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Inside the Living Cell

Inside
the Living Cell

SOME SECRETS OF LIFE

J. A.V.BUTLER

D.SC, F.R.S.

New York
BASIC BOOKS, INC.

©

George Allen and Unwin Ltd 1959

Library of Congress Catalog Card Number 59-8434
printed in the united states of america

ACKNOWLEDGMENTS

I am greatly indebted to the following for providing the excellent
photographs reproduced in this book: my colleagues, Messrs E. J.
Ambrose, M. S. C. Birbeck, Drs O. G. Fahmy, E. H. Mercer and
S. H. Revell and Professor P. KoUer, of the Chester Beatty Research
Institute; Dr J. C. Kendrew of the mrc Unit for Molecular Biology,
Cambridge; Professor J. Z. Young, Dr H. E. Huxley and Dr D. Sholl
of University College, London; Dr F. Sjostrand of the Department of
Anatomy, Karolinska Institute, Stockholm; Dr R. W. G. Wyckoff;
and Professor F. Jacob of the Institut Pasteur, Paris. The following
gave me permission to use hne drawings: Dr Lorente de No and the
Oxford University Press; Dr W. F. Floyd of the Middlesex Hospital
Medical School; Professor G. W. Beadle and Professor G. A. Baitsell
and the University Presses of Oxford and Yale. I am indebted to The
Pergamon Press for the loan of a block.
To all I tender my sincere thanks.

J.A.V.B.

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PREFACE

'Wot I like in that 'ere style of writin',' said the elder Mr Weller,
'is that there ain't no callin' names in it.' There is plenty of 'callin'
names' in this book and it is doubtful if Mr Weller would have
approved of it. Many names are inevitable, even in a popular book
about the nature of life, and the writer's chief problem is to know
where to draw a line between what may be expected to be intelligible
to general readers and technicalities. I have tried to make the style of
this book reasonably simple while still maintaining a level of informa-
tion and comment which, I hope, will not be without interest to
students of science and also scientists who are not experts. No doubt
readers will find some portions more difficult than others, but no
harm will be done if they skip passages which they do not like.

It would be impossible to give authority for all the facts quoted
without overburdening the text with references. Some indications as
to where further information can be found are given in an Appendix,
for those readers who would like to follow up particular subjects. The
text is also supplemented by an Appendix containing a collection of
chemical formulae of some of the compounds mentioned.

In writing this book I have made use, when appropriate, of
passages from a previous book, Man is a Microcosm, which is now out
of print. But the scope of the present book is much wider, quite apart
from the fact that many parts of the subject have advanced radically
in the last few years.

I am much indebted to Dr P. Rosbaud, who read the ms and made
useful suggestions, and for his friendly assistance. I am also grateful
to Miss Jill Emmerson, for coping with an untidy manuscript.

London
July 20, 1958

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CONTENTS

PREFACE 9

I Living Cells and What They Are Made Of 15

II How the Cell Lives 22

III Vitamins and Antivitamins 28

IV Cell Division 38
V Inside the Cell 46

VI Genes and Mutations 54
VII Effects of X-rays and Atomic Radiations on

Living Cells 58

VIII Viruses — Cell Parasites 67

IX A Community of Cells 75

X Chemical Messengers 80

XI Antibodies and Immunity 86

XII Cancer 93

XIII The Origin of Life and Photosynthesis 101

XIV Specialized Cells: Muscles, Nerves and Sense

Organs 110

XV The Brain and What It Does 121

XVI Actions and How They Are Performed 131

XVII Using Tools and Symbols 137

XVIII Is the Brain a Calculating Machine? 143

XIX Ageing and Death 150

XX Life in the Universe 156

APPENDIX I— Chemical Formulae 163

APPENDIX II — Selected References and Books for further

Reading 167

INDEX 171

ILLUSTRATIONS

1 A model of a protein molecule 16

2 An interference photograph of chromosomes in living

cells 17

3 Chromosomes in the nucleus of cells from the growing

tip of an onion root 32

4 The giant banded chromosomes of the saUvary glands of

the larva of the fly 33

5 A section of a rat Uver taken with the electron microscope 48

6 A portion of a liver cell taken with the electron micro-

scope at a higher magnification 49

7 Fine structure in the cell interior 64

8 Electron micrographs of Influenza A virus adhering to a

red fowl cell 65

9 (a) The eff*ect of a chemical substance (a nitrogen mustard)

on the chromosomes of a tumour cell 80
(b) The eff'ect of X-rays on the chromosomes of a broad

bean cell 80

10 (a) Turnip yellow virus 81
(b) Two dissimilar bacterial cells (B. coli) in conjugation 81

1 1 Bacteriophage attacking a bacterium 96

12 A later stage of the attack of Bacteriophage 97

13 A cross-section of a chloroplast from Aspidistra elatior 1 12

14 Electron micrographs of muscle fibrils 113

15 A nerve cell of the mammalian spinal chord and the

associated nerve fibre 128

16 Nerve cells in the visual cortex of the brain of a cat 129

and 28 figures in text /f^^L ^ <)\

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Living Cells and
What They Are Made Of

H

Since all living things are either single cells or colonies of cells, the cell
is the real unit of life and we shall have gone far in the understanding
of life if we can find out what a single cell is made of and how it works.
However, this is a colossal undertaking. Although it is tiny — the
average cell is not more than 1 / 100 of a millimetre in diameter — we
shall see that it contains a very large number of chemical substances,
all connected by beautiful and complex mechanisms into a function-
ing whole. I shall try to explain something about these substances and
about these mechanisms — in other words, about the life of the living
cell.

We do not usually need to specify, at least for a bird's-eye view,
what organism our cell belongs to. The remarkable fact has been
established — and this is one of the most significant findings of the
study of the chemistry of life — that all living organisms are very simi-
larly constructed and operate in very similar ways. There are naturally
quite large differences, e.g. between the cells of a woody plant, which
contain much cellulose, and those of, say, a jellyfish in which the cells
are entirely made of a jelly-like substance called protoplasm; but it
is found that the basic organization of all cells has much in common.
They are constructed to a considerable extent out of similar materials
and the chemical operations which occur in them are basically the
same. In other words all living things, from amoeba to man, are one
family. The smallest body which exhibits all the properties of living
things is the cell and it is only by studying life at the level of cells and
their contents that we can hope to understand the secrets of living
processes.

First of all, then, we must know something of what cells are made
of. This study only really began in this century because it was impos-
sible to describe adequately the characteristic substances present in
cells until chemistry had advanced sufficiently to cope with them. In

16 Inside the Living Cell

fact, only during the last twenty or thirty years has much progress
been made in this study.

It has been known for a long time that living things are made of the
same elementary substances as we are familiar with in ordinary life —
carbon, oxygen, hydrogen, nitrogen and phosphorus and calcium are
the commonest elements, but there are smaller quantities of many
others.

A great profusion of ^organic' substances, like oils, fats, waxes,
starch, resins, alcohols, sugars, had been obtained from living things.
They were termed 'organic' to distinguish them from the 'inorganic'
substances found in the earth, such as salt, ores, rocks, etc. An exact
chemistry of 'organic' substances is a fairly recent development of
science, going back only about one hundred years. Before that time,
they were thought to be an entirely different type of material from
'inorganic' substances. But the distinction broke down when a Ger-
man chemist, Wohler, prepared in his laboratory from purely 'in-
organic' materials (ammonia and carbon dioxide) the substance urea,
which had previously been regarded as a typical product of life. Since
then chemists have been able to synthesize in their laboratories an
enormous number of 'organic' substances, including not only many
which occur in nature, but also many thousands of others for which
no natural counterpart exists. They were guided in this work by a
knowledge of the rules of combination of atoms which were estab-
lished by Van't Hoff and Kekule. On the basis of these rules, it is pos-
sible to derive a formula for a compound which shows how its atoms
are joined together. The formula is usually established by studying
the reactions of the compound and by breaking it down into its com-
ponent parts. It can be confirmed by building up a compound as rep-
resented by the formulae, to see if it corresponds with the original
compound.

It soon appeared that it would be only a matter of time before all
the compounds present in living cells could be synthesized in this way.
However, it was eventually found that the more characteristic com-
pounds of living cells, such as proteins, had a much greater com-
plexity for which chemists were hardly prepared.

It had been recognized early in the nineteenth century that the most
characteristic of the substances of living things were a type of nitro-
genous compound called 'proteins'. These are familiar to everyone.
They constitute a large part of animal tissues, e.g. they are the prin-
cipal constituent of muscle, skin, nails, hair, the red cells of blood and
the soluble substances present in blood serum, the casein of milk.

Every form of life — even plants, corals, jellyfish and bacteria —
are rich in proteins of many kinds, and no organism has been found

1 A model of a protein molecule — myoglobin, a protein present in
muscle and other tissues, which serves as an oxygen store. Poly-
peptide chains are white; the grey disc is the haem group. The small
spheres are marker atoms attached to the molecule. Marks on the
scale are 1 Angstrom Unit (10"* cm.) apart. (Photograph by Dr J.
C. Kendrew)

2 An interference photograph of chromosomes in living cells of
Schistocerea gregaria (grasshopper). (From Proc. Royal Soc, by
permission of Mr E. J. Ambrose)

Living Cells and What They Are Made Of 17

which does not contain protein in every part. We are justified in re-
garding proteins as the characteristic typical substances of life.

The constitution of proteins, therefore, became a matter of great
interest. It was found that they are composed of simpler substances,
known as amino acids. The characteristic feature of these compounds
is that they contain both 'acidic' and 'basic' parts.^ Because of this they
are capable of uniting with each other, the basic group of one amino
acid combining with the acidic group of another. (Fig. 1.) The

Five

different
amino
acids

A

peptide

chain

FIG. 1. A diagrammatic representation of the molecules of five different amino
acids, showing how they combine together (with the elimination of water,
HOH) to form a peptide chain. This is the basic structure of proteins. The
different characteristic groups of the amino acids are represented by con-
ventional signs. For their chemical formulae see Appendix 1

amino acids have, as it were, two hands : an acidic hand and a basic
hand. Suppose all the acidic hands wear red gloves and all the basic
hands wear black gloves; then the protein can be pictured as like a
row of people in which every hand v/ith a red glove clasps his neigh-
bour's black glove.

In this way long chains of amino acid groups are formed. As there
are found in proteins about twenty separate kinds of amino acids,
which can be joined together in this way, it is easy to see that the num-
ber of ways in which they can be arranged is almost endless. It is thus
possible to have a very large number of different 'chains' of amino

^ The acid group is CO. OH and the basic group H:>N
R

18 Inside the Living Cell

acids, according to the proportions of the amino acids which they
contain and the order in which they come.

The study of proteins was delayed because their molecules were
found to be very much larger than the molecules with which the
chemist had been accustomed to deal. This presented problems of
purification of a novel kind, and for a long time it was not known
whether typical proteins were composed of many molecules of one
kind or were mixtures of numerous different molecules. This could
not be settled until ways were discovered of analysing mixtures of
proteins into their component parts.

This analysis was greatly assisted by the discovery that some
soluble proteins could be induced to crystallize. It had been dis-
covered by Hofmeister, in 1890, that egg albumin, the principal con-
stituent of the white of eggs, could be obtained as crystals. The red
pigment of the blood of animals had also been obtained in a crystalline
state by very simple means as early as 1867.

Now a crystalline form is only possible when all the molecules are
either identical or have very similar shapes. So the crystallization of a
protein is by itself an act of purification and means that we have ob-
tained a substance in which all the particles are either the same, or at
least very similar in size and shape.

Physical methods of studying the molecules of proteins were also
developed. Professor The Svedberg of Uppsala, Sweden, invented an
instrument called the 'ultracentrifuge' in which a solution of the
protein was whirled in a small tube at a very high speed — about one
thousand revolutions per second. The heavier the particle, the more
rapidly does it move away from the centre of rotation under the
influence of the centrifugal force. From the rate of movement, which
could be observed by a rather complicated optical arrangement, it
was possible to deduce the weights of the molecules of proteins.

In these experiments, it was found first that well-purified proteins
(and especially those which had been crystallized) were composed
of molecules of the same size. Secondly, the weights of the molecules
of proteins were much greater than those of simple organic sub-
stances like sugar, which chemists were accustomed to handle.
Taking the weight of the hydrogen atom as unity, simple organic
substances like alcohol or sugar have molecular weights between
50 and 200. Larger molecules had been made with weights up to a
thousand or even more, but with proteins molecular weights of many
thousands are common. One of the simplest protein-like substances
known is insulin, with a molecular weight of about 5,000. But most
protein molecules are much larger than this and have weights of
60,000-100,000, while in others the molecular weight is a million or

Living Cells and What They Are Made Of

19

more. Fig. 2 shows the comparative sizes of some protein molecules
and some of the larger particles we shall meet later.

It is evident that the task of determining how the constituent
amino acids are arranged in molecules of this magnitude is a very
difficult one. Up to the present it has only been completely achieved
in the one case of insulin, which has been studied by Dr F. Sanger, of
Cambridge. Analysis showed that the molecule of insulin contained

Foot and
mouth virus

Tobacco mosaic virus (half-length) 15,000,000 to 20,000,000 I

Hemocyanin molecule (6,000,000)

mZD Hemoglobin molecule (63,000)
l^" Albumin molecule (40,000)

— Amino acid chain -10 units (1,300)

• Sugar molecule (350)

• Single amino acid molecule (130)

0.1 ;x=100 m^jL = jo^oo
of a millimeter

FIG. 2. Relative sizes of molecules of simple substances and proteins
and of virus particles and bacteria

fifty-one amino acids of fifteen different kinds. These were arranged
in two distinct chains, one containing twenty-one amino acids and
the other with thirty amino acids. The exact order of the amino acids
in both of these chains have been determined by Dr Sanger and is
shown in Fig. 3. The two chains are joined together in two places by
cystine, which contains sulphur, through which the junction is made.
Insulin is a protein which is extracted from the pancreas of
animals. It is of great importance because it is involved in the utiliza-
tion of sugar in the body. The human disease, diabetes — an inability

20

Inside the Living Cell

phe

I

val

I
asp

I

glu

I

his

I

leu

I

cys

I
glu

I
ser

I

his

I

leu

val
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glu

I
ala

I
leu

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tyr

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leu

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val

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cys ■

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glu

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arg
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g'y
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phe

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phe

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tyr

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thr

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pro

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hys

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ala

— S S —

\

S —

g'y
I

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val

glu

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cys

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thr

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tyr

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glu-

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leu

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glu

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asp'

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tyr

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asp-

NH2

S

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S

B

NH2

NH-

•NH-

Variants in position B

Cattle

ala
I

g'y
I

val

Sheep

ala

I
ser

I
val

FIG. 3. The arrangement of the fifty-one amino acids in the molecule of pig
insulin (for full names, see Appendix, p. 162). Insulins from other species
differ slightly in the short section marked B. asp — NH2 is asparigine and
glu — ^NH2 is glutamine.

Living Cells And What They Are Made Of 21

to utilize sugar, which then accumulates in the blood — is due to a
deficiency of insulin. It is well known that this deficiency in human
beings can be made good by injecting insuHn of animal origin; and
for this reason large quantities of animal insulins are prepared.

Sanger's experiments showed that the arrangement of amino
acids in beef insulin is quite unique. Every molecule has the same
amino acids in precisely the same order. Experiments have also
shown that a very slight modification of insulin, e.g. the removal or
modification of only one of the amino acids, may spoil its activity
as an anti-diabetic agent. When a comparison was made of insulins
prepared from different species, the ox, pig and sheep, they were
found to be very similar, differing only in a small section of three
amino acids only, in which some variation of the insulins from
different sources was found. ^

We meet here one of the most characteristic features of living
organisms — their ability to construct long protein chains, in which
the order of the amino acids is precisely maintained. We shall see
that this ability is the basic characteristic of living things, which
enables them to make the large numbers of highly specific and
exactly constructed proteins which they need for their purposes.

Plate 1 shows a model of a much more complicated protein, myo-
globin, which contains several peptide chains, as worked out by
Dr J. C. Kendrew.

' The variations are in the section of chain marked B which is bridged by -S-S-
(Fig. 3), i.e. in the positions 8, 9 and 10 from the top.

n

How the Cell Lives
H

Before we go any further, we must consider a little what goes on
inside the living cell. All living cells are necessarily the seat of
intense chemical activity. In order to live they must have food, which
enters into the cell and is transformed in various ways. It may be
burnt to provide energy or changed into substances needed for
growth, and the products which are not wanted are given out.

Plants are extremely modest in their requirements. From carbon
dioxide taken from the atmosphere, water and simple inorganic
nutrients, which contain nitrogen, phosphorus and a few more in-
organic elements, they are able, with the help of the energy of light,
to construct all the chemical compounds, including proteins, which
they require. Animal cells have to be provided with a greater variety
of foods. As a rule they require, at least, the simpler compounds
synthesized in plants; but from these they obtain what they need to
live. From the oxidation of sugars they obtain the energy which is
required to keep them alive. While single living cells like bacteria
are able to carry out all the processes necessary for living, in higher
organisms there is some specialization, i.e. some cells perform one
task and others mainly another.

It is obvious that the life of all cells involves a multitude of
chemical actions which are carried and with a speed and smoothness
which the laboratory chemist cannot approach. In practice, chemical
synthesis in the laboratory is often very diflScult. Compounds have
to be transformed one step at a time, often by wasteful processes,
using powerful chemicals, heat, and sometimes electrical actions, to
bring about the desired changes. It may take months to build up a
compound, by a complicated sequence of actions, which a cell can
make in a matter of minutes. Many bacteria completely reproduce
themselves in half an hour, and in that time everything they contain
is duplicated.

