In order to understand clearly the course of human embryology, we must
select the more important of its wonderful and manifold processes for
fuller explanation, and then proceed from these to the innumerable
features of less importance. The most important feature in this sense,
and the best starting-point for ontogenetic study, is the fact that
man is developed from an ovum, and that this ovum is a simple cell.
The human ovum does not materially differ in form and composition from
that of the other mammals, whereas there is a distinct difference
between the fertilised ovum of the mammal and that of any other
animal.
(FIGURE 1.1. The human ovum, magnified 100 times. The globular mass of
yelk (b) is enclosed by a transparent membrane (the ovolemma or zona
pellucida [a]), and contains a noncentral nucleus (the germinal
vesicle, c). Cf. Figure 1.14.)
This fact is so important that few should be unaware of its extreme
significance; yet it was quite unknown in the first quarter of the
nineteenth century. As we have seen, the human and mammal ovum was not
discovered until 1827, when Carl Ernst von Baer detected it. Up to
that time the larger vesicles, in which the real and much smaller ovum
is contained, had been wrongly regarded as ova. The important
circumstance that this mammal ovum is a simple cell, like the ovum of
other animals, could not, of course, be recognised until the cell
theory was established. This was not done, by Schleiden for the plant
and Schwann for the animal, until 1838. As we have seen, this cell
theory is of the greatest service in explaining the human frame and
its embryonic development. Hence we must say a few words about the
actual condition of the theory and the significance of the views it
has suggested.
In order properly to appreciate the cellular theory, the most
important element in our science, it is necessary to understand in the
first place that the cell is a UNIFIED ORGANISM, a self-contained
living being. When we anatomically dissect the fully-formed animal or
plant into its various organs, and then examine the finer structure of
these organs with the microscope, we are surprised to find that all
these different parts are ultimately made up of the same structural
element or unit. This common unit of structure is the cell. It does
not matter whether we thus dissect a leaf, flower, or fruit, or a
bone, muscle, gland, or bit of skin, etc.; we find in every case the
same ultimate constituent, which has been called the cell since
Schleiden's discovery. There are many opinions as to its real nature,
but the essential point in our view of the cell is to look upon it as
a self-contained or independent living unit. It is, in the words of
Brucke, "an elementary organism." We may define it most precisely as
the ultimate organic unit, and, as the cells are the sole active
principles in every vital function, we may call them the "plastids,"
or "formative elements." This unity is found in both the anatomic
structure and the physiological function. In the case of the protists,
the entire organism usually consists of a single independent cell
throughout life. But in the tissue-forming animals and plants, which
are the great majority, the organism begins its career as a simple
cell, and then grows into a cell-community, or, more correctly, an
organised cell-state. Our own body is not really the simple unity that
it is generally supposed to be. On the contrary, it is a very
elaborate social system of countless microscopic organisms, a colony
or commonwealth, made up of innumerable independent units, or very
different tissue-cells.
In reality, the term "cell," which existed long before the cell theory
was formulated, is not happily chosen. Schleiden, who first brought it
into scientific use in the sense of the cell theory, gave this name to
the elementary organisms because, when you find them in the dissected
plant, they generally have the appearance of chambers, like the cells
in a bee-hive, with firm walls and a fluid or pulpy content. But some
cells, especially young ones, are entirely without the enveloping
membrane, or stiff wall. Hence we now generally describe the cell as a
living, viscous particle of protoplasm, enclosing a firmer nucleus in
its albuminoid body. There may be an enclosing membrane, as there
actually is in the case of most of the plants; but it may be wholly
lacking, as is the case with most of the animals. There is no membrane
at all in the first stage. The young cells are usually round, but they
vary much in shape later on. Illustrations of this will be found in
the cells of the various parts of the body shown in Figures 1.3 to
1.7.
