PREFACE. In the present monograph an attempt has been made to present the subject of the lipins in as simple a farm as the state of our knowledge permits. The task has been a difficult one, owing chiefly to the chaotic state of the literature dealing with these bodies the loose manner in which the term lipoid has been used has also added not a little to the general corfusion. A good deal of space has been given to a description of such alleged lipin substarlces as protagon and jecorin it was felt that a detailed discussion of these bodies was necessary in order that the reader might fully appreciate the very insecure and incoclusive evidence on which their existence as chemical entities had been based. Much of the monograph was written under adverse conditions, and more especially was this the case with Chapter VIII. which deals with the biological aspect of the subject. Before this chapter could be finished I was called to France, and was faced with the alternative of postponing the appearance of the volume indefinitely or of publishing it with this part treated much less fully than I could wish and with the omission of the section dealing with the phenomerla of hzrnolysis and immunity. The fact that little or nothing is known with certainty of the physiologicaI functions of the lipins suggested that no great loss would ensue from a curtailment of this part, and, after consultation with the Editors, it was resolved to publish the manuscript as it stood. If a second edition should ever be called for, some of the part dealing with the alleged lipins might be curtailed, and a more complete description of the biological aspect of the subject inserted. Our knowledge of the lipins has been materially increased within the last few years, but, strange as it may seem, we really know little to-day beyond that which was known and published by Thudichum over twenty years ago. Indeed, recent advances are in many cases but corroborations of Thudichums wonderful experimental work. That the brilliant investigations of this observer should have been so long neglected and despised is an anomaly which perhaps was in some measure due to Thudichurns more or less obscure literary style his pugnacity towards his opponents may also have contributed to produce this result. Whatever the cause, it is only justice to his memory to affirm that he has done more for lipin chemistry than has been done by any other individual.