How are these manifold changes brought about? We will take a

How the Cell Lives 23

simple living thing — the yeast cell — a unicellular organism which
causes fermentation of sugar. It is a jelly-like bag, containing a vis-
cous sticky fluid. It is in this fluid that the chemical changes take
place. Yeast will grow in a liquid containing a few simple compounds
like sugar, nitrogen compounds, phosphate, and a few other essential
nutrients. These enter the cell through its skin or membrane, and
when inside are transformed into many other compounds. The
nitrogen compounds are built up into the proteins required for
growth and reproduction, the sugars are partly elaborated into more
complex carbohydrates and partly burnt to provide the energy
required for the other transformations, yielding either carbon dioxide
or alcohol, according to the circumstances.

This property of yeast was discovered in the early stages of pastoral
civilization. The carbon dioxide was used to leaven the dough in the
baking of bread; and the production of alcohol has been used for the
manufacture of alcoholic drinks from time immemorial.

If you were to ask a chemist to take sugar and turn it into alcohol,
he could do it, but only with great difficulty and through a long and
complex series of changes. In fact most of the alcohol of commerce
is manufactured by the fermentation of sugars by the humble yeast
cell. How does it bring about this change with such surprising ease?
At one time it was thought that only the living cells could bring
about such changes, which were connected with the essential vital
activity of the cell. Then a German chemist, Biichner, showed that
it was possible to squeeze the juice out of the living cells, and this
juice, freed of cell fragments, was capable of fermenting sugar. It
contained substances, which he called enzymes^ which bring about
these changes.

Before this, substances which assist chemical change had been
discovered and called catalysts; for example, platinum, which makes
hydrogen and oxygen combine. Enzymes were evidently vital cata-
lysts or bio-catalysts\ but for many years, indeed until quite recently,
their nature has been quite unknown. Many chemists tried unsuccess-
fully to purify and isolate them. They were found to be both varied
and ubiquitous.

Before we examine their nature, let me gives a few examples of
enzymes. When food passes through the digestive tract of animals,
it encounters enzymes throughout its passage which break it down
and make it suitable for absorption. In the stomach there is pepsin,
which begins the break-down of proteins. Into the intestine, the
pancreas pours other enzymes, which continue the digestion of
protein and break down starch into sugars, digest the fats and so on.
A very large number of enzymes, which facilitate a multitude of

24 Inside the Living Cell

chemical changes, have been distinguished in every kind of living
organism and no doubt there are many others to be discovered. All
living things contain them, not only in their digestive organs, but
also in every tissue. The sap of the growing plant contains a host of
them. Even in the seed there are enzymes packed away, which will
act on the starch when germination begins and produce the sugar
which feeds the plant until it has roots and leaves.

A large number of enzymes are also concerned with the oxidation
of carbohydrates. When conducted in the cell this is by no means a
simple process like the burning of wood. In the cell it is necessary
that the energy given out on oxidation should be obtained in a form
in which it is available for use. A great deal of work has been done
to elucidate the very complex sequence of reactions which occur.
The main result has been to find that the oxidation processes are
coupled with reactions involving phosphates, as the result of which
'high-energy' phosphate compounds are formed. In these compounds
the energy is available for bringing about the reactions which the
cell has to perform, and they are also used in muscle cells in the pro-
cesses by which mechanical work is performed.

What are the enzymes which have these remarkable properties?
In 1926, Professor J. B. Sumner, of Ithaca, obtained from jack beans
a crystalline substance which had the property of breaking down
urea. It appeared to be the enzyme called urease, and further investi-
gation has confirmed this. It was the first enzyme to be prepared in a
crystalline condition as a pure substance, so that its nature could be
determined. It was found to be a protein. Since then a considerable
number of enzymes has been isolated and many have been obtained
in the crystalline state. Pepsin, the protein-splitting enzyme of the
stomach, was crystallized by Dr J. H. Northrop at Princeton in
1929. Later the animal pancreas was found to be a rich mine of
proteins, and Drs Northrop and Kunitz and their collaborators
isolated from it a succession of crystalline enzymes, such as trypsin
and chymotrypsin, which are poured into the intestine and complete
the digestion of the food. Enzymes have also been isolated from
varied materials such as yeast, malt, potatoes, fruit juices and milk.

All these substances are very powerful bio-catalysts, i.e. they
bring about rapid chemical changes when present in very small
amounts, without being changed themselves. A solution containing a
ten millionth of an ounce of pepsin has a powerful effect on clotting
of milk; rennin, another enzyme present in calves' stomachs and
used for making junkets and cheese, can clot ten million times its
weight of milk in ten minutes. Urease crystals produce a hundred
times their weight of ammonia from urea in five minutes; it is said

How the Cell Lives 25

that one molecule of catalase can decompose over two million mole-
cules of hydrogen peroxide every minute.

All these enzymes turned out to be proteins. In this way it became
evident that besides their other functions, Nature makes use of
proteins as 'biocatalysts'. Enzymes are indeed highly speciahzed
agents. There is usually a separate enzyme for each distinct chemical
process. Sometimes an enzyme will bring about the change in a
single substance only; but others are capable of changing a number
of closely related substances.

Very little is known about how enzymes bring about these
chemical actions. It has been shown that the first step is the com-
bination of the enzyme with the substance undergoing change. This
combination is very specific. The enzyme will combine with one, or
perhaps a few similar molecules, but with no others. There must
thus be a close fit between the enzyme and the molecule or mole-
cules it acts on. The molecules must fit exactly into some part of the
surface of the enzyme molecule.

What happens then is largely a matter of conjecture, but it is not
difficult to see that if the enzyme is combined firmly with the two
halves of a molecule, the bond joining them may be weakened so
that the molecule easily divides into the two halves. The binding of
the molecule with the enzyme will necessarily weaken the strength
of the bonds holding the molecule together. In this way the enzyme
can facilitate the rupture of a molecule into two parts. Readers who
are interested will see the idea worked out in more detail in Fig. 4.

The synthesis of a molecule from its parts will also be facilitated
if they fit closely into adjacent parts of the enzyme surface, so that
they are brought into juxtaposition under conditions in which they
will then unite. The enzyme provides a kind of *jig' which holds the
bits together while a bond between them is formed.

In either case the important thing is that the enzyme must provide
a structure which is exactly adapted to the molecules it has to deal
with.

Life is largely a matter of enzymes. All organisms contain large
numbers of them — all working together and bringing about the
extraordinary changes which life produces in its environment. Many
kinds of them exist in living things. Not only is it found that every
organism has a considerable number of enzymes, but it also appears
that an enzyme of one species is not exactly the same as an enzyme
with similar functions in another. The enzymes are therefore not
only highly adapted to the chemical changes they have to bring
about; they are also stamped in some way with the individuality of
the species they belong to. We do not know how this is achieved.

26

Inside the Living Cell

But in the amino acid chains of proteins, which were described in
Chapter I, we have seen that there exists the possibility of almost
innumerable different patterns. Nature has apparently taken advan-
tage of this to produce the many specialized patterns of enzymes, but

/'compound^

Stage I. The compound

approaches the enzyme surface

ENZYME SURFACE

Stage ll.The compound combines
with the enzyme surface

COMPOUND BROKEN

Stage III. The compound

is broken into two parts

\_/PRODUCTS

ENZYME SURFACE

Stage IV. The products are

released from the enzyme surface

FIG. 4. How an enzyme acts. Characteristic of enzyme action is the high
degree of specificity, i.e., a close fit between the enzyme and the compound
it acts on

at present we are ignorant as to the nature of the patterns which
constitute the active groupings in the enzymes.

It is an extraordinary fact — and an unexpected one — that the
simplest organisms seem to be provided with enzymes which are
very similar to those of the highest organisms. Even the simplest
forms of Hfe which have survived to this day have practically all the
types of enzymes which we find in the higher animals. It is possible
that new types may be discovered in the higher animals but so far

How the Cell Lives 27

we have no clear evidence in favour of this possibility. If life has
evolved from simple unicellular organisms to complex ones, there
does not seem to have been any great improvement in the enzymes.
There has merely been an increase in specialization and in the possi-
bilities of cooperation and of division of labour between cells.

In fact, as we have seen, it is usually the case that the simpler
organisms, in order to live, have to carry out a greater range of
chemical actions, so that they are very rich in enzymes — perhaps
richer than the more highly developed animals.

Let us take a look at a specimen of a very simple organism — a
single cell of a small bacterium, of which the solid parts weigh 10~"
of a gram (i.e. about 300 million million will make up one ounce).
In this tiny volume it can be calculated that there will be about
200,000 protein molecules altogether. A moderate estimate suggests
that the bacterium must have several hundred distinct enzymes at
least for all its activities, or perhaps a thousand or more. Obviously
there cannot be very many molecules of each enzyme — an average of
200 or so of each, but in some cases perhaps there will only be a
few molecules of a particular enzyme.

Our picture of the smallest living ceUs is thus already one of a
complexity which the mind finds difficulty in grasping. How all
these separate and complex enzyme molecules are packed away in a
tiny fragment of protoplasm, how they work in harmony with each
other, producing that result which we recognize as life, we hardly
know. How they are made out of the food which the organism lives
on, we shall discuss in a later chapter.

m

Vitamins and Antivitamins

It would be wrong to give the impression that living things contain
only proteins. An enormous number of smaller molecules have been
found, usually in rather small quantities, in living tissues. They are
the intermediates, the partly completed structures which will eventu-
ally enter into the larger aggregates, or the messengers which go to
and fro, carrying energy or otherwise facilitating the working of the
machinery of the cells.

The organisms make many of these substances themselves, but
different organisms differ enormously in their ability to synthesize
compounds. Some, like plants, can create everything they need from
very simple nutrients — a few inorganic salts, ammonia or nitrate,
phosphate and carbon dioxide from the air. General speaking, the
higher an organism is in scale of complexity, the less is its synthetic
ability. Micro-organisms usually have much greater power of syn-
thesis than animals, but even they are found to differ greatly in their
nutritional requirements. Some are perhaps even less exacting than
the plants; for example, the nitrogen-fixing bacteria which exist in
nodules of leguminous plants have the ability to use the nitrogen
directly from the air.

The basic food requirements of the higher animals are certain
inorganic materials like salt, phosphate, calcium, iron (the mineral
elements of diet), proteins or their constituent parts, from which they
are able to build new protein, and carbohydrates, and perhaps also
some fat as sources of energy. It was found, however, in many
laboratories engaged in nutritional studies that animals fed with
what appeared to be completely satisfactory diets, containing all
these constituents in a purified form but nothing else, failed to grow
and often developed severe symptoms of disease.

One of the earliest experiments of this kind was made in 1912 by
Sir Frederick Gowland Hopkins. He found that young rats would
not grow properly on a diet made from the purified constituents of

Vitamins and Antivitamins 29

milk, but even small quantities of whole natural milk restored the
rate of growth. Much earlier than this, disease in both humans and
animals were suspected to be due to deficiencies of diet. Beri-beri,
a disease of the Orient, which was prevalent in the Japanese navy
up to 1882, was almost eliminated by a change of diet involving a
reduction of the amount of rice consumed. Eijkman in 1897 found
that hens and pigeons fed on white polished rice developed a type of
paralysis, which disappeared when the rice polishings were added to
the diet. The inference was that the polishings contained something
necessary for health. Such auxiliary substances were named vitamins
by Funk, a scientist who tried to separate the essential factor from
the rice polishings.

Following the observations of Hopkins, experiments were made to
try to distinguish the factor which was present in raw milk, but was
not present in the purified milk constituents. It was found in fact
that there were two factors, and the lack of either produced growth
disorders. The first, which was present in the fat, was called vitamin
A, and the second, present in the watery part of the milk, vitamin B.
This was the beginning of the discovery of the vitamins, and since
that time at least thirty such substances which are necessary, in
small quantities, for the growth of various animals have been
distinguished.

Many of them have been isolated in the pure form and their
chemical structures have been determined, and in many cases this
has been followed by their synthesis by chemists. The first to be
identified and reconstructed in the laboratory was vitamin C. It had
been known for many years that people who lived for a considerable
time on a diet which contained no fresh vegetables or fruit juices
developed scurvy. Vitamin C was identified by Szent Gyorgyi with a
substance called ascorbic acid which is present in large amounts in
pepper, and it was then synthesized chemically by Sir Norman
Haworth and his co-workers.

Another somewhat similar deficiency disease was pellagra, which
was common in the southern part of the United States among the
poorer people whose diet was mainly com meal, sugar, molasses,
and sometimes pork. Elvehjem discovered in 1937 that pellagra
could be cured in a few days by a substance called nicotinic acid or
niacin,

Hopkins suggested, as long ago as 1906, that rickets was a
deficiency disease, and it was found that cod liver oil had an excellent
effect. The active substance, which was called vitamin D, has been
shown to consist of a group of rather similar complex molecules
calbd sterols. Sterols exist in many foods, as well as in skin, in an

30 Inside the Living Cell

inactive form, and it has been found that if this is exposed to a
source of ultra-violet light, such as bright sunlight, out of doors, the
active vitamin is formed. This is one of the reasons why sunshine is
health-giving.

Vitamin B has been found to contain quite a number of different
substances, many of which have been identified and some synthe-
sized. Vitamin Bl, now called aneurin or thiamine, is the substance
of which a deficiency causes beri-beri. B2 was identified by Kuhn
and Karrer with riboflavin, a substance present in milk, eggs, or
liver, the lack of which produces dermatitis, facial sores, etc. B3 is
possibly a substance called pantothenic acid, which was first found
to be required by yeasts and later recognized as a food requirement
of animals and insects. B6 was identified with pyridoxine in 1938,
and in 1 944 a more active form of the vitamin was discovered in the
compound pyridoxal.

Other substances which are essential to the life and growth of
many organisms are choline, a simple substance required by rats
and birds and possibly by human beings; biotin, originally recog-
nized as a food requirement of yeasts, but since known to be
required by the higher animals; inositol, a particular type of sugar,
and folic acid, a substance abundantly present in green leaves, which
stimulates growth.

Having discovered all these substances, which must be present
in our food in minute quantities, to keep us healthy, working
organisms, scientists naturally began to wonder what their action is.
If the lack of them produces such serious effects, they must perform
a very vital function.

The clue to this came from quite a different direction. It will be
remembered that the juice squeezed out of living yeast cells contains
enzymes which can by themselves ferment sugars. It was discovered
by Harden and Young in 1906 that if this juice was divided into two
parts, one containing the big (protein) molecules and the other small
molecules, neither part separately could bring about the fermenta-
tion, but when they were again mixed the fermentation went on as in
the original juice. Hence both the big molecules and the smaller
molecules were necessary to bring about the fermentation. The big
molecules, as we have seen, are proteins and some of them enzymes.
The smaller molecules include some which are also needed for
enzyme action, and these were called co-enzymes.

In the course of time a number of co-enzymes have been separated
and distinguished. It has been found that frequently one co-enzyme
is shared by several different enzymes. This appears to be a device of
Nature to economize materials which are rather scarce. The yeas'

Vitamins and Antivitamins 31

juice is like a machine-shop containing many large and complicated
machines. The tools used by these machines are, however, in short
supply, so the management, instead of firmly attaching each tool to a
machine, arranges to have them shared, so that each machine can
have the use of the tools when needed.

Enzymes are so active that only small quantities of them are
required, and it is not surprising that minute quantities of the co-
enzymes are also sufficient. Prof H. Mcllwain has found that some
bacteria only have a few molecules of some of them in each cell.

The co-enzymes are small molecules, and some of them have
been purified and their chemical structures ascertained (see Ap-
pendix). It has been found that they are closely associated with the
vitamins, being either the vitamins themselves or closely related to
them. This has been proved in at least eight cases so far, in which
the enzyme system with which the vitamin functions has been identi-
fied. For example, nicotinic acid is a constituent of the co-enzymes
of yeast juice; aneurin (vitamin Bl) is required by an enzyme called
carboxylase; riboflavin (vitamin B2) is the co-enzyme of several
coloured enzymes which bring about oxidations in the cell.

The vitamins are required by animals, firstly because they cannot
make them themselves, and secondly because they are unable to
perform many of the vital processes of life without them. If the
vitamin is not present, the corresponding enzyme system is paralysed,
and the animal organisms function only imperfectly or not at all.

Micro-organisms such as bacteria and fungi often need the same
vitamins which are required by animals. Eight essential substances
have been distinguished as essential to the life of micro-organisms —
and they all serve as vitamins for the higher animals. They are
aneurin (vitamin Bl), riboflavin (vitamin B2), pyridoxin (vitamin B6),
biotin (vitamin H), pantothenic acid, folic acid, co-enzyme I or its
components, and choline. Readers who are interested will find the
formulae of these compounds in the Appendix (p. 163). The ability
to synthesize these substances is very haphazardly distributed among
micro-organisms. Some, like yeasts, have a considerable ability of
making vitamins and are in fact rich sources of some of them. The
wild mould Neurospora crassa needs only glucose, ammonia, and
biotin in order to live; all other necessary substances it can make for
itself. Other micro-organisms can only grow if they are provided
with certain of these substances; they are unable to make them for
themselves, and they must have them if they are to grow. It was
found that some animals seemed to get along quite well on diets
which lacked some of the B vitamins. There was no evidence that
the animal itself was capable of synthesizing them. The mystery was

32 Inside the Living Cell

solved when it was discovered that the microflora of the animal's
digestive tract were manufacturing vitamins and so contributing to
the nutrition of their host.

