come into general use; he called the outer layer the ectoderm
("outer-skin"), and the inner the entoderm ("inner-skin"). But in 1867
it was shown, particularly by Kowalevsky, from comparative
observation, that even in invertebrates, also, of the most different
classes--annelids, molluscs, echinoderms, and articulates--the body is
developed out of the same two primary layers. Finally, I discovered
them (1872) in the lowest tissue-forming animals, the sponges, and
proved in my gastraea theory that these two layers must be regarded as
identical throughout the animal world, from the sponges and corals to
the insects and vertebrates, including man. This fundamental "homology
[identity] of the primary germinal layers and the primitive gut" has
been confirmed during the last thirty years by the careful research of
many able observers, and is now pretty generally admitted for the
whole of the metazoa.

As a rule, the cells which compose the two primary germinal layers
show appreciable differences even in the gastrula stage. Generally (if
not always) the cells of the skin-layer or ectoderm (Figures 1.36 c
and 1.37 e) are the smaller, more numerous, and clearer; while the
cells of the gut-layer, or entoderm (i), are larger, less numerous,
and darker. The protoplasm of the ectodermic (outer) cells is clearer
and firmer than the thicker and softer cell-matter of the entodermic
(inner) cells; the latter are, as a rule, much richer in yelk-granules
(albumen and fatty particles) than the former. Also the cells of the
gut-layer have, as a rule, a stronger affinity for colouring matter,
and take on a tinge in a solution of carmine, aniline, etc., more
quickly and appreciably than the cells of the skin-layer. The nuclei
of the entoderm-cells are usually roundish, while those of the
ectoderm-cells are oval.