the permanent kidneys. Professor E. Ray-Lankester suggested three
years afterwards (1875) the name archenteron for the primitive gut,
and blastoporus for the primitive mouth.)

(FIGURE 1.36. Gastrula of a lower sponge (olynthus). A external view,
B longitudinal section through the axis, g primitive-gut cavity, a
primitive mouth-aperture, i inner cell-layer (entoderm, endoblast,
gut-layer), e external cell-layer (outer germinal layer, ectoderm,
ectoblast, or skin-layer).

The two layers of cells which line the gut-cavity and compose its wall
are of extreme importance. These two layers, which are the sole
builders of the whole organism, are no other than the two primary
germinal layers, or the primitive germ-layers. I have spoken in the
introductory section (Chapter 1.3.) of their radical importance. The
outer stratum is the skin-layer, or ectoderm (Figures 1.30 to 1.35 e);
the inner stratum is the gut-layer, or entoderm (i). The former is
often also called the ectoblast, or epiblast, and the latter the
endoblast, or hypoblast. FROM THESE TWO PRIMARY GERMINAL LAYERS ALONE
IS DEVELOPED THE ENTIRE ORGANISM OF ALL THE METAZOA OR MULTICELLULAR
ANIMALS. The skin-layer forms the external skin, the gut-layer forms
the internal skin or lining of the body. Between these two germinal
layers are afterwards developed the middle germinal layer (mesoderma)
and the body-cavity (coeloma) filled with blood or lymph.

The two primary germinal layers were first distinguished by Pander in
1817 in the incubated chick. Twenty years later (1849) Huxley pointed
out that in many of the lower zoophytes, especially the medusae, the
whole body consists throughout life of these two primary germinal
layers. Soon afterwards (1853) Allman introduced the names which have