The wide occurrence of vitamins in all kinds of living things is
another proof of the essential unity of life. From micro-organisms to
man, they are all buUt on much the same plan. They all have
extremely similar enzyme systems, and require the same auxiliary
substances. But some organisms are dependent on others for some
of these vital substances, as they have lost the power of making their
own. Broadly speaking, the higher the organism is in the scale of
living things the less is its synthetic ability and its dependence on
other forms of life for the compounds it needs is greater.

As I said before, many of the vitamins can now be constructed in
the laboratory. Chemists can do more than this. Not only can they
construct the actual vitamin, the molecule which has been selected in
Nature to do a particular job; they can also make a variety of
similar molecules, which may differ from the vitamin in a slightly
different arrangement of atoms or in the slight additions here and
there, which do not spoil the general plan of the molecule. What
happens when an organism is presented with these pseudo vitamins?
Does it reject them or can it make use of them? Actually it tries to
make use of them, but often unsuccessfully. The 'unnatural' vitamin
combines with its enzyme. The protein part of the enzyme does not
notice that the vitamin is not quite right. It accepts it as a co-enzyme
but, alas, the combination does not function properly. As we have
seen, enzymes act by achieving a very exact fit with the substances
they act on — like the fit of a lock and key. If a part of the enzyme,
the co-enzyme for example, is changed slightly, the key will not go
into the lock. Moreover, even if the natural vitamin is also present,
it may be partly prevented from doing its work, for the unnatural
vitamin competes for the available enzyme molecules and put a
proportion of them, at least, out of action.

So the result is that 'unnatural' vitamin molecules act as
antagonists to the real vitamins. They often prevent the proper
functioning of the real vitamin and produce the same symptoms as a
severe vitamin deficiency.

This seems to provide a wonderful way of preventing the growth of
harmful organisms, like bacteria. All you need do is to introduce a
suitable antagonist which will put out of action a vital enzyme in
the bacteria, which will then be unable to grow.

Unfortunately, the hosts and the parasites are usually very much
alike and possess very similar enzymes, and it is frequently found
that they require the same vitamins and respond similarly to their

■ The chromosomes are seen
as a tangle of fine threads

2

The chromosomes are
separating from each other
and are thickening

•

3

Each chromosome has
split into two distinct
threads

4

'^ Ac ^

S

\

The newly divided chromosomes are moving away from each other

L

^ Pk

S'^^'X

•^^^TP»#

H

9

Both sets of chromosomes collect together at opposite sides of the cell and cell division takes
place. At the end of the process there are two distinct cells. The cell walls are not visible

3 Chromosomes in the nucleus of cells from the growing tip of
onion root in different stages of cell division. (Photographs of fixed
and stained preparations reproduced by courtesy of Dr S. H. Revell,
Chester Beatty Research Institute)

*#»

. ^^^'^^

X

^*

#

%

'

*

*

•

.^^

%*

k

'V

-

#*

•

M

^

. %*

4

\

%^

>

SI

/

^/

4

•♦

m

f

>

# ^"^

I

t

%

4 The giant banded chromosomes of the saHvary glands of the
larva of the fly, Drosophila melanogaster. In these chromosomes the
threads have multiplied side by side so that the final chromosome
consists of many similar threads lying alongside each other. (Photo-
graph by Dr O. G. Fahmy)

Vitamins and Antivitamins 33

antagonists. A large number of vitamin antagonists have been made,
which do in fact interfere with the growth of bacteria; but in most
cases they are nearly as harmful to animals as to the bacteria.

There is, however, one case in which successful use is made of an
antagonist, and this is the basis of the well-known 'sulphur' drugs
(sulphanilamides). These were discovered by trial and error. Until
recent years there was very little to guide chemists in the search for
valuable drugs. Enormous numbers of compounds have been made
and tested; some had curative properties, but no one knew why. This
group of drugs was discovered after long research to be extremely
effective in some kinds of infections. The reason for this was not
clear at once. Then it was discovered that a somewhat similar sub-
stance, p- amino-benzoic acid (pab for short), interfered with the
bactericidal action of the drug (for formulae, see Appendix, p. 164).
If PAB is administered with the drug, much larger doses have to be
used to produce the same effect. This suggested to Fildes and Woods
that the 'sulphur' drugs were really antagonists of pab. At this time
PAB had not been recognized as a vitamin, but its presence in many
micro-organisms has since been confirmed. It is now known to be a
component of fohc acid, a substance which occurs abundantly in
green leaves and has a stimulating effect on the growth of some
bacteria. Folic acid seems to be concerned in the formation of the
highly specialized substances required in the nuclei of cells. The
higher animals as well as many of the lower organisms require pab;
but it is not known definitely why certain bacteria are more sensitive
to PAB antagonists than are the hosts. More recently it has been found
that another compound of the same kind (fas = p. -aminosalicylic
acid) is very effective in arresting some types of tuberculosis.

If it were possible to find enzymes in the disease-producing organ-
isms which were more sensitive to antagonists than those of the
animals, the problem of finding drugs to kill the bacteria would be
very simple; but unfortunately this is not usually possible. Drugs
which harm the parasite usually injure the host as well.

The problem of producing substances which will harm bacteria
more than their hosts is one which has been solved more effectively
by many organisms than by the conscious efforts of scientists. Many
living things must have encountered the problem of how to repel the
attacks of bacteria and other parasitic forms, and it is perhaps not
surprising that those which have survived are provided with chemical
protection. The best known of these substances is penicillin, which
is produced by a fungus, Penicilliiim notatum, the greenish mould
which grows on damp bread. The story of the discovery of penicillin
by Sir Alexander Fleming in 1929 has often been told. He noticed

34 Inside the Living Cell

that there was a clear patch around a growth of the fungus on a
plate on which colonies of bacteria were growing. He recognized that
the fungus was producing a substance which prevented the growth
of the bacteria. The isolation and identification of this substance, as
the result first of the efforts of a group of workers headed by Sir
Howard Florey and Professor Chain in Oxford and the co-operative
effort of scientists on both sides of the Atlantic during the 1939-45
war, and its large-scale production and use in medicine have been
one of the greatest triumphs of intensive research. Penicillin is unique
in having an extremely powerful effect on some types of bacteria and
in doing no harm to animals, and is one of the most powerful means
of combating many kinds of bacteria which is available at the present
time. The nature of its action is not clearly understood.

But many other 'antibiotics', as they are called, have also been
discovered in recent years. Many of them are too harmful to human
beings to find use in medicine. Others, like gramicidin, and strepto-
mycin, both prepared from soil bacteria, are extremely useful.

Streptomycin, which is produced by a micro-organism first isolated
from the soil of a field near Caracas in Venezuela, seems to be
effective in some cases of tuberculosis. This family of micro-
organisms {Streptomyces) — in this case a type isolated from a com-
post heap in Illinois — has also yielded Chloromycetin, which has
been found to be a most successful curative agent for typhus — a
louse-carried fever which is common in overcrowded conditions in
the tropics. It almost completely fulfils the two requirements of an
effective drug — it is deadly against the agent of the disease, and
harmless to the human body. Although it is also effective against
some bacteria, it is of great interest because typhus is a virus disease
and this is the first drug to be discovered which is effective in such
cases. Virus diseases have hitherto been very refractory to treatment
by chemical agents, so that this discovery opens up great possibilities
of new treatments. Chloromycetin has been found to be a compara-
tively simple compound and has been synthesized. Another useful
agent of the same type is known as aureomycin or duomycin, which
is very effective in rocky mountain fever and other virus diseases of
the same type.

All these substances may be similar to fragments of proteins —
probably warped fragments, which will not fit into the normal protein
molecule.

Competition in the world of micro-organisms, which multiply at a
very rapid rate, must be very intense, and any species which can
produce something which kills, or merely prevents the multiplication
of, organisms in its neighbourhood, secures for itself the chance to

Vitamins and Antivitamins 35

multiply. Such substances, which may have been produced originally
by the faulty working of a single cell, would immediately be of
value to the organism, and would ensure its survival. It is not sur-
prising that in the innumerable generations through which they
must have passed in the course of ages, the micro-organisms have
been found effective chemical weapons for self-preservation. At one
time or another individuals which are faulty in some way may
produce unusual chemical compounds which may be of value in this
way.

THE USE OF METALS BY LIVING CELLS

For the want of a nail the kingdom was lost — Nursery Rhyme

Vitamins are not the only minerals, besides the basic amino acids,
which are required in constructing the large molecules of living cells.
Life seems to have explored every possibility and has made use
somewhere or other of nearly every material available on the surface
of the earth. The common metals sodium, potassium, and calcium
are ubiquitous. There are a fair number of proteins which contain
other metals. Iron, for example, is an important constituent of
haemoglobin, the red protein of the blood of mammals. The iron,
present in a particular combination, enables the haemoglobin to pick
up oxygen in the lungs and to carry it into the tissues, where it is
released. Lobsters and other crustaceans offer a variation on the same
theme. Their respiratory protein contains copper instead of iron, and
as a result their blood is literally blue.

Even more extraordinary is the fact that Ascidia (star fish) have
green, blue, and orange corpuscles in their blood, as well as colourless
ones. The coloured ones contain appreciable quantities of vanadium,
although its concentration in the sea water in which they live is so
small it cannot be detected by any known test. Other types of animal,
the beautiful annelid worms called SabelUds and SerpuUds, have
green blood, the colour of which is due to a haemoglobin-like protein
called chlorocruorin.

Plant life uses the same kind of compound in chlorophyll — the all-
important pigment of green leaves — but the metal is now magnesium.
It might be thought that the difference represents the great division
between animal and plant life, the animals being based taking haemo-
globin or similar oxygen-carrying proteins and the plants using
chlorophyll : but as a matter of fact haemoglobin-like proteins have
been detected in the nitrogen-fixing nodules of leguminous plants.
So it would appear that both chlorophyll proteins and haemoglobins
were present before the division into plant and animal life took
place, but plants have relied increasingly on chlorophyll and many

36 Inside the Living Cell

of them have lost ability to make haemoglobin-like proteins.

Enzymes have also been isolated which contain copper, zinc,
manganese, iron, and vanadium. The amount of the metal in the
enzyme is usually quite small, less than 1 per cent, and as only small
quantities of the enzymes are present in the tissues, the amounts of
the metals used in this way are very small — mere traces are sufficient
to satisfy the needs of organisms. Living things usually get what they
need without any difficulty, but sometimes the supply falls below
the level required.

It has been found in experiments with plants growing in carefully
purified solutions, that iron, manganese, boron, copper, and zinc are
essential for plant growth, at concentrations of less than one part in a
million. No more than this is necessary, and much larger quantities
may be harmful. If the soil is seriously deficient in any of the metals,
plant growth suffers, but it can be improved by making good the
deficiency. Thus, the application of manganese and zinc compounds
to marshland fields in Kent increased the yield of potatoes from
seven to eleven tons per acre. In south-east Australia there is an
extensive coastal strip in which a copper deficiency limits vegetation.
Oats, wheat, and lucerne all failed without added copper, but grew
luxuriantly when a copper salt was added to the soil. Further inland,
zinc is also needed, but extra copper was also required before the
clover would set seed. It has been found that plants often grow quite
well without much copper up to blossoming time, but seed formation
is interfered with. The effect of copper on clover growth is greater if
the copper salt is applied a season in advance, due apparently to its
effect on nitrification bacteria. Fruit trees, which will not fruit, have
been made to do so by inserting capsules of a manganese salt into
holes bored into the trunks.

Even when plants can grow well, they do not always satisfy the
mineral requirements of the animals feeding on them. Diseases due to
cobalt — and copper — deficiencies in sheep and cattle are well known
in various regions. In Scotland and AustraHa sheep 'pining' occurs
on soils with less than five parts per million of cobalt. It has been
found that this condition is remedied if cobalt, together with copper,
in the form of salts, of course, is added to the food. Apparently only
ruminants suffer from this deficiency; horses, kangaroos, and rabbits
do quite well under the same conditions.

Copper is utilized, rather oddly, in putting the 'crimp' into sheep's
wool. If the herbage is deficient in copper, the wool grows straight.
The reason, apparently, is that the molecules of the wool protein are
first formed in a straight parallel form. The 'crimp' is produced
when the long protein chains are joined by -S-S- bonds, an operation

Vitamins and Antivitamins 37

performed by an enzyme containing copper. It has also been found
that black sheep are unable to grow black wool on a copper-
deficient diet, a copper enzyme being the agent of the formation of
the black pigment.

Another not very common metal which is found in many living
organisms is molybdenum. It has been found recently that this is
required for certain enzyme systems, particularly xanthine oxidase,
which is concerned with the breakdown of 'purines' and performs a
step in a series of reactions which ultimately leads to their excretion
as uric acid. Xanthine oxidase is present in quite appreciable
quantities in cows' milk.

Until recently it was not known in what substances the animals
made use of cobalt, but in 1948 two teams of workers, one headed
by Dr Lester Smith in England and the other by Dr Rickes in New
Jersey, isolated a cobalt-containing substance (sometimes known as
vitamin B12 from the liver; this substance ameliorates pernicious
anaemia. If the animal is unable to make enough of this substance,
red blood cells are not manufactured in the marrow. But injecting
sheep with either a cobalt salt or this pernicious anaemia factor, did
not cure the deficiency condition. Cobalt taken by the mouth is
effective, but not if injected directly into the blood. It was therefore
thought that even in cobalt-deficient pastures, sheep get enough
cobalt to make the anti-anaemic factor; but insufficient to satisfy the
micro-organisms which play so large a part in the digestive apparatus
of ruminants.

How the micro-organisms use cobalt is not known at present; but
it may be that they synthesize for their own purposes compounds
similar to the pernicious anaemia factor. In fact tiiis substance has
been isolated from a mould Streptomyces griseus — the same one
which yielded streptomycin. What an astonishing thing ! After years
of great effort, scientists succeed in isolating, from liver, the substance
which is necessary for the formation of red blood corpuscles in the
bone marrow. No sooner is it isolated than it is also discovered as
the product of a mould.

If all this is correct, the emaciation and eventual death of the
animals on cobalt-deficient pastures is due to the lack of a milligram
or so of cobalt daily, which is apparently required in the enzyme
systems of certain micro-organisms, which synthesize vitamins or
other substances required by the animal in small amounts. The
whole situation rather reminds one of the disastrous consequences
of the loss of a nail in the nursery rhyme — but in this case the nail is a
cobalt one, which is required as a necessary part of the structure of
an essential vitamin.

IV

Cell Division
H

At this point it is necessary to introduce another great family of
substances, which are important constituents of all living cells, viz.
the nucleic acids. They are so called because they were first found in
the cell nucleus — a specialized part of many cells which contains
thread-like bodies called chromosomes. They were given this name
because they can be stained by certain dyes and made visible. How-
ever, in recent years optical techniques have been developed which
enable the chromosomes to be photographed in living cells (see
Plate 2).

There is no doubt that the chromosomes are intimately concerned
with the processes of cell division, whereby a single cell becomes
two cells, because they undergo changes during cell division which
suggest that they either control the whole process or are an essential
part of it.

Just before the cell is ready to divide, the contents of the nucleus
can be seen, rather indistinctly, as a tangle of fine threads. They
become thicker and more definite and finally collect themselves into
distinct chromosomes. At a later stage in the processes (see Plate 3),
each chromosome is seen to be split into two threads more or less
parallel with each other. The members of each pair move away from
each other and finally collect together at opposite sides of the cell.

In this process, the original set of chromosomes is duplicated, so
that when the cell divides into two, giving two daughter cells, each
contains a complete set of chromosomes like the original cell. It is
not surprising that cytologists see in this process the basic mechanism
of cell division, which ensures that each new ceU has in it a full set of
the chromosomes. Later workers on genetics were able to correlate
the behaviour of the chromosomes with the laws of heredity, so that
they regarded the chromosomes as the carriers of the unit hereditary
characters called genes. The genes were believed to be arranged in a
linear order along the chromosomes and in some cases (particularly

Cell Division

39

in the salivary glands from the fruit fly larva, which have enormous
chromosomes with a banded structure [Plate 4] can be identified
with a particular place in the chromosome.

The process just described is that of simple cell division. It is a
common method by which unicellular organisms multiply and it
occurs in the growth of larger organisms. Sexual reproduction is

Two chromosomes, one
coming from male and

one from female parent.

The chromosomes line
up near each other

/■.

♦ *

^

/

...

>> '

and exchange parts

so that each thread

contains contributions

from both parents.

FIG. 5. The reduction process by which the germ cells are formed

One of the"mixed"chromo-
somes is rejected and the

germ cell then is left with
a single chromosome.

more complicated. The organism sets aside germ cells, viz- the
sperm in the male and the egg in the female, which unite in fertiUza-
tion. In the ordinary body cells of the organism there are two of
each kind of chromosome, one coming from the male parent and one
from the female. When these germ cells are formed, this number is
reduced to one by a process of reduction (see Fig. 5). This is itself a
very wonderful provision which ensures that the chromosomes in the
germ cells contain parts derived from both parents. The two similar

40

Inside the Living Cell

chromosomes line up close to each other and exchange parts with
each other. A new chromosome is then formed which contains parts
from each of the original pair of chromosomes. This remarkable
process, which is known as crossing over, ensures that the single
chromosome of the germ cell contains parts from both parents, but
as the exchange can take place in many different ways, each germ
cell contains a different selection of the genes which the individual
carries and which it has received from its two parents.

When fertilization occurs, the male and female germ cells unite

Sperm

Egg cell

Fertilized
egg cell

FIG. 6. Fertilization of the egg by the
sperm restores two chromosomes of
each kind to each cell (only one
chromosome in each germ cell is
shown)

and the normal number of chromosomes of each kind — two — in the
cell is then restored. (Fig. 6.) A new individual is formed by many
divisions of the fertilized cell. In every division each chromosome is
replicated, so that every cell of the new organism contains two of
each kind of chromosome, which have been derived from the two
parents. This continues to be the case until the germ cells of the new
individual are formed, when the reduction process occurs again,
which again mixes in a single chromosome the genes derived from
both parents.

