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microscopic power (Figure 1.22 a, b), has detected some further
details in the finer structure of the ciliated cell, and these are
common to man and the anthropoid ape. The head (k) encloses the
elliptic nucleus in a thin envelope of cytoplasm; it is a little
flattened on one side, and thus looks rather pear-shaped from the
front (b). In the central piece (m) we can distinguish a short neck
and a longer connective piece (with central body). The tail consists
of a long main section (h) and a short, very fine tail (e).

The process of fertilisation by sexual conception consists, therefore,
essentially in the coalescence and fusing together of two different
cells. The lively spermatozoon travels towards the ovum by its
serpentine movements, and bores its way into the female cell (Figure
1.23). The nuclei of both sexual cells, attracted by a certain
"affinity," approach each other and melt into one.

The fertilised cell is quite another thing from the unfertilised cell.
For if we must regard the spermia as real cells no less than the ova,
and the process of conception as a coalescence of the two, we must
consider the resultant cell as a quite new and independent organism.
It bears in the cell and nuclear matter of the penetrating
spermatozoon a part of the father's body, and in the protoplasm and
caryoplasm of the ovum a part of the mother's body. This is clear from
the fact that the child inherits many features from both parents. It
inherits from the father by means of the spermatozoon, and from the
mother by means of the ovum. The actual blending of the two cells
produces a third cell, which is the germ of the child, or the new
organism conceived. One may also say of this sexual coalescence that
the STEM-CELL IS A SIMPLE HERMAPHRODITE; it unites both sexual
substances in itself.

(FIGURE 1.23. The fertilisation of the ovum by the spermatozoon (of a
mammal). One of the many thread-like, lively spermidia pierces through
a fine pore-canal into the nuclear yelk. The nucleus of the ovum is

FIGURE 1.24. An impregnated echinoderm ovum, with small homogeneous
nucleus (e k). (From Hertwig.))

I think it necessary to emphasise the fundamental importance of this
simple, but often unappreciated, feature in order to have a correct
and clear idea of conception. With that end, I have given a special
name to the new cell from which the child develops, and which is
generally loosely called "the fertilised ovum," or "the first
segmentation sphere." I call it "the stem-cell" (cytula). The name
"stem-cell" seems to me the simplest and most suitable, because all
the other cells of the body are derived from it, and because it is, in
the strictest sense, the stem-father and stem-mother of all the
countless generations of cells of which the multicellular organism is
to be composed. That complicated molecular movement of the protoplasm
which we call "life" is, naturally, something quite different in this
stem-cell from what we find in the two parent-cells, from the
coalescence of which it has issued. THE LIFE OF THE STEM-CELL OR

The admirable work done by recent observers has shown that the
individual development, in man and the other animals, commences with
the formation of a simple "stem-cell" of this character, and that this
then passes, by repeated segmentation (or cleavage), into a cluster of
cells, known as "the segmentation sphere" or "segmentation cells." The
process is most clearly observed in the ova of the echinoderms
(star-fishes, sea-urchins, etc.). The investigations of Oscar and
Richard Hertwig were chiefly directed to these. The main results may
be summed up as follows: -

Conception is preceded by certain preliminary changes, which are very
necessary - in fact, usually indispensable - for its occurrence. They
are comprised under the general heading of "Changes prior to
impregnation." In these the original nucleus of the ovum, the germinal
vesicle, is lost. Part of it is extruded, and part dissolved in the
cell contents; only a very small part of it is left to form the basis
of a fresh nucleus, the pronucleus femininus. It is the latter alone
that combines in conception with the invading nucleus of the
fertilising spermatozoon (the pronucleus masculinus).

The impregnation of the ovum commences with a decay of the germinal
vesicle, or the original nucleus of the ovum (Figure 1.8). We have
seen that this is in most unripe ova a large, transparent, round
vesicle. This germinal vesicle contains a viscous fluid (the
caryolymph). The firm nuclear frame (caryobasis) is formed of the
enveloping membrane and a mesh-work of nuclear threads running across
the interior, which is filled with the nuclear sap. In a knot of the
network is contained the dark, stiff, opaque nuclear corpuscle or
nucleolus. When the impregnation of the ovum sets in, the greater part
of the germinal vesicle is dissolved in the cell; the nuclear membrane
and mesh-work disappear; the nuclear sap is distributed in the
protoplasm; a small portion of the nuclear base is extruded; another
small portion is left, and is converted into the secondary nucleus, or
the female pro-nucleus (Figure 1.24 e k).

