The Act of Creation (59 page)
Read The Act of Creation Online
Authors: Arthur Koestler
The nuclear changes can actually be observed under the microscope. The
salivary gland cells of midge larvae for instance possess large bundles of
chromosomes, which are seen as sausage-like structures, with occasional
swellings or puffs -- the so-called Balbiani rings. The puffs are the
sites of intense RNA production; it is therefore assumed that they
indicate active genes. The pattern of these swellings changes according
to the age of the larva and the type of cell.*
The action of the cytoplasm on the nucleus has also been directly
demonstrated. When the nucleus from the salivary gland cell of a
drosophila larva is transplanted into the cytoplasmic environment of a
cell at an earlier stage of development, the chromosomes again undergo
very marked changes: certain swellings disappear and others appear in
their place. This clearly indicates the existence of a feedback mechanism
whereby the development of the cytoplasm as a result of gene activity
in its turn calls forth the activity of particular genes. [6]
The reverse type of experiment demonstrates the action of nuclei of
different ages on the same cytoplasmic environment. The nucleus of
an unfertilized frog-egg is removed, and replaced by a nucleus from
a developing frog-embryo. If the transplant nucleus was taken from an
embryo in the early blastular stage, the result will be a clone of normal
tadpoles; if it was obtained from embryos in later stages of development,
it will produce abnormal forms.
These experiments indicate not only that the nuclei undergo changes
in the course of differentiation, but also that in the course of these
changes they progressively lose their erstwhile 'totipotentiality'. The
more the cell specializes in the role of a part, the less it is capable
of creating a new whole.
This does not mean that the DNA chains which contain the genetic code of
the whole organism are lost in the differentiated cell -- it only means
that as differentiation progresses, more and more genes are debarred from
activity. Those which remain active are genetic sub-codes, governing
the particular sub-skills which the specialized cell is called on to
perform. Only the future germ-cells, segregated and protected from the
beginning, retain their total creative potential to continue the genetic
line -- they specialize in immortality as it were.
Specialization,
in morphogenesis as in other fields, exacts its price in creativity.
The transplant experiments which I have briefly mentioned, and other
evidence clearly show that while on the one hand the nuclear code
governs the activities of the cell-matrix, the cytoplasm, on the
other hand, is in communication with the cells' outer environment,
and by feeding information on the total state of affairs back to the
nucleus, co-determines which sub-code should be switched on next. The
code as a whole is unalterable; but the choice of the sub-codes to be
activated depends on the 'lie of the land', as in other skills. The
destiny of a cell depends on the composition of the cytoplasm which
it inherited from its mother cell (e.g. more animal or vegetal stuff)
and on its spatial position in the growing embryo.* We have here the
equivalent of a flexible strategy in morphogenesis: the development
of the individual cell is determined by its invariant code and by the
hazards of environment. The code represents the fixed rules of the game:
if you get into the ectoderm, you must do this; if into the endoderm,
you must do that. Both the fixity of the code and the flexibility of
strategy become more evident as we turn to later stages of development --
the matrices of morphogenetic fields, which differentiate, in hierarchic
order, into organ-systems, organs, and organ-parts.
Regulative and Mosaic Development
In the five-day-old salamander embryo, whose development is fairly typical
for vertebrates in general, transplantation experiments make it possible
to distinguish well-defined areas which will give rise to the eye, gill,
limbs, kidney, etc., although not the faintest indication of these organs
is as yet visible. At this stage, the tissue of a limb-area transplanted
into a different position on the embryo, or on another embryo, will form
a complete limb; even a heart can be formed on an embryo's flank, Such
autonomous, 'self-determining' tissue-areas are called
morphogenetic
fields
. If half of the heart, limb, or eye area is cut away, the
remainder of the field will form not half an organ, but a complete heart,
limb or eye -- just as, at the earlier, cleavage state, each half of a
frog-egg, mechanically broken up, will form a complete frog. Moreover,
if the tissue of, say, the kidney area is (by centrifuging) completely
disintegrated into freely floating separate cells distributed at random,
these cells, suspended in a proper medium, will in due time produce
rudimentary kidneys -- just as the dissociated cells of a living sponge,
which has been broken up by straining through a filter, will start to form
new cell aggregates and end up by forming a complete, normal sponge. [7]
Thus a morphogenetic field behaves 'as a unit or a whole and not merely
the sum of the cellular materials of which it is composed. The field
with its organizing capacities remains undisturbed if the cellular
material which it controls under normal circumstances is diminished or
enlarged. The unit character of the field finds its clearest manifestation
in these regulative properties.' (Hamburger. [8])
The various fields of the future organs and limbs form a mosaic in the
embryo as a whole; at the same time they display remarkable regulative
properties towards their own parts; -- they are again Janus-faced
entities. Each organ primordium is, when 'looking upward', a member of
the total matrix; when 'looking downward', a self-governing, autonomous
sub-whole. Although the future of the field in its entirety is clearly
predetermined on the mosaic principle, the future of its parts is still
dependent on regulative factors. The cell-populations which constitute
an organ-primordium have lost their genetic totipotentiality, but they
still possess a sufficient amount of multi-potentiality to keep the
matrix of the field flexible. The shape of the future organ is fixed,
but the part which a given cell-group or single cell will play in it is
again dependent on biochemical gradients and inducers in the environment,
which will trigger off the appropriate genes in the cells' genetic code.