Hence the essential point in the modern idea of the cell is that it is
made up of two different active constituents--an inner and an outer
part. The smaller and inner part is the nucleus (or caryon or
cytoblastus, Figure 1.1 c and Figure 1.2 k). The outer and larger
part, which encloses the other, is the body of the cell (celleus,
cytos, or cytosoma). The soft living substance of which the two are
composed has a peculiar chemical composition, and belongs to the group
of the albuminoid plasma-substances ("formative matter"), or
protoplasm. The essential and indispensable element of the nucleus is
called nuclein (or caryoplasm); that of the cell body is called
plastin (or cytoplasm). In the most rudimentary cases both substances
seem to be quite simple and homogeneous, without any visible
structure. But, as a rule, when we examine them under a high power of
the microscope, we find a certain structure in the protoplasm. The
chief and most common form of this is the fibrous or net-like "thready
structure" (Frommann) and the frothy "honeycomb structure" (Butschli).
(FIGURE 1.2. Stem-cell of one of the echinoderms (cytula, or "first
segmentation-cell" = fertilised ovum), after Hertwig. k is the nucleus
or caryon.)
The shape or outer form of the cell is infinitely varied, in
accordance with its endless power of adapting itself to the most
diverse activities or environments. In its simplest form the cell is
globular (Figure 1.2). This normal round form is especially found in
cells of the simplest construction, and those that are developed in a
free fluid without any external pressure. In such cases the nucleus
also is not infrequently round, and located in the centre of the
cell-body (Figure 1.2 k). In other cases, the cells have no definite
shape; they are constantly changing their form owing to their
automatic movements. This is the case with the amoebae (Figures 1.15
and 1.16) and the amoeboid travelling cells (Figure 1.11), and also
with very young ova (Figure 1.13). However, as a rule, the cell
assumes a definite form in the course of its career. In the tissues of
the multicellular organism, in which a number of similar cells are
bound together in virtue of certain laws of heredity, the shape is
determined partly by the form of their connection and partly by their
special functions. Thus, for instance, we find in the mucous lining of
our tongue very thin and delicate flat cells of roundish shape (Figure
1.3). In the outer skin we find similar, but harder, covering cells,
joined together by saw-like edges (Figure 1.4). In the liver and other
glands there are thicker and softer cells, linked together in rows
(Figure 1.5).
The last-named tissues (Figures 1.3 to 1.5) belong to the simplest and
most primitive type, the group of the "covering-tissues," or
epithelia. In these "primary tissues" (to which the germinal layers
belong) simple cells of the same kind are arranged in layers. The
arrangement and shape are more complicated in the "secondary tissues,"
which are gradually developed out of the primary, as in the tissues of
the muscles, nerves, bones, etc. In the bones, for instance, which
belong to the group of supporting or connecting organs, the cells
(Figure 1.6) are star-shaped, and are joined together by numbers of
net-like interlacing processes; so, also, in the tissues of the teeth
(Figure 1.7), and in other forms of supporting-tissue, in which a soft
or hard substance (intercellular matter, or base) is inserted between
the cells.
(FIGURE 1.3. Three epithelial cells from the mucous lining of the
tongue.
FIGURE 1.4. Five spiny or grooved cells, with edges joined, from the
outer skin (epidermis): one of them (b) is isolated.
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
a thin yelk-membrane (a). The nucleus or germinal vesicle is seen
above in the cicatrix or "tread" (b). From that point the white yelk
penetrates to the central yelk-cavity (d apostrophe). The two kinds of
yelk do not differ very much.
FIGURE 1.16. A creeping amoeba (highly magnified). The whole organism
is a simple naked cell, and moves about by means of the changing arms
which it thrusts out of and withdraws into its protoplasmic body.
Inside it is the roundish nucleus with its nucleolus.)
When the mature bird-ovum has left the ovary and been fertilised in
the oviduct, it covers itself with various membranes which are
secreted from the wall of the oviduct. First, the large clear
albuminous layer is deposited around the yellow yelk; afterwards, the
hard external shell, with a fine inner skin. All these gradually
forming envelopes and processes are of no importance in the formation
of the embryo; they serve merely for the protection of the original
simple ovum. We sometimes find extraordinarily large eggs with strong
envelopes in the case of other animals, such as fishes of the shark
type. Here, also, the ovum is originally of the same character as it
is in the mammal; it is a perfectly simple and naked cell. But, as in
the case of the bird, a considerable quantity of nutritive yelk is
accumulated inside the original yelk as food for the developing
embryo; and various coverings are formed round the egg. The ovum of
many other animals has the same internal and external features. They
have, however, only a physiological, not a morphological, importance;
they have no direct influence on the formation of the foetus. They are
partly consumed as food by the embryo, and partly serve as protective
envelopes. Hence we may leave them out of consideration altogether
here, and restrict ourselves to material points--TO THE SUBSTANTIAL
IDENTITY OF THE ORIGINAL OVUM IN MAN AND THE REST OF THE ANIMALS
(Figure 1.13).
