W. T. (William Thompson) Sedgwick.

An introduction to general biology online

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feathers, claws, and beaks of birds ; the
fur of animals — these are a few of the
countless instances of structures com-
posed wholly or in part of lifeless mat-
FiG.5.(AfterRanvier)-Mns-tej. ^j^j^i^ nevertheless enter into the

cle-ceUs. A, from the mtes- ^ ^ ^

tine of a dog, in cross-sec- Composition of living animals.

tion; B, single isolated cell, A,^^^,,^ -.^l^^-t ^'^ £ j.

from the intestine of a rab- Among plants like facts are even

bit, viewed from the side, more COllSpicUOUS. 'No OUC Cail doubt

that the outer bark of an oak is devoid
of life. The heart-wood of a tree is entirely dead, and even
in the so-called live wood, through which the sap flows, not only
is the solid part of the wood lifeless, but also the sap itself.



Fig. 6 (After Schafer.) — Human cartilage (from head of metatarsal bone), c, cells;

ni, lifeless matrix. (X 600.)

Fig. 7. (Modified from Ran vier.)— Blood of frog, showing two forms of eells (cor-
puscles), one flattened and oval, one branched, floating in the lifeless plasma.



Lifeless Matter in the Living Tissues. In the tissues the Hv-
ing cells are seldom in contact one with another, but are more or
less completely separated by partitions of lifeless matter. This
may be seen in a section through some rapidly growing organ
like a young shoot (Fig. 1). The wliole mass is formed of
nearly similar, closely crowded units or cells separated by very
narrow partitions. Each cell consists of a mass of granular,
viscid, living substance known 2,'^ jprotoijlasm^ and a more solid,
rounded body, the micleus.

In such a group of cells no tissues can be distinguished ; or,
rather, the whole mass consists of a "single tissue (meristem),
wliich is almost entirely composed of living matter (protoplasm).
In older tissues the partitions often increase in thickness, as
shown in Fig. 2. In every case the ])artitwns are composed of
lifeless matter which has heen niannfactitred and deposited hy
the living jprotoplasni constituting the hodies of the cells. In
still older parts of the plant certain of the lifeless walls may
become extremely thick, the protoplasm entirely disappears, and

the whole tissue (wood) consists of
Kf eless matter enclosing spaces filled
with air or water (Figs. 3 and 4).

Among animals analogous cases
are common. The nuiscles of the
small intestine, for instance, (Fig.
5,) consist of bundles of elongated
cells {fihres) eacli of which is com-
posed of living matter surrounded
by a very tliin covering (sheath) of
lifeless matter. In cartilao^e or
gristle, which covers tlie ends of
many bones (Fig. 6), the oval cells
are very widely separated by the
deposition between them of large
quantities of solid lifeless matter

Fig. 8. (Modified from Schenk.)— Sec- /? • i < • i ^^

tion of bone from the human femur lOrmmg what IS kuown aS the

showing the living branching bone- matrix. In blood (Fig. 7) the
cells lying inthe bony life less ma- n ,, i . in/

trix. Diagramatic. flattened , or irregular cells {cor-

jpuscles) are separated by a lifeless
fluid {j>lasind) in which they float. In bone (Fig. 8) the cells

^J.::;^]:^:-: '■^t?^l^;^^i^:'^-;/i;^^:[^iyi^



have a branching, irregular form, and are separated by solid
calcareous matter whicli is unmistakably lifeless. These ex-
amples show that the lifeless matters of the body often occur in
the form of deposits between living cells by which tliey have
been produced. In all such cases the embryonic tissue consists
at first of living cells in direct contact, or separated by only a
very snmll quantity of lifeless matter. In later stages the
cells may manufacture additional lifeless substance which
appears in the form of firm partition-walls between the cells,
or as a matrix, solid or liquid, in which the cells lie. AVhen
solid walls are present they are often perforated by narrow clian-
nels through which the protoplasmic cell-bodies remain in con-
nection. (See Figs. 4, 8, and 50.)

Lifeless Matter within Living Cells. Equally important with
the deposit of lifeless matter hetioeen cells is the formation of Kfe-
less matter within cells, either {ct) by the deposition of various sub-
stances in the protoplasm, or (b) by the direct transformation of
the whole mass of protoplasm. Examj)les of the first kind are

Fig. 9.— a group of cells from the stem of a geranium
(Pelargonium), showing lifeless substances (starch
and crystals) within the protoplasm. As in Fig. 2,
each cell contains a large central vacuole, filled
with sap ; c, groups of crystals of calcium oxalate ;
i.e., intercellular space ; n, nucleus ; s, granules of
starch, (x 300.)

