n
enterocoel (_Quart. Journ. Micr. Sci._, April, 1875).... My theory of
the coelom as an enterocoel was accepted by Balfour and was greatly
strengthened by his observations on the derivation of both notochord and
mesoblastic somites from archenteron in the Elasmobranchs, and by the
publication in 1877 by Kowalevsky of his second paper on the development
of Amphioxus--in which the actual condition which I had supposed to
exist in the Vertebrata was shown to occur, namely, the formation of the
mesoblast as paired pouches in which a narrow lumen exists, but is
practically obliterated on the nipping-off of the pouch from the
archenteron, after which process it opens out again as coelom" (pp.
16-18).

The enterocoelic theory was taken up by O. and R. Hertwig as an
essential part of their _Coelomtheorie_.[447] In a lengthy series of
monographs these workers made a comparative study of the mode of
formation of the middle layer, and arrived at a coherent theory of its
origin. They distinguished in the middle layer two quite distinct
elements, the mesoblast proper, formed by the evagination of the walls
of the archenteron, and the mesenchyme, formed by free cells budded off
from the germ-layers. The following passage gives a good idea of their
views and of the phylogenetic implications involved:--"Ectoblast and
entoblast are the two primary germ-layers which arise from the
invagination of the blastula; they are always the first to be laid down,
and they can be directly referred back to a simple ancestral form, the
Gastraea; they form the limits of the organism towards the exterior and
towards the archenteron. The parietal and visceral mesoblast, or the two
middle layers, are always of later origin, and arise through evagination
or plaiting of the entoblast, the remainder of which can now be
distinguished as secondary entoblast from the primary. They form the
walls of a new cavity, the enterocoel, which is to be regarded as a
nipped-off diverticulum of the archenteron. Just as the two-layered
animals can be derived from the Gastraea, so can the four-layered animals
be derived from a Coelom form. Embryonic cells, which become singly
detached from their epitheliar connections we consider to be something
quite different from the germ-layers, and accordingly we call them by
the special name of mesenchyme germs or primary cells of the mesenchyme.
They may develop both in two-layered and in four-layered animals. Their
function is to f