Page:Popular Science Monthly Volume 50.djvu/660

640 of India, is believed to be due to a similar parasitic protozoan (Trypanosma Evansi). According to Lingard, this infusorium exists as an innocuous parasite in the blood of rats in India. It is not pathogenic, or only feebly so, for the native ox of India, but gives rise to a fatal infectious disease in horses, dogs, and camels.

Another pathogenic micro-organism belonging to the protozoa is the Amœba coli, which is found in great numbers in the large intestine in cases of tropical dysentery and also in liver abscess secondary to this disease.

The rapid progress of our knowledge of the bacteria has been due to the fact that satisfactory methods (staining) have been devised for detecting these minute micro-organisms in the blood and tissues of infected individuals and for cultivating them in artificial media. Unfortunately, these methods have only a limited utility when applied to investigations relating to the protozoa. The bacterial cell has considerable stability, owing to its cellulose envelope (cell wall), and it is readily stained by the aniline dyes. The protozoa, on the contrary, very readily undergo disintegration, and the more fluid protoplasm of these unicellular organisms is not so easy to demonstrate by the usual staining reagents. It has also been found very difficult, and in many cases quite impossible, to obtain pure cultures in artificial media. Again, the recognition of protozoa in the blood of infected animals by means of the microscope requires special skill in making preparations, in the management of the light, etc., and expert knowledge of the normal elements of the blood and of the changes they undergo as a result of various methods of preparation. This is illustrated by the fact that many persons, more or less familiar with the use of the microscope, have failed to discover the malarial parasite in blood which undoubtedly contained it, while others have evidently mistaken vacuoles in normal blood-corpuscles for the Plasmodium, the crenated red corpuscles for pigmented cells, and deformed corpuscles for malarial crescents.

Notwithstanding the painstaking researches which have been made during the past few years for the purpose of determining the nature of certain bodies which may be demonstrated by special staining methods in the cells of carcinomatous tumors, we are still uncertain as to the nature and ætiological import of these bodies. Some investigators believe them to be protozoa, and from their location infer that they are the specific ætiological agents in the development of malignant growths of this character. But, so far as we are informed, this view has not as yet received any very substantial support, and has not been accepted by the leading pathologists of the world.

The presence of amœboid micro-organisms in the contents of