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Ernst Heinrich Philipp August Haeckel.

The Evolution of Man — Volume 1 online

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FIGURE 1.5. Ten liver-cells: one of them (b) has two nuclei.)

The cells also differ very much in size. The great majority of them
are invisible to the naked eye, and can be seen only through the
microscope (being as a rule between 1/2500 and 1/250 inch in
diameter). There are many of the smaller plastids - such as the famous
bacteria - which only come into view with a very high magnifying power.
On the other hand, many cells attain a considerable size, and run
occasionally to several inches in diameter, as do certain kinds of
rhizopods among the unicellular protists (such as the radiolaria and
thalamophora). Among the tissue-cells of the animal body many of the
muscular fibres and nerve fibres are more than four inches, and
sometimes more than a yard, in length. Among the largest cells are the
yelk-filled ova; as, for instance, the yellow "yolk" in the hen's egg,
which we shall describe later (Figure 1.15).

Cells also vary considerably in structure. In this connection we must
first distinguish between the active and passive components of the
cell. It is only the former, or active parts of the cell, that really
live, and effect that marvellous world of phenomena to which we give
the name of "organic life." The first of these is the inner nucleus
(caryoplasm), and the second the body of the cell (cytoplasm). The
passive portions come third; these are subsequently formed from the
others, and I have given them the name of "plasma-products." They are
partly external (cell-membranes and intercellular matter) and partly
internal (cell-sap and cell-contents).

The nucleus (or caryon), which is usually of a simple roundish form,
is quite structureless at first (especially in very young cells), and
composed of homogeneous nuclear matter or caryoplasm (Figure 1.2 k).
But, as a rule, it forms a sort of vesicle later on, in which we can
distinguish a more solid nuclear base (caryobasis) and a softer or
fluid nuclear sap (caryolymph). In a mesh of the nuclear network (or
it may be on the inner side of the nuclear envelope) there is, as a
rule, a dark, very opaque, solid body, called the nucleolus. Many of
the nuclei contain several of these nucleoli (as, for instance, the
germinal vesicle of the ova of fishes and amphibia). Recently a very
small, but particularly important, part of the nucleus has been
distinguished as the central body (centrosoma) - a tiny particle that
is originally found in the nucleus itself, but is usually outside it,
in the cytoplasm; as a rule, fine threads stream out from it in the
cytoplasm. From the position of the central body with regard to the
other parts it seems probable that it has a high physiological
importance as a centre of movement; but it is lacking in many cells.

The cell-body also consists originally, and in its simplest form, of a
homogeneous viscid plasmic matter. But, as a rule, only the smaller
part of it is formed of the living active cell-substance (protoplasm);
the greater part consists of dead, passive plasma-products
(metaplasm). It is useful to distinguish between the inner and outer
of these. External plasma-products (which are thrust out from the
protoplasm as solid "structural matter") are the cell-membranes and
the intercellular matter. The internal plasma-products are either the
fluid cell-sap or hard structures. As a rule, in mature and
differentiated cells these various parts are so arranged that the
protoplasm (like the caryoplasm in the round nucleus) forms a sort of
skeleton or framework. The spaces of this network are filled partly
with the fluid cell-sap and partly by hard structural products.

(FIGURE 1.6. Nine star-shaped bone-cells, with interlaced branches.

FIGURE 1.7. Eleven star-shaped cells from the enamel of a tooth,
joined together by their branchlets.)

The simple round ovum, which we take as the starting-point of our
study (Figures 1.1 and 1.2), has in many cases the vague, indifferent
features of the typical primitive cell. As a contrast to it, and as an
instance of a very highly differentiated plastid, we may consider for
a moment a large nerve-cell, or ganglionic cell, from the brain. The
ovum stands potentially for the entire organism - in other words, it
has the faculty of building up out of itself the whole multicellular
body. It is the common parent of all the countless generations of
cells which form the different tissues of the body; it unites all
their powers in itself, though only potentially or in germ. In
complete contrast to this, the neural cell in the brain (Figure 1.9)
develops along one rigid line. It cannot, like the ovum, beget endless
generations of cells, of which some will become skin-cells, others
muscle-cells, and others again bone-cells. But, on the other hand, the
nerve-cell has become fitted to discharge the highest functions of
life; it has the powers of sensation, will, and thought. It is a real
soul-cell, or an elementary organ of the psychic activity. It has,
therefore, a most elaborate and delicate structure. Numbers of
extremely fine threads, like the electric wires at a large telegraphic
centre, cross and recross in the delicate protoplasm of the nerve
cell, and pass out in the branching processes which proceed from it
and put it in communication with other nerve-cells or nerve-fibres (a,
b). We can only partly follow their intricate paths in the fine matter
of the body of the cell.

