n might be
masked by cenogenetic modifications caused chiefly by the presence of
yolk. The gastrula stage was the palingenetic repetition of the
ancestral form of all Metazoa, the Gastraea.

From the Gastraea theory there followed at once two consequences, (1)
that ectoderm and endoderm, invagination-cavity (_Urdarm_) and
gastrula-mouth (_Urmund_ or _Protostoma_), were, with all their
derivatives, homologous, because homogenous, throughout the Metazoa, and
(2) that the descent of the Metazoa had been monophyletic, since all
were derived from the ancestral Gastraea. Huxley's suggestion (_supra_,
p. 208) that the outer and inner layers in Coelentera were homologous
with the ectoderm and endoderm of the germ was thus fully confirmed and
greatly extended.

The great importance of the Gastraea theory lay in the fact that it
linked up, by means of the biogenetic law, the germ-layer theory with
the doctrine of evolution. It supplied an evolutionary interpretation of
the earliest and most important of embryogenetic events, the process of
layer-formation. Upon the Gastraea theory or its implications were
founded most of the phylogenetic speculations which subsequently
appeared.

Upon the Gastraea theory Haeckel based a system of phylogenetic
classification which was intended to replace Cuvier's and von Baer's
doctrine of Types. This took the form of a monophyletic ancestral tree.
Its main outlines are given on p. 290 in graphic form, combined and
modified from the table on p. 53 of the 1874 paper and the genealogical
tree given in the _Kalkschwaemme_.[434]

_Monophyletic Genealogical Tree of the Animal Kingdom, based upon the
Gastraea Theory and the Homology of the Germ Layers_.

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| m | _Arthropoda_. | | e |
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| d |_Echinoderma_. | | _Mollusca_. | |
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| n | | | Sagi