Fig. 10. (After Ranvier.) —
Group of "adipose cells"
from the tissue beneath the
skin (" subcutaneous con-
nective tissue") of an em-
bryo calf, showing drops of
fat in the protoplasm. /, fat-
drops (black) ; n, nuclei

mineral crystals (Fig. 9), grains of starch (Fig. 9), drops of
water, and many other substances found witliin tlie cells of
plants. Among animals drops of fat (Fig. 10) and calcareous



or siliceous deposits are similarly produced. Indeed, there is
scarcely any limit to the number of lifeless substances which
may thus appear within the cells both of plants and animals.

The second case is of less importance, though of common
occurrence. A good examj^le is found in the lining membrane
of the cesopliagus of the dog (Fig. 11), which like the human
skin is almost entirely made up of closely crowded cells. Those


Fig. 11.— Section through the inner coat of the gullet of a dog, showing : p, living
cells of the deeper layers ; s, lifeless cells of the superficial layers; 71, nucleus.

in the deepest part consist chiefly of living protoplasm very
similar to that of the young pine shoot (compare Fig. 1).
Above them the cells gradually become flattened until at the
surface they have the form of flat scales. As the cells become
flattened their substance changes. The protoplasm diminishes
in quantity and dies; so that near the surface the cells are
wholly dead, and finally fall oft'. In a similar manner are
formed the lifeless parts of nails, claws, beaks, feathers, and
many related structures. A hair is composed of cells essentially
like those of the skin. At the root of the hair they are alive,
but as they are pushed outwards by continued growth at the
root, they are transformed bodily into a dead, horny substance
forming the free portion of the hair. Feathers are only a com-
plicated kind of hair and are formed in the same way.

It is a significant fact that the quantity of lifeless matter in
the organism tends to increase with age. The very young plant
or animal probably possesses a maximum proportion of proto-
plasm, and as life progresses lifeless matter gradually accumulates
within or about it, — sometimes for support, as in tree-trunks and


bony skeletons ; sometimes for protection as in oyster- and snail
shells ; sometimes apparently from sheer inability on the part of
the protoplasm to get rid of it. Thns we see that youth is lit-
erally the period of life and vigor, and age the period of com-
parative lifelessness.

Summary. The bodies of higher animals and plants are
subdivided into various parts {organs) having different structure
and functions. These mav be resolved into one or more tissues^
each of which consists of a mass of similar cells (or their deriva-
tives) having a similar function. The cells are small masses of
living matter, or protoplasm, which dej)Osit more or less lifeless
matter either around (outside) them or within their substance.
In the former case the protoplasm may continue to live, or it
may die and be absorbed. In the latter case it may likewise live
on for a time, or may die, either disappearing altogether or leav-
ing behind a residue of lifeless matter.

The Organism as a Whole. Up to this point we have con-
sidered living organisms from an anatomical and analytical stand-
point, and have observed their natural subdivisions into organs,
tissues, and cells. We have now only to remark that these parts
are mutually interdependent, and that the organism as a whole
is greater than any of its parts. Precisely as a chronometer is
superior to an aggregate of wheels and springs, so a living organ-
ism is superior in the solidarity of its parts to a mere aggregate of
organs, tissues, and cells. We shall soon see that in the living
body these have had a common ancestry and still stand in the
closest relationship both in respect to structural continuity and
community of interest.



It lias been sliown in the last chapter that life is inherent in
a peculiar substance, protoiylasm^ occurring in definite masses or
cells. In other words, protoj)lasm is the physical basis of life,
and the cell is the ultimate visible structural unit. Protoplasm
and the cell deserve therefore the most careful consideration;
but because of the technical difficulties involved in their study
only such characteristics as are either obvious or indispensable to
the beginner will here be dwelt upon.