This device, which permits two parents to contribute to the
heredity of a single individual, seems to have a crucial one for the

Cell Division 41

development of the higher organisms. In simple cell division, all lines
of descent are distinct; any change occurring in a cell which
improves its chance of survival, can only affect its own lineal
descendants. In sexual reproduction, every individual has many
ancestors. It may seem a very small change to have two parents
instead of one; but the effect over a number of generations is over-
whelming. If one individual receives contributions to his genes from
two parents, four grandparents, and eight great grandparents, it is
easy to see that the number of ancestors, say twenty or thirty genera-
tions back, which may be represented in one or more of the genes,
may be enormous.

Ancestral trees which show the ^descendants' of an individual are
extremely misleading. They leave out a great many of the collateral
ancestors of succeeding generations. They would be much more
informative if written backwards, so that a man or woman could
see who are his four grandparents, his eight great-grandparents, and
so on, up to 1,048,576 ancestors of the twentieth generation. Even
allowing for a considerable amount of duplication, as for example
the likelihood that not all these lines of descent are distinct, it is
obvious that most of us are descended from practically the whole
population of our country twenty generations back, or a mere five or
six hundred years ago.

It is not difficult to see that sexual reproduction is in the long run
a very effective device for spreading and uniting all the useful and
desirable characteristics in a population. The fact that all the higher
forms of life use sexual reproduction shows that it has been an
important factor in their development.

In the chromosomal filaments we come near to the core of the
processes of life. Strung along them are the genes which we must
regard as molecular patterns which are capable of

(1) maintaining their identity through many generations of living
things,

(2) of being duplicated in the processes of cell division,

(3) of controlling the processes whereby a new individual is formed
from a fertilized cell.

We may now ask what is the nature of the substance which has
these properties. It was discovered by Miescher in 1871 that the
nuclei of cells contain a substance which he called nucleic acid. It
was not a protein, but a phosphate of a particular and, at the time, a
rather unusual, kind of sugar, called deoxyribose. The whole com-
pound is called deoxyribonucleic acid, abbreviated to dna. The
sugar and phosphate groups occur in equal numbers and it is now
known that they are arranged in long chains, consisting of alternate

42

Inside the Living Cell

phosphate and sugar groups (Fig. 7). In addition each sugar molecule
has an organic *base' attached to it, of which there are commonly
four kinds, known as guanine (G), adenine (A), cytosine (C) and
thymine (T). It was at first thought that these four bases were present
in equal amounts, but careful analyses by Chargaff and others estab-
lished that this is not so, but they do occur in pairs, i.e. the amount
of adenine is always equal to the amount of thymine and the amount
of guanine is equal to the amount of cytosine.

The reason for this was discovered by Crick and Watson in the
fact that DNA is made up of two threads in which the adenine of one
is always paired with thymine in the other and, similarly, guanine in
one thread is always paired with thymine in the other (see Fig. 8).

/\

FIG. 7. Chemical composition of one thread of deoxyribonucleic acid

They can do this because their chemical structures are exactly
complementary. Guanine can combine in a regular twin-threaded
structure with cytosine and only with cytosine, and the same is true
of adenine and thymine.^

In the structure proposed by Crick and Watson, the two threads
are wound round each other to form a spiral, but this is probably
not an important feature.

What is much more important is that this structure provides a
basis for the reduplication of threads having bases in any particular
order. This is because, when they are combined in the way I have
described, each thread is the exact complement of the other, i.e.
wherever there is A in one thread, there is T in the other and
wherever there is G there is C in the other.

A chromosome contains a large number of particles of dna and
it is tempting to identify each one with an independent gene. At the
> See Appendix (p. 165) for chemical formulae.

Cell Division

43

present time this identification cannot be made with certainty, but
there is no doubt that one chromosome contains many genes and
also many particles of dna. We might ask how they are assembled
in the chromosome. The results of genetics requires that they should
be arranged in a Unear order, i.e. in one long line. How the individual
particles are joined up is not precisely known, but it is known that
the DNA is not the sole constituent of the chromosomes. They also
contain some rather specialized kinds of protein called protamines
(present in fish sperm) and histones (present in many mammalian
cells). These proteins are of a basic character, i.e. they are capable of
combining firmly with the acidic nucleic acid. Their precise function

FIG. 8. Double-threaded structure of deoxyribonucleic acid
of Crick and Watson

is unknown. It is Hkely that, by combining with and neutralising the
phosphate groups of the dna, they protect it and perhaps also shield
it from other cell chemicals until the time comes for it to function.
It is also possible that they are used to effect the junctions between
the separate dna particles. Another possibility, which has been sug-
gested, is that the junctions are effected by the ions of metals such as
calcium. However the junction between the individual dna particles
is made, there is no doubt that when the whole chromosome is
duplicated, a replica of every dna particle is formed in the new
chromosome.

Experiments have been made using radioactive materials as
markers to find in more detail how the replication of the chromo-
some occurs. The experiments of Taylor and others seem to show

44

Inside the Living Cell

that when repUcation occurs, each new chromosome consists one
half of old material and one half of newly synthesized material. This
is exactly what would be expected if replication occurs according to
the Watson-Crick model, i.e. if each dna particle divides into two
single threads and each half re-forms a complete double thread (see
Fig. 9).

H

i4

...I

t

■
• •I

...I

Original DNA*
double thread

The two
threads
separate

Each single thread

acquires a new

complementary

thread

FIG. 9. How the DNA particle is reduplicated

All that has to be added to this picture to make it true of the whole
chromosome is that the dna particles should remain attached to
each other in a hnear order, even when replication occurs, so that
the replication of all the individual dna particles means the replica-
tion of the whole chromosome. (Fig. 10.)

^mMm^^^^^oc^^^m

A D N A particles joined
by protein

Each single fibre retains
its position when the
double fibre is split and
then attracts its
complement.

FIG. 10. How the whole chromosome might reduplicate itself

In the whole chromosome the dna threads must be coiled up and
there must also be an apparatus which will pull the new particles
apart towards the poles of the cell when cell division occurs. It is
possible that the histones are involved to some extent as minute
muscles in bringing about these processes, but very little is known

Cell Division 45

of the detailed mechanisms of cell division. However it is brought
about, the mechanism is a beautiful example of an intricate
mechanism, which functions with great precision. It ensures that
when a cell divides into two, not only does one gene of each kind go
into the two daughter cells, but they also remain in the same order
as in the original cell.^ Nevertheless very occasionally mistakes
happen. A gene may get out of its correct order, or may even be
omitted from one of the cells or by accident be reduplicated twice.
Such events, as we shall see, may be of great importance in the life
of the organism, because they introduce modifications which may
sometimes be advantageous.

^ Dr Lajtha has suggested that the dna in the chromosome exists as one
single continuous thread, which becomes broken into smaller pieces during
the processes of preparation of dna, so that the particle size of the latter,
when isolated, does not represent its original condition. However, this
view has not been proven.

Inside the Cell

fi

We can now turn our attention from the reproductive apparatus of
the nucleus to the other parts of the cell, known as the cytoplasm,
in which its ordinary everyday business of living takes place. As we
have seen, the cell is a chemical factory which takes in food materials
and uses them for its own purposes. One of its needs is energy, which
it gets by burning sugars. This requires a whole series of enzymes
by which controlled oxidations are performed. As I have already
said, these oxidative processes are coupled with other processes
which give rise to 'high energy' phosphate compounds. The most
important of these is adenosine triphosphate (atp), which is the main
carrier of chemical energy within the organism\ It not only provides
the energy required for the functioning of muscles, but it also, as we
shall see, takes a direct part in the synthetic activities of the cells,
including those which give rise to the proteins and nucleic acids.

A typical sugar such as glucose contains six carbon atoms. The
first stage is its degradation to two molecules of lactic acid, a 'three-
carbon' compound which is oxidized to pyruvic acid. Pyruvic acid
is one of the main sources of energy in the cell. The way in which it
is used was worked out by Krebs, who showed that it is the fuel of
a kind of chemical engine, which operates through a cyclic series of
operations known as the Krebs or 'citric acid' cycle. The result of
this is that the pyruvic acid is oxidized to carbon dioxide and water;
while the energy obtained in this process is stored in adenosine
triphosphate (atp), which goes into the pool available for all syn-
thetic processes and for the performance of muscular work.

A rather general picture, due to H. Lettre, showing how this fuel
station within the cell operates, is given in Fig. 11. atp is obtained not
only from the oxidation of pyruvic acid, but also, to a lesser extent,
from the initial conversion of the sugar to pyruvic acid. No less than

^ ATP consists of adenine and d-ribose, combined with three molecules of phos-
phoric acid. See right-hand side of Appendix 1, Fig. 2.

Inside the Cell

47

thirty-eight molecules of atp are formed for the oxidation of one
molecule of glucose.

It is of interest to compare the efficiency of the oxidation of fuel
in a power station with that in the cell. The overall efficiency of
even large mechanical power stations rarely exceeds 15 per cent.
Measurements have been made of the yield of atp from glucose in
chopped up pigeon-breast muscle. It was found that at least 70 per

glycolytic
cycle

(non-oxidative)

oxidative cycle

carbon dioxide
and
water

carbo-
hydrates -

>\oxygeji

proteins

nucleic

acids

represents ATP

FIG. 11. How the cell obtains its energy. A flow sheet of the fuel station
within the cell. (Modified from a diagram by Professor H. Lettre)

cent of the free energy of the glucose was obtained as high-energy
ATP : the fuel station in the cell is evidently highly efficient.

But the cell is self-contained. It not only provides its own power
supply, but it also uses within itself the energy provided. Some of this
energy may be used to perform muscular work, but it is mainly used

48 Inside the Living Cell

in chemical operations and these include the manufacture of all the
proteins which are necessary to the life of the cell. The cell thus not
only has a power station, but it is also continually manufacturing new
parts for its own use including the power station 'engines', besides
many other mechanisms. The main outcome of all the chemical
operations is thus to produce more of the instruments, such as
enzymes, which bring about these operations. It is this feature which
gives life its militant and aggressive character — living things are
always changing their environment, as far as they can, into more
living material.

It is obvious that all this chemical activity requires a great deal of
organization. However, only very recently has much been learnt about
the structure of the cytoplasm in which many of these activities
occur, and the way in which the multiple chemical activities are
organized so that they are in harmony with each other. The fact
that a dead organism soon destroys itself by enzyme degradations
is a proof of the existence of co-ordinating factors in living cells.

One of the first explorations of the constituents of cytoplasm was
carried out by Dr A. Claude.^ The walls of the cells were broken by
gentle rubbing processes in such a way as to liberate the contents
without damaging them severely. The different structures present in
the broken cell 'mush' could then be separated according to their size
and density by careful centrifuging at different speeds.

The first structure to be deposited is the nucleus, together with cell
wall debris, which can be removed. Next come large granules from
the cytoplasm, known as mitochondria. Then at a still higher speed
we can separate submiscroscopic particles, known as microsomes.
This leaves a 'cell sap' containing some still smaller particles and also
proteins and compounds of proteins and nucleic acid, with molecular
weights up to a few millions.

This structure of the cell has been confirmed by electron microscope
studies of cell sections. The cells are first 'fixed' with a staining
material of a heavy metal, osmium, which makes the cell structures
visible when the sections are examined in the electron microscope.
The cells are then impregnated with a plastic substance, which sets
hard and can be cut into very thin slices. Plates 5 and 6 are examples of
a cell section, which shows the main structures present. What we have
referred to as the microsome appears here as membranes lying
parallel with each other in pairs with small massive particles (sub-
microsomes) attached to their inner surfaces (see er in upper part
of Plate 7). Fig. 12 gives a diagrammatic representation of the con-
stitution of a typical cell according to these findings.
1 Earlier studies had been made bv R. R. Bensley and Robert Chambers.

<7-:-'

i,,.^:^'

v> i.

~9

.f3

If :

. • ■ V

^* /*5^V

^«

*'0 -"^^

'im^-

^'Kt

5 A section of rat liver taken with the electron microscope.
(Mr M. S. C. Birbeck.) Magnification x 13,500. The larger central
body is the nucleus. The oval and kidney shaped bodies outside the
nucleus are mitochondria. Below the nucleus can be seen the double
membranes which form the microsomes

*

i/

< V

\

^ M.^

#.

■%

r

6 A portion of a liver cell taken with the electron microscope at a
higher magnification (x 48,000) by Mr M. S. C. Birbeck. It shows
the oval and kidney shaped mitochondria and also the microsomal
membranes

Inside the Cell

49

From these experiments we leam that the constituents of the
cytoplasm are not all mixed up, but arranged in particles or struc-
tures of different kinds. Much work has been done to explore the
functions of these particles. It has been found that the mitochondria
contain the oxidizing enzymes of the cells. They are in fact the power
houses of the cells. The electron microscope has shown them to
possess an internal structure of membranes, by means of which the
enzyme processes are controlled and organized, but their detailed
constitution is still unknown.

/ Contents of
this sector ^

I

I are magnifiedy

Microsomal filaments
with submicrosomes
(black dots) attached

f •

Fat globule

Mitochondrion

Nucleolus

-Nucleus

FIG. 12. Diagram showing some of the structures usually present
in a cell

Mitochondria are relatively large bodies. A typical one is about 2//
long and \ jx wide (1 /x = 1 / 1000 of a millimetre) and contains about
a milHon protein molecules of typical size. They appear in the elec-
tron microscope pictures (see m, Plate 7) to be crossed by thin
membranes arranged in pairs. It would seem that the enzymes are
organized on these membranes. A considerable number of enzymes
is present in a single mitochondrion, to bring about the many chemical
changes necessary, but even if there were five hundred distinct
enzymes, it would still be possible to have a large number of molecules
of each.

D

50 Inside the Living Cell

The small microsomal granules are much smaller. They are about
200 A (1/100/^), i.e. the diameter is only about one per cent of the
average length of the mitochondrion. They are of great interest
because they have been found to be the main site of protein synthesis
in the cell. They are therefore the central point of the cell's opera-
tions— a sort of inner sanctum where the essential components of the
cell are formed.

PROTEIN SYNTHESIS

We have now come to the heart of the matter and we must consider
how proteins are made. As proteins are perhaps the most essential
and typical of all the constituents of living cells, this is clearly one
of the basic questions about life, which has attracted an enormous
amount of attention in recent years.

As we have seen, the proteins are highly complex chains of amino
acids arranged in a particular order. There are about twenty separate
amino acids and we know that there must be in the cell a mechanism
which reproduces exacdy and without error the specific order of the
amino acids in every natural protein. The number of possible ways of
arranging say a chain of one hundred units chosen from twenty
different kinds is enormous. The cell selects the amino acids and
places them in the correct order with great ease and speed. In many
bacteria a new generation is produced in thirty minutes or even less
time. In this period, the full complement of proteins for a new cell
must be synthesized. It is evident that the protein synthesizing mecha-
nism works with great speed and efficiency. What is its nature?

In the first instance we need to know something of the nature of
microsomes and other cytoplasmic constituents. It has been found
that the cytoplasmic constituents, as well as the nucleus, contain a
nucleic acid component known as ribonucleic acid (rna), which is
somewhat similar to the dna of the nucleus, but differs from it in
using a different sugar and also one of the component bases is
different.

Nucleic acids of this type are probably present in most of the
cytoplasmic constituents, and occur to a marked extent in the
microsomes; but they are present in a highly concentrated form in the
sub-microsomes, which were mentioned above. As the initial site of
protein synthesis appeared to be the microsomes and it was suspected
that RNA was implicated, it was natural to consider the sub-micro-
somes as a possible site of the synthesizing mechanism. It has long
been thought that the synthesis of a protein with amino acids in a
particular order requires a 'template' which will act as a guide or

Inside the Cell 51

determiner of the order of the amino acids and it was therefore sug-
gested that the rna provides a template to guide protein synthesis.
The great difficulty was to see how it could function in such a way.

Very recently suggestions have been made which may offer the
solution of this puzzle. It appears that the synthesis of protein occurs
in two stages. It has been shown by Lippman that in the first stage
the amino acids are 'activated' by combination with the high energy
phosphate compounds, which, as was mentioned above, are the final
products of the oxidation of sugars. It has also been found that, for
each amino acid there is in the cell a specific enzyme which can acti-
vate it and it alone and this enzyme enables it to be combined with
some of the component parts of the nucleic acid (rna) present in the
microsomes.

A very interesting suggestion as to how the specificity is brought
about has been made by Drs Crick, Orgel and Griffiths, of Cambridge
University.

It has long been thought that the order of the bases in nucleic
acid provided a 'code' which is stored up in the dna of the chromo-
somes and transmitted to give rise to the actual templates on which
the proteins are formed. The great difficulty was to see how such a
code could be constructed making use only of the four different bases
which are present in the nucleic acids. With these bases there can
only be four different ways of filling one place of the nucleic acid
threads, so that a code based on single places can only give four
different signals and this is not enough to indicate the order in struc-
tures of twenty amino acids. However, if we can allow two places
in the nucleic acid thread for each signal, we can fill them in sixteen
different ways and if we allow three places in the nucleotide thread
for a signal, we can have sixty-four different combinations, because
there are sixty-four different ways in which four kinds of units can
be arranged in groups of three.