The differentiation of organ systems, organ parts, etc., is a stepwise
affair which has been compared to the way a sculptor carves a statue
out of a block of wood. With each step in development, the functions
assigned to each group of cells become more precise, and more of its
genetic potential is suppressed -- until in the end most cells lose
even their basic freedom to divide. By the time the fertilized ovum has
developed into an adult organism, the individual cell has been reduced
from totipotentiality to almost nullipotentiality. It still carries
the coded blue-print of the whole organism in its chromosomes, but all,
except that tiny fraction of the code which regulates its specialized
activities, has been permanently switched off.
Organizers and Inducers
The embryo grows; the adult behaves. Growth is controlled by the
genetic code; adult behaviour by the nervous (and hormonal) systems.*
But in between the initial and the final stage there are some transient
controlling agencies at work, which catalyze development by a mechanism
as yet incompletely understood: the organizers or evocators.
During the earliest stages of development the growth of the embryo
takes place in a fairly stable environment, so that feedback-controls
play a relatively minor part. But with the beginning of gastrulation
the situation changes: from now on each differentiating tissue acts as
'environment' on adjacent tissues; the various types of cell-population
interact within the embryo.
A particularly important cell-population originates in the grey crescent
of the zygote; reappears as an analogous crescent on the blastula; gives
rise to the dorsal lip, migrates into the internal cavity of the gastru]a,
where it ties its place in the chordamesoderm, and becomes the so-called
'primary organizer' of the embryo, specifically concerned with initiating
its nervous system -- to which it will eventually hand over control
The tissue in the ectoderm which lies directly above it is destined to
become the neural plate -- but only if it remains in physical contact
with the organizer. If that contact is prevented, the ectoderm will not
form a neural plate and there will be no nervous system. If, on the other
hand, organizer tissue from the dorsal lip is grafted on to the flank
of another salamander embryo which is in the process of gastrulation,
it will invade the host and produce a complete Siamese twin, composed
partly of the invader's tissue and partly of host tissue. It was this
remarkable experiment, first performed by Spemann and Hilde Mangold
in 1925, which earned the privileged region of the dorsal lip in the
gastrula the name of 'primary organizer'.
At a later stage, the organizer tissue seems to differentiate into head-,
trunk-, and tail-organizers; and with the appearance of organ primordia,
its inductive functions are further divided up and handed over to centres
located in the organs themselves. A classic example of induction is the
formation of the vertebrate eye. The rudimentary brain has two sacs, or
vesicles, attached to it: the future eyes. The brain and its eye primordia
originate as thickenings of the surface area which, after the in-folding
of the neural tube, come to lie
under
the surface. So the eye
vesicles must now move outward again to make contact with the surface,
but at the same time remain attached to the brain by the optic stalks
(which will develop into the optic nerves). In the process, the vesicles
assume the shape of concave saucers, the optic cups. When these make
contact with the surface, the skin areas overlaying the contact areas
fold neatly into the hollow cups, thicken, detach themselves from the
surface, and eventually become the transparent lenses. It can be shown
that it is the optic cup which induces the skin to make a lens, for if
the cup of a frog embryo is removed, no lens will form; and vice versa,
if the eye vesicle is grafted under the embryo's belly, the belly skin
will form a lens.