The small portion of the nuclear base which is extruded from the
impregnated ovum is known as the "directive bodies" or "polar cells";
there are many disputes as to their origin and significance, but we
are as yet imperfectly acquainted with them. As a rule, they are two
small round granules, of the same size and appearance as the remaining
pro-nucleus. They are detached cell-buds; their separation from the
large mother-cell takes place in the same way as in ordinary "indirect
cell-division." Hence, the polar cells are probably to be conceived as
"abortive ova," or "rudimentary ova," which proceed from a simple
original ovum by cleavage in the same way that several sperm-cells
arise from one "sperm-mother-cell," in reproduction from sperm. The
male sperm-cells in the testicles must undergo similar changes in view
of the coming impregnation as the ova in the female ovary. In this
maturing of the sperm each of the original seed-cells divides by
double segmentation into four daughter-cells, each furnished with a
fourth of the original nuclear matter (the hereditary chromatin); and
each of these four descendant cells becomes a spermatozoon, ready for
impregnation. Thus is prevented the doubling of the chromatin in the
coalescence of the two nuclei at conception. As the two polar cells
are extruded and lost, and have no further part in the fertilisation
of the ovum, we need not discuss them any further. But we must give
more attention to the female pro-nucleus which alone remains after the
extrusion of the polar cells and the dissolving of the germinal
vesicle (Figure 1.23 e k). This tiny round corpuscle of chromatin now
acts as a centre of attraction for the invading spermatozoon in the
large ripe ovum, and coalesces with its "head," the male pro-nucleus.
The product of this blending, which is the most important part of the
act of impregnation, is the stem-nucleus, or the first segmentation
nucleus (archicaryon) - that is to say, the nucleus of the new-born
embryonic stem-cell or "first segmentation cell." This stem-cell is
the starting point of the subsequent embryonic processes.

Hertwig has shown that the tiny transparent ova of the echinoderms are
the most convenient for following the details of this important
process of impregnation. We can, in this case, easily and successfully
accomplish artificial impregnation, and follow the formation of the
stem-cell step by step within the space of ten minutes. If we put ripe
ova of the star-fish or sea-urchin in a watch glass with sea-water and
add a drop of ripe sperm-fluid, we find each ovum impregnated within
five minutes. Thousands of the fine, mobile ciliated cells, which we
have described as "sperm-threads" (Figure 1.20), make their way to the
ova, owing to a sort of chemical sensitive action which may be called
"smell." But only one of these innumerable spermatozoa is
chosen - namely, the one that first reaches the ovum by the serpentine
motions of its tail, and touches the ovum with its head. At the spot
where the point of its head touches the surface of the ovum the
protoplasm of the latter is raised in the form of a small wart, the
"impregnation rise" (Figure 1.25 A). The spermatozoon then bores its
way into this with its head, the tail outside wriggling about all the
time (Figure 1.25 B, C). Presently the tail also disappears within the
ovum. At the same time the ovum secretes a thin external yelk-membrane
(Figure 1.25 C), starting from the point of impregnation; and this
prevents any more spermatozoa from entering.

Inside the impregnated ovum we now see a rapid series of most
important changes. The pear-shaped head of the sperm-cell, or the
"head of the spermatozoon," grows larger and rounder, and is converted
into the male pro-nucleus (Figure 1.26 s k). This has an attractive
influence on the fine granules or particles which are distributed in
the protoplasm of the ovum; they arrange themselves in lines in the
figure of a star. But the attraction or the "affinity" between the two
nuclei is even stronger. They move towards each other inside the yelk
with increasing speed, the male (Figure 1.27 s k) going more quickly
than the female nucleus (e k). The tiny male nucleus takes with it the
radiating mantle which spreads like a star about it. At last the two
sexual nuclei touch (usually in the centre of the globular ovum), lie
close together, are flattened at the points of contact, and coalesce
into a common mass. The small central particle of nuclein which is
formed from this combination of the nuclei is the stem-nucleus, or the
first segmentation nucleus; the new-formed cell, the product of the
impregnation, is our stem-cell, or "first segmentation sphere" (Figure

