which are the direct consequence of physiological principles. What His
takes to be a simple physical process--for instance, the folding of
the germinal layers (in the formation of the medullary tube,
alimentary tube, etc.)--is, as a matter of fact, the direct result of
the growth of the various cells which form those organic structures;
but these growth-motions have themselves been transmitted by heredity
from parents and ancestors, and are only the hereditary repetition of
countless phylogenetic changes which have taken place for thousands of
years in the race-history of the said ancestors. Each of these
historical changes was, of course, originally due to adaptation; it
was, in other words, physiological, and reducible to mechanical
causes. But we have, naturally, no means of observing them now. It is
only by the hypotheses of the science of evolution that we can form an
approximate idea of the organic links in this historic chain.

All the best recent research in animal embryology has led to the
confirmation and development of Baer and Remak's theory of the
germinal layers. One of the most important advances in this direction
of late was the discovery that the two primary layers out of which is
built the body of all vertebrates (including man) are also present in
all the invertebrates, with the sole exception of the lowest group,
the unicellular protozoa. Huxley had detected them in the medusa in
1849. He showed that the two layers of cells from which the body of
this zoophyte is developed correspond, both morphologically and
physiologically, to the two original germinal layers of the
vertebrate. The outer layer, from which come the external skin and the
muscles, was then called by Allman (1853) the "ectoderm" (outer layer,
or skin); the inner layer, which forms the alimentary and reproductory
organs, was called the "entoderm" (= inner layer). In 1867 and the
following years the discovery of the germinal layers was extended to