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112 the homology of the two primary layers, ecto- and endoderm, throughout the Metazoa. The attempt to do the same for the four secondary layers, as made in the second part of his 'Gastræa-theory,' failed. It caused an enormous amount of research, hitherto without a satisfactory solution of the problem.

'Studien zur Gastræa-Theorie.' Jena, 1874. The transformation of the single primitive egg-cell by cleavage into a globular mass of cells (Morula)—which latter, becoming hollow (and then known as the Blastula), turns ultimately by invagination or by delamination into the Gastrula—is a series of processes which applies to all Metazoa. The Gastrula is, therefore, the ancestral form of the Metazoa; and the Gastræa-theory, founded by Haeckel, throws light, on the one hand, upon the mystery of the phyletic connection of the various animal groups, while, on the other hand, it connects the Metazoa, or multicellular organisms, with the lowest Protozoa. We come to this conclusion be-