(FIGURE 1.25. Impregnation of the ovum of a star-fish. (From Hertwig.)
Only a small part of the surface of the ovum is shown. One of the
numerous spermatozoa approaches the "impregnation rise" (A), touches
it (B), and then penetrates into the protoplasm of the ovum (C).

FIGURES 1.26 AND 1.27. Impregnation of the ovum of the sea-urchin.
(From Hertwig.) In Figure 1.26 the little sperm-nucleus (sk) moves
towards the larger nucleus of the ovum (ek). In Figure 1.27 they
nearly touch, and are surrounded by the radiating mantle of

Hence the one essential point in the process of sexual reproduction or
impregnation is the formation of a new cell, the stem-cell, by the
combination of two originally different cells, the female ovum and the
male spermatozoon. This process is of the highest importance, and
merits our closest attention; all that happens in the later
development of this first cell and in the life of the organism that
comes of it is determined from the first by the chemical and
morphological composition of the stem-cell, its nucleus and its body.
We must, therefore, make a very careful study of the rise and
structure of the stem-cell.

The first question that arises is as to the two different active
elements, the nucleus and the protoplasm, in the actual coalescence.
It is obvious that the nucleus plays the more important part in this.
Hence Hertwig puts his theory of conception in the principle:
"Conception consists in the copulation of two cell-nuclei, which come
from a male and a female cell." And as the phenomenon of heredity is
inseparably connected with the reproductive process, we may further
conclude that these two copulating nuclei "convey the characteristics
which are transmitted from parents to offspring." In this sense I had
in 1866 (in the ninth chapter of the General Morphology) ascribed to
the reproductive nucleus the function of generation and heredity, and
to the nutritive protoplasm the duties of nutrition and adaptation.
As, moreover, there is a complete coalescence of the mutually
attracted nuclear substances in conception, and the new nucleus formed
(the stem-nucleus) is the real starting-point for the development of
the fresh organism, the further conclusion may be drawn that the male
nucleus conveys to the child the qualities of the father, and the
female nucleus the features of the mother. We must not forget,
however, that the protoplasmic bodies of the copulating cells also
fuse together in the act of impregnation; the cell-body of the
invading spermatozoon (the trunk and tail of the male ciliated cell)
is dissolved in the yelk of the female ovum. This coalescence is not
so important as that of the nuclei, but it must not be overlooked;
and, though this process is not so well known to us, we see clearly at
least the formation of the star-like figure (the radial arrangement of
the particles in the plasma) in it (Figures 1.26 to 1.27).

The older theories of impregnation generally went astray in regarding
the large ovum as the sole base of the new organism, and only ascribed
to the spermatozoon the work of stimulating and originating its
development. The stimulus which it gave to the ovum was sometimes
thought to be purely chemical, at other times rather physical (on the
principle of transferred movement), or again a mystic and
transcendental process. This error was partly due to the imperfect
knowledge at that time of the facts of impregnation, and partly to the
striking difference in the sizes of the two sexual cells. Most of the
earlier observers thought that the spermatozoon did not penetrate into
the ovum. And even when this had been demonstrated, the spermatozoon
was believed to disappear in the ovum without leaving a trace.
However, the splendid research made in the last three decades with the
finer technical methods of our time has completely exposed the error
of this. It has been shown that the tiny sperm-cell is NOT
SUBORDINATED TO, BUT COORDINATED WITH, the large ovum. The nuclei of
the two cells, as the vehicles of the hereditary features of the
parents, are of equal physiological importance. In some cases we have
succeeded in proving that the mass of the active nuclear substance
which combines in the copulation of the two sexual nuclei is
originally the same for both.

