ll and cell-contents, show the same relations and
behave in the same manner in these two types of animal cells as in the
plant-cells studied by Schleiden. The types of cell which he chose for
this comparison are the most plant-like of all animal cells, and he was
even able to point to a thickening of the cell-wall in certain cartilage
cells, analogous to the thickening which plays so important a part in
the outward modification of plant-cells. The analogy indeed in structure
and development between chorda and cartilage cells and the cells of
plants seemed to him complete. The substance of the notochord consisted
of polyhedral cells having attached to their wall an oval disc similar
in all respects to the nucleus of the plant-cell, and like it containing
one or more nucleoli. Inside the mother-cell were to be found young
developing cells of spherical shape, lacking however a nucleus.
Cartilage was even more like plant tissue. It was composed of cells,
each with its cell membrane. The cells lay close to one another,
separated only by their thickened cell-wall and the intercellular
matrix, showing thus even the general appearance of the cellular tissue
of plants. They contained a nucleus with one or two nucleoli, and the
nucleus was often resorbed, as in plants, when the cell reached its full
development. Other nuclei were in many cases present in the cell, round
which young cells could be seen to develop, in exactly the same manner
as in plants. These nuclei had accordingly the same significance as the
nuclei of plants, and deserved the same name of cytoblasts or
cell-generators. The true nucleus of the cartilage cell was probably in
the same way the original generator of the mother-cell.

Having proved the identity in structure and function of the cells of
these selected tissues with the cells of plants, as conceived by
Schleiden, Schwann had still to show that the generality of animal
tissues consisted either in their adult or in their embryonic state of
similar cells. This demonstration occupies the second and longest
section of his book.

His method is throughout genetic; he seeks to show, not so much that all
animal tissues are actually in their finished state composed of cells
and modifications of cells, as that all tissues, even the most complex,
are developed from cells analogous in structure and growth with the
cells of plants.

All animals develop from an ovum; it was his first task to discover
whether the ov