Now, let us for the first time make use of our biogenetic law; and
directly apply this fundamental law of evolution to the human ovum. We
reach a very simple, but very important, conclusion. FROM THE FACT
THAT THE HUMAN OVUM AND THAT OF ALL OTHER ANIMALS CONSISTS OF A SINGLE
CELL, IT FOLLOWS IMMEDIATELY, ACCORDING TO THE BIOGENETIC LAW, THAT
ALL THE ANIMALS, INCLUDING MAN, DESCEND FROM A UNICELLULAR ORGANISM.
If our biogenetic law is true, if the embryonic development is a
summary or condensed recapitulation of the stem-history--and there can
be no doubt about it--we are bound to conclude, from the fact that all
the ova are at first simple cells, that all the multicellular
organisms originally sprang from a unicellular being. And as the
original ovum in man and all the other animals has the same simple and
indefinite appearance, we may assume with some probability that this
unicellular stem-form was the common ancestor of the whole animal
world, including man. However, this last hypothesis does not seem to
me as inevitable and as absolutely certain as our first conclusion.
This inference from the unicellular embryonic form to the unicellular
ancestor is so simple, but so important, that we cannot sufficiently
emphasise it. We must, therefore, turn next to the question whether
there are to-day any unicellular organisms, from the features of which
we may draw some approximate conclusion as to the unicellular
ancestors of the multicellular organisms. The answer is: Most
certainly there are. There are assuredly still unicellular organisms
which are, in their whole nature, really nothing more than permanent
ova. There are independent unicellular organisms of the simplest
character which develop no further, but reproduce themselves as such,
without any further growth. We know to-day of a great number of these
little beings, such as the gregarinae, flagellata, acineta, infusoria,
etc. However, there is one of them that has an especial interest for
us, because it at once suggests itself when we raise our question, and
it must be regarded as the unicellular being that approaches nearest
to the real ancestral form. This organism is the amoeba.
For a long time now we have comprised under the general name of
amoebae a number of microscopic unicellular organisms, which are very
widely distributed, especially in fresh-water, but also in the ocean;
in fact, they have lately been discovered in damp soil. There are also
parasitic amoebae which live inside other animals. When we place one
of these amoebae in a drop of water under the microscope and examine
it with a high power, it generally appears as a roundish particle of a
very irregular and varying shape (Figures 1.16 and 1.17). In its soft,
slimy, semi-fluid substance, which consists of protoplasm, we see only
the solid globular particle it contains, the nucleus. This unicellular
body moves about continually, creeping in every direction on the glass
on which we are examining it. The movement is effected by the
shapeless body thrusting out finger-like processes at various parts of
its surface; and these are slowly but continually changing, and
drawing the rest of the body after them. After a time, perhaps, the
action changes. The amoeba suddenly stands still, withdraws its
projections, and assumes a globular shape. In a little while, however,
the round body begins to expand again, thrusts out arms in another
direction, and moves on once more. These changeable processes are
called "false feet," or pseudopodia, because they act physiologically
as feet, yet are not special organs in the anatomic sense. They
disappear as quickly as they come, and are nothing more than temporary
projections of the semi-fluid and structureless body.
(FIGURE 1.17. Division of a unicellular amoeba (Amoeba polypodia) in
six stages. (From F.E. Schultze.) the dark spot is the nucleus, the
lighter spot a contractile vacuole in the protoplasm. The latter
reforms in one of the daughter-cells.)