These morphological facts are in perfect harmony with the familiar
physiological truth that the child inherits from both parents, and
that on the average they are equally distributed. I say "on the
average," because it is well known that a child may have a greater
likeness to the father or to the mother; that goes without saying, as
far as the primary sexual characters (the sexual glands) are
concerned. But it is also possible that the determination of the
latter - the weighty determination whether the child is to be a boy or
a girl - depends on a slight qualitative or quantitative difference in
the nuclein or the coloured nuclear matter which comes from both
parents in the act of conception.

The striking differences of the respective sexual cells in size and
shape, which occasioned the erroneous views of earlier scientists, are
easily explained on the principle of division of labour. The inert,
motionless ovum grows in size according to the quantity of provision
it stores up in the form of nutritive yelk for the development of the
germ. The active swimming sperm-cell is reduced in size in proportion
to its need to seek the ovum and bore its way into its yelk. These
differences are very conspicuous in the higher animals, but they are
much less in the lower animals. In those protists (unicellular plants
and animals) which have the first rudiments of sexual reproduction the
two copulating cells are at first quite equal. In these cases the act
of impregnation is nothing more than a sudden GROWTH, in which the
originally simple cell doubles its volume, and is thus prepared for
reproduction (cell-division). Afterwards slight differences are seen
in the size of the copulating cells; though the smaller ones still
have the same shape as the larger ones. It is only when the difference
in size is very pronounced that a notable difference in shape is
found: the sprightly sperm-cell changes more in shape and the ovum in

Quite in harmony with this new conception of the EQUIVALENCE OF THE
TWO GONADS, or the equal physiological importance of the male and
female sex-cells and their equal share in the process of heredity, is
the important fact established by Hertwig (1875), that in normal
impregnation only one single spermatozoon copulates with one ovum; the
membrane which is raised on the surface of the yelk immediately after
one sperm-cell has penetrated (Figure 1.25 C) prevents any others from
entering. All the rivals of the fortunate penetrator are excluded, and
die without. But if the ovum passes into a morbid state, if it is made
stiff by a lowering of its temperature or stupefied with narcotics
(chloroform, morphia, nicotine, etc.), two or more spermatozoa may
penetrate into its yelk-body. We then witness polyspermism. The more
Hertwig chloroformed the ovum, the more spermatozoa were able to bore
their way into its unconscious body.

(FIGURE 1.28. Stem-cell of a rabbit, magnified 200 times. In the
centre of the granular protoplasm of the fertilised ovum (d) is seen
the little, bright stem-nucleus, z is the ovolemma, with a mucous
membrane (h). s are dead spermatozoa.)

These remarkable facts of impregnation are also of the greatest
interest in psychology, especially as regards the theory of the
cell-soul, which I consider to be its chief foundation. The phenomena
we have described can only be understood and explained by ascribing a
certain lower degree of psychic activity to the sexual principles.
They FEEL each other's proximity, and are drawn together by a
SENSITIVE impulse (probably related to smell); they MOVE towards each
other, and do not rest until they fuse together. Physiologists may say
that it is only a question of a peculiar physico-chemical phenomenon,
and not a psychic action; but the two cannot be separated. Even the
psychic functions, in the strict sense of the word, are only complex
physical processes, or "psycho-physical" phenomena, which are
determined in all cases exclusively by the chemical composition of
their material substratum.

The monistic view of the matter becomes clear enough when we remember
the radical importance of impregnation as regards heredity. It is well
known that not only the most delicate bodily structures, but also the
subtlest traits of mind, are transmitted from the parents to the
children. In this the chromatic matter of the male nucleus is just as
important a vehicle as the large caryoplasmic substance of the female
nucleus; the one transmits the mental features of the father, and the
other those of the mother. The blending of the two parental nuclei
determines the individual psychic character of the child.

