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the germinal layers, and that by differentiation of the cells of these layers are formed all the tissues of the body.
Kowalevsky, of St. Petersburg, found the two primary germinal layers also in Worms, Echinoderms, Articulata, and other animals.
Haeckel, in 1872, found the same in the Sponges. He stated that these two germinal layers occur in all animals, except in the Protozoa; and that they are homologous, or equivalent, in all the groups of animals, from the Sponges up to Man. In 1873, in his 'Gastræa-theorie,' he explained the phylogenetic significance, and tried to show the homology, of the four secondary germinal layers.
