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.