true, quite
different in shape and consistency from crystals, and they grow by
intussusception, not by apposition--their plastic or attractive forces
seem therefore to be different. A still more important difference is
that the metabolic force is peculiar to the cell. Yet there are
important analogies between crystals and cells. They agree in the
important respect that they both grow in solutions at the cost of the
dissolved substance, according to definite laws, and develop a definite
and characteristic shape. It might even be maintained, Schwann thinks,
that the attractive force of crystals is really identical with that of
cells, and that the difference in result is due merely to the difference
between the substance of the cell and the substance of the crystal. He
points out how organic bodies are remarkable for their powers of
imbibition, and he seeks to show that the cell is the form under which a
body capable of imbibition must necessarily crystallise, and that the
organism is an aggregate of such imbibition-crystals. The analogy
between crystallisation and cell-formation he works out in the following
manner:--"The substance of which cells are composed possesses the power
of chemically transforming the substance with which it is in immediate
contact, in somewhat the same way as the well-known preparation of
platinum changes alcohol into acetic acid. Each part of the cell
possesses this property. If now the cytoblastem is altered by an already
formed cell in such a way that a substance is formed that cannot become
part of the cell, it crystallises out first as the nucleolus of a new
cell. This in its turn alters the composition of the cytoblastem. A part
of the transfomed substance may remain in solution in the cytoblastem or
may crystallise out as the beginning of a new cell; another part, the
cell-substance, crystallises round the nucleolus. The cell-substance is
either soluble in the cytoblastem and crystallises out only when the
latter is saturated with it, or it is insoluble and crystallises as soon
as it is formed, according to the aforementioned laws of the
crystallisation of imbibition-bodies; it forms thus one or more layers
round the nucleolus, etc. If one imagines cell-formation to take place
in this way, one is led to think of the plastic force of the cell as
identical with the force by means of which a crystal grows" (pp.
249-50).

Two difficulties have to be faced by this theory--(1) the origin of the