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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 complexity.


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