Historical Sketch. Organs and tissues are readily visible, but
in order to resolve tissues into cells something more than the
naked eye was necessary. The compound microscope came into
use about 1650, and in 1665 the English botanist Robert Hooke
announced that a familiar vegetal tissue, cork, is made up of
''^little hoxes or cells distinct from one another ^ Many other
observers described similar cells in sections of wood and other
vegetal tissues, and the word soon came into general use. It
was not until 1838, however, and as a consequence of a most
important improvement in the compound microscope, viz., the
invention of the achromatic objective, that cellular structure
came to be recognized as an invariable and fundamental charac-
teristic of li\dng bodies. At this time the botanist Schleiden
brought forward proof that the higher plants do not simply con-
tain cells but are wholly made up of them or their products ; and
about a year later the zoologist Schwann demonstrated that the
same is true of animals. This great generalization, known as
the ' ' cell-theory ' ' of Schleiden and Schwann^ laid the basis for
all subsequent biological study. The cell-theory was at first de-
veloped upon a purely morphological basis. Its application to

the phenomena of physiological action was for a time retarded



hv the misleading character of the term "cell." The word itself
shows that cells were at first regarded as cavities (hke tlie cells
of a honeycomb or of a prison) surrounded by solid walls ; and
even Schleiden and Schwann had no accurate conception of their
true nature. Soon after the promulgation of the cell-theorv,
however, it was shown that both the walls and the cavity miirlit
be wanting, and that therefore the remaining portion, namely,
the protoplasm with its nucleus, must be the active and essential
part. The cell was accordingly defined by Yirchow and Max
Schultze as "a mass of protoplasm surrounding a nucleus," and
in this sense the word is used to-day. ^^ The word cell became
thereafter as inappropriate as it would be if applied to the honey
within the honeycomb or to the living prisoner in a prison -cell,
Nevertheless, by a curious conservatism, the term was and is re-
tained to designate these structures whether occurring in masses,
as segments of the plant or animal body, or leading independent
lives as unicellular organisms.

Protoplasm was observed long before its significance was
understood. The discovery of its essential identity in plants and
animals and, ultimately, the general recognition of the extreme
importance of the role which it everywhere plays, must be reck-
oned as one of the greatest scientific achievements of this cen-
tury. It was Dujardin w^ho in 1835 first distinctly called atten-
tion to the importance of the ''primary animal substance" or
''sarcode" which forms the bodies of the simplest animals.
Without clearly recognizing this substance as the seat of life, or
using the word protoplasm, he nevertheless described it as en-
dowed with the powers of spontaneous movement and con-
tractility. The word protoplasm (rrpcSTOs, first; TrXdajua,
form) was apparently first used for animal substance by Purkinje
in 1839-40, and next by H. von Mold, in 18-16, to designate
the granular viscid substance occurring in plant-cells, although
both workers were ignorant of its full significance. In 1850
Cohn definitely maintained not only that animal sarcode and
vegetal protoplasm were essentially of the same nature, but
also that this substance is the real seat of vitality and hence to
be regarded as the physical basis of life. To Max Schultze

* It is possible that in some of the lowest and simplest organisms even tbe
nucleus may be wanting as a distinctly differentiated body. See p. 193.


(1860) is generally assigned the credit of having finally placed
this conclusion upon a secure basis ; and by him the meaning of
the word Protoplasm was so extended as to include all living
matter, whether animal or vegetal. In this sense the word is
now universally employed.

Appearance and Structure. Protoplasm and cells differ
gi'eatly in appearance in different plants and animals, as well as
in different parts and different stages of development of the
same individual. The appearance of protoplasm and the consti-

,„ ,. tution of the cell are as a rule

m^ >!^>r^\v-.r.T^-sJV most easily made out in very

young structures, such as the
eggs of some animals or in
the cells of young vegetal
slioots. The egg of the star-
fish, for example, (Fig. 12), is
I^^ ^<^'l'- '':■ "^';'V'^' ^'^?-''>^ a sin2:le isolated cell of nearly

tyj^ical form and structure.
It is a minute, nearly spheri-
FiG. i2.-siightiy diagrammatic figure of gal bodv U^ inch diameter)

the egg or ovum of a star-fish, showing the . , . ' ,

structure of a typical ceU. w, membrane; m wllich three parts may be

n, nucleus; p, protoplasm (cytoplasm). disthlguislied, viz. I (1) the

cell-hody^ which forms the bulk of the cell ; (2) the nucleus^ a
rounded vesicular body suspended in the cell -body ; (3) the Tnein-
hrane or cell-icall^ which immediately surrounds the cell-body.
Of these three, the nucleus and cell- body are mainly composed
of protoplasm, while the membrane is a lifeless dej)Osit upon the
exterior. The protoplasm of the cell-body is generally called
cell-plasm, or cytoj>lcts7}i^ that of the nucleus nucleoplasm'j that
is, the living matter of the cell is differentiated into two different
but closely related forms of protoplasm, cytoplasm and nucleo-

