orm between the epithelial limiting layers a secreted
tissue (_Secretgewebe_) or connective tissue with scattered cells, which
cells can undergo, like the epithelial elements, the most varied
modifications.... This secreted tissue in its simple or in its
differentiated state, with all its derivatives, we call the mesenchyme"
(p. 122).

The important point for us is that, just as all Metazoa were considered
by Haeckel to be descended from the Gastraea, so all Coelomati were held
by the Hertwigs to be derived from an original coelomate _Urform_. In
both cases an embryological archetype becomes a hypothetical ancestral
form.

The Coelom theory was considerably modified, extended and developed by
later workers, particularly as regards the relations to the coelom of
the genital organs and ducts and the nephridia, but no special
methodological interest attaches to these further developments.[448] We
shall here focus attention upon one interesting line of speculation
followed out in this country particularly by Sedgwick--the theory of the
Actinozoan ancestry of segmented animals. Its relation to the Coelom
theory lies in the fact that Sedgwick regarded the segmentation of the
body as moulded upon the segmentation of the mesoblast, which in its
turn, as Kowalevsky and Hatschek had shown, was a consequence of its
mode of origin as a series of pouches of the archenteron. In other
respects Sedgwick's speculations link on more closely to the Gastraea
theory, for one of his main contentions is that the blastopore or
_Urmund_ is homologous throughout at least the three metameric phyla. In
following up Balfour's observations on the development of
_Peripatus_,[449] Sedgwick was struck with the close resemblance existing
between the elongated slit-like blastopore of this form (giving rise to
both mouth and anus), with its border of nervous tissue, and the
slit-like mouth of the Actinozoan (functioning both as mouth and anus),
round which, as the Hertwigs had shown, there lies a special
concentration of nerve cells and nerve fibres. He found another point of
resemblance in the gastric pouches of the Actinozoa, which he
homologised directly with the enterocoelic pouches of the Coelomati. He
was led to enunciate the following theses:--[450] (1) that the mouth and
anus of Vermes, Mollusca, Arthopoda, and probably Vertebrata, is derived
from the elongated mouth of an ancestor resembling the Actinozoa; (2)
that somites are derived from a se