512 The American Naturalist. [June, 
to make up muscular tissue, where their united contraction causes 
movements of organs of which they may forma part. This 
movement is, of course, not voluntary with the cell; but there are 
certain cells, even in the bodies of the most highly differentiated 
animals, which still possess this power of acting voluntarily 
without external stimuli. 
If a drop of blood be taken from the body of any of the ver- 
tebrated animals, with one or two exceptions, and the microscope 
again called into use, its fluid part will be seen to be filled with 
motionless, pale-yellow discs. Among these red blood corpuscles 
will be seen, here and there, transparent bodies, larger or smaller 
than these, as the case may be, which we recognize by their slow 
movement as amceboid cells. These are the white corpuscles of 
the blood. In lymphoid tissue, such as that of the spleen, and small 
lymph nodules occurring in different parts of the body, larger 
amceboid cells of the same nature are collected in great masses, 
which are richly supplied from the general vascular system. 
Though these amceboid cells in the blood are called white cor- 
puscles, and in the lymph glands lymph corpuscles, and so on, 
they are all included in the general term “leucocytes.” Their func- 
tions in these different tissues will be more readily understood if we 
first examine cells of the same nature in some of the lower forms 
of animal life; for in these their habits and relations to the bodies 
of which they are a part are more easily observed and classified. 
The student of biology will recognize a distinct morphological 
bearing in the observations about to be recorded concerning these 
wandering cells. Our knowledge of them has come mainly | 
through a study of the embryonic development of lower animals, 
and foremost among investigators of this subject must be placed 
the name of the great Russian morphologist, Metschnikoff. It 
was he who first noticed their similarity of function in the whole 
animal series, and he formulated his great mass of observations in. 
what is called his phagocyte theory. Almost all the examples to 
be given here are the results of his researches. On account of 
their very great interest to all classes, these observations are very 
widely known, and we may briefly consider a few of them. In 
so doing, also, we will be better able to comprehend the important 