The minimum signal which has enough possibilities to cope with
about twenty amino acids is therefore one which uses three places
in the nucleic acid thread as a code for the placing of a single amino
acid. However, if we were to adopt some such code as this, we should
need some way of distinguishing the three places which constitute
a signal from adjacent places. So far as we know in a nucleic acid
thread, all the places are equivalent to each other and there is no way
in which one group of three are distinguished from another group of
three. For example, if the four bases are represented by A, B, C and D,
we might have the series :

ABCCADBCACDBAC
If groups of three are to constitute a signal, how are we to distinguish

52 Inside the Living Cell

one group of three from others? We need to know where the groups
of three which belong to the code start. Thus, in the example given
above, the signal might be given either by ABC or the adjacent BCA
or CCA. It would be impossible to make use of both ABC and BCA
as signals for separate amino acids, as they overlap.

Crick, Orgel and Griffiths have offered a solution of this difficulty.
They pointed out that out of the sixty-four possible combinations of
A, B, C and D taken three at a time, it is possible to pick out twenty
which are distinctive in the sense that they will not overlap with any
others. They suggest that these twenty combinations are the ones
which Nature uses as code signals for the twenty amino acids of
which, with few exceptions, proteins are made up.

The whole picture of protein synthesis which is beginning to emerge
is thus a very complex one.^ The genes or hereditary particles are com-
posed of the DNA of the cell nucleus and these carry the code for the
synthesis of the specific proteins of the cell. But the code is not ex-
pressed directly. In the first place it is transferred to the rna of the cell
particles. How this is done is not clearly known, as the process has
never been actually demonstrated. But we must suppose that the
two complementary strands of dna are separated and a complemen-
tary strand which is composed of rna is built up on similar principles
on one of them. This rna enters the microsomal granules where it
forms a 'template' for the construction of protein molecules on the
lines discussed above. Here the code functions by determining the
order of the amino acids in the way I have already mentioned. That is,
each amino acid is 'activated' by a specific enzyme which attaches
to it the 'nucleotide' groups, which are going to enable it to find its
place by means of the code signal. Thus, suppose that the code on
the RNA for a particular amino acid X is ABC. X is attached by its
enzyme, not to ABC itself, but to the exact complement of ABC
which we shall call A'B'C. This grouping can only combine with
its complement ABC on the rna template and thus the position of X
in the peptide chain is fixed by the position of ABC in the nucleic
acid template (Fig. 13). Similarly the amino acids Y and Z find their
positions by being combined with the code group which we may
suppose to be A'B'D' and B'B'A'. The next step is the combination of
X with its neighbours Y and Z. This will liberate the code groups
A'B'C, etc. which are free to act again in the same way.

It should be made clear that this proposed mechanism has not been
completely established — many parts of it have not yet been found
experimentally. In fact, all we have are a series of hints. But im-
portant experiments carried out by Dr Zamecnik and his collaborators
1 Readers who do not want to go into details can omit this and turn to p. 54.

Inside the Cell 53

at Boston have isolated some parts of the process. They have shown
that the amino acids are first attached to small unidentified nucleo-
tide fragments by the agency of enzymes in the cell sap. From this
state they have shown that transfer to the microsomal granules can
take place.

A-B-C-A-B-D-B-B-A-A-C-D Template code of R N A

: I I I I ■ : I I I II
iV </ •/ 1/ 1/ !/ !/ 1/ 1/ I I I - . .J . ,

A— B-C A— B-D B-B— A Ammo acids with groups

I I I which are complementary

X Y Z to the code

I

A— B-C-A-B— D-B-B-A-A— C-D- Template code

— X — Y—Z— Small piece of protein chain

f / / /// ///

A- B-C A- B-D B-B-A Liberated complementary

groups

FIG. 13. How the code carried by the RNA template might operate.
Each amino acid is attached to the complementary code grouping
which enables it to find its place in the sequence

These experiments appear to provide the beginning of a solution of
one of the greatest mysteries of life — how specific proteins are made.
The mechanism suggested is extremely complicated and we can only
guess as to hov/ it was established to begin with, but it is clear that
nothing less complicated would be able to explain the appearance in
the cell of such a host of highly individual proteins.

VI

Genes and Mutations

n

We have already seen that there is much evidence that the genes,
which carry the unit hereditary characters from one generation to
another, are located in the chromosomes and as the chromosomes are
composed of nucleic acid (dna) and proteins, it was believed that dna
must be at least an important constituent of the gene. The first con-
crete evidence in support of this view was the discovery of 'transform-
ing principles' which consist entirely of dna. The initial observation
which led to the discovery of these factors was made by Griffith. The
bacterium, Pneiimococcus, occurs in two forms, one of which is
covered by a capsule of starchy material and the other is bare. Grif-
fith found that when live 'bare' bacteria were injected into mice to-
gether with dead bacteria of the encapsulated type, the former
acquired the ability to grow a capsule and this ability, when once
acquired, was transmitted to succeeding generations. In other words,
the bare variety which had gone through numerous generations with-
out any change, acquired the ability, when treated with non-living
material, to grow a capsule of its own. Later it was found by Dawson
and Sia that it was not necessary to grow the bacteria in an animal.
The same transformation could be brought about in the same way
in a test tube.

Alioway showed that the whole of the dead encapsulated bac-
terium was not necessary, but extracts from it were effective. Finally,
in 1944, three American workers, Avery, McCleod and McCarty, iso-
lated the active agent or transforming principle and showed that it
had all the properties of a nucleic acid (dna). Similar observations
have since been made with a considerable number of other bacteria
which exist in two well-defined forms. The transformation from one
form to another can be effected by material, which in every case has
turned out to be dna. Once transformed the bacteria continue to re-
produce in the transformed state, so that their genetic constitution
must have been altered. It is concluded from this that the material

Genes and Mutations 55

added contains a gene, and the conclusion is inevitable that in these
instances the gene is in fact a nucleic acid (dna).

How the different types of one kind of bacteria arise in the first in-
stance is not very clear. But it is known that if colonies of bacteria
are kept, occasionally a change will occur in a bacterium which is
transmitted to its progeny. Such a change, known as a mutation, is
not a very infrequent phenomenon and is responsible for the many
strains of bacteria which exist.

Mutations of this kind are probably responsible for the resistance
to drugs which bacteria develop. For example, it is well known that
common bacteria present in infections may develop a resistance to
penicillin. This means that a change has occurred, with the develop-
ment of a new strain which can multiply in the presence of penicillin,
because it has a somewhat different constitution.

Hotchkiss, in 1951, made the surprising discovery that penicillin-
sensitive strains of Pneumococcus can be transformed into penicillin-
resistant strains simply by adding to the former dna derived from
the latter. In other words the property of being able to grow and mul-
tiply in the presence of penicillin is conferred by addition of dna, and
again, once acquired, this ability is transmitted to descendants. The
same phenomenon has been found in other cases, e.g. the streptomy-
cin-resistance which is developed by a certain type of influenza
bacillus.

All these experiments supported the idea that dna alone was
capable of behaving as a gene. This demonstration in higher organ-
isms is, of course, much more difficult because there is no way in
which we can introduce extraneous dna into the germ cells of ani-
mals. It would certainly be fascinating if it were possible to remove
one chromosome and replace it by another with a different hereditary
background, or if it were possible to change the characteristics of an
animal by introducing dna from another species. The difficulty is to
get the dna into appropriate cells and as we shall see later animals re-
ject nucleoproteins even from other individuals of their own kind. If
this were not so it might be possible to make all sorts of composite
animals. It has been reported recently by J. Benoit, P. Leroy and C.
and R. Vendrely that the injection of dna made from ducks of one
variety (Khaki Campbells) into ducks of a second variety (Pekin)
from the eighth day after hatching, caused definite changes in the
latter. Usually the Khaki Campbells are large birds with yellow
beaks and creamy-yellow plumage, while the Pekins are smaller
ducks with greenish-black beaks. The beaks of the treated ducks de-
veloped bold black smudges against a background which was either
yellow or rose pink. It was suggested that these modifications

56 Inside the Living Cell

were also transmitted to the progeny; but further experiments are
required before it can be regarded as certain that dna injected into
animals can really change the genetic characteristics.

The biological method of doing this experiment is to mate male
and female organisms of the same species which have a different
genetic constitution. As I have mentioned above, 'crossing over' of
material derived from the two parents occurs and the progeny will
carry different combinations of the genes available. From a study of
these, it is possible to distinguish the genes of different individuals.

An enormous amount of work of this kind has been done with the
fruit fly, or drosophila, which has been bred through many genera-
tions. These flies are very convenient. A new generation appears in
about a week and they are simple to keep and to feed. If a large num-
ber of flies is studied, every now and then a spontaneous change or
mutation occurs, which is transmitted to the progeny of this indivi-
dual in accordance with the laws of genetics. In the course of many
years of work by Morgan and his colleagues in Columbia University,
no less than 800 distinct mutations were observed, which could be
bred true to type and were undoubtedly due to distinct changes in the
genes. The chromosomes of the flies were examined at the same time.
This is easy to do because it happens that, in the larva state, the
chromosomes of their salivary glands are enormously developed and
can be seen even with a low power microscope (Plate 4) to have a
banded structure. Changes in the chromosome corresponding to the
different mutations can easily be seen, so that it is possible to identify
a mutation with a particular part of the chromosome. In fact chromo-
some maps showing the positions of many genes have been worked
out.

The essential characteristic of a gene is that it has a permanence
which enables the same character to be transmitted through innumer-
able generations. But at the same time new characters sometimes
appear (or old characters disappear). It has been estimated that in
the fruit fly, on the average, an individual gene may undergo a muta-
tion in something like a million generations. This means that no mis-
take is made in the copying of the original gene until on the average
correct reproduction has occurred a million times— which would
occupy, with the fruit fly, between a thousand and ten thousand years.
Professor J. B. S. Haidane has calculated that with human genes the
likelihood of a mutation may be only of the order of once in a mil-
lion years. Nevertheless the fact that such changes in the genie
material can occur is of great importance because without it evolu-
tion could not take place. The majority of gene changes will be dis-
advantageous and will cause the death of the organism or their non-

Genes and Mutations 57

survival. But very occasionally valuable mutations may occur some-
where in the population of a species and the sexual process ensures
that the new gene will be combined with the others present. In fact,
sexual combination of genes, together with natural selection even-
tually leads to the combination of all the best genes in the species.

According to this view, evolution is the result of two qualities of
the genie substance; it has to be permanent and pass on the same
characteristic through many generations. Some species, e.g. the
primitive coelacanth fish Latimeria chaliimnae, have remained essen-
tially unchanged since the tertiary times. If it were not so, a species
would not continue to exist and exhibit recognizable characters. But
the ability to change must also be present or evolution would not
occur at all.

VII

Ejfects of X-rays and Atomic Radiations

on Living Cells

H

The great permanence of the gene shows that the material it is made
of must be well shielded from external influences, and at one time it
was thought that the genes were so well protected that they could not
be modified by any external agency applied to the organism.

However, it was discovered by H. J. Miiller in 1928 that the fre-
quency with which gene mutations occurred could be definitely
increased by exposing the germ cells to X-rays and similar pene-
trating radiations, which are produced by radioactive substances.
The characteristic property of all these radiations is their ability to
penetrate into tissue and produce chemical changes within it. The
dangerous nature of X-rays was discovered very early by the pioneer
workers, who often suffered serious and frequently fatal X-ray burns.
The widespread use of radioactive substances and, even more, the
possible exposure of large populations to the effects of radiations
from atomic bombs, has greatly increased the importance and in-
terest of these effects and in recent years an enormous amount of
scientific research has been devoted to them.

The radiations concerned are called ionizing radiations because
they are sufficiently powerful to disrupt the molecules of matter
through which they pass, with the formation of electrically charged
fragments or ions. They usually bring this about by knocking one or
more electrons out of the neutral molecules they pass through. The
intensity of the radiation is measured by the number of ions primarily
formed in the material through which the radiation passes. It is ex-
pressed in terms of a unit, the roentgen, which is the quantity of radia-
tion which brings about 10^" (a million milHon) ionizations in a cubic
centimetre of the substance. This may seem to be a very large num-
ber, but it is small compared with the number of molecules present in
the same volume. For example, one cubic centimetre of water con-

Effects of X-Rays and Atomic Radiations on Living Cells 59

tains roughly 3 x 10" water molecules/ so that one roentgen (1 r)
ionizes only one water molecule in every 3 X 10^° (i.e. 30,000 million).
The primary ions formed have a very short life and change into re-
active substances called radicals (often H or OH in water) which are
capable of producing chemical changes in other molecules present.

There is no doubt that all ionizing radiations have a very damaging
effect on all living cells. In the case of a rat the average lethal dose is
about 700 roentgens. The lethal dose for a human being is probably
less, not more than 400-500 roentgens over the whole body. We can
easily calculate how many ionizations a typical rat cell receives with
this lethal dose. The volume of a typical cell (taken as a sphere about
10"^ cms. across) will be about 5 X 10~" cubic centimetres (i.e. about
2,000 million to 1 cubic centimetre). From this it follows that the
lethal dose, viz. 700 roentgens, will give rise to 350,000 ionizations^
in every typical cell. This may seem to be quite a large number, but
it is small compared with the total number of protein molecules pre-
sent in such a cell.

Careful studies have shown that only a small proportion of the
protein molecules in the cell are damaged by a lethal dose. The reason
for the death of the animal in such cases is still unknown — it may be
that there are cell constituents which are both essential to the life of
the cell and are also damaged by very small doses of radiation, but
they have not been identified with certainty. Some organs in the body,
such as lymphatic glands, are more sensitive to radiation than others.
The bone marrow, in which the red blood corpuscles are made, is
particularly sensitive.

It has been found possible by adding certain protective substances
like cysteine or cystamine to increase the lethal dose of radiation, i.e.
the animal will tolerate a greater dose and still recover. However,
these substances have to be present in the body before exposure to
the radiation — addition after the exposure has very little effect.

The bone marrow of animals has the very important function of
being the source of the red blood corpuscles. It is also a highly sensi-
tive organ and an important effect of radiation has been to decrease
the power which the animal has of replacing its red blood corpuscles.
Dr Jacobson of Chicago discovered that, if bone marrow from an un-
exposed animal is injected into the exposed animal, it helps it to
recover from the effects of radiation. It is also possible to use bone
marrow from another species to some extent. For example, bone
marrow from mice will help an irradiated rat to recover. It does this
by taking the place temporarily of the damaged bone marrow mech-

^ This means 3 followed by 22 noughts.
2 700 X 5 X 10-10 X 1012 = 350,000.

60 Inside the Living Cell

anism and keeping the rat going until its own bone marrow is func-
tioning again.

Another important finding, made by the workers in the atomic re-
search laboratory at Oak Ridge, Tennessee, and also in Germany,
that although animals which have received less than the lethal dose
of radiation recover, their average life is shortened. A shortening of
life is caused by even small doses of radiation. With rats, the shorten-
ing of the average life is approximately 0-61 days for each roentgen
received. This is an important phenomenon, the meaning and cause
of which are rather obscure at present. It is very remarkable that a
similar shortening of life of the offspring occurs if the spermatozoa of
the father are exposed to the radiation! It follows that factors which
determine the length of life are carried by the spermatozoa.

Very little information is available about the actual effects of ex-
posure to radiations on the life span of human beings; but it has been
observed in the United States that, while the average life of physicians
having no known contact with radiation is 65-7 years, that of radio-
logists, who may be exposed to radiations in the course of their work,
is only 60-5 years. The average exposure in these cases is unknown.

Another serious effect of ionizing radiations, which may be men-
tioned briefly here, is that they give rise in some cases to malignant
growths (cancers). Many miners in the mines of Joachimsthal, in
which radioactive minerals are extensively worked, die of cancer of
the lung, probably due to inhaling radioactive gases such as radon
and also radioactive dust particles. Workers with radioactive paints,
used for producing luminous watch dials, etc, are liable to contract
various diseases, one of which is cancer of bones. Radium, which is
used in these paints, is similar to calcium in its properties and finds
its way into the bones and may remain there for many years. A cor-
relation has been found between the incidence of bone cancer in these
cases and the amount of radium present in the bones. It has been esti-
mated that OT microcurie of radium in the bones of an individual
gives a probability of 0-5 per cent of a bone sarcoma occurring {Bull.
Atomic Scientists, June 1957).

A similar effect has become of importance in connection with the
explosion of atomic bombs. One of the products of the explosion is
radioactive strontium which has a long life and may find its way into
bones. Radioactive strontium is disseminated through the upper at-
mosphere following an explosion and is slowly carried to the surface
of the earth in rain. It is taken up by plants from rain water and, when
the plants are eaten by animals, a part (5 per cent) is retained in the
animal. In this way, at two or three removes, human beings acquire
strontium originating in atomic bomb explosions. The retention is

Effects of X-Rays and Atomic Radiations on Living Cells 61

greater in the growing bones of young people than of older people.
However at the present time the effect is not serious as the amount of
strontium present in the bones of human beings has reached about a
thousandth of a microcurie and one microcurie is regarded as a rea-
sonably safe permissible amount. However, even this small amount
is expected to produce a certain incidence of bone cancer.

Another well-established effect of radiation is a disease of the bone
marrow known as leukemia. This is a kind of cancer in which an un-
controlled excessive formation of white blood cells occurs. Many
cases of leukemia have occurred among the survivors of the atomic
bomb at Hiroshima and the observations recorded in the following
Table show that there is a definite correlation between the incidence
of the disease and the amount of radiation received:

Incidence of leukemia between
Distance from explosion January 1947 — A ugust 1955

(per 10,000 persons)
more than 2,000 metres 2

1,500 — 2,000 metres 3-4

1,000—1,500 metres 28

below 1,000 metres 128

The possible effect of ionizing radiations from radioactive sub-
stances on the genie material is of even greater moment than the
effect on the health and life of the individual. As we have seen, ioniz-
ing radiations are capable of giving rise to mutations and it has been
found that a great majority of the mutations are deleterious, i.e. result
in the loss of a useful character.

