Later workers had the advantage of using stock sera belonging to known groups, so that the number of observations to be made was very greatly reduced. Brem introduced in 1916 a method of testing in which he mixed the serum and suspension of washed corpuscles in very small quantities on a coverslip, which was inverted over an ordinary cell slide rimmed with petroleum jelly. The results could then be observed macroscopically or under the microscope, and the presence or absence of agglutination could be determined within fifteen minutes. The detection of hæmolysis by the hanging drop method requires that the cells should be incubated and observed at intervals for several hours, but it is not always easy to see the disintegrated corpuscles unless the process has taken place extensively. The diagram on p. 105 gives in a tabulated form some idea of the appearances presented by the corpuscles of the different groups when mixed with the stock sera and observed in a hanging drop under a microscope. Agglutination must be distinguished from the formation of rouleaux, which may be seen in any of the mixtures. For scientific purposes these very careful tests are necessary, but it seems to be clear that for clinical purposes a much rougher and quicker test is adequate. In the clinical determination of blood groups it is superfluous to carry the test to the point of watching for hæmolysis, for it is upon the presence of agglutinins in the serum and the corresponding iso-agglutinins in the corpuscles that the determination of the groups depends. Further, no error is introduced by neglecting the hæmolysis, since it has been shown that hæmolysis is invariably preceded by agglutination. It is the occurrence of agglutination therefore that is of prime clinical importance. If that is excluded, hæmolysis is necessarily excluded also, and the prolonging of the test is seen to be only of academic interest. In the methods described above the corpuscles were always tested in the form of a washed suspension. This precaution was taken on the supposition that the presence of any of the serum belonging to the corpuscles might interfere with the reaction. If, however, the amount of this serum be small relatively to the amount of the test serum, then no such interference takes place. [Illustration: Fig. 7.--TABULATION OF SERUM REACTIONS AS SEEN IN HANGING DROPS.]