etazoa
was a two-layered closed sac formed typically by delamination, less
often by invagination. He denied that the invagination opening (which he
named the blastopore) represented the primitive mouth,[443] holding that
this was typically formed by an "inruptive" process at the anterior end
of the planula, which led to the formation of a "stomodaeum." A similar
process at the posterior end gave rise to the anus and the "proctodaeum."

The question as to whether delamination or invagination was to be
considered the more primitive process was discussed in detail by
Balfour,[444] without, however, any very definite conclusion being
reached. He held that both processes could be proved in certain cases to
be purely secondary or adaptive, and that accordingly there was nothing
to show that either of them reproduced the original mode of transition
from the Protozoa to the ancestral two-layered Metazoa (p. 342). He by
no means rejected the theory that the Gastraea, "however evolved, was a
primitive form of the Metazoa," but, having regard to the great
variations shown in the relation of the blastopore to mouth and anus
(pp. 340-1), he was inclined to think that if the gastrula had any
ancestral characters at all, these could only be of the most general
kind. Balfour's attitude perhaps best represents the general consensus
of opinion with regard to the Gastraea theory.

From the same origins as the Gastraea theory arose the theory of the
coelom. The term dates back to Haeckel in 1872, and the observations
which first led up to the theory were made by the men who supplied the
foundations of the Gastraea theory--A. Agassiz, Metschnikoff and
Kowalevsky. But it was not Haeckel himself who enunciated the coelom
theory.

It will be remembered that Remak introduced in 1855 the conception of
the mesoderm as an independent layer derived from the endoderm. The
pleuro-peritoneal or body-cavity was formed as a split in the "ventral
plates" of the mesoderm. Haeckel's "coelom" corresponded to the
"pleuro-peritoneal cavity" of Remak, but his view of the origin of the
mesoderm brought him much closer to von Baer's conception of the origin
of _two_ secondary layers from ectoderm and endoderm respectively than
to Remak's conception of the mesoderm as a single independent layer.

Much uncertainty reigned at the time as to the exact manner of origin of
the mesoderm;[445] some held that it developed from the ectoderm, others
that it originated