It must be remembered that such mutations may remain concealed
for several generations. This is because all the body cells contain two
chromosomes, one from each parent. Thus, if one of the chromo-
somes is complete, the fact that the other is damaged may be hidden
and only come to light when two individuals with the same mutated
gene produce offspring.

It has been of great importance to make an estimate of the fre-
quency with which mutations are likely to occur as a result of ex-
posure to ionizing radiations. The first estimate of this kind was made
by H. J. Mliller, working with fruit flies, who found that the exposure
of the spermatozoa of the fruit fly to only one roentgen of X-rays in-
creased the mutation frequency (for certain mutations) no less than
ten times. W. L. Russell, at Oak Ridge, has made studies on mice ex-
posed to ionizing radiation and found an effect at least ten times that
observed with fruit flies. However, the natural mutation frequency is

62 Inside the Living Cell

very low, so that it is better to express the result directly in terms of
the number of mutations observed in the whole population. It was
found that the average frequency of a particular kind of mutation was
of the order of 25 X 10~^ per roentgen. This means that if one hun-
dred million mice received one roentgen each, twenty-five mutations
of this particular type would occur. The figure for the fruit fly is about
one tenth as much.

We can apply this figure to human beings on the assumption that
the human mutation rate is the same as that of mice. Many of the
survivors of Hiroshima must have received 200 roentgens of radia-
tion. The chance of mutation of one particular gene in one of these
individuals may thus be expected to be about 200 X 25 X 10"^ i.e.
5,000 X 10-' or 0-5 in 10,000. As each individual probably has at
least 10,000 genes in all, which are equally liable to be affected, it fol-
lows that there is a high probability that each individual who survived
Hiroshima and received 200 roentgens of radiation carries at least
one mutated gene of one kind or another in the chromosomes of each
of his germ cells.

We can now ask how ionizing radiations bring about mutations. It
is easy to observe that exposure to these radiations has a marked
effect on chromosomes. When the chromosomes of cells are
examined under the microscope after exposure to radiations, visible
effects are frequently found, such as breakage of a chromosome into
two portions (Plate 9). This is sometimes followed by more complex
happenings. If two chromosomes are broken, the two sets of broken
ends may join up with the wrong partners. When replication of the
chromosomes occurs, it often happens that the two new daughter
chromosomes do not separate from each other completely, but a
'bridge' forms between them which continues to hold them together.
Such effects are observed in a proportion of the cells, even after fairly
small doses of radiations. With plant material, Swanson observed at
least one chromosome aberration per cell with 150 roentgens of
radiation.

An important feature of the genetic damage due to radiations is
that it is cumulative. It is believed that the effect is the same whether
a dose is delivered all at one time or in very small quantities over a
considerable period. It follows that exposure to very weak sources
of radiation for a long period may be as important as a large intensity
for a short time.

It is therefore of great interest to estimate the prospects of genetic
damage to human beings from natural and other sources of radiation.
We are in fact all the time exposed to radiations from (1) cosmic rays;
(2) radioactive gases or dust in the ground and atmosphere produced

Effects of X-Rays and Atomic Radiations on Living Cells 63

by natural sources. In addition, we may receive radiations from (3)
artificial sources of radiations such as X-rays used for medical exami-
nations and radioactive paints used on watch dials; (4) radioactivity
derived from the 'fallout' from the explosion of atomic bombs and of
thermonuclear weapons. A very careful study of these effects has
been made by government committees in both the United Kingdom
and in the United States of America.

It is estimated that from natural sources of radiation including
cosmic rays and naturally occurring radioactive substances, an indi-
vidual in the United Kingdom will receive a total dose in the repro-
ductive organs of about three roentgens during a period of thirty
years, which is taken as the average reproductive period. This appears
to be less than the amount of exposure which would be required to
double the natural spontaneous rate at which some mutations occur.
The average dose received by the reproductive organs from the diag-
nostic use of X-rays is estimated to be about 20 per cent of the dose
from natural sources. The dose of radiation which individuals are
likely to receive from the radioactive 'fallout' from atomic or thermo-
nuclear explosions is comparatively small and even if continued in-
definitely at the same rate as during the last few years, would only
amount to one per cent of the radiation from natural sources.

To find the actual chemical changes which are responsible for the
biological effects of radiation much work has been done on the effects
of the radiations on nucleic acids. It is only necessary to say here that
the radiations do bring about numerous different kinds of chemical
changes. They not only destroy the bases on which, as we have seen,
the genetic properties ultimately depend; but they are also capable of
breaking a dna particle into two or more parts. There is little doubt
that these chemical changes are sufficient to give rise to the observed
effects of the radiations on the chromosome and consequently to the
genetic effects which they produce.

If the genes can be damaged by chemical reactions which pene-
trating radiations bring about within the cell, should we not expect
that chemical substances could be introduced, which would bring
about similar effects? It used to be thought that anything which hap-
pened to the body could not affect the germinal cells, so well
were they protected. However, following Muller's experiments with
X-rays it was found by Dr C. Auerbach and Drs Robson and Carr in
Edinburgh that mutations could be produced in the fruit fly by the
application of certain chemicals. The chemical substances which
they used as known as 'mustards', a reactive type of substance which
was developed for use in chemical warfare. It was noticed that the
wounds produced by 'mustard gas' were in some respects similar to

64 Inside the Living Cell

X-ray burns, and this suggested the possibihty of these substances
causing mutations.

Since the original experiments of Dr Auerbach and her colleagues,
a great many substances have been discovered which are capable of
producing mutations. They are not usually very highly reactive sub-
stances. One of the necessary features of such a substance will be its
ability to reach the nucleus and react there with the chromosomal
filaments. If it is too reactive it will react with the first proteins and
other substances it meets when it enters the cell. One property of the
chemical mutagens, as they are called, is their ability to deliver a de-
layed punch in the cell nucleus. But some of the mutagens are appar-
ently only very weakly reactive (e.g. certain hydrocarbons) and the
nature of their action is still unknown. But they have one feature
which is shared by all or nearly all of them; they are also *carcino-
genic' agents — they are capable of causing some types of cancer (see
p. 95).

They cause changes in the chromosomes which are very similar
to those produced by ionizing radiations. Not only are chromosome
breaks observed; but a 'stickiness' similar to that caused by X-rays,
i.e. in cell division the two sets of chromosomes do not pull away
from each other cleanly but leaves 'bridges' of chromosome material
between them. In fact it appears that the two sets of chromosomes
are joined together at one or more places. It is significant that many
of the substances which give rise to these effects are known as 'cross-
linking' agents, i.e. they can join two protein molecules together. It
is likely that they do join in this way the two dividing strands of the
chromosome fibre.

STUDYING MUTATIONS WITH MICRO-ORGANISMS

Many genetic experiments have also been carried out with micro-
organisms— fungi or bacteria. Work with these organisms has the
great advantage that the time taken to pass through one generation is
very short so that very large numbers of individuals can be dealt with,
and it is possible to make a thorough analysis of the frequency of
mutations and mutations which only occur very infrequently can be
discovered.

A favourite material for experiments of this kind is the fungus
Neurospora. Beadle and Tatum exposed the spores of this fungus to
X-rays. Many of them are killed by this treatment, but among those
which survive mutants are found. These have lost the ability to per-
form one or more of the chemical reactions which the original 'wild
type' neurospora could perform.

7 Fine structure in the cell interior. Electron micrograph of a cell
from the pancreas, showing sub-microsomes (small round bodies
attached to the microsomal membranes). The large sausage-shaped
object is a mitochondrion. (From Sjostrand and Hanson, Exp. Cell
Research, 7, 393, 1954)

8 Electron micrographs of Influenza A virus adhering to a red fowl
cell. The upper photograph shows the round virus particles and the
lower photograph the long filaments they sometimes form adhering
to the edge of the cell. Photographs by Drs Chu, Dawson and Elford

Effects of X-Rays and Atomic Radiations on Living Cells 65

The original 'wild' neurospora is an organism with remarkable
synthetic ability. All it requires to live and to grow, besides water and
some inorganic elements, are glucose or some other source of car-
bon, ammonia and the vitamin biotin. Out of these simple materials
it makes everything it requires. It makes twenty amino acids, a variety
of purines or pyrimidines, nucleic acids and proteins, aneurin, ribo-
flavin, pyridoxin, pantothenic acid, folic acid, choline, inositol, and
no doubt numerous other substances.

The mutants produced by X-rays often lack the ability to make
one or other of these compounds and growth will not take place un-
less the necessary compound is supplied. It is thus quite easy to iso-
late a mutant which lacks the ability to make one compound (see

C9

i

Wild type

I*'

V5/

Minimal
medium

X-rays or
ultraviolet

//////

Conidia
(asexual spores)

O

O

oi

Complete

medium

(with vitamins,

amino acids,

etc.)

U

Crossed with
wild type of
opposite sex

Fruiting body

I

Sexual spore

(9 O

^

1^

V^ V5^ Vi/ Vi/

Vitamins Amino Minimal Complete
acids

FIG. 14. How mutants of the fungus, Neurospora, are made and tested for
their nutritional requirements. Spores of the 'wild type' of the fungus are
exposed to X-rays or ultra-violet light. They are then 'crossed' with spores
of the original wild type and the progeny is examined (this ensures that the
changes observed are genuine effects on the genes or mutations). The spores
so obtained are tested for their ability to grow in various solutions. In the
first place those which cannot grow in a minimal medium which supports
the wild type, but can grow when a complete set of supplements (vitamins,
amino acids, etc.) are isolated. Separate tests are then made to discover which
of the supplementary substances they require

66 Inside the Living Cell

Fig. 14) and then to grow it in the presence of this compound. Quite
a large number of such mutants have been produced, all of which
lack the enzyme which is required to make the missing substance.

It is inferred from this that in each case one gene has been damaged
by X-rays and this gene controls the formation of one enzyme. Occa-
sionally strains of the fungus are produced where two or more sub-
stances must be added to the basal medium to enable them to grow.
But it can be shown by cross-breeding experiments that the genes are
distinct and it is only a matter of chance that two have been damaged
in the same spore. A gene that has been lost can be replaced by cross-
breeding with a strain which possesses it. This is the clearest evidence
we have for a close, and probably a one-to-one, relation between
genes and enzymes. According to this the gene essentially contains
the pattern which guides the formation of the enzyme. It is probable
that it carries this pattern as a sequence of bases in dna, as described
in Chapter V. If this is so, it is evident that damage to the pattern of
bases in dna will clearly interfere with the ability of this material to
pass on its message. The damage might be only damage to the bases
at one or two critical points which would interfere with the code; or
it might be a break of the dna thread which would prevent an intact
protein being formed. Many of the stages by which genes bring about
their effects still remain to be worked out. It has been sometimes pos-
sible in micro-organisms to track down the whole chain of reactions
by which a particular chemical substance is formed, in a number of
stages, each of which require a special enzyme. It is found that each
step is controlled by a separate gene. If this gene is damaged by
X-rays, the progeny of this individual is unable to perform the cor-
responding process, so that the whole sequence of reactions is broken
at this point. If the substance which the organism is now unable to
make is added artificially, growth will proceed normally; but other-
wise the substance which is formed as an intermediate, just before the
break in the sequence, accumulates because the organism is unable
to make use of it. By experiments of this kind, many complex
sequences of reactions have been worked out.

Another example of the control of protein synthesis by genes has
been found in connection with *sickle cell anaemia', a human disease
which is inherited according to the Mendelean rules and is therefore
under genie control. It has been found by V. M. Ingram that the
haemoglobin extracted from sickle cells differs from normal human
haemoglobin in a single amino acid only out of the 300 present in the
molecule — one glutamic acid group in the normal haemoglobin being
replaced by valine.
1 Nature 180, 326, 1957.

VIII

Viruses — Cell Parasites

A good many years ago it was discovered that there are agents of
disease which are much smaller than bacteria, since they pass through
filters that retain bacteria and also cannot be seen under the micro-
scope. As long ago as 1892, Ivanowski observed that the sap of
tobacco plants which were infected with tobacco mosaic disease
still had the power of infecting healthy plants, when rubbed on their
leaves, even after it had been passed through a fine porcelain filter.
In 1898, Loeffler and Frosch showed that the infective agent of foot
and mouth disease of cattle could also pass through such filters.

The importance of these observations was not at first understood,
but gradually a distinction was drawn between bacteria, which were
usually visible under the microscope and were stopped by fine filters,
and filter-passing agents which were submicroscopic and were called
viruses.

The list of virus diseases, even of man, is now very long and includes
measles, mumps, influenza, poliomyelitis, chickenpox, rabies and
yellow fever.

For many years viruses remained in the realm of the invisible.
Estimates of their size were made by various means. For example,
Dr W. J. Elford made collodion membranes having pores of different
sizes and was able to estimate the sizes of the different agents by their
ability to pass through the pores. It was found that, although sub-
microscopic, they were much larger than most protein molecules (see
Fig. 15).

Methods of concentrating and purifying the virus material were
also worked out. Dr W. M. Stanley, in 1935, applied to infected
tobacco plant juice the methods of salt precipitation which had been
successfully applied to the separation of the enzymes. He obtained
a protein of high molecular weight, which was highly infective and
could be prepared in a semi-crystalline condition. Here was a pure,
or comparatively pure chemical substance, which had the abiUty of

V

68

Inside the Living Cell

infecting healthy plants. When introduced into the plant, an extremely
small quantity infects the plant with the disease and it also multiplies
extensively so that, after a time, a much larger quantity of infective
material than that introduced can be obtained from the plant cells.
It therefore appeared that this substance had one of the characteristic
features of life, viz., the ability to multiply in an appropriate en-
vironment.

1^= Tooo^*
a millimeter

FIG. 15. Relative sizes of viruses and bacteria

A considerable number of viruses has now been isolated in what
appears to be a pure condition, some of them in a crystalline state.
Crystallinity simply means that the particles all have the same well-
defined regular shape so that they will pack together in a lattice
arrangement, forming crystals which have regular geometrical shapes.
More recently many viruses have been photographed under the elec-
tron microscope and their shapes and sizes made visible (see Plates
8 and 10). It is found that they vary greatly in size and shape. Some
are spheres; others, like the tobacco mosaic virus, are cylindrical
rods; others resemble a tadpole with a tail. The spherical ones easily
pack into a crystalline form, and the long cylinders line up in one
direction.

Viruses — Cell Parasites 69

It was shown by Pine that they are not simple proteins, but nucleo-
proteins, i.e. compounds of a nucleic acid with proteins. The nucleic
acid is sometimes similar to the dna of the chromosomes (but the
bases present are sometimes different, e.g. T2 bacteriophage contains
hydroxymethylcytosine instead of the usual cytosine); in others the
nucleic acid is similar to that present in the cytoplasm (rna). Plant
viruses frequently contain rna and many animal viruses dna.

The infectivity is usually highly specific in that a strain of the virus
will only live and multiply in one particular host species, e.g. tobacco
mosaic virus only attacks tobacco plants, and the common polio-
myelitis virus attacks very few animal species. Sometimes the virus
can multiply only in some strains of a species and not in others, e.g.
some varieties of potato such as the King Edward are resistant to
potato mosaic virus. They cannot usually be made to multiply in an
'artificial' medium, although some will grown when introduced into
living eggs. Either their nutritional requirements are so exacting that
proper mixtures which will support tliem have not yet been found,
or they require to make use of the whole apparatus for providing
energy and growth substances, which is present in living cells.

Are we to class these substances as living or non-living? They have
the basic properties of living things — the ability to reproduce them-
selves in a suitable environment. But the size and crystallizability
of their molecules brings them close to the inanimate world. They
seem to be on the dividing line between the two worlds. Dr Stanley
has said : 'As we go from the admittedly non-living to the admittedly
living, I think there must be a transition stage where there are entities
that may possess some properties of the non-living and of living
things. What could fill this place more simply and logically than the
high molecular weight virus proteins that are intermediate in com-
plexity betwen the protein enzymes and hormones, the wonderful
properties of which we already recognize, and the system of proteins
that we call protoplasm and which constitutes life? There is evidence
that even within the virus group there is a gradual increase of com-
plexity of structure from the small nucleo-proteins to the more
"elementary-body" type of virus.^ There is, however, no sharp
break. . . .*

It seems that a certain degree of complexity is needed before an
organism can maintain itself as an independent living thing with the
ability to live on its environment and grow and multiply. As we have
seen, this ability involves a most complicated apparatus of reactions
which will extract raw materials and energy sources from the food
and use them not only to maintain life but to make all the enzymes
^ Large round virus particles, about 175 m/x across.

70 Inside the Living Cell

and proteins required. The virus particles do not have this ability,
but they can make use of the apparatus which is provided by their
host cells. They must therefore be regarded as parasites and, like
parasitic animals, they are intimately related to their host species.

It is possible that the virus particles have arisen as a mutation of a
normal self-reproducing constituent of the cells in which they grow.
Suppose that a self-reproducing particle in a cell makes an error of
reproduction and produces particles which, although they can 'live'
and multiply in the cell, do not function correctly. They will be
*rogue' particles which interfere to a greater or less degree with the
normal working of the cells. If they are introduced into healthy cells,
they will continue to reproduce themselves there. But if they go on
multiplying, they may cause such disturbances of the normal working
of the cells that the organism eventually dies. This might mean the
destruction of the virus as well, but the natural viruses have often
developed efficient means of transferring themselves from infected
to fresh organisms. They make use of all sorts of agents such as
insects and birds to help them to disperse themselves. This implies
a long period of adaptation.

