The Last Link/Theory of Cells

The vegetable cell was discovered by Schleiden, Professor of Botany at Jena, in 1838. Next year Schwann found the animal cell.

In 1844 Koelliker discovered that the egg cell, by division and multiplication, becomes an aggregation—a heap of new cells.

In 1849 Huxley found the two primary layers (observed long before by Pander and Baer in the chick) also in certain Invertebrata, the Medusæ; and he called these layers 'ectoderm' and 'endoderm' respectively.

In 1851 Remak, in his 'Untersuchungen über die Entwicklung der Thiere,' showed the egg to be a simple cell, and that from it, by repeated division or multiplication, arise ​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.