The Cytoplasm appears as a clear semifluid or viscid sub-
stance, containing numerous minute granules and of a watery
appearance, though it shows no tendency to mix with water.
Under very high powers of the microscope, especially after treat-
ment with suitable reagents, the clear substance is found to have
a definite structure, the precise nature of which is in dispute.
By some observers it is described as a fibrous meshwork or retic-



Ilium, like a sponge; l)y others as more neai-ly like an emulsion
or foam, consisting of a more solid framework enclosing innu-
merable minute separate spherical cavities tilled with li(piid ; hy
others still as comj)osed of unbranched threads running in all
directions through a more liquid basis ; but its real nature is still

It is evident that the visible structure of protoplasm gives no
hint of its marvellous powers as the seat of vital action, and we
are therefore compelled to infer that it is endowed with a chemi-
cal and molecular constitution extremely complex, and probably
far exceeding in comj)lexity that of any lifeless substance.

The Nucleus is a rounded body suspended in the cell-sul)-
stance ; it is distinguishable from the latter by its higher refrac-
tive powder, and by the intense color it assumes when treated
with staining fluids. It is surrounded by a very thin membrane,
and consists internally of a clear substance {acliromatin)^ through
which extends an irregular network of fibres (cliromath}). It
is especially these fibres which are stained by dyes. In the

Fig. 13. (After Sachs.)— Young growing cells from the extreme tip of a stonewort
{Oiara), m, membrane; ?i, nuclei; p, protoplasm; v, vacuole filled with sap.

meshes of the network is suspended in many cases a second
rounded body known as the nucleolus, which stains even more
deeply than the network itself.

The Membrane or Wall of the cell forms a rather thick sac,


composed of a soft, lifeless material closely surrounding the cell

As a second example we clioose tlie growing jDoint of a com-
mon water-plant (Cliarci)^ Fig. 13. This structure is composed
of cells which are more or less angular in outline as a result of
mutual pressure, but show otherwise an unmistakable similarity
to the egg-cell just described. They differ mainly in the fact
that the protoplasm of the larger cells contains rounded cavities,
known as vacuoles^ filled with sap ((v) ; also in the chemical com-
position cf the cell-walls (here consisting of "cellulose," a sub-
stance of rare occurrence among animals).

Origin of Cells and Genesis of the Body. The body of every
higher plant or animal arises from a single germ -cell (" ^ggt^"^
" spore," etc.) more or less nearly similar to that of the star-
fish, described above, and originally forming a part of tlie parent
body. The germ-cell, therefore, in spite of endless variations in
detail, shows us the model after which all others are built ; for
it gives rise to all the cells of the body by a continued j^rocess
of segmentation as follows :

The first step (Fig. 14) consists in the division of the Qg^g
into two similar halves, which differ from the original cell only
in lacking membranes, both being surrounded by the membrane
of the original cell. Each of the halves divides into two, mak-
ing four in all ; these again into two, making eight, and so on
throughout the earlier part of the development. By this jDrocess
(known as the cleavage or segtnentation of the ^g^ the germ-
cell gives rise successively to 2, 4, 8, 16, 32, 64, etc., de-
scendants, forming a primitive body composed of a mass of
nearly similar cells, out of which, by still further division and
growth, the fully-formed body of the future animal is to be
built up. These cells are only slightly modified, but differ in
most animals from the typical germ-cell in having at first no sur-
rounding membranes. The membrane of the original germ-
cell meanwhile disappears.

^ The word cell lias been used in Chap. I and elsewhere to denote the
living matter within the membrane, the latter being considered a product of
the cell rather than an integral part of it. It is more usual to include the
membrane in a definition of the cell, and as a matter of convenience it is so
included here.

pupLRii imtARr


The enibryoiiic body or emhryo of every liiglier })lant and ani-
mal is derived from the germ-cell by a process essentially like that
just described, though both the form of the cells and the order of
division are usually more or less irregular. In animals the cells

Fig. 14.— Cleavage or segmentation of an ovum, showing successive division of the
germ-cell (a) into two (b), four (c), and eight (r/). Later stages are shown ate
and /. The first four figures are diagrammatic ; e and / are after Hatschek's fig-
ures of the development of a very simple vertehrate {AmpMoxxui).

