metabolic power of the cells, (2) the reason why the cells arrange
themselves so as to form an organism of complex and definite structure.
Schwann tries to explain the origin of the "metabolic" action, the
analogy of which with the contact-action of colloidal platinum he
recognises, by attributing it to the peculiar structural arrangements of
molecules. In attempting to account for the harmonious structure of the
organism he points to the analogy of ordinary crystals, which often form
complex and regular tree-like arrangements; plants in particular
resemble these regularly shaped crystal-aggregates.

The whole ingenious theory is offered merely as an hypothesis and a
guide to research. It is interesting as one of the most carefully
thought-out attempts ever made to give a thorough-going materialistic
account of the origin and development of organic form, and it arose
directly out of the cell-theory.

Schleiden and Schwann started out from an erroneous theory of the origin
and development of cells, which impaired to some extent the value of
their results. It was not long, however, before their theory of the
origin of cells by "crystallisation" from an intra- or extra-cellular
cytoblastem was challenged and overthrown, and the generalisation that
cells originate by division from pre-existing cells put in its place.

This was established for plant cells by Meyen, Unger, von Mohl, Naegeli
and Hofmeister in or about the forties.[261] Criticism of the
Schwann-Schleiden theory from the zoological side was suggested by the
study of the segmentation of the ovum--the developmental process in
which the multiplication of cells is most easily observed. The
segmentation of the ovum was well known to Schwann, for the process had
been described in the frog by Prevost and Dumas in 1824,[262] in the frog
and newt by Rusconi,[263] and an elaborate study of the process in the
frog had been made by von Baer.[264] Schwann indeed suspected that there
must be some connection between the segmentation of the ovum and the
formation of cells, but he did not realise that the cellular blastoderm
of the chick was formed by the division or segmentation of the egg-cell.

Segmentation was soon found to be of widespread occurrence. Von Siebold
in 1837 described the process in Entozoa,[265] and in the same year Loven
saw segmentation in _Campanularia_,[266] and Sars in the starfish and in
Nudibranchs.[267]

In 1838 Bischoff[268] observed segmentation