Here we have a most elaborate apparatus, the delicate structure of
which we are just beginning to appreciate through our most powerful
microscopes, but whose significance is rather a matter of conjecture
than knowledge. Its intricate structure corresponds to the very
complicated functions of the mind. Nevertheless, this elementary organ
of psychic activity - of which there are thousands in our brain - is
nothing but a single cell. Our whole mental life is only the joint
result of the combined activity of all these nerve-cells, or
soul-cells. In the centre of each cell there is a large transparent
nucleus, containing a small and dark nuclear body. Here, as elsewhere,
it is the nucleus that determines the individuality of the cell; it
proves that the whole structure, in spite of its intricate
composition, amounts to only a single cell.

(FIGURE 1.8. Unfertilised ovum of an echinoderm (from Hertwig). The
vesicular nucleus (or "germinal vesicle") is globular, half the size
of the round ovum, and encloses a nuclear framework, in the central
knot of which there is a dark nucleolus (the "germinal spot").

FIGURE 1.9. A large branching nerve-cell, or "soul-cell," from the
brain of an electric fish (Torpedo), magnified 600 times. In the
middle of the cell is the large transparent round nucleus, one
nucleolus, and, within the latter again, a nucleolinus. The protoplasm
of the cell is split into innumerable fine threads (or fibrils), which
are embedded in intercellular matter, and are prolonged into the
branching processes of the cell (b). One branch (a) passes into a
nerve-fibre. (From Max Schultze.))

In contrast with this very elaborate and very strictly differentiated
psychic cell (Figure 1.9), we have our ovum (Figures 1.1 and 1.2),
which has hardly any structure at all. But even in the case of the
ovum we must infer from its properties that its protoplasmic body has
a very complicated chemical composition and a fine molecular structure
which escapes our observation. This presumed molecular structure of
the plasm is now generally admitted; but it has never been seen, and,
indeed, lies far beyond the range of microscopic vision. It must not
be confused - as is often done - with the structure of the plasm (the
fibrous network, groups of granules, honey-comb, etc.) which does come
within the range of the microscope.

But when we speak of the cells as the elementary organisms, or
structural units, or "ultimate individualities," we must bear in mind
a certain restriction of the phrases. I mean, that the cells are not,
as is often supposed, the very lowest stage of organic individuality.
There are yet more elementary organisms to which I must refer
occasionally. These are what we call the "cytodes" (cytos = cell),
certain living, independent beings, consisting only of a particle of
plasson - an albuminoid substance, which is not yet differentiated into
caryoplasm and cytoplasm, but combines the properties of both. Those
remarkable beings called the monera - especially the chromacea and
bacteria - are specimens of these simple cytodes. (Compare Chapter
2.19.) To be quite accurate, then, we must say: the elementary
organism, or the ultimate individual, is found in two different
stages. The first and lower stage is the cytode, which consists merely
of a particle of plasson, or quite simple plasm. The second and higher
stage is the cell, which is already divided or differentiated into
nuclear matter and cellular matter. We comprise both kinds - the
cytodes and the cells - under the name of plastids ("formative
particles"), because they are the real builders of the organism.
However, these cytodes are not found, as a rule, in the higher animals
and plants; here we have only real cells with a nucleus. Hence, in
these tissue-forming organisms (both plant and animal) the organic
unit always consists of two chemically and anatomically different
parts - the outer cell-body and the inner nucleus.

In order to convince oneself that this cell is really an independent
organism, we have only to observe the development and vital phenomena
of one of them. We see then that it performs all the essential
functions of life - both vegetal and animal - which we find in the
entire organism. Each of these tiny beings grows and nourishes itself
independently. It takes its food from the surrounding fluid;
sometimes, even, the naked cells take in solid particles at certain
points of their surface - in other words, "eat" them - without needing
any special mouth and stomach for the purpose (cf. Figure 1.19).

