In presenting this preliminary communication it is necessary to explain that the term leucocyte is used in the widest sense, and is intended to include all the wandering nucleated cells and their immediate ancestors. After many endeavours to make the observations here recorded compatible with the current classification of these cells, it has been found imperative to use a term that will include them all, and, at any rate in most cases, to put aside for the moment the question as to which particular kinds of leucocyte are being dealt with. The tissues used have been chiefly bone-marrow, lymphatic glands and spleen, but the leucocytes in various other tissues and the free leucocytes in the blood have also been examined. Most of the work has been done with material derived from the guinea-pig and the rat, but tissues from man, rabbit, mouse, crocodile, frog, Triton and Axolotl have also been used. In every case the tissues have been normal and, with one exception, derived from adult or nearly adult animals, the one exception being the testis of the early embryo of the guinea-pig.