However, the docility of embryonic tissue has its limits. The tissue
must be 'competent' [9] to react to the inductor; and 'competence' is
determined by the degree of differentiation the tissue has reached -- or,
put in another way, by the amount of genetic multipotential which it still
retains. An inductor 'cannot make any cell produce any specific response
unless the cell is intrinsically prepared to do so'. [10] A given
region of the ectoderm at a given stage of differentiation may retain
enough genetic flexibility to become either a lens or skin-tissue; it
will not be prepared to form a kidney. In the experimental laboratory, a
transplanted eye-vesicle can be used to induce a lens on the salamander's
belly. But under normal conditions the inductor's function is to catalyze
or 'evocate' the actualization of the genetic potentials present in the
appropriate tissue. Hence the term 'evocator-substance' for the chemical
agent responsible for induction.
A curious fact about inductors is that they seem to be organ-specific
but not species-specific. The optic cup of a frog transplanted under a
salamander embryo's skin will cause it to produce a lens; the primary
organizer of the salamader will induce brain structures not only in frogs
but even in fish; [11] and the organizer of a frog, even of a fish, can
induce secondary embryos in the obliging salamander. But the induced
embryo will be a salamander, not a frog or a fish; and the frog-skin
transplanted on to the salamander's head will form a frog-mouth, not a
salamander-mouth. In this respect, too the evocator seems to act merely
as a trigger-releaser on the genetic potential of the cell.
This assumption was confirmed when Holtfreter, J. Needham, and others
discovered that rudimentary nervous systems could be inducted in
salamander embryos by a great variety of living or dead organizers. These
include most tissues of the adult salamander itself; mouse-liver and
insect organs, molluscs, acidified salt solutions, sterols, and dye
stuffs. Moreover, it was found that some tissues (such as embryonic skin
and intestine) which cannot act as inductors when alive can do so when
killed (in alcohol or by heating). All this points to the conclusion
that the evocator of the nervous system is a non-specific chemical
agent whose function is merely 'to release the true active substance
from neighbouring cytolyzing cells'; [12] and that the substance thus
released is RNA, the carrier of the cell's genetic instructions. It
has indeed been shown that there exist distinct RNA gradients in the
inducted tissues, and that the highest RNA concentrations are found in
nervous-systems induction. Since the evocator substances, unlike hormones,
act only by direct contact, i.e. by diffusion from cell to cell, it
seems that their function is merely
to activate those RNA sub-codes
which will specify the tissues' destiny
. This is in keeping with
Hamburger's definition of embryonic induction as 'a process in which
one developing structure, the
inductor
, stimulates an adjacent
structure to undergo a specific differentiation'. [13] Artificial
induction through transplant experiments would then amount to drastic
changes in the environment of a cell-population, which interfere with
its biological time-clocks (the pre-set sequence of gene-activities);
just as the transplantation of a nucleus into a different cytoplasmic
environment causes a change in the pattern of its chromosome-puffs.
Induction is a transient method of regulating development, where
the genetic potentials of certain cell-populations, or morphogenetic
fields, are activated by chemical agencies diffused in their immediate
environment. Although the chemistry of induction is still a problematic
affair, it seems safe to assume, as Mittasch has pointed out, that
'organic catalyzers also show a rank order: beginning with the enzymes,
which are adjusted most specifically to carry through a single reaction,
to biocatalyzers such as the . . . organizer substances in animals which
regulate more or less wide complexes of processes, and up to directing
biocatalyzers, such as many hormones, that influence to a large extent
the whole organism psychophysically.' [14]
Past the early and transitory phase of induction, more advanced methods
of co-ordination and control make their appearance. In the human embryo
the' heart begins to beat at the end of the third week, controlled by its
own pace-maker, when the whole creature is less than a fifth of an inch
long. Muscle contractions in response to external stimuli can be elicited
after the eighth week, and spontaneous movements may begin in the tenth
week. They are myogenic reactions of the muscle tissue to direct local
stimulation, while the nervous system is still in the making. But the
conspicuous readiness of the neural plate to start growing in response to
non-specific 'evocators' designates it, as it were, as the heir apparent
to the earlier forms of integration.