The common viruses must have gone through innumerable genera-
tions since they first originated. They are in fact very variable. Even
in short periods of observation, changes in their behaviour are often
noticed and new strains appear. In fact, their successful maintenance
may depend on this ability to change and so keep one step ahead of
defensive mechanisms which the hosts may also develop.

Not only large organisms but also bacteria have their virus enemies.
It was noticed by Twort in 1915 that bacterial cultures sometimes
cleared suddenly and the bacteria appeared to break up and dissolve.
A few drops from this liquid added to another culture caused it to
behave in the same way. Two years later d'Herelle made similar
observations. The agent which destroyed the bacteria could not only
be transferred from one culture to another, but its activity increased
with each transfer. This phenomenon is now recognized as being due
to bacterial viruses, which are known as bacteriophages. Their form
has been definitely demonstrated by electron micrographs of the
particles. A number of different types have been found which differ
considerably in size and shape. Plates 1 1 and 12 show some of the fine
pictures of Dr R. W. G. Wyckoff of successive stages of the attack
of Bacterium coli. The 'phage particles are easily recognized by their
tadpole-like shapes, with short rod-shaped tails. The attack begins
when one or two of these particles attach themselves to a bacterium.
The material inside the 'phage particle then passes through the wall
and multiplies within the cell until, finally, the whole of the interior

Viruses — Cell Parasites 71

appears to be filled with new particles which escape through the dis-
rupted cell membrane.

It has been found that only the nucleic acid of the bacteriophage
passes into the bacterial cell, the protein sack in which it was con-
tained remaining outside. (Hershey and Chase, 1952.)

Once inside the bacterium, a very extraordinary series of changes
occurs. The bacteriophage takes over, as it were, the whole chemistry
of the cell for its own purpose. The cell begins to synthesize
substances to make new bacteriophage. In some cases this involves
making compounds which were not even present in the normal cell;
thus T2 bacteriophage contains the base hydroxymethylcytosine
instead of cytosine which is normally present in cells.

With the bacteriophage called T2, it has been shown that, for the
first seven minutes after the entry of the agent into the bacterium,
little can be observed to happen. The bacteriophage material appears
to 'dissolve' and for a short time cannot be detected in the cell. After
nine minutes the synthesis of new bacteriophage particles begins
and continues until about the thirteenth minute, when all the avail-
able material in the cells has been used up and an average of 130 new
particles of bacteriophage are formed in each bacterium. The bac-
teriophage particle has succeeded in imposing its pattern on the cell
substances and has used them for its own purposes.

In the case of tobacco mosaic virus, it has been found by Fraenkel-
Conrat and by G. Schramm that, if the protein is stripped off the
virus particle, the nucleic acid (rna) is still capable of causing the
disease. It is true that the infectivity of the nucleic acid is a good deal
less than that of the intact virus, but it is possible that the rna is
damaged in the preparative processes.

This experiment shows that the nucleic acid alone is capable of
causing the synthesis of the whole virus in the infected plant. How-
ever, it has not yet been possible to cause infection of bacteria by
using the nucleic acids prepared from bacteriophages. The probable
reason for this is that the nucleic acid by itself is unable to enter the
bacteria. The tail of the bacteriophage has a 'tip' which attaches itself
to the bacterium which is being attacked and the nucleic acid is then
injected into the bacterium through the tube. This is itself a very
remarkable process and it is not understood how the nucleic acid
from the head of the bacteriophage is pushed through the narrow
tube which constitutes the tail (Fig. 16). It has recently been found by
Dean Eraser and others, however, that if the bacteriophage dna is
added to broken bacterial protoplasts (i.e. bacteria without a cell
wall) synthesis of new bacteriophage particles occurs.

This shows that the nucleic acid (dna) of bacteriophage is capable

72

Inside the Living Cell

not only of reproducing itself but also of forming the proteins present
in the complete bacteriophage particles.

The bacteriophages offer a very favourable opportunity for the
study of the elementary processes of reproduction as there is no
doubt that replication occurs within the bacterium. M. Delbriick
and W. T. Bailey discovered that bacteriophage particles are made
up of still smaller units. They found that two different related strains
of bacteriophage can be crossed with each other so as to produce

D N A

Tail protein
l,000A long

:LSite of
attachment

FIG. 16. A bacteriophage particle
(slightly diagrammatic) (from E. A.
Evans, Texas Reports Biol. Medicine,
15, 783 [1957])

a kind of hybrid. For example, one strain of virus known as T2
produces a small colony and can destroy a strain of the bacterium
called A; while another strain known as T4r produces a large colony
and can destroy a mutant strain called C. When both types are added
together to a culture, not only are both of these original types released
when the bacteria burst, but also two intermediate types, in which
the characteristics are combined in a different way. One produces
a large colony and destroys bacterium A. The other produces a small
colony and destroys bacterium C. It appears that the characteristics
of T2 and T4r have got mixed up in the new types, each of which
has features drawn from both of the two parent types. The inference

Viruses — Cell Parasites 73

to be drawn from this is that bacteriophages break up in the cell into
smaller particles which multiply independently and reassemble so as
to give rise to all their possible combinations in the new individuals.
Thus we find a kind of sexual reproduction even below the level of
cells.

Another experiment by S. E. Luria of Indiana University also
indicates the composite nature of bacteriophages. Viruses can be
'killed' by exposure to ultra-violet light. When infected by a single
'killed' bacteriophage particle the bacterium usually dies, but no mul-
tiplication of the virus takes place; but if two or more of the 'killed'
virus particles enter the bacterium, it was found that they often
multiplied as well as the undamaged virus. It looks as if the two
damaged particles can share their parts and make up one complete
individual between them. Luria estimated from his results that the
'phage particle has at least twenty independent units. When only one
of these is damaged by ultra-violet light, the particle is incapable of
multiplication, but the damaged unit can be made good from other
particles.

This reconstitution of bacteriophages from damaged parents has
been carried very much further by S. Benzer. By treatment with ultra-
violet light or by ionizing radiations, it is possible to obtain a con-
siderable number of mutants of a bacteriophage, which can be de-
tected by their effects on their original host cell; or on slightly different
strains of the bacterium, which react in different ways to infection.
It is possible also to study the recombination of these different forms
so as to reconstitute the original bacteriophage.

In these experiments it has been possible to construct a 'map'
showing the order in which the different genes occur. This must mean
that although recombination of the genes derived from two different
'phages can occur, it cannot be a haphazard process. That is, we
must not think of the bacteriophage being dissolved into its com-
ponent units which are then reassembled in a haphazard way.
Although recombination of the parts occurs, it must be carried out
in such a way that the order in which the genes are arranged is not
lost. We must picture the bacteriophage particles as exchanging parts
with each other, without losing their overall structure. This is similar
to what happens in chromosomes, where as we have seen, in the
formation of the germ cells, two chromosomes line up near each
other and exchange. But the bacteriophage particle is much smaller
than the chromosome and in fact it has been found that forty per
cent of the dna inside it is a single dna particle, having a molecular
weight probably of about fifteen million. The fact that exchange of
genes between two bacteriophages can occur, means that a single

74 Inside the Living Cell

DNA molecule can exchange parts with another one, during the course
of the duplication process. Of course this may not be a simple
exchange of parts. It is quite likely that it occurs during the duplica-
tion of the nucleotide threads, in that the replication process which
is forming a new nucleic acid particle along one fibre, jumps to a
second one which happens to be in the vicinity.

Another remarkable discovery is that bacteria can exchange
material carrying genie characteristics with each other. Plate 10, by
E. L. Wollman, F. Jacob, and W. Hayes of the Pasteur Institute, Paris,
shows two bacteria in conjunction. This is a kind of primitive sexual
process. A bridge is formed between the bacteria, through which genie
particles are exchanged. It has even been found possible to control
the number of genes which are transferred by stopping the conjunc-
tion (by agitation) after different times. But this conjunction is a rare
process affecting not more than one cell in a million — often even less
— at any one time.

IX

A Community of Cells

H

By and large, all living cells have underlying mechanisms which are
basically similar; they are all variations of the same theme. Thus to
some extent the activities we have described so far are possessed by
every living cell but, of course, some features are emphasized in some
forms of life and repressed in others. The cells of unicellular organ-
isms, like many bacteria, carry out all the functions of living by them-
selves. But in organisms consisting of many cells, there is usually
specialization — some cells perform one function, some another. The
organism becomes a society or community of cells in which each
cell has a trade of its own. So we must now consider the life of the
cell as a member of a community.

The first thing to note is that most of the organisms we are familar
with contain very large numbers of cells. Even a small fly contains
many millions of cells of many kinds. So however specialization
started, it has obviously gone very far and we are not likely to learn
much about how it came about from highly developed plants and
animals.

The animal organism is particularly complex. There are cells set
aside for the manufacture of special enzymes. In a mammal, the pan-
creas produces the digestive enzymes required in the intestine. The
liver accumulates starch and also breaks it down to sugar. It also
manufactures the main proteins of the blood serum. The red blood
corpsucles are highly specialized structures for carrying oxygen and
for regulating the carbon dioxide content of the blood.

So we have to ask how all these highly specialized cells come into
existence. It is obvious that different forms of life have very different
abilities to reproduce specialized structures. As a rule a plant is able
to reproduce the whole plant from a small part of it. If you break off
part of the stem of a growing plant and put it in the ground, it will
often develop into a complete plant with roots and the ability to
form flowers and seeds. The whole of the hereditary character of the

76 Inside the Living Cell

plant is thus contained in every part of it — or at least in many of the
parts.

This ability is much less developed in animals and the more highly
developed the animal, the less is the power of reproducing lost parts.
The higher animals like man only possess the power to a limited de-
gree. If you cut your finger, the cells on either side of the cut will
begin to multiply and spread over the wound, reproducing the main
superficial features in doing so. In man, however, even the area of
skin which can be replaced is rather limited; but animals often have
a greater power of regenerating lost skin. Animals which are lower
in the scale of life have much more extensive powers of regenera-
tion. If a lizard loses its tail, it can grow a new one; so can a fish. A
starfish will reproduce a ray which is broken off; somfe species of
worms will grow a new individual from a small segment. In these
cases the pattern of the whole individual is inherent in the parts and
if the pattern is broken it is reformed.

These are very difficult facts to account for. Why are the cells of
a mutilated starfish stimulated to growth and why does growth cease
when it has just replaced the missing ray? We must ask how the pat-
tern is carried in the organism.

The answer to these questions is not known. Indeed, little is known
about how large organisms are formed, as they must be in all cases
of sexual reproduction from a single fertilized egg cell, by successive
divisions. Most of the higher organisms go back to a single fertilized
egg cell at one stage of their life cycle, and the complex individual is
formed by its successive divisions. About fifty successive divisions of
the fertilized human ovum make a human being, with all the tissues
of the human body, muscles, gut, liver, bone and brain. They are not
merely made in sufficient quantities but precisely fashioned and ex-
actly fitted together to make a functioning organism. It is obvious
that, when cells in the growing embryo divide, they do not always
give rise to two identical daughter cells. There must be many points
at which differentiation occurs; the two daughter cells formed at one
division will have different destinies; one may give rise to muscle and
the other to nerve.

Very little is known about how this process of differentation
occurs. It is certainly one of the darkest chapters of scientific know-
ledge— rather, one should say ignorance — at the present time. It
would seem that, although in every cell division the chromosomes
are replicated and one pair goes into each of the new daughter cells,
yet there is a difference. One possibility is that the genes present in
the chromosomes are covered, or inhibited, until they are required
to carry out their function. This covering of the genes might be

A Community of Cells 11

effected by the proteins present in the chromosome and it may be that
after a certain number of subdivisions a gene becomes bare and there-
fore effective. But this is all very speculative and at the present time
we can only be amazed at the precision with which the process of de-
velopment occurs. Not only are cells produced which have distinct
functions, but shape and form is accurately reproduced. The fingers
grow at similar rates until the adult hand is formed. The characteristic
features of the face are even passed down from one generation to
another. We have to admit our almost complete ignorance of the
controlling factors.

The ability of tissues to produce cells of differentiated types is
greatest in the early stages of the development of the embryo and is
lost as the completed organism takes shape. In fact, in the very early
embryo, the future destiny of the cells seems to be largely undeter-
mined. H. Spemann studied the early stages of development of the
newt, before the stage known as gastrulation was reached. When the
developing embryos were cut in two, a whole embryo developed
from each half. However, this possibility is lost very quickly, and dif-
ferent parts are formed which have destinies (e.g. to form heart tissue,
or intestine) which cannot be altered. Indeed these 'committed' cells
are able to modify and impress their destiny on any other tissue im-
planted in them. It is necessary to suppose that effects of this kind are
brought about by chemical substances, called organizers, but very
little is known as to their nature. If we 'transplant a young and undif-
ferentiated cell group into the region of the head,' says Paul Weiss.^
*it will form eye or brain; transplant it to the anterior trunk and it will
form limb; or further back, kidney; and transplant it to the rear and
it will form tail — the same cells forming different structures depend-
ing on the location. We may say: organizing factors take hold of the
cells and direct them to appropriate functions.'

However, we know little about these organizing factors. Indeed,
many biologists see more in an organism than a collection of cells.
The cells all fit into each other and make a whole. If the whole is dis-
rupted, it is reconstructed. 'Each cell develops in conformity with its
surroundings. ... It becomes a cone or a rod when in the retina, a
cartilage cell when in the centre of the limb bud, a neurone when in
the brain. Intrinsically capable of a variety of performances the cell
receives some definite cue from the locality indicating which trend it
is to follow. These cues are decidedly of supracellular origin.'

It follows from this that we must regard the organism as more than
a mere collection of cells. It undoubtedly has a unity of its own, which
in some ways impresses its character and needs on the individual

^ American Naturalist, 24, 43, 1946.

78 Inside the Living Cell

cells. How this unity or continuity is maintained is still unknown. Dr
V. B. Wiggles worth put this point of view as follows :

'What is the nature of this continuity which the cells are at such
pains to restore? May we not regard it as a chemical organism? On
this view the difference of organization of unicellular and multi-
cellular animals disappears. The latter, presumably for reasons of
size, are subdivided into cellular units. But we may picture the organ-
ism as a chemical continium; a prodigious molecule . . . with active
centres requiring specific hormones to enable them to exert their acti-
vity. . . . The cells do not "co-operate to mould the body form" — they
merely carry and care for a small segment of the organism of which
they are the servants.'

So we get glimpses of the organism as something more than a mere
collection of cells; but we do not know what the additional factors or
agents are. The whole organism is like a factory which is made up of
many specialized workshops, each having a task of its own. Into each
workshop or cell, raw materials come in and products are made and
sent out. But the factory as a whole is more than a collection of work-
shops; there must be planning and co-ordination which ensures that
everything works at the right rate. There must be services, supplies,
transport and flow sheets.

Little is known of these 'organizers'. Substances which promote
growth have been isolated from plants and animals. Plant growth-
promoting substances called auxins were isolated by Kogl and are
of a comparatively simple nature. It was then found that quite simple
synthetic substances which may not exist in plants, such as deriva-
tives of indoleacetic and phenoxyacetic acids, also have this ability.
They are now manufactured on a large scale and used as selective
weed killers. This is because they cause dicotyledons to grow so
rapidly that some phases of the growth process outstrip others and
the plant dies. Another growth promoting substance is ghiberellic
acid, which, when introduced in extraordinarily small amounts, causes
dwarf varieties of plants, such as peas, to develop into giants. It does
this by stimulating the growth of cells in the direction of the stem.
The formation of flower-buds in plants is also controlled by chemical
substances. In many cases the formation of the flower-bud is deter-
mined by the length of the day, and not as we might think by the
average temperature. The flower bud is often formed during the
winter period and it has been found that a dark period of a certain
length is necessary at the time when flower buds are formed. If this
period is broken by a single flash of light, flowering may not occur.
In this case the formation of the flower bud is stimulated by a sub-
stance which accumulates in the dark. If the dark period is broken

A Community of Cells 79

the active substance is removed by the sequence of 'light' reactions.
Growth-promoting substances or hormones are also found in animals
(see next Chapter).

This account of development may be concluded by mentioning
the extraordinary transformations which insects, etc, undergo in
their life cycles. The larva which hatches out from the egg may be an
entirely different creature from the final insect, different in form,
habits, food and construction. A butterfly or moth comes out of the
egg as a gaily coloured caterpillar, which, as it eats and grows, sheds
its hard skin several times. When it is fully grown, it winds itself into
a cocoon and goes into a dormant state. Its tissues are dissolved and
refashioned; and it ultimately emerges as an entirely different
creature, the finished butterfly, which can lay eggs from which new
larvae emerge. The genetic material carried by the chromosomes re-
tains its integrity throughout these changes.

The moulting stages and the final metamorphosis into the adult
insect have been carefully studied by Dr V. B. Wigglesworth in a
blood-sucking bug, Rhodnius prolixus. This goes through five stages
as a larva, with a moult between each, and then metamorphosis into
the complete adult occurs. Between each moult it needs a single large
meal of blood, and this starts off the train of events leading to the
moulting, which takes place 12-15 days later. It has been shown that
the moulting process is caused by a hormone, which is secreted into
the blood by a gland situated in the brain.

It has also been found that the premature change into the adult is
prevented by another hormone (the juvenile hormone) which is pro-
duced by the corpus allatum, a gland just behind the brain. If these
glands are removed by chopping off the head of the larva at an appro-
priate interval after feeding, metamorphosis into the adult form takes
place. On the other hand, if the head of a young larva is implanted
into the fifth stage larva, which is on the point of turning into the
adult insect, the metamorphosis is suppressed and a giant sixth stage
larva is formed instead of an adult.

