MICHIGAN ACADEMY OF SCIENCE. 
31 
Just as the plasma of the blood forms the special habitat for the 
spirochetes and trypanosomes, so the corpuscles of the blood act the part 
of hosts for other types of protozoa, collectively designated as Hemocy- 
tozoa. The organisms thus brought together because of a common 
habitat may be extremely different in other respects. In a rather rough 
way, they have been separated into a number of genera, the most im- 
portant of which is the genus Plasmodium which includes the malarial 
organisms of man and the similar parasites found in the red blood cells 
of birds, monkeys, bats, etc. These organisms are transmitted by cer- 
tain mosquitoes, the parasite undergoing in the insect host a complex 
cycle of development. An enormous amount of work has been done dur- 
ing the past few years on these and the related parasites, such as the 
Halteridium, Leucocytozoon, Hemogregarina, and Piroplasma. To give 
an adequate account of these studies woqld require considerably more 
time than is available. 
ULTRASOMES. 
Even a short review of the advances which have been made within 
recent years in the study of parasites would, be incomplete without 
some reference to the existence of the so-called invisible or filterable or- 
ganisms. The ordinary bacteria and protozoa are relatively large, and 
can be seen under the microscope by the trained observer. Their size is 
such that, as a rule, they will not pass through the porcelain and other 
filters. For years, it has been customary to speak of bacteria as the 
smallest of living beings but this is now no longer true. We now know 
of the existence of living organisms which are smaller than the smallest 
known bacterial cell; so small, indeed, as to be invisible under the highest 
powers of the microscope. Because of this fact they are often referred 
to as ultramicroscopic. 
A number of very important human diseases have been shown to be due 
to organisms of this type. Mention need only be made of Yellow Fever, 
Acute Anterior Poliomyelitis, Dengue, Pupataci Fever, Foot and Mouth 
Disease, and Rabies. It is quite probable that this list will be extended 
materially in the near future. Similar diseases are known to exist 
among animals; chicken pest, hog cholera and pleuro-pneumonia of cattle 
are examples of this kind. It lias been our good fortune to encounter a 
disease of this nature among out laboratory rats and a brief account of 
several facts recently ascertained will serve to emphasize the general 
character of this type of organisms. 
The rat ultrasome is invariably fatal to this animal but is without 
effect in all other laboratory animals. It is present in the blood in large 
numbers as is indicated by the fact that the injection of a billionth of a 
cubic centimeter (and at times even less than this amount) of the blood 
is sufficient to kill a rat. The smallness is demonstrated by the ease 
with which it passes through the Berkfield, Pasteur-Chamberland and 
Doulton filters. Indeed, it has been possible to pass it through appar- 
ently homogeneous membranes, such as the collodium sac and agar- 
soaked filters. When submitted to centrifugation at a speed of 8,000 
revolutions per minute for 12 hours, even when the serum is diluted with 
a thousand times its volume of distilled water, the organism is not 
thrown down. 
This rat ultrasome is probably an extreme example of this group, and 
