time observations were made and again after the tubes had stood for twelve hours in an ice chest. Varying degrees of agglutination and hæmolysis were then accurately recorded, and far-reaching results were obtained.