To sum up: at various stages of embryonic development, and at various
structural levels, we find different biochemical mechanisms, but analogue
principles at work. At every stage and level the game is played according
to fixed rules but with flexible strategies (although their flexibility
is normally hidden from the eye and revealed only by the transplantation
and grafting techniques of experimental embryology). The overall rules of
the game are laid down in the complete set of instructions of the genetic
code; but the particular set of instructions operative at any level at
any time is triggered off by messages from the inter- and extra-cellular
environment, which vary in character according to structural level
and developmental stage: fertilizing agents, cytoplasmic feedbacks,
direct-contact evocators, hormones, and other catalysts.
salivary gland cells of midge larvae for instance possess large bundles of
chromosomes, which are seen as sausage-like structures, with occasional
swellings or puffs -- the so-called Balbiani rings. The puffs are the
sites of intense RNA production; it is therefore assumed that they
indicate active genes. The pattern of these swellings changes according
to the age of the larva and the type of cell.*
The action of the cytoplasm on the nucleus has also been directly
demonstrated. When the nucleus from the salivary gland cell of a
drosophila larva is transplanted into the cytoplasmic environment of a
cell at an earlier stage of development, the chromosomes again undergo
very marked changes: certain swellings disappear and others appear in
their place. This clearly indicates the existence of a feedback mechanism
whereby the development of the cytoplasm as a result of gene activity
in its turn calls forth the activity of particular genes. [6]
The reverse type of experiment demonstrates the action of nuclei of
different ages on the same cytoplasmic environment. The nucleus of
an unfertilized frog-egg is removed, and replaced by a nucleus from
a developing frog-embryo. If the transplant nucleus was taken from an
embryo in the early blastular stage, the result will be a clone of normal
tadpoles; if it was obtained from embryos in later stages of development,
it will produce abnormal forms.
These experiments indicate not only that the nuclei undergo changes
in the course of differentiation, but also that in the course of these
changes they progressively lose their erstwhile 'totipotentiality'. The
more the cell specializes in the role of a part, the less it is capable
of creating a new whole.
This does not mean that the DNA chains which contain the genetic code of
the whole organism are lost in the differentiated cell -- it only means
that as differentiation progresses, more and more genes are debarred from
activity. Those which remain active are genetic sub-codes, governing
the particular sub-skills which the specialized cell is called on to
perform. Only the future germ-cells, segregated and protected from the
beginning, retain their total creative potential to continue the genetic
line -- they specialize in immortality as it were.
Specialization,
in morphogenesis as in other fields, exacts its price in creativity.
The transplant experiments which I have briefly mentioned, and other
evidence clearly show that while on the one hand the nuclear code
governs the activities of the cell-matrix, the cytoplasm, on the
other hand, is in communication with the cells' outer environment,
and by feeding information on the total state of affairs back to the
nucleus, co-determines which sub-code should be switched on next. The
code as a whole is unalterable; but the choice of the sub-codes to be
activated depends on the 'lie of the land', as in other skills. The
destiny of a cell depends on the composition of the cytoplasm which
it inherited from its mother cell (e.g. more animal or vegetal stuff)
and on its spatial position in the growing embryo.* We have here the
equivalent of a flexible strategy in morphogenesis: the development
of the individual cell is determined by its invariant code and by the
hazards of environment. The code represents the fixed rules of the game:
if you get into the ectoderm, you must do this; if into the endoderm,
you must do that. Both the fixity of the code and the flexibility of
strategy become more evident as we turn to later stages of development --
the matrices of morphogenetic fields, which differentiate, in hierarchic
order, into organ-systems, organs, and organ-parts.
Regulative and Mosaic Development
In the five-day-old salamander embryo, whose development is fairly typical
for vertebrates in general, transplantation experiments make it possible
to distinguish well-defined areas which will give rise to the eye, gill,
limbs, kidney, etc., although not the faintest indication of these organs
is as yet visible. At this stage, the tissue of a limb-area transplanted
into a different position on the embryo, or on another embryo, will form
a complete limb; even a heart can be formed on an embryo's flank, Such
autonomous, 'self-determining' tissue-areas are called
morphogenetic
fields
. If half of the heart, limb, or eye area is cut away, the
remainder of the field will form not half an organ, but a complete heart,
limb or eye -- just as, at the earlier, cleavage state, each half of a
frog-egg, mechanically broken up, will form a complete frog. Moreover,
if the tissue of, say, the kidney area is (by centrifuging) completely
disintegrated into freely floating separate cells distributed at random,
these cells, suspended in a proper medium, will in due time produce
rudimentary kidneys -- just as the dissociated cells of a living sponge,
which has been broken up by straining through a filter, will start to form
new cell aggregates and end up by forming a complete, normal sponge. [7]
Thus a morphogenetic field behaves 'as a unit or a whole and not merely
the sum of the cellular materials of which it is composed. The field
with its organizing capacities remains undisturbed if the cellular
material which it controls under normal circumstances is diminished or
enlarged. The unit character of the field finds its clearest manifestation
in these regulative properties.' (Hamburger. [8])
The various fields of the future organs and limbs form a mosaic in the
embryo as a whole; at the same time they display remarkable regulative
properties towards their own parts; -- they are again Janus-faced
entities. Each organ primordium is, when 'looking upward', a member of
the total matrix; when 'looking downward', a self-governing, autonomous
sub-whole. Although the future of the field in its entirety is clearly
predetermined on the mosaic principle, the future of its parts is still
dependent on regulative factors. The cell-populations which constitute
an organ-primordium have lost their genetic totipotentiality, but they
still possess a sufficient amount of multi-potentiality to keep the
matrix of the field flexible. The shape of the future organ is fixed,
but the part which a given cell-group or single cell will play in it is
again dependent on biochemical gradients and inducers in the environment,
which will trigger off the appropriate genes in the cells' genetic code.
