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.
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