Picture - Cosmic Polymath

140 DESIGN IN NATURE
and all the actions of plants and animals to cells, appears from this, that they are not agreed as to what consti-
tutes a cell. . 1 rpi, 11
The value of the cell in the vegetable and animal kingdoms cannot, nevertheless, be over estimated, ine cell
in biology forms the structural and physiological centre around which life in all its forms may be said to revolve.
It exhibits in its multifarious changes well-marked lines of communication and force. The cell has a Hterature
of its own, but this is so extensive as to forbid my dealing with more than a fringe of it. I will therefore confine
myself to a consideration of what I regard as the more important parts. I will also limit my observations on the
cell itself to its position as a separate living entity, as a centre of reproduction, nourishment, growth, movement, &c.
A cell may be defined as a microscopic object measuring from the three-thousandth to the four-thousandth of
an inch in diameter, which is variously constituted and shaped. Thus it varies in chemical composition : it also
varies in shape, being round, oval, caudate, branched, polygonal, cyhndrical, stelUform, fusiform, &c. The cell,
as a rule, is round to begin mth, but its shape is modified by pressure and various other conditions.
A typical cell consists of three parts, each of which is porous and permeable, namely, the cell wall or envelope,
the nucleus contained within the cell wall, and the matter contained between the cell wall and the nucleus._ These
FiG. 32.—Typical cell consisting of a cell
wall or envelope, cell contents, a nucleus, and
a nucleolus, a, Cell wall ; b, cell contents ;
c, nucleus ; d, nucleolus. The darts, r, f, represent
the endosmotic or ingoing nutrient
currents, by which the cell is fed ; the darts
g, h, the exosmotic or outgoing currents,
whereby the cell rids itself of waste products
and injiuious substances.
The cell wall, cell contents, nucleus, and
nucleolus are osmotic media, and, when the
cell is placed in suitable fluids, its vital and
mechanical properties are at once evoked
(the Author).
three parts are essentially different in structure and ultimate composition, but
are necessary to each other ;
the cell taking in nourishment, giving out waste
products, reproducing itself, and carrying on all the functions of hfe. The
matter contained between the cell wall and the nucleus is variously designated
protoplasm, cell sap, cell contents, &c. The nucleus is, for the most part,
round or oval. It varies in size, and is sometimes soUd, sometimes hollow,
and sometimes granular. It may contain one or several granules, and these,
when they exist, are known as nucleoh. I subjoin an original sketch of a cell
in which the several points referred to above are indicated (Fig. 32).
The manner in which cells take in nutrient materials and extrude waste
products by the aid of endosmotic and exosmotic currents will be readily
understood by a reference to the subjoined figure, where the darts pointing to
the interior of the cell indicate the ingoing centripetal nourishing currents,
and the darts pointing to the exterior of the cell the outgoing centrifugal
waste -product currents (Fig. 32).
The normal life of the adult cell affords a striking example of vital and
mechanical forces working side by side and in harmony to bring about a
common result. The osmotic currents are essentially and intrinsically mechani-
cal in their nature, but the cell hves, and determines, within limits, the nature
and the extent of the currents. In other words, it selects and takes in by
the endosmotic currents certain materials which it absorbs and assimilates
the assimilated materials being useful and necessary—wliile it rejects and
gives out by the exosmotic currents other substances which are inimical to its well-being and which, if retained,
would prove injurious.
That cells can and do discriminate, and that they select and reject within limits, is proved by the behaviour
of animal cells as a class. Thus, some cells secrete and others excrete : the secreting and excreting cells are supphed
with, and act upon, the same blood. Of the secreting cells some produce saliva, others gastric juice, others bile,
and so on. Of the excreting cells some furnish perspiration, some urine and other waste products. The aggregates
of cells form plants and animals ; they also form the great majority of the vegetable and animal tissues—cellular
tissue, woody fibre, muscle, nerve, bone, hairs, feathers, &c. ;
but these several and diverse structures are the product
of one and the same vegetable sap, or, in the case of animals, one and the same blood, from which it follows that
the several kinds of cells are endowed with special properties to bring about certain predetermined results. In
the economy of cells, the division of labour is carried to an extreme. Cells, as structural units, are invested with
high powers. They hve, grow, and reproduce themselves. They perform the bulk of the work in plants and
animals. They are conditioned, and work to given ends, singly and in combination. Their work is duly appor-
tioned : they do nothing in a hap-hazard way. They reproduce, build up, and keep the organism going in all its
parts. They, however, do this according to a fixed plan and under supervision. Cells can only work within
prescribed Umits. They have no power, in the normal or healthy condition, to change either their constitutions
or the role to be played by them. Least of all can thej^ change or abrogate the function assigned them in the
great scheme of organic nature.
The importance of the cell in life and organisation is universally admitted. Indeed the majority of physio-