The differentiation of organ systems, organ parts, etc., is a stepwise
affair which has been compared to the way a sculptor carves a statue
out of a block of wood. With each step in development, the functions
assigned to each group of cells become more precise, and more of its
genetic potential is suppressed -- until in the end most cells lose
even their basic freedom to divide. By the time the fertilized ovum has
developed into an adult organism, the individual cell has been reduced
from totipotentiality to almost nullipotentiality. It still carries
the coded blue-print of the whole organism in its chromosomes, but all,
except that tiny fraction of the code which regulates its specialized
activities, has been permanently switched off.
Organizers and Inducers
The embryo grows; the adult behaves. Growth is controlled by the
genetic code; adult behaviour by the nervous (and hormonal) systems.*
But in between the initial and the final stage there are some transient
controlling agencies at work, which catalyze development by a mechanism
as yet incompletely understood: the organizers or evocators.
During the earliest stages of development the growth of the embryo
takes place in a fairly stable environment, so that feedback-controls
play a relatively minor part. But with the beginning of gastrulation
the situation changes: from now on each differentiating tissue acts as
'environment' on adjacent tissues; the various types of cell-population
interact within the embryo.
A particularly important cell-population originates in the grey crescent
of the zygote; reappears as an analogous crescent on the blastula; gives
rise to the dorsal lip, migrates into the internal cavity of the gastru]a,
where it ties its place in the chordamesoderm, and becomes the so-called
'primary organizer' of the embryo, specifically concerned with initiating
its nervous system -- to which it will eventually hand over control
The tissue in the ectoderm which lies directly above it is destined to
become the neural plate -- but only if it remains in physical contact
with the organizer. If that contact is prevented, the ectoderm will not
form a neural plate and there will be no nervous system. If, on the other
hand, organizer tissue from the dorsal lip is grafted on to the flank
of another salamander embryo which is in the process of gastrulation,
it will invade the host and produce a complete Siamese twin, composed
partly of the invader's tissue and partly of host tissue. It was this
remarkable experiment, first performed by Spemann and Hilde Mangold
in 1925, which earned the privileged region of the dorsal lip in the
gastrula the name of 'primary organizer'.
At a later stage, the organizer tissue seems to differentiate into head-,
trunk-, and tail-organizers; and with the appearance of organ primordia,
its inductive functions are further divided up and handed over to centres
located in the organs themselves. A classic example of induction is the
formation of the vertebrate eye. The rudimentary brain has two sacs, or
vesicles, attached to it: the future eyes. The brain and its eye primordia
originate as thickenings of the surface area which, after the in-folding
of the neural tube, come to lie
under
the surface. So the eye
vesicles must now move outward again to make contact with the surface,
but at the same time remain attached to the brain by the optic stalks
(which will develop into the optic nerves). In the process, the vesicles
assume the shape of concave saucers, the optic cups. When these make
contact with the surface, the skin areas overlaying the contact areas
fold neatly into the hollow cups, thicken, detach themselves from the
surface, and eventually become the transparent lenses. It can be shown
that it is the optic cup which induces the skin to make a lens, for if
the cup of a frog embryo is removed, no lens will form; and vice versa,
if the eye vesicle is grafted under the embryo's belly, the belly skin
will form a lens.
