organisation of the coelomaria, I was unable to deal satisfactorily
with the difficult question of the mode of their origin. This was done
eight years afterwards by the brothers Oscar and Richard Hertwig in
their careful and extensive comparative studies. In their masterly
Coelum Theory: An Attempt to Explain the Middle Germinal Layer [not
translated] (1881) they showed that in most of the metazoa, especially
in all the vertebrates, the body-cavity arises in the same way, by the
outgrowth of two sacs from the inner layer. These two coelom-pouches
proceed from the rudimentary mouth of the gastrula, between the two
primary layers. The inner plate of the two-layered coelom-pouch (the
visceral layer) joins itself to the entoderm; the outer plate
(parietal layer) unites with the ectoderm. Thus are formed the
double-layered gut-wall within and the double-layered body-wall
without; and between the two is formed the cavity of the coelom, by
the blending of the right and left coelom-sacs. We shall see this more
fully in Chapter 1.10.

The many new points of view and fresh ideas suggested by my gastraea
theory and Hertwig's coelom theory led to the publication of a number
of writings on the theory of germinal layers. Most of them set out to
oppose it at first, but in the end the majority supported it. Of late
years both theories are accepted in their essential features by nearly
every competent man of science, and light and order have been
introduced into this once dark and contradictory field of research. A
further cause of congratulation for this solution of the great
embryological controversy is that it brought with it a recognition of
the need for phylogenetic study and explanation.

Interest and practice in embryological research have been remarkably
stimulated during the past thirty years by this appreciation of