FIGURE 1.18. Ovum of a sponge (Olynthus). The ovum creeps about in a
body of the sponge by thrusting out ever-changing processes. It is
indistinguishable from the common amoeba.)
If you touch one of these creeping amoebae with a needle, or put a
drop of acid in the water, the whole body at once contracts in
consequence of this mechanical or physical stimulus. As a rule, the
body then resumes its globular shape. In certain circumstances--for
instance, if the impurity of the water lasts some time--the amoeba
begins to develop a covering. It exudes a membrane or capsule, which
immediately hardens, and assumes the appearance of a round cell with a
protective membrane. The amoeba either takes its food directly by
imbibition of matter floating in the water, or by pressing into its
protoplasmic body solid particles with which it comes in contact. The
latter process may be observed at any moment by forcing it to eat. If
finely ground colouring matter, such as carmine or indigo, is put into
the water, you can see the body of the amoeba pressing these coloured
particles into itself, the substance of the cell closing round them.
The amoeba can take in food in this way at any point on its surface,
without having any special organs for intussusception and digestion,
or a real mouth or gut.
The amoeba grows by thus taking in food and dissolving the particles
eaten in its protoplasm. When it reaches a certain size by this
continual feeding, it begins to reproduce. This is done by the simple
process of cleavage (Figure 1.17). First, the nucleus divides into two
parts. Then the protoplasm is separated between the two new nuclei,
and the whole cell splits into two daughter-cells, the protoplasm
gathering about each of the nuclei. The thin bridge of protoplasm
which at first connects the daughter-cells soon breaks. Here we have
the simple form of direct cleavage of the nuclei. Without mitosis, or
formation of threads, the homogeneous nucleus divides into two halves.
These move away from each other, and become centres of attraction for
the enveloping matter, the protoplasm. The same direct cleavage of the
nuclei is also witnessed in the reproduction of many other protists,
while other unicellular organisms show the indirect division of the
cell.
Hence, although the amoeba is nothing but a simple cell, it is
evidently able to accomplish all the functions of the multicellular
organism. It moves, feels, nourishes itself, and reproduces. Some
kinds of these amoebae can be seen with the naked eye, but most of
them are microscopically small. It is for the following reasons that
we regard the amoebae as the unicellular organisms which have special
phylogenetic (or evolutionary) relations to the ovum. In many of the
lower animals the ovum retains its original naked form until
fertilisation, develops no membranes, and is then often
indistinguishable from the ordinary amoeba. Like the amoebae, these
naked ova may thrust out processes, and move about as travelling
cells. In the sponges these mobile ova move about freely in the
maternal body like independent amoebae (Figure 1.17). They had been
observed by earlier scientists, but described as foreign
bodies--namely, parasitic amoebae, living parasitically on the body of
the sponge. Later, however, it was discovered that they were not
parasites, but the ova of the sponge. We also find this remarkable
phenomenon among other animals, such as the graceful, bell-shaped
zoophytes, which we call polyps and medusae. Their ova remain naked
cells, which thrust out amoeboid projections, nourish themselves, and
move about. When they have been fertilised, the multicellular organism
is formed from them by repeated segmentation.
It is, therefore, no audacious hypothesis, but a perfectly sound
conclusion, to regard the amoeba as the particular unicellular
organism which offers us an approximate illustration of the ancient
common unicellular ancestor of all the metazoa, or multicellular
animals. The simple naked amoeba has a less definite and more original
character than any other cell. Moreover, there is the fact that recent
research has discovered such amoeba-like cells everywhere in the
mature body of the multicellular animals. They are found, for
instance, in the human blood, side by side with the red corpuscles, as
colourless blood-cells; and it is the same with all the vertebrates.
They are also found in many of the invertebrates--for instance, in the
blood of the snail. I showed, in 1859, that these colourless
blood-cells can, like the independent amoebae, take up solid
particles, or "eat" (whence they are called phagocytes =
"eating-cells," Figure 1.19). Lately, it has been discovered that many
different cells may, if they have room enough, execute the same
movements, creeping about and eating. They behave just like amoebae
(Figure 1.12). It has also been shown that these "travelling-cells,"
or planocytes, play an important part in man's physiology and
pathology (as means of transport for food, infectious matter,
bacteria, etc.).