However, the docility of embryonic tissue has its limits. The tissue
must be 'competent' [9] to react to the inductor; and 'competence' is
determined by the degree of differentiation the tissue has reached -- or,
put in another way, by the amount of genetic multipotential which it still
retains. An inductor 'cannot make any cell produce any specific response
unless the cell is intrinsically prepared to do so'. [10] A given
region of the ectoderm at a given stage of differentiation may retain
enough genetic flexibility to become either a lens or skin-tissue; it
will not be prepared to form a kidney. In the experimental laboratory, a
transplanted eye-vesicle can be used to induce a lens on the salamander's
belly. But under normal conditions the inductor's function is to catalyze
or 'evocate' the actualization of the genetic potentials present in the
appropriate tissue. Hence the term 'evocator-substance' for the chemical
agent responsible for induction.
A curious fact about inductors is that they seem to be organ-specific
but not species-specific. The optic cup of a frog transplanted under a
salamander embryo's skin will cause it to produce a lens; the primary
organizer of the salamader will induce brain structures not only in frogs
but even in fish; [11] and the organizer of a frog, even of a fish, can
induce secondary embryos in the obliging salamander. But the induced
embryo will be a salamander, not a frog or a fish; and the frog-skin
transplanted on to the salamander's head will form a frog-mouth, not a
salamander-mouth. In this respect, too the evocator seems to act merely
as a trigger-releaser on the genetic potential of the cell.
This assumption was confirmed when Holtfreter, J. Needham, and others
discovered that rudimentary nervous systems could be inducted in
salamander embryos by a great variety of living or dead organizers. These
include most tissues of the adult salamander itself; mouse-liver and
insect organs, molluscs, acidified salt solutions, sterols, and dye
stuffs. Moreover, it was found that some tissues (such as embryonic skin
and intestine) which cannot act as inductors when alive can do so when
killed (in alcohol or by heating). All this points to the conclusion
that the evocator of the nervous system is a non-specific chemical
agent whose function is merely 'to release the true active substance
from neighbouring cytolyzing cells'; [12] and that the substance thus
released is RNA, the carrier of the cell's genetic instructions. It
has indeed been shown that there exist distinct RNA gradients in the
inducted tissues, and that the highest RNA concentrations are found in
nervous-systems induction. Since the evocator substances, unlike hormones,
act only by direct contact, i.e. by diffusion from cell to cell, it
seems that their function is merely
to activate those RNA sub-codes
which will specify the tissues' destiny
. This is in keeping with
Hamburger's definition of embryonic induction as 'a process in which
one developing structure, the
inductor
, stimulates an adjacent
structure to undergo a specific differentiation'. [13] Artificial
induction through transplant experiments would then amount to drastic
changes in the environment of a cell-population, which interfere with
its biological time-clocks (the pre-set sequence of gene-activities);
just as the transplantation of a nucleus into a different cytoplasmic
environment causes a change in the pattern of its chromosome-puffs.
Induction is a transient method of regulating development, where
the genetic potentials of certain cell-populations, or morphogenetic
fields, are activated by chemical agencies diffused in their immediate
environment. Although the chemistry of induction is still a problematic
affair, it seems safe to assume, as Mittasch has pointed out, that
'organic catalyzers also show a rank order: beginning with the enzymes,
which are adjusted most specifically to carry through a single reaction,
to biocatalyzers such as the . . . organizer substances in animals which
regulate more or less wide complexes of processes, and up to directing
biocatalyzers, such as many hormones, that influence to a large extent
the whole organism psychophysically.' [14]
Past the early and transitory phase of induction, more advanced methods
of co-ordination and control make their appearance. In the human embryo
the' heart begins to beat at the end of the third week, controlled by its
own pace-maker, when the whole creature is less than a fifth of an inch
long. Muscle contractions in response to external stimuli can be elicited
after the eighth week, and spontaneous movements may begin in the tenth
week. They are myogenic reactions of the muscle tissue to direct local
stimulation, while the nervous system is still in the making. But the
conspicuous readiness of the neural plate to start growing in response to
non-specific 'evocators' designates it, as it were, as the heir apparent
to the earlier forms of integration.
To sum up: at various stages of embryonic development, and at various
structural levels, we find different biochemical mechanisms, but analogue
principles at work. At every stage and level the game is played according
to fixed rules but with flexible strategies (although their flexibility
is normally hidden from the eye and revealed only by the transplantation
and grafting techniques of experimental embryology). The overall rules of
the game are laid down in the complete set of instructions of the genetic
code; but the particular set of instructions operative at any level at
any time is triggered off by messages from the inter- and extra-cellular
environment, which vary in character according to structural level
and developmental stage: fertilizing agents, cytoplasmic feedbacks,
direct-contact evocators, hormones, and other catalysts.
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