Our subject falls naturally into two main periods.
The first dates from the discovery of the cell in 1665 to the foundation
of the Cell Theory in 1839. It is a period full of interest,
during which microscopists were constantly learning more and more
regarding the intimate structure of plants and animals,so that ultimately
it became possible to say definitely what were the elementary
fundamental units of the structure of living organisms. It is
thus an important period,although the significance of the observations
made during it were often obscured by fantastic representation.
The second period ranges from the foundation of the Cell Theory to
the present day. During it the cell was studied as an individual,and
it is during this period that our knowledge of its function has
evolved. Lines of study have inevitably become more and more widely
separated as each different cell -organ and each different function
has come into prominence,so that there has developed around nearly
every branch of the subject a special bibliography.
In endeavouring to unravel this mass of material no
strictly systematic order has been adhered to,the various subdivisions being treated in a sequence as natural as possible. Thus,after
considering the nucleus and cell- division,we have turned to the work
on the structure of protoplasm; the theories of the ultramicroscopic
structure of protoplasm have led us on to investigate the theories
of heredity:,and while on the subject we have found it opportune to
investigate the other cell- organs. The vital phenomena of the assimilation
of the cell have next attracted our attention; from it we
have turned to the process of secretion; and lastly we have considered
that function which,above all others,seems to be a pressing
question of the day - the phenomena of growth and the laws regulating
The writer offers no apology for having considered
some subjects more fully than others. Subjects which have attracted
most. attention in the past have been most fully investigated. While
the physical structure of protoplasm was a subject of great interest
fifty years ago,very little was done to discover its chemical composition
and the mechanism of its vital activities until much later.
So it happens that while we have fully investigated the former we
have scarcely mentioned the latter.