Ernst Heinrich Philipp August Haeckel.

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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
(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
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

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.


The recognition of the fact that every man begins his individual
existence as a simple cell is the solid foundation of all research
into the genesis of man. From this fact we are forced, in virtue of
our biogenetic law, to draw the weighty phylogenetic conclusion that
the earliest ancestors of the human race were also unicellular
organisms; and among these protozoa we may single out the vague form
of the amoeba as particularly important (cf. Chapter 1.6). That these
unicellular ancestral forms did once exist follows directly from the
phenomena which we perceive every day in the fertilised ovum. The
development of the multicellular organism from the ovum, and the
formation of the germinal layers and the tissues, follow the same laws
in man and all the higher animals. It will, therefore, be our next
task to consider more closely the impregnated ovum and the process of
conception which produces it.

The process of impregnation or sexual conception is one of those
phenomena that people love to conceal behind the mystic veil of
supernatural power. We shall soon see, however, that it is a purely
mechanical process, and can be reduced to familiar physiological
functions. Moreover, this process of conception is of the same type,
and is effected by the same organs, in man as in all the other
mammals. The pairing of the male and female has in both cases for its
main purpose the introduction of the ripe matter of the male seed or
sperm into the female body, in the sexual canals of which it
encounters the ovum. Conception then ensues by the blending of the

We must observe, first, that this important process is by no means so
widely distributed in the animal and plant world as is commonly
supposed. There is a very large number of lower organisms which
propagate unsexually, or by monogamy; these are especially the sexless
monera (chromacea, bacteria, etc.) but also many other protists, such
as the amoebae, foraminifera, radiolaria, myxomycetae, etc. In these
the multiplication of individuals takes place by unsexual
reproduction, which takes the form of cleavage, budding, or
spore-formation. The copulation of two coalescing cells, which in
these cases often precedes the reproduction, cannot be regarded as a
sexual act unless the two copulating plastids differ in size or
structure. On the other hand, sexual reproduction is the general rule
with all the higher organisms, both animal and plant; very rarely do
we find asexual reproduction among them. There are, in particular, no
cases of parthenogenesis (virginal conception) among the vertebrates.

Sexual reproduction offers an infinite variety of interesting forms in
the different classes of animals and plants, especially as regards the
mode of conception, and the conveyance of the spermatozoon to the
ovum. These features are of great importance not only as regards
conception itself, but for the development of the organic form, and
especially for the differentiation of the sexes. There is a
particularly curious correlation of plants and animals in this
respect. The splendid studies of Charles Darwin and Hermann Muller on
the fertilisation of flowers by insects have given us very interesting
particulars of this.* (* See Darwin's work, On the Various
Contrivances by which Orchids are Fertilised (1862).) This reciprocal
service has given rise to a most intricate sexual apparatus. Equally
elaborate structures have been developed in man and the higher
animals, serving partly for the isolation of the sexual products on
each side, partly for bringing them together in conception. But,
however interesting these phenomena are in themselves, we cannot go
into them here, as they have only a minor importance - if any at
all - in the real process of conception. We must, however, try to get a
very clear idea of this process and the meaning of sexual

In every act of conception we have, as I said, to consider two
different kinds of cells - a female and a male cell. The female cell of
the animal organism is always called the ovum (or ovulum, egg, or
egg-cell); the male cells are known as the sperm or seed-cells, or the
spermatozoa (also spermium and zoospermium). The ripe ovum is, on the
whole, one of the largest cells we know. It attains colossal
dimensions when it absorbs great quantities of nutritive yelk, as is
the case with birds and reptiles and many of the fishes. In the great
majority of the animals the ripe ovum is rich in yelk and much larger
than the other cells. On the other hand, the next cell which we have
to consider in the process of conception, the male sperm-cell or
spermatozoon, is one of the smallest cells in the animal body.
Conception usually consists in the bringing into contact with the ovum
of a slimy fluid secreted by the male, and this may take place either
inside or out of the female body. This fluid is called sperm, or the
male seed. Sperm, like saliva or blood, is not a simple fluid, but a
thick agglomeration of innumerable cells, swimming about in a
comparatively small quantity of fluid. It is not the fluid, but the
independent male cells that swim in it, that cause conception.

(FIGURE 1.20. Spermia or spermatozoa of various mammals. The
pear-shaped flattened nucleus is seen from the front in I and sideways
in II. k is the nucleus, m its middle part (protoplasm), s the mobile,
serpent-like tail (or whip); M four human spermatozoa, A spermatozoa
from the ape; K from the rabbit; H from the mouse; C from the dog; S
from the pig.

FIGURE 1.21. Spermatozoa or spermidia of various animals. (From Lang).
a of a fish, b of a turbellaria worm (with two side-lashes), c to e of
a nematode worm (amoeboid spermatozoa), f from a craw fish
(star-shaped), g from the salamander (with undulating membrane), h of
an annelid (a and h are the usual shape).

FIGURE 1.22. A single human spermatozoon magnified 2000 times; a shows
it from the broader and b from the narrower side. k head (with
nucleus), m middle-stem, h long-stem, and e tail. (From Retzius.))

The spermatozoa of the great majority of animals have two
characteristic features. Firstly, they are extraordinarily small,
being usually the smallest cells in the body; and, secondly, they
have, as a rule, a peculiarly lively motion, which is known as
spermatozoic motion. The shape of the cell has a good deal to do with
this motion. In most of the animals, and also in many of the lower
plants (but not the higher) each of these spermatozoa has a very
small, naked cell-body, enclosing an elongated nucleus, and a long
thread hanging from it (Figure 1.20). It was long before we could
recognise that these structures are simple cells. They were formerly
held to be special organisms, and were called "seed animals"
(spermato-zoa, or spermato-zoidia); they are now scientifically known
as spermia or spermidia, or as spermatosomata (seed-bodies) or
spermatofila (seed threads). It took a good deal of comparative
research to convince us that each of these spermatozoa is really a
simple cell. They have the same shape as in many other vertebrates and
most of the invertebrates. However, in many of the lower animals they
have quite a different shape. Thus, for instance, in the craw fish
they are large round cells, without any movement, equipped with stiff
outgrowths like bristles (Figure 1.21 f). They have also a peculiar
form in some of the worms, such as the thread-worms (filaria); in this
case they are sometimes amoeboid and like very small ova (Figure 1.21
c to e). But in most of the lower animals (such as the sponges and
polyps) they have the same pine-cone shape as in man and the other
animals (Figure 1.21 a, h).

When the Dutch naturalist Leeuwenhoek discovered these thread-like
lively particles in 1677 in the male sperm, it was generally believed
that they were special, independent, tiny animalcules, like the
infusoria, and that the whole mature organism existed already, with
all its parts, but very small and packed together, in each
spermatozoon (see Chapter 1.2). We now know that the mobile
spermatozoa are nothing but simple and real cells, of the kind that we
call "ciliated" (equipped with lashes, or cilia). In the previous
illustrations we have distinguished in the spermatozoon a head, trunk,
and tail. The "head" (Figure 1.20 k) is merely the oval nucleus of the
cell; the body or middle-part (m) is an accumulation of cell-matter;
and the tail (s) is a thread-like prolongation of the same.

Moreover, we now know that these spermatozoa are not at all a peculiar
form of cell; precisely similar cells are found in various other parts
of the body. If they have many short threads projecting, they are
called ciliated; if only one long, whip-shaped process (or, more
rarely, two or four), caudate (tailed) cells.

Very careful recent examination of the spermia, under a very high

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