Introduction As one of the lesser problems of physiology, the nature of blood coagulation has received more than its share of investigation. There is now an insurmountable mass of literature dealing with its various aspects, contributed over the greater part of a century by some of the foremost experimentalists of their time, and embodying theories, even whole schoolsof thought, that have produced little but acrimony and confusion. The reasons for this activity are not at once apparent, though

more »

... o doubt there is something provocative to the curious mind in the spontaneous conversion of fluid blood to solid clot. More attractive, perhaps, is the ease with which experiments in blood coagulation can be devised and carried out. The requirements for such work in technique, apparatus, and even in time are seemingly not exacting. The real or imaginary components of a theoretical mechanism can be separated to the investigator's satisfaction with little difficulty and allowed to interact in endless permutations and combinations. Each experiment suggests another; always the intangible solution seems just within reach and the experimenter is led deeper and deeper into his own, often unjustified, interpretations of his findings. Thus it is that elaborate theories have grown up which, aided by the confusion of rival terminologies, have so obscured-and entangled basic facts that these are now scarcely recognizable. Within the last decade this hitherto rather academic problem has taken on a more practical aspect; the discovery of vitamin K and the use of anticoagulants to reduce the danger of post-operative thrombosis, both advances of fundamental importance, have demanded the reliable quantitative assay of coagulation factors. This development has necessarily clarified