surrounded by a wide zone of protoplasm. The nucleus. takes a fainter stain than that of the lymphocyte.

The transitional large mononuclear cell differs from the large mononuclear only in having a saddle-bag-shaped nucleus. Variations in size are frequently observed in the small mononuclear as well as in the large mononuclear cells, so that under some circumstances it is difficult to distinguish to which type of cell a given cell belongs.

The neutrophile or polymorpho-nuclear leucocyte is about three times as large as a red corpuscle. This cell is the socalled polynuclear leucocyte found in pus. The nucleus, as the name implies, is more or less subdivided, and often resembles various letters, as S, Z, V, M, etc. The nucleus is surrounded by a relatively large amount of protoplasm which is more or less completely filled with fine neutrophilic granules.

The normal eosinophile resembles the neutrophile in size and in the character of its nucleus. It differs from the neutrophile, at least chemically, by containing the eosinophilic a granules.

The large mononuclear, transitional, and neutrophilic cells. are regarded as the same cell in different stages of development. The younger form is the large mononuclear, and the oldest is the neutrophile. During the process of " ripening," as it is called, the fine granules are formed in the protoplasm, and the nucleus becomes more or less subdivided.

The so-called mastzellen—that is, cells which contain basophilic granules-were supposed by Ehrlich to originate from fixed connective-tissue corpuscles and from the spleen, and not to be present normally in the blood. Later investigations show that an occasional mastzelle may be found in normal blood.

Markzellen, or myelocytes, is the name applied to large mononuclear cells containing neutrophilic, and sometimes eosinophilic, granules. These cells are present in the medullary cavity of the long bones, but are never found in the blood normally.

Hematoblasts (blut-plättchen) are small round or oval discs

from 1.5 to 3.5 in diameter. They do not contain hemoglobin and have no central depression. They are so rapidly disintegrated on exposure to the air that it is necessary to use special precautions in order to preserve them. The best way to study them is to put a drop of some preserving fluid on the skin from which the blood is to be obtained, and then puncture the skin through the fluid. A 1 per cent. aqueous solution of osmic acid can be used, or, if it is desired to stain them, I part of methyl-violet in 5000 parts salt solution may be used.

Methods of Staining.-Of the many staining fluids. which have been employed to differentiate the white corpuscles, it is necessary to mention only those which are commonly used and which have been found to answer all purposes.

Ehrlich's triple stain possesses the advantage of staining both the eosinophilic and neutrophilic granules in addition to the nuclei. The formula is as follows:

The various ingredients are prepared separately as indicated by the dotted lines, and are afterward mixed gradually. The mixture must stand for several weeks before using. It is advisable to withdraw by means of a pipette some of the staining fluid from the middle portion without disturbing the bottom.

The cover-glass preparations should be stained from six to

eight minutes, washed thoroughly with water, dried, and mounted in Canada balsam. The neutrophilic granules are stained violet; the eosinophilic, a bright red; the nuclei of the neutrophilic and the eosinophilic cells are a greenishblue; the nuclei of the lymphocytes, a deep blue; the nuclei of the large mononuclear cells, a pale blue; the red corpuscles, copper color: and the nuclei of the red corpuscles, if any be present, a more intense blue than the nuclei of the lymphocytes. For some unexplained reason this stain is not always uniform in its action.

It is sometimes difficult to distinguish a nucleated red corpuscle from a lymphocyte. It is well to remember, therefore, that the nuclei of red corpuscles stain more intensely than other nuclei, and have very sharply defined outlines, and by careful focussing it is seen that the surrounding stroma is stained the same color as the other red corpuscles.

The mixture remains from four to six weeks in the sun, and then about I per cent. of eosin is added. Stain for from twelve to twenty-four hours in a covered glass dish in the sun. Wash thoroughly in water, dry, and mount in Canada balsam.

The red corpuscles are stained red, with at times a tinge of orange; the nuclei of the red corpuscles, a deep black; the protoplasm of the leucocytes, a light lilac; the nuclei, a dark lilac; the eosinophilic granules, a bright red; the nuclei of the lymphocytes, black, with not quite so deep a tinge as the nuclei of the red corpuscles. The protoplasm scarcely stains.

Eosin and Methylene-blue Stains.-Separate stain. The eosin solution, a per cent. alcoholic solution, is heated and

the cover-glasses are stained in it from two to three minutes, and are then stained in a saturated aqueous solution of methylene-blue for from two to three minutes; thoroughly washed, dried, and mounted in Canada balsam.

The red corpuscles are stained eosin red; the nuclei of the red corpuscles, a deep-blue; the nuclei of the leucocytes are stained a light blue; the eosinophilic granules, a bright red.

Chenzinsky-Plein Solution.-Saturated aqueous solution of methylene-blue, 40 c.c.; per cent. (in 70 per cent, alcohol) eosin solution, 20 c.c.; distilled water, 40 c.c.

The best results are obtained by staining the specimens for twenty-four hours. A fairly good stain may be obtained by heating the solution fifteen minutes. The red corpuscles are stained eosin red; the eosinophilic granules, a bright red; and the nuclei, blue.

The basic granules may be stained by a concentrated solution of methylene-blue. The specimens should remain in the solution from five to ten minutes.

METHODS OF FIXING AND EXAMINING SPECIAL ORGANS AND TISSUES.

TISSUES which are to be hardened should be obtained as fresh as possible. For this reason autopsies rarely furnish such perfect material as is obtainable from experimental lesions in animals or from surgical operations. Still, most of the pathological material comes from autopsies, and it is encouraging to know that very good work can often be done with tissues not fixed until twenty-four hours or even more after death. The most valuable autopsies are those which are freshest, and in which but one etiological factor has been concerned, so that the relation between the cause and the lesion produced is uncomplicated and can be readily grasped and understood.

The choice of the proper fixing reagent varies with the tissue, the lesion, and the use to which the material is to be

put. For simple diagnosis alcohol is usually sufficient. For special investigations other fixatives may be required. In general it may be said that alcohol is best suited for bacteria and for many micro-chemical color reactions; Zenker's or Orth's fluids for red blood-globules, nuclear figures, and protoplasm; formaldehyde for the central nervous system. More attention will be paid to this point in the consideration of the separate organs and tissues.

It is important that pieces of tissue for histological study should be placed in the proper fixative as soon after the removal of the organs from the body as possible, so that the surface will not dry or the blood and other fluids escape from the vessels. Do not wash off the surface with water. The tissues should almost invariably be cut into thin slices, not over to 1 cm. thick. For the finer fixatives 2 mm. should not be exceeded.

Frozen sections of fresh tissues will often show whether the material is worth saving, and suggest what fixatives had best be used-such as Flemming's solution, for instance, if fat is present.

Acute Inflammatory Exudations; Granulationtissue. The elements in acute inflammatory exudations which require preservation are polynuclear leucocytes, serum, fibrin, and red blood-globules. The best general fixative for them all is Zenker's fluid. It not only preserves perfectly the characteristic nuclei of the leucocytes, but also the protoplasm which stands out sharply in contrast-staining with eosin. The albumin of the serum is coagulated into a finely granular material. The fibrin and red blood-globules stain brilliantly with eosin. Alcohol is generally preferred for the fixation of any organisms associated with the exudation, but after fixation in Zenker's fluid they can be perfectly stained with Unna's alkaline methylene-blue solution. Occasionally other fixatives, such as Flemming's or Pianese's, will be found useful, especially for the study of any attendant degenerations.

Zenker's fluid will also be found invaluable for fixing granulation-tissue where a similar exudation is generally