But there is another important psychological question - the most
important of all - that has been definitely answered by the recent
discoveries in connection with conception. This is the question of the
immortality of the soul. No fact throws more light on it and refutes
it more convincingly than the elementary process of conception that we
have described. For this copulation of the two sexual nuclei (Figures
1.26 and 1.27) indicates the precise moment at which the individual
begins to exist. All the bodily and mental features of the new-born
child are the sum-total of the hereditary qualities which it has
received in reproduction from parents and ancestors. All that man
acquires afterwards in life by the exercise of his organs, the
influence of his environment, and education - in a word, by
adaptation - cannot obliterate that general outline of his being which
he inherited from his parents. But this hereditary disposition, the
essence of every human soul, is not "eternal," but "temporal"; it
comes into being only at the moment when the sperm-nucleus of the
father and the nucleus of the maternal ovum meet and fuse together. It
is clearly irrational to assume an "eternal life without end" for an
individual phenomenon, the commencement of which we can indicate to a
moment by direct visual observation.

The great importance of the process of impregnation in answering such
questions is quite clear. It is true that conception has never been
studied microscopically in all its details in the human
case - notwithstanding its occurrence at every moment - for reasons that
are obvious enough. However, the two cells which need consideration,
the female ovum and the male spermatozoon, proceed in the case of man
in just the same way as in all the other mammals; the human foetus or
embryo which results from copulation has the same form as with the
other animals. Hence, no scientist who is acquainted with the facts
doubts that the processes of impregnation are just the same in man as
in the other animals.

The stem-cell which is produced, and with which every man begins his
career, cannot be distinguished in appearance from those of other
mammals, such as the rabbit (Figure 1.28). In the case of man, also,
this stem-cell differs materially from the original ovum, both in
regard to form (morphologically), in regard to material composition
(chemically), and in regard to vital properties (physiologically). It
comes partly from the father and partly from the mother. Hence it is
not surprising that the child who is developed from it inherits from
both parents. The vital movements of each of these cells form a sum of
mechanical processes which in the last analysis are due to movements
of the smallest vital parts, or the molecules, of the living
substance. If we agree to call this active substance plasson, and its
molecules plastidules, we may say that the individual physiological
character of each of these cells is due to its molecular
AND THE MALE SPERM-CELL.* (* The plasson of the stem-cell or cytula
may, from the anatomical point of view, be regarded as homogeneous and
structureless, like that of the monera. This is not inconsistent with
our hypothetical ascription to the plastidules (or molecules of the
plasson) of a complex molecular structure. The complexity of this is
the greater in proportion to the complexity of the organism that is
developed from it and the length of the chain of its ancestry, or to
the multitude of antecedent processes of heredity and adaptation.)


There is a substantial agreement throughout the animal world in the
first changes which follow the impregnation of the ovum and the
formation of the stem-cell; they begin in all cases with the
segmentation of the ovum and the formation of the germinal layers. The
only exception is found in the protozoa, the very lowest and simplest
forms of animal life; these remain unicellular throughout life. To
this group belong the amoebae, gregarinae, rhizopods, infusoria, etc.
As their whole organism consists of a single cell, they can never form
germinal layers, or definite strata of cells. But all the other
animals - all the tissue-forming animals, or metazoa, as we call them,
in contradistinction to the protozoa - construct real germinal layers
by the repeated cleavage of the impregnated ovum. This we find in the
lower cnidaria and worms, as well as in the more highly-developed
molluscs, echinoderms, articulates, and vertebrates.

In all these metazoa, or multicellular animals, the chief embryonic
processes are substantially alike, although they often seem to a
superficial observer to differ considerably. The stem-cell that
proceeds from the impregnated ovum always passes by repeated cleavage
into a number of simple cells. These cells are all direct descendants
of the stem-cell, and are, for reasons we shall see presently, called
segmentation-cells. The repeated cleavage of the stem-cell, which
gives rise to these segmentation-spheres, has long been known as
"segmentation." Sooner or later the segmentation-cells join together
to form a round (at first, globular) embryonic sphere (blastula); they
then form into two very different groups, and arrange themselves in
two separate strata - the two primary germinal layers. These enclose a
digestive cavity, the primitive gut, with an opening, the primitive
mouth. We give the name of the gastrula to the important embryonic
form that has these primitive organs, and the name of gastrulation to

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Online LibraryErnst Heinrich Philipp August HaeckelThe Evolution of Man — Volume 1 → online text (page 11 of 26)