clinical diagnosis, and must never be taken as positive in itself.

The bacillus grows similarly upon blood-serum and Löffler's mixture. Upon glycerin agar-agar and agar-agar the colonies are much larger, more translucent, always

FIG. 64.-Diphtheria bacilli (from photographs taken by Prof. E. K. Dunham, Carnegie Laboratory, New York): a, pseudo-bacillus; b, true bacillus ; c, pseudo-bacillus.

without the yellowish-white or china-white color of the blood-serum cultures, and generally are distinctly divided into a small elevated centre and a flatter surrounding zone with indented edges, sometimes with a distinctly radiated appearance. It must be remarked that when sudden transplantations are made from blood-serum to agaragar the growth resulting is meagre, but the oftener this growth is transplanted to fresh agar-agar the more luxuriant it becomes.

The growth in gelatin puncture-cultures is characterized by small spherical colonies which develop along the entire length of the needle-track. The gelatin is not liquefied.

Upon the surface of gelatin plates the colonies that develop do not attain anything like the size of the colonies upon Löffler's mixture. They appear to the naked

FIG. 65. Bacillus diphtheria, colony twenty-four hours old upon agar-agar; × 100 (Fränkel and Pfeiffer).

eye as whitish points with smooth contents and regular though sometimes indented borders. Under the microscope they appear as granular, yellowish-brown colonies with irregular borders (Fig. 65).

When planted in bouillon the organism causes a diffuse cloudiness at first, but, not being motile, soon settles to the bottom in the form of a rather flocculent precipitate which has a tendency to cling to the sides of the glass. Sometimes a delicate irregular mycoderma forms upon the surface, especially when the cultivation is made by the method of Fernbach with a passing current of air. This mycoderma, which may appear quite regular when the flask is undisturbed, is so brittle that it at once falls to pieces if the flask be moved.

Spronck has recently determined that the characteristics of the growth of the diphtheria bacillus in bouillon, as well as the amount of toxin-production, vary according to the amount of glucose in the bouillon. He divides the cultures into three types:

Type A. The reaction of the bouillon becomes acid and remains acid, the acidity increasing. The bacilli accumulate at the bottom of the clear liquid. The toxin-production is meagre.

Type B. There is no change from alkalinity to acidity, but the original alkalinity of the bouillon steadily increases. The culture is very rich, the bottom of the flask shows a considerable sediment, the liquid is cloudy, and a delicate growth occupies the surface. The toxicity is very great.

Type C. In a few days the reaction of the culture becomes acid, and then later on changes to alkaline. During the acid period the liquid is clear, with a white surface-growth. When the alkalinity returns the bouillon clouds and the surface-growth increases in thickness. Sediment accumulates at the bottom of the flask. The toxicity of the culture is much less than in Type B. Spronck regards the varying reaction as due to the fermentation of the glucose, and asserts that the most luxuriant and toxic cultures are those in which no glucose is present. To exclude as much of the undesirable sugar as possible, he makes the bouillon from the stalest meat obtainable, preferring it when just about to putrefy. Of the meats experimented with, beef was

found to be the best.

Upon potato the bacillus only develops when the reaction is alkaline. The potato growth is not characteristic. Welch and Abbott always secured a growth of the organism when planted upon potato, but do not mention the reaction of the medium they employed.

Milk is an excellent medium for the cultivation of the Bacillus diphtheria, and is possibly at times a means of infection. Litmus milk is an excellent medium for ob

serving the changes of reaction brought about by the growth of the bacillus. At first the alkalinity, which is always favorable to the development of the bacillus, is destroyed by the production of an acid. When the culture is old the acid is replaced by a strong alkaline reaction.

Diphtheria as it occurs in man is generally a disease characterized by the formation of a pseudo-membrane upon the fauces. It is a local infection, due to the presence and development of the bacilli on the pseudomembrane, but is accompanied by a general toxemia resulting from the absorption of a violently poisonous substance produced by the bacilli. The bacilli are found only in the membranous exudation, and most plentifully in its older portions. As a rule, they do not distribute themselves through the circulation of the animal, though at times they may be found in the heart's blood.

The most malignant cases of the disease seem to be due to pure infection by the diphtheria bacillus, though such cases are more rare than those in which the Streptococcus pyogenes and Staphylococcus aureus and albus are found in association with it.

It may be well to remark that all pseudo-membranous diseases of the throat are not diphtheria, but that some of them result from the activity of the pyogenic organisms alone.

No more convincing proof of the existence of a powerful poison in diphtheria could be desired than the evidences of general toxemia resulting from the absorption of material from a comparatively small number of bacilli situated upon a little patch of mucous membrane.

In animals artificially inoculated the lesions produced are not identical with those seen in the human subject, yet they have the same general features of local infection with general toxemia.

Guinea-pigs, kittens, and young pups are susceptible animals. When half a cubic centimeter of a twenty-fourhour-old bouillon culture is injected beneath the skin of

such an animal, the bacilli multiply at the point of inoculation, with the production of a patch of inflammation associated with a distinct fibrinous exudation and the presence of an extensive edema. The animal dies in twenty-four to thirty-six hours. The liver is enlarged, and sometimes shows minute whitish points, which in microscopic sections prove to be necrotic areas in which the cells are completely degenerated and the chromatin of their nuclei is scattered about in granular form. Similar necrotic foci, to which attention was first called by Oertel, are present in nearly all the organs in cases of death from the toxin. The bacilli are constantly absent from these lesions. Flexner has shown these foci to be common to numerous irritant poisonings, and not peculiar to diphtheria alone.

The lymphatic glands are usually enlarged; the adrenals are also enlarged, and, in cases into which the live bacilli have been injected, are hemorrhagic.

Sometimes the bacilli themselves are present in the internal organs, and even in the blood, but generally this is not the case.

It might be argued, from the different clinical pictures presented by the disease as it occurs in man and in animals, that they were not expressions of the same thing. A careful study, however, together with the evidences adduced by Roux and Yersin, who found that when the bacilli were introduced into the trachea of animals opened by operation a typical false membrane was produced, and that diphtheritic palsy often followed, and of hundreds of investigators, who find the bacilli constantly present in the disease as it occurs in man, must satisfy us that the doubt of the etiological rôle of the bacillus rests on a very slight foundation.

One reason for skepticism in this particular is the supposed existence of a pseudo-diphtheria bacillus, which has so many points in common with the real diphtheria bacillus that it is difficult to distinguish between them. We have, however, come to regard this pseudo-bacillus as