Further, each cell is able to reproduce itself. This multiplication,
in most cases, takes the form of a simple cleavage, sometimes direct,
sometimes indirect; the simple direct (or "amitotic") division is less
common, and is found, for instance, in the blood cells (Figure 1.10).
In these the nucleus first divides into two equal parts by
constriction. The indirect (or "mitotic") cleavage is much more
frequent; in this the caryoplasm of the nucleus and the cytoplasm of
the cell-body act upon each other in a peculiar way, with a partial
dissolution (caryolysis), the formation of knots and loops (mitosis),
and a movement of the halved plasma-particles towards two mutually
repulsive poles of attraction (caryokinesis, Figure 1.11.)

(FIGURE 1.10. Blood-cells, multiplying by direct division, from the
blood of the embryo of a stag. Originally, each blood-cell has a
nucleus and is round (a). When it is going to multiply, the nucleus
divides into two (b, c, d). Then the protoplasmic body is constricted
between the two nuclei, and these move away from each other (e).
Finally, the constriction is complete, and the cell splits into two
daughter-cells (f). (From Frey.))

FIGURE 1.11. Indirect or mitotic cell-division (with caryolysis and
caryokinesis) from the skin of the larva of a salamander. (From
Rabl.).
A. Mother-cell (Knot, spirema), with Nuclear threads (chromosomata)
(coloured nuclear matter, chromatin), Cytosoma, Nuclear membrane,
Protoplasm of the cell-body and Nuclear sap.
B. Mother-star, the loops beginning to split lengthways (nuclear
membrane gone), with Star-like appearance in cytoplasm, Centrosoma
(sphere of attraction), Nuclear spindle (achromin, colourless matter)
and Nuclear loops (chromatin, coloured matter).
C. The two daughter-stars, produced by the breaking of the loops of
the mother-star (moving away), with Upper daughter-crown, Connecting
threads of the two crowns (achromin), Lower daughter-crown and
Double-star (amphiaster).
D. The two daughter-cells, produced by the complete division of the
two nuclear halves (cytosomata still connected at the equator)
(Double-knot, Dispirema), with Upper daughter-nucleus, Equatorial
constriction of the cell-body and Lower daughter-nucleus.)

The intricate physiological processes which accompany this "mitosis"
have been very closely studied of late years. The inquiry has led to
the detection of certain laws of evolution which are of extreme
importance in connection with heredity. As a rule, two very different
parts of the nucleus play an important part in these changes. They
are: the chromatin, or coloured nuclear substance, which has a
peculiar property of tingeing itself deeply with certain colouring
matters (carmine, haematoxylin, etc.), and the achromin (or linin, or
achromatin), a colourless nuclear substance that lacks this property.
The latter generally forms in the dividing cell a sort of spindle, at
the poles of which there is a very small particle, also colourless,
called the "central body" (centrosoma). This acts as the centre or
focus in a "sphere of attraction" for the granules of protoplasm in
the surrounding cell-body, and assumes a star-like appearance (the
cell-star, or monaster). The two central bodies, standing opposed to
each other at the poles of the nuclear spindle, form "the double-star"
(or amphiaster, Figure 1.11, BC). The chromatin often forms a long,
irregularly-wound thread - "the coil" (spirema, Figure A). At the
commencement of the cleavage it gathers at the equator of the cell,
between the stellar poles, and forms a crown of U-shaped loops
(generally four or eight, or some other definite number). The loops
split lengthwise into two halves (B), and these back away from each
other towards the poles of the spindle (C). Here each group forms a
crown once more, and this, with the corresponding half of the divided
spindle, forms a fresh nucleus (D). Then the protoplasm of the
cell-body begins to contract in the middle, and gather about the new
daughter-nuclei, and at last the two daughter-cells become independent
beings.

Between this common mitosis, or indirect cell-division - which is the
normal cleavage-process in most cells of the higher animals and
plants - and the simple direct division (Figure 1.10) we find every
grade of segmentation; in some circumstances even one kind of division
may be converted into another.

