It is still impossible to judge with certainty the rate at which the ripening of the mononuclear to the polynuclear cells proceeds, or further to decide if the ripening of the granules always runs parallel in point of time with that of the whole cell. On the grounds of our observations we would suppose that in general the two processes run their course side by side, but that in special cases the morphological ripening of the cell may proceed more rapidly than that of the granules. It is particularly easy to observe this point in eosinophil cells. Ehrlich had already mentioned in his first paper (1878) that side by side with the typical eosinophil granules isolated granules are often found which shew a deviation in tinctorial properties: for instance, they stain more of a black colour in eosine-aurantia-nigrosin; in eosine-methylene-blue, bluish-red to pure blue. Ehrlich had already described these as young elements in his first paper. The same differences are found more sharply marked in leukaemia even in the circulating blood, in the neutrophil as well as in the eosinophil group. Ehrlich has repeatedly found in leukaemic blood polynuclear eosinophil cells, whose granules must almost exclusively be regarded as young forms[15]. Ehrlich regarded these as typical examples of a relative acceleration of the morphological ripening of the cells, as compared with the development of the granules. =In normal blood we find only the ripe forms of the specific granulated cells of the bone-marrow. The mononuclear and transitional forms of the neutrophil group, do not under normal circumstances pass over into the blood-stream.= Ehrlich regarded the mononuclear neutrophil granulated cells as characteristic for the bone-marrow, since they are found exclusively in the bone-marrow, never in the spleen or lymph glands, and for this reason named them "=myelocytes=," [Greek: kat' exochen][16]. When myelocytes, no matter of what size, appear in considerable numbers in the blood of an adult, a leukaemia of myelogenic nature is nearly always present. (For the very rare exceptions to this rule, which it may be added can never be confused with leukaemia, see pages 77, 78.) Exactly similar conditions hold good for the eosinophil cells, in as much as the singly nucleated forms, which one may call eosinophil myelocytes, occur, almost exclusively, in leukaemic blood. These forms, which were first recognised by H. F. Mueller, are however of less importance, for in myelogenic leukaemia the chief part of the foreign admixture of the blood is made up of Ehrlich's myelocytes.