a sufficiently high microscopic power, there will be seen, among the
innumerable multitude of little, circular, discoidal bodies, or
corpuscles, which float in it and give it its colour, a comparatively
small number of colourless corpuscles, of somewhat larger size and very
irregular shape. If the drop of blood be kept at the temperature of the
body, these colourless corpuscles will be seen to exhibit a marvellous
activity, changing their forms with great rapidity, drawing in and
thrusting out prolongations of their substance, and creeping about as if
they were independent organisms.

The substance which is thus active is a mass of protoplasm, and its
activity differs in detail, rather than in principle, from that of the
protoplasm of the nettle. Under sundry circumstances the corpuscle dies
and becomes distended into a round mass, in the midst of which is seen a
smaller spherical body, which existed, but was more or less hidden, in
the living corpuscle, and is called its _nucleus_. Corpuscles of
essentially similar structure are to be found in the skin, in the lining
of the mouth, and scattered through the whole framework of the body.
Nay, more: in the earliest condition of the human organism, in that
state in which it has but just become distinguishable from the egg in
which it arises, it is nothing but an aggregation of such corpuscles,
and every organ of the body was, once, no more than such an aggregation.

Thus a nucleated mass of protoplasm turns out to be what may be termed
the structural unit of the human body. As a matter of fact, the body, in
its earliest state, is a mere multiple of such units; and in its perfect
condition, it is a multiple of such units, variously modified.

But does the formula which expresses the essential structural character
of the highest animal cover all the rest, as the statement of its powers
and faculties covered that of all others? Very nearly. Beast and fowl,
reptile and fish, mollusk, worm, and polype, are all composed of
structural units of the same character, namely, masses of protoplasm
with a nucleus. There are sundry very low animals, each of which,
structurally, is a mere colourless blood-corpuscle, leading an
independent life. But, at the very bottom of the animal scale, even this
simplicity becomes simplified, and all the phenomena of life are
manifested by a particle of protoplasm without a nucleus. Nor are such
organisms insignificant by reason of their want of comple