The power of the naked cell to execute these characteristic
amoeba-like movements comes from the contractility (or automatic
mobility) of its protoplasm. This seems to be a universal property of
young cells. When they are not enclosed by a firm membrane, or
confined in a "cellular prison," they can always accomplish these
amoeboid movements. This is true of the naked ova as well as of any
other naked cells, of the "travelling-cells," of various kinds in
connective tissue, lymph-cells, mucus-cells, etc.
We have now, by our study of the ovum and the comparison of it with
the amoeba, provided a perfectly sound and most valuable foundation
for both the embryology and the evolution of man. We have learned that
the human ovum is a simple cell, that this ovum is not materially
different from that of other mammals, and that we may infer from it
the existence of a primitive unicellular ancestral form, with a
substantial resemblance to the amoeba.
The statement that the earliest progenitors of the human race were
simple cells of this kind, and led an independent unicellular life
like the amoeba, has not only been ridiculed as the dream of a natural
philosopher, but also been violently censured in theological journals
as "shameful and immoral." But, as I observed in my essay On the
Origin and Ancestral Tree of the Human Race in 1870, this offended
piety must equally protest against the "shameful and immoral" fact
that each human individual is developed from a simple ovum, and that
this human ovum is indistinguishable from those of the other mammals,
and in its earliest stage is like a naked amoeba. We can show this to
be a fact any day with the microscope, and it is little use to close
one's eyes to "immoral" facts of this kind. It is as indisputable as
the momentous conclusions we draw from it and as the vertebrate
character of man (see Chapter 1.11).
(FIGURE 1.19. Blood-cells that eat, or phagocytes, from a naked
sea-snail (Thetis), greatly magnified. I was the first to observe in
the blood-cells of this snail the important fact that "the blood-cells
of the invertebrates are unprotected pieces of plasm, and take in
food, by means of their peculiar movements, like the amoebae." I had
(in Naples, on May 10th, 1859) injected into the blood-vessels of one
of these snails an infusion of water and ground indigo, and was
greatly astonished to find the blood-cells themselves more or less
filled with the particles of indigo after a few hours. After repeated
injections I succeeded in "observing the very entrance of the coloured
particles in the blood-cells, which took place just in the same way as
with the amoeba." I have given further particulars about this in my
Monograph on the Radiolaria.)
We now see very clearly how extremely important the cell theory has
been for our whole conception of organic nature. "Man's place in
nature" is settled beyond question by it. Apart from the cell theory,
man is an insoluble enigma to us. Hence philosophers, and especially
physiologists, should be thoroughly conversant with it. The soul of
man can only be really understood in the light of the cell-soul, and
we have the simplest form of this in the amoeba. Only those who are
acquainted with the simple psychic functions of the unicellular
organisms and their gradual evolution in the series of lower animals
can understand how the elaborate mind of the higher vertebrates, and
especially of man, was gradually evolved from them. The academic
psychologists who lack this zoological equipment are unable to do so.
This naturalistic and realistic conception is a stumbling-block to our
modern idealistic metaphysicians and their theological colleagues.
Fenced about with their transcendental and dualistic prejudices, they
attack not only the monistic system we establish on our scientific
knowledge, but even the plainest facts which go to form its
foundation. An instructive instance of this was seen a few years ago,
in the academic discourse delivered by a distinguished theologian,
Willibald Beyschlag, at Halle, January 12th, 1900, on the occasion of
the centenary festival. The theologian protested violently against the
"materialistic dustmen of the scientific world who offer our people
the diploma of a descent from the ape, and would prove to them that
the genius of a Shakespeare or a Goethe is merely a distillation from
a drop of primitive mucus." Another well-known theologian protested
against "the horrible idea that the greatest of men, Luther and
Christ, were descended from a mere globule of protoplasm."
Nevertheless, not a single informed and impartial scientist doubts the
fact that these greatest men were, like all other men--and all other
vertebrates--developed from an impregnated ovum, and that this simple
nucleated globule of protoplasm has the same chemical constitution in
all the mammals.