other groups of the invertebrates. In particular, the indefatigable
Russian zoologist, Kowalevsky, found them in all the most diverse
sections of the invertebrates--the worms, tunicates, echinoderms,
molluscs, articulates, etc.

In my monograph on the sponges (1872) I proved that these two primary
germinal layers are also found in that group, and that they may be
traced from it right up to man, through all the various classes, in
identical form. This "homology of the two primary germinal layers"
extends through the whole of the metazoa, or tissue-forming animals;
that is to say, through the whole animal kingdom, with the one
exception of its lowest section, the unicellular beings, or protozoa.
These lowly organised animals do not form germinal layers, and
therefore do not succeed in forming true tissue. Their whole body
consists of a single cell (as is the case with the amoebae and
infusoria), or of a loose aggregation of only slightly differentiated
cells, though it may not even reach the full structure of a single
cell (as with the monera). But in all other animals the ovum first
grows into two primary layers, the outer or animal layer (the
ectoderm, epiblast, or ectoblast), and the inner or vegetal layer (the
entoderm, hypoblast, or endoblast); and from these the tissues and
organs are formed. The first and oldest organ of all these metazoa is
the primitive gut (or progaster) and its opening, the primitive mouth
(prostoma). The typical embryonic form of the metazoa, as it is
presented for a time by this simple structure of the two-layered body,
is called the gastrula; it is to be conceived as the hereditary
reproduction of some primitive common ancestor of the metazoa, which
we call the gastraea. This applies to the sponges and other zoophyta,
and to the worms, the mollusca, echinoderma, articulata, and
vertebrata. All these animals may be comprised under the general