THE Microscope. 267 
its flow of protoplasm and leaves it behind. There must be 
chemical products present for the purpose of dissolving and ef- 
fecting changes in this raw material taken as food. These must 
be regarded as secretions. The ameceba is certainly excretory. 
In man, the digestive, urinary, and pulmonary tracts, and the 
epithelia represent this physiological property of the amceba. 
It is further noticed that the amcebais metabolic, constantly 
undergoing chemical change. Certain cells in the human body 
are specially reserved for carrying on chemical changes. Their 
material is derived from the blood and their products are finally 
returned to it. Such cells are the fat cells, liver cells, also the 
lymphatic and ductless glands and in one sense, the blood. 
The ameeba is also reproductive. After attaining a certain 
size or living a longer or shorter life, it may by division resolve 
itself into two parts, each of which is capable of living as a 
complete unit. The amceba divides by becoming constricted in 
its centre, by its protoplasm flowing in opposite directions, or by 
a pseudopodium detaching itself from the body of the cell. Cer- 
tain cells are set apart in the human body for the accomplish- 
ment of this purpose. Such collections of cells are the ovary 
and the testis. The amceba, therefore, has the following prop- 
erties: 
It is contractile. 
It is irritable and automatic. 
It is secretory and excretory. 
It is metabolic. 
It is reproductive. 
Man, then, is but afederation of amcebiform units. Certain 
of these units have been exclusively set apart for the manifes- 
tation of certain of the properties of protoplasmic matter. These 
groups have received the name of ¢ésswes. With this grouping 
there has come a change in structure in order that the part 
might better perform its function. At one period in the history 
of these cells they were as simple as our amoeboid unit ; in fact, 
for that matter, at an early period in the history of every life 
the whole being, the embryo, was but a mass of units as simple 
in their structure as theamoeba. Suchcellsremain in this ame- 
boid condition in the body for a considerable time; such cells 