The plastid is also endowed with the functions of movement and
sensation. The single cell can move and creep about, when it has space
for free movement and is not prevented by a hard envelope; it then
thrusts out at its surface processes like fingers, and quickly
withdraws them again, and thus changes its shape (Figure 1.12).
Finally, the young cell is sensitive, or more or less responsive to
stimuli; it makes certain movements on the application of chemical and
mechanical irritation. Hence we can ascribe to the individual cell all
the chief functions which we comprehend under the general heading of
"life" - sensation, movement, nutrition, and reproduction. All these
properties of the multicellular and highly developed animal are also
found in the single animal-cell, at least in its younger stages. There
is no longer any doubt about this, and so we may regard it as a solid
and important base of our physiological conception of the elementary
organism.

Without going any further here into these very interesting phenomena
of the life of the cell, we will pass on to consider the application
of the cell theory to the ovum. Here comparative research yields the
important result that EVERY OVUM IS AT FIRST A SIMPLE CELL. I say this
is very important, because our whole science of embryology now
resolves itself into the problem: "How does the multicellular organism
arise from the unicellular?" Every organic individual is at first a
simple cell, and as such an elementary organism, or a unit of
individuality. This cell produces a cluster of cells by segmentation,
and from these develops the multicellular organism, or individual of
higher rank.

When we examine a little closer the original features of the ovum, we
notice the extremely significant fact that in its first stage the ovum
is just the same simple and indefinite structure in the case of man
and all the animals (Figure 1.13). We are unable to detect any
material difference between them, either in outer shape or internal
constitution. Later, though the ova remain unicellular, they differ in
size and shape, enclose various kinds of yelk-particles, have
different envelopes, and so on. But when we examine them at their
birth, in the ovary of the female animal, we find them to be always of
the same form in the first stages of their life. In the beginning each
ovum is a very simple, roundish, naked, mobile cell, without a
membrane; it consists merely of a particle of cytoplasm enclosing a
nucleus (Figure 1.13). Special names have been given to these parts of
the ovum; the cell-body is called the yelk (vitellus), and the
cell-nucleus the germinal vesicle. As a rule, the nucleus of the ovum
is soft, and looks like a small pimple or vesicle. Inside it, as in
many other cells, there is a nuclear skeleton or frame and a third,
hard nuclear body (the nucleolus). In the ovum this is called the
germinal spot. Finally, we find in many ova (but not in all) a still
further point within the germinal spot, a "nucleolin," which goes by
the name of the germinal point. The latter parts (germinal spot and
germinal point) have, apparently, a minor importance, in comparison
with the other two (the yelk and germinal vesicle). In the yelk we
must distinguish the active formative yelk (or protoplasm = first
plasm) from the passive nutritive yelk (or deutoplasm = second plasm).

(FIGURE 1.12. Mobile cells from the inflamed eye of a frog (from the
watery fluid of the eye, the humor aqueus). The naked cells creep
freely about, by (like the amoeba or rhizopods) protruding fine
processes from the uncovered protoplasmic body. These bodies vary
continually in number, shape, and size. The nucleus of these amoeboid
lymph-cells ("travelling cells," or planocytes) is invisible, because
concealed by the numbers of fine granules which are scattered in the
protoplasm. (From Frey.))

In many of the lower animals (such as sponges, polyps, and medusae)
the naked ova retain their original simple appearance until
impregnation. But in most animals they at once begin to change; the
change consists partly in the formation of connections with the yelk,
which serve to nourish the ovum, and partly of external membranes for
their protection (the ovolemma, or prochorion). A membrane of this
sort is formed in all the mammals in the course of the embryonic
process. The little globule is surrounded by a thick capsule of
glass-like transparency, the zona pellucida, or ovolemma pellucidum
(Figure 1.14). When we examine it closely under the microscope, we see
very fine radial streaks in it, piercing the zona, which are really
very narrow canals. The human ovum, whether fertilised or not, cannot
be distinguished from that of most of the other mammals. It is nearly
the same everywhere in form, size, and composition. When it is fully
formed, it has a diameter of (on an average) about 1/120 of an inch.
When the mammal ovum has been carefully isolated, and held against the
light on a glass-plate, it may be seen as a fine point even with the
naked eye. The ova of most of the higher mammals are about the same
size. The diameter of the ovum is almost always between 1/250 to 1/125
inch. It has always the same globular shape; the same characteristic
membrane; the same transparent germinal vesicle with its dark germinal
spot. Even when we use the most powerful microscope with its highest
power, we can detect no material difference between the ova of man,
the ape, the dog, and so on. I do not mean to say that there are no
differences between the ova of these different mammals. On the
contrary, we are bound to assume that there are such, at least as
regards chemical composition. Even the ova of different men must
differ from each other; otherwise we should not have a different
individual from each ovum. It is true that our crude and imperfect
apparatus cannot detect these subtle individual differences, which are
probably in the molecular structure. However, such a striking
resemblance of their ova in form, so great as to seem to be a complete
similarity, is a strong proof of the common parentage of man and the
other mammals. From the common germ-form we infer a common stem-form.
On the other hand, there are striking peculiarities by which we can
easily distinguish the fertilised ovum of the mammal from the
fertilised ovum of the birds, amphibia, fishes, and other vertebrates
(see the close of Chapter 2.29).

