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The Evolution of Man — Volume 1 by Ernst Heinrich Philipp August Haeckel
page 162 of 358 (45%)
forms of animal life; these remain unicellular throughout life. To
this group belong the amoebae, gregarinae, rhizopods, infusoria, etc.
As their whole organism consists of a single cell, they can never form
germinal layers, or definite strata of cells. But all the other
animals--all the tissue-forming animals, or metazoa, as we call them,
in contradistinction to the protozoa--construct real germinal layers
by the repeated cleavage of the impregnated ovum. This we find in the
lower cnidaria and worms, as well as in the more highly-developed
molluscs, echinoderms, articulates, and vertebrates.

In all these metazoa, or multicellular animals, the chief embryonic
processes are substantially alike, although they often seem to a
superficial observer to differ considerably. The stem-cell that
proceeds from the impregnated ovum always passes by repeated cleavage
into a number of simple cells. These cells are all direct descendants
of the stem-cell, and are, for reasons we shall see presently, called
segmentation-cells. The repeated cleavage of the stem-cell, which
gives rise to these segmentation-spheres, has long been known as
"segmentation." Sooner or later the segmentation-cells join together
to form a round (at first, globular) embryonic sphere (blastula); they
then form into two very different groups, and arrange themselves in
two separate strata--the two primary germinal layers. These enclose a
digestive cavity, the primitive gut, with an opening, the primitive
mouth. We give the name of the gastrula to the important embryonic
form that has these primitive organs, and the name of gastrulation to
the formation of it. This ontogenetic process has a very great
significance, and is the real starting-point of the construction of
the multicellular animal body.

The fundamental embryonic processes of the cleavage of the ovum and
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