thus formed are usually naked at first, though they often ac-
quire a membrane in later stages. Among plants, on the con-
trary, the cells usually possess membranes from the first, prob-
ably because their need for a firm outer support is greater than
the need for free movement demanded by animals. -

Modification of the Embryonic Cells. Differentiation. The
close similarity of the embryonic cells does not long persist. As
development proceeds, the cells continually increasing in munber
by division become modified in difl:'erent ways, or fliffereiifiatefl^
to fit them for the many different kinds of work which they have
to do. Those which are to become muscle-cells gradually assume
an entirely different form and structure from those which are to
become skin-cells; and the future nerve- or gland-cells take
on still other forms and structures. The embrvonic cells are
gradually converted into the elements of the difi'ereiit tissues —
this process being the differentiation of the tissues which has

* For a more precise account of cell-division see p. 83.


already been mentioned on p. 11 — and are in tins way enabled
to effect a physiological division of labor.

Tlie variations in form and structure which thus appear are
endlessly diversified. Cells may assume almost any conceivable
form, and there are even cells (e.g., Amoebce^ or the colorless
corpuscles of the blood) which continually change their form
from moment to moment. The variations in structure may in-
volve any or all of the three characteristic j^arts of the typical
cell, being at the same time accompanied by variations of form.
It is easy to understand, therefore, how cells may vary endlessly
in appearance, wliile conforming more or less closely to the same
general type.

Meanwhile the protoplasm itself undergoes extensive altera-
tion. Even in young cells, or in the germ-cell itself, it may
=ssF7;;^Sn:i?^fSmn^iv-^ contain an admixture of other substances,

lS^li;,;;;;;;;iuu,',i^;;;;;;:;;iii;; and these may entirely change their

ii^/ji^a, iiiHWjiiifiiv^i : ,,,,,1^ character or (as is especially common in

'IttfSiitifHS^^ plant-cells) may become more abun-

dant as the cell grows older, taking tlie
^?^'SI;if;;" shape of fluid, solid, or even gaseous de-


'Prnvtsmm-rrt, ..."



Jf;i.;,-';:,„r,.,rt»:!'PaTfyo,; 'y,! W'iMiMifflii

,.„ , posits. Common examples of such de-

^^^^^^^maftmn^m posits are drops of water, oil, and resin,
^mmmm0mmmmmm': granules oi piscment, starcli, and solid

mf^mmmm^mkr proteid matters, and crystals of mineral

Fig. 15. (After Ranvier.)— ^ im i •

Part of a single fibre of vol- substaiiccs like calciuiii Oxalate, phos-
nntary muscle from the leg .^^^ ^^^^ carbonate, and siKca. Bub-

of a rabbit, p, protopiasin ; i: '

71, nucleus. (x700.) blcs of gas somctimcs appear in the pro-

toplasm, but this is exceptional. The living substance itself
often changes in appearance as the cells become differentiated.
The protoplasm of voluntary muscles (Fig. 15) is firm, clear,
non-granular, highly refractive, and arranged in alternating
bands or stripes of darker and lighter substance. In some cases
(e.g., the outer portions of the skin, or of a hair, as explained
in Chap. II) the modifications of tlie cell-substance becomes so
great that both its physical and chemical constitution are entirely
altered, and it is no longer protoplasm, but some form of lifeless

Protoplasm in Action. "We may now briefly consider proto-
plasm from the dynamical or physiological point of view. We



know that living tilings are the seat of active changes, wliicli
taken together constitute tlieir life. In tlie last analysis tliese
changes are undoubtedly chemical actions taking ])lace in the
protoplasm, which may or may not produce visible i-esults.
There is no doubt that extensive and probably very complex
molecular actions go on in the protoplasm of young growing
cells, tliough it may appear absolutely quiescent to the eye, even
under a powerful microscope. In other cases, the chemical
action produces perceptible clianges in the protoplasm, — for in
stance, some form of motion, — just as the invisible chemical
action in an electrical battery may be made to produce visible
effects (light, locomotion, etc.) through the agency of an electrical

A familiar instance of protoplasmic movement is the contrac-
tion of a muscle. This process is most likely a change of molec-
ular arrangement, causing the nmscle, while keeping its exact

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Online LibraryW. T. (William Thompson) SedgwickAn introduction to general biology → online text (page 3 of 20)