(FIGURE 1.13. Ova of various animals, executing amoeboid movements,
highly magnified. All the ova are naked cells of varying shape. In the
dark fine-grained protoplasm (yelk) is a large vesicular nucleus (the
germinal vesicle), and in this is seen a nuclear body (the germinal
spot), in which again we often see a germinal point. Figures A1 to A4
represent the ovum of a sponge (Leuculmis echinus) in four successive
movements. B1 to B8 are the ovum of a parasitic crab (Chondracanthus
cornutus), in eight successive movements. (From Edward von Beneden.)
C1 to C5 show the ovum of the cat in various stages of movement (from
Pfluger); Figure P the ovum of a trout; E the ovum of a chicken; F a
human ovum.)

The fertilised bird-ovum (Figure 1.15) is notably different. It is
true that in its earliest stage (Figure 1.13 E) this ovum also is very
like that of the mammal (Figure 1.13 F). But afterwards, while still
within the oviduct, it takes up a quantity of nourishment and works
this into the familiar large yellow yelk. When we examine a very young
ovum in the hen's oviduct, we find it to be a simple, small, naked,
amoeboid cell, just like the young ova of other animals (Figure 1.13).
But it then grows to the size we are familiar with in the round yelk
of the egg. The nucleus of the ovum, or the germinal vesicle, is thus
pressed right to the surface of the globular ovum, and is embedded
there in a small quantity of transparent matter, the so-called white
yelk. This forms a round white spot, which is known as the "tread"
(cicatricula) (Figure 1.15 b). From the tread a thin column of the
white yelk penetrates through the yellow yelk to the centre of the
globular cell, where it swells into a small, central globule (wrongly
called the yelk-cavity, or latebra, Figure 1.15 d apostrophe). The
yellow yelk-matter which surrounds this white yelk has the appearance
in the egg (when boiled hard) of concentric layers (c). The yellow
yelk is also enclosed in a delicate structureless membrane (the
membrana vitellina, a).

As the large yellow ovum of the bird attains a diameter of several
inches in the bigger birds, and encloses round yelk-particles, there
was formerly a reluctance to consider it as a simple cell. This was a
mistake. Every animal that has only one cell-nucleus, every amoeba,
every gregarina, every infusorium, is unicellular, and remains
unicellular whatever variety of matter it feeds on. So the ovum
remains a simple cell, however much yellow yelk it afterwards
accumulates within its protoplasm. It is, of course, different, with
the bird's egg when it has been fertilised. The ovum then consists of
as many cells as there are nuclei in the tread. Hence, in the
fertilised egg which we eat daily, the yellow yelk is already a
multicellular body. Its tread is composed of several cells, and is now
commonly called the germinal disc. We shall return to this
discogastrula in Chapter 1.9.

(FIGURE 1.14. The human ovum, taken from the female ovary, magnified
500 times. The whole ovum is a simple round cell. The chief part of
the globular mass is formed by the nuclear yelk (deutoplasm), which is
evenly distributed in the active protoplasm, and consists of numbers
of fine yelk-granules. In the upper part of the yelk is the
transparent round germinal vesicle, which corresponds to the nucleus.
This encloses a darker granule, the germinal spot, which shows a
nucleolus. The globular yelk is surrounded by the thick transparent
germinal membrane (ovolemma, or zona pellucida). This is traversed by
numbers of lines as fine as hairs, which are directed radially towards
the centre of the ovum. These are called the pore-canals; it is
through these that the moving spermatozoa penetrate into the yelk at
impregnation.

FIGURE 1.15. A fertilised ovum from the oviduct of a hen. the yellow
yelk (c) consists of several concentric layers (d), and is enclosed in



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