Chemical Messengers

H

As we have seen, a large community of cells must have controlling
mechanisms, not only while the organism is growing but also to
secure its harmonious functioning. The different kinds of cells must
carry out their functions at the right time. A highly developed animal
has of course very delicate mechanisms like nerves which help to
control its actions, but it is probable that the earliest methods of
control were chemical ones, such as are still found in simple organisms
which have no nervous system. Although the nervous system is used
for types of control in which speed is essential, nevertheless many of
the chemical controls have been retained and are still made use of
for many purposes, even in highly developed animals.

It has been known for many years that animals possess a variety
of glands. Some have ducts leading into various organs, other are
called ductless glands, because they release their secretions directly
into the blood vessels. The importance of these glands began to be
appreciated at the end of the nineteenth century, when it was found
that the loss of one or more of them had serious effects on the
behaviour of the body. Experiments were made on the effects of the
removal of the glands of animals and it was found that the symptoms
produced could be relieved by injecting extracts obtained from the
glands of other animals. It was inferred from this that the glands
produced chemical agents, which were called 'hormones' (chemical
messengers) many of which have since been isolated. They differ
greatly in chemical complexity. Some are quite simple substances,
others are proteins. The study of hormones has become one of the
largest and also one of the most successful branches of biochemistry.
I shall not attempt to give an exhaustive description of these, but a
few examples will illustrate their importance.

As long ago as 1884 it was recognized by Horsley that the removal
or atrophy of the thyroid glands, a pair of small glands in the neck,
produced cretinism, a kind of atrophy of the body and also of the

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Chemical Messengers 81

mental faculties. It was found by Murray in 1891, that this condition
could be relieved by feeding sheep's thyroid. The active substance in
the thyroid extracts was isolated by E. C. Kendall in 1916, and in
the course of time its structure was established, and it was finally
synthesized in 1927 by Barger and Harington. It is a comparatively
simple substance, a compound of iodine and a common amino-acid,
tyrosine. Because of this, the thyroid gland is the reservoir of most
of the iodine in the body. If the water supply is deficient in iodine, as
happens in some limestone districts, the thyroid is unable to make
enough thyroxine and it increases in size in an effort to make up the
deficiency. This produces goitre, once a common disease in mountain
areas.

Thyroxine is concerned with the oxidation of fats and proteins.
When it is absent, the body does not develop properly, giving rise to
cretinism. Over-activity of the thyroid causes protuberant eyes and
a kind of feverish activity. It is necessary for the metamorphosis of
the lower animals. With the thyroid gland removed, tadpoles cannot
metamorphose into frogs. On the other hand, when small quantities
of thyroxine are added to the water, tadpoles will change prematurely
into miniature frogs. It is intimately connected with the secretion of
milk. Injection of thyroxine into cows markedly increases both their
milk and their fat production — at least temporarily; and it has been
found that feeding iodinated proteins, which probably help the cow
to make thyroxine, has a similar efi'ect.

The pancreas is another organ which produces substances necessary
for health, as well as the digestive enzymes it pours into the intestine.
If it is damaged or removed, serious bodily changes occur, especially
an inability to oxidize sugar, which then accumulates in the blood.
This is what happens in the disease diabetes, which is due to an
impaired action of the pancreas. It was found that this condition was
relieved by extracts of pancreas from animals, and in 1921 Banting
and Best in Toronto isolated from the pancreas a protein called
insulin. Actually it is present in separate parts of the pancreas, distinct
from the enzyme-producing parts, known as the islets of Langerhans.
Only a fraction of a milligram of insulin is required each day by a
healthy person, and diabetics can be kept going indefinitely by in-
jections of ox or swine insulin.

One of the first to benefit from insulin was Dr R. D. Lawrence,
who has since become one of the foremost authorities on diabetes.
In 1922 during an operation, a bone splinter flew into and injured his
right eye. The wound turned septic and he became very ill. His illness
was eventually diagnosed as diabetes, and he was considered a hope-
less case. He went to Florence to spend what time he had left and

82 Inside the Living Cell

while there, in the spring of 1923, he heard of the discovery of
insuUn and at once went to obtain the treatment. He soon recovered
and has been able to lead a normal and productive life during the
last 26 years.

As we have seen (Fig. 3), insulin is a very complex substance and
has not yet been synthesized, and until this has been achieved dia-
betics will remain dependent on the supplies of insulin which can be
obtained from animals.

Above the kidneys are two small glands, known as the suprarenals.
Each suprarenal consists of two distinct glands, as their inner parts
have quite a different function from the outer layers. It was found
in 1895 by Oliver and Schafer that extracts of the inner part pro-
duced very marked physiological actions, and some years afterwards
an active substance called adrenaline was isolated from them. In due
course the structure of adrenaline was found; it has been synthesized,
and the synthetic product has exactly the same action as the natural
substance. It is also quite a simple substance. Introduced into the
blood-stream, it causes an increase in the blood-pressure, a quicken-
ing of the heart-beat, and an increase of blood sugar. It tones up the
body and makes it ready to meet unusual stresses. The gland is
stimulated by the sympathetic nervous system. Strong mental
stresses such as fear, anger, and strong emotion, all cause it to
secrete adrenaline into the bloodstream. The result is to stimulate the
heart and prepare the body to meet unusual demands upon it. The
gland thus acts as a kind of relay station — an intermediary between
the brain and the chemical regulating mechanisms. The tiny nervous
impulses from the brain enter the gland and cause it to release the
hormone, which is carried by the blood to all parts of the body. This
is naturally a relatively slow process, and when adrenaline has been
released its effects only wear off slowly, so that when you have had a
fright your heart continues to palpitate for some time after the cause
has disappeared.

The outer layers of this gland produces quite different substances.
It was not until 1931 that Swingle and Pfiffner succeeded in preparing
stable extracts of the cortical hormones which would maintain life in
an animal deprived of the gland. Soon after that, crystalline sub-
stances were obtained in several laboratories and their chemical
nature was worked out. They belong to a group of substances called
steroids which have the same rather unusual hydrocarbon nucleus,
which is the central structural feature of a number of apparently
unrelated compounds (see Appendix, Fig. 7, III). It turns up in the
bile-acids, in vitamin D, and in the sex hormones which are mentioned
below.

Chemical Messengers 83

At least six of these compounds have been isolated from the sup-
rarenal cortex. Some of them seem to be concerned with the break-
down of carbohydrates and proteins. When given in excess they have
an anti-insulin effect and cause a rise of blood sugar which amounts
to temporary diabetes. Others are concerned in maintaining the
balance of inorganic salts in the body — the preponderance of sodium
and chloride in the fluids of the body and of potassium and phosphate
in the cells.

It was found by Dr Philip S. Hench, of the Mayo Clinic, that one
of the first group of substances, namely, cortisone or compound E
originally isolated by E. S. Kendall, has a palliative effect on some
types of rheumatoid arthritis. Unfortunately the supply of cortisone
from suprarenal glands is very small and the expense of treating only
a few patients with gland extracts was enormous. The substance can
be obtained synthetically from a bile-acid but the development of a
method of manufacture has been shortened by the discovery that
many plants contain steroids which can be converted into cortisone
in fewer steps.

One important function of cortisone and other steroids made by
the adrenal cortex is to protect the tissues against the effects of stress.
The mechanism of this is unknown, but the increased cellular action
required after shock and injuries depends upon the presence of these
substances.

We come now to the hormones secreted by the male and female
sexual organs. They govern the sexual cycles and all the processes
of reproduction. They also determine the secondary sexual characters.
A hundred years ago Berthoud showed that the sex glands of a cock
transplanted into a hen caused her to develop a cock's comb. The
capon develops only a rudimentary comb, but the male sex hormone
from any species, even in minute amounts, causes its full develop-
ment. Males have been known to turn into females, and vice versa,
when their sex glands are atrophied and injured. Maleness and
femaleness is the result of a hormonal balance which is easily
disturbed.

Similarly extracts from the ovaries have a powerful effect in causing
females to 'come on heat'. A number of substances (oestrogens) have
been isolated which produce this effect to a greater or lesser degree.
The sex hormones are, in general, very similar to each other — in
fact the differences between the characteristic male and female hor-
mones are minor ones.

Finding out how these compounds are related to each other is like
solving a complicated jig-saw puzzle in which many of the pieces are
missing. Quite a number of synthetic compounds which are not

84 Inside the Living Cell

present in the living organism also act as oestrogens. Some are sur-
prisingly simple — for example, the compound stilboestrol, which was
discovered by Lawson and Dodds to be more active than the natural
hormone. What common characteristics these molecules have has
not been discovered.

The last source of hormones I shall mention is the pituitary gland —
a complex gland situated in a bony hollow at the base of the skull.
The posterior portion of the gland controls the amount of water
retained by the body by regulating its flow through the kidneys. The
effects of the anterior portion are so extensive that it has been called
by Sir Walter Langdon Brown *the conductor of the whole endocrine
orchestra'. It stimulates, and indeed is necessary for, the action of
most of the other glands, since it is found that if the anterior part of
the gland is removed, the thyroid, the suprarenal cortex, the islets
of Langerhans, and the ovaries or testes all atrophy. It controls
growth, in that an excess of pituitary action causes gigantism, while a
deficiency is dwarfing. A number of distinct hormones have been
isolated having distinct actions in stimulating the various hormone-
producing glands.

There is a hormone which stimulates the sex glands; others which
stimulate milk production, and still others which control various
phases of the sexual cycle and control growth. One of them (adreno
cortico tropin — acth) stimulates the adrenal cortex to liberate the
hormones which protect the body from the effects of stress, and
relieve the symptoms of rheumatism. This pituitary substance is
much too rare and costly for general use in relieving rheumatism —
400,000 pigs being required to yield one pound of the hormone!
Unlike most of the others, the pituitary hormones are proteins, but
Dr C. H. Li has found that in some of them the protein molecule is
made up of smaller parts which possess the activity.

The chemical control of the body thus presents a very complicated
picture and much remains to be learnt about it. The ductless glands
produce a wide variety of substances and their actions are far from
simple. A single hormone may be concerned in a great variety of
different processes, and the glands also interact with each other. The
pituitary, as we have seen, acts as a stimulator of the others; the
proper working of the body always depends on a delicate balance
of the working of the various glands.

Since they are often quite simple compounds, and are also,
generally speaking, indifferent to species, the hormones seem to be
relics of a primitive system of control. The thyroxin of the frog is
the same as that of cows and human beings. The sex hormones of
the fowl are much the same as those of the horse. Very little is known

Chemical Messengers 85

about how they work and what constituents of the body they act
on. The only clue in this direction so far is the observations by Prof
and Mrs Cori in St Louis, that insulin and the secretion of the
pituitary gland both affect the action of an enzyme, hexokinase, which
is concerned with oxidizing sugar. They act on it in opposite direc-
tions, so that the functioning of hexokinase depends on a correct
balance between the two. This is one of the first glimpses we have of
a complex interplay of enzymes and hormones through which the
bodily functions are regulated and controlled.

XI

Antibodies and Immunity
H

Without special means of protection large organisms could not live j
very long in a world full of parasitic bacteria and viruses. It is diffi-
cult to think of a more perfect breeding ground for them than animal
tissues and fluids. One method of protection developed by bacteria ■
themselves and by fungi is the use of antibiotics — substances which
prevent the growth of foreign bacteria and so secure for their pos- i
sessor a certain amount of living space. It is possible that animals and i
plants have on their outer surfaces antibiotic substances which in- I
hibit fungal growths — but such have not been clearly demonstrated.

The skin or outer layer of cells is hard and impervious which helps
to keep parasitic particles out of the organism, but it is not sufficient
to prevent the accidental entry of micro-organisms through scratches
and cuts and through the softer tissues of the alimentary canal. For
this reason, it has been necessary for animals to develop other means
of protection against the invasion of the body by foreign organisms.

The facts which imply the existence of internal protective agents
have been known from early times. It is common knowledge that
when an individual has recovered from an infectious disease like
measles or mumps, he is immune from a second attack even if he re-
ceives another infection. It was also found that with certain diseases
which occur in two forms, one mild and the other virulent, infection
with the mild form often confers immunity against the virulent form.
This was the case with cowpox and smallpox and in some countries
it has long been a practice to seek to be infected with cowpox for the
sake of immunity against the more serious disease. In England vac-
cination with cowpox to confer immunity against smallpox was in-
troduced successfully by Edward Jenner. The same line of thought
led to the attempt to produce mild strains of a disease for the specific
purpose of conferring immunity.

Experiments with laboratory animals showed that the injection of

Antibodies and Immunity 87

killed or damaged bacteria or viruses (vaccines) sometimes conferred
immunity against infection by living bacteria. In making prepara-
tions of this kind it is necessary to be sure that all the bacteria are de-
stroyed, to the extent that they are incapable of multiplication; but at
the same time their ability to cause immunity must be retained. This
is obviously a rather tricky business and instead of killing the bac-
teria by heating, treatment with chemicals such as formalin is used
in the preparation of vaccines. For example a successful vaccine
against poliomyelitis has been produced by treatment of the virus
with formalin.

From innumerable experiments of this kind it was concluded that
the body responds to the introduction of foreign organisms by pro-
ducing substances called antibodies, which are able in some way to
neutralize or destroy the foreign bodies. The course of a typical in-
fectious disease is (1) infection, (2) multiplication of the infective
agent, (3) antibody formation which, if it is extensive enough, may
lead to the neutraUzation and destruction of the agent.

In some cases once formed, the antibodies or the ability to make
them may remain in the body of the recovered person for consider-
able periods and even for all the rest of his life and, in that case, he
will probably be immune from the disease if he receives another in-
fection. However, in other diseases, such as influenza, immunity is
only conferred for a limited period, i.e. the antibodies formed are lost
or become ineffective.

It is obvious that in this reaction animals have developed a very
effective means of destroying invading micro-organisms. A great deal
of research has been done to discover its mechanism and although
much still remains to be learnt about it, the main features are now
known.

Antibodies are formed in the animal organism when a foreign pro-
tein (i.e. a protein from another species) or certain other substances
are injected into an animal. These have the power of combining with
the foreign substance and in many cases of causing it to be preci-
pitated. Substances which are capable of invoking the production of
antibodies are called antigens. They include not only proteins (and
some proteins are better antigens than others) but also complex
carbohydrates. The reaction between an antigen and its antibody is
highly specific and also quantitative, i.e. a quantity of antibody will
combine with a definite quantity of its own antigen, but not with
others.

Bacteria, of course, contain many substances which could act as
antigens, but the most effective ones are probably the substances of
the cell walls. When antibodies to these are formed they combine

88 Inside the Living Cell

with the cell surface and render the bacterium open to attack by
scavenger cells in the blood called phagocytes.

Bacteria also produce harmful proteins called toxins, which are
often responsible for some of the effects of the disease. These toxins
also elicit the formation of antibodies — called antitoxins. Antitoxins
can be prepared by inoculating a suitable animal such as the horse
with the toxin and they can be used to help a patient — especially in
the period before his own antibodies have come into action.

Antibodies are proteins and it has been found that they are present
among the blood serum proteins in the fraction which is distinguished
as y-globulin. It is possible that this globulin is made up entirely of the
different antibodies which the body possesses. If a person has had
measles, his y-globulin contains the measles antibody and in fact this
antibody can be concentrated from the y-globulin of the blood. This
y-globulin has been prepared for human use from the stocks of blood
plasma which are kept for emergency uses and confers immunity
against measles. However, it is not possible to separate the measles
antibody from others present, and it may be that there are less de-
sirable substances with it so it is not used too freely.

Much research has been carried out in efforts to discover the
mechanism of antibody formation. As the antibody is a protein and
is produced as a response to the presence in the blood of foreign
antigens, it is reasonable to suppose that the antigen is present when
its antibody is made. The site of the synthesis of antibody is known
to be certain cells of the lymphatic system which are present in
various parts of the body. The antibodies pass into the blood through
the lymphatic vessels.

As I mentioned above, the relation between an antibody and its
antigen is highly specific. Much work has been done to discover the
limits of the specificity of the antigen-antibody reaction. An antigen
can be modified slightly by having small chemical groups added to
it. When antibodies were mixed with slightly changed antigens it was
found by Landsteiner that they were capable of detecting very small
modifications of the antigen, such as the addition of a small chemical
group.

However, the exact way in which the antigen influences the syn-
thesis of the antibody protein is not known. It has been suggested
that as the antibodies are all proteins of a similar nature, they are
kept, as it were, in a semi-fabricated state, like suits of clothes in cer-
tain *ready-to-wear' tailoring shops, waiting for the final touches
which will ensure an exact fit, to be put on in the presence of the
antigen. It has also been thought that this modification of the anti-
bodies to suit particular antigens consists only in local modifications

Antibodies and Immunity 89

of the folding of the protein chains. Attempts have been made by
PauHng and Campbell to produce artificial antibodies by incubating
'denatured' ox globulin with an antigen. A material was obtained
which was capable of precipitating the antigenic substance, but it is
not certain that this was a real antibody similar to those produced
in life.

If the antigen has to be present when the antibody is formed, we
have to suppose that in those cases in which immunity lasts for life,
the antigen also remains in the tissues as a 'pattern' all this time —
not withstanding the fact that we know that the lymph cells in which
the antibodies are formed are changed many times.

DISORDERS ARISING FROM THE IMMUNITY MECHANISM

The immunity mechanism is in some ways too efficient, because in
some rather special cases it leads to undesired reactions. For ex-
ample, there is the phenomenon known as 'allergy'. An individual
may become sensitive to a foreign protein, which may be quite com-
mon in his surroundings. His blood will contain antibodies to this
substance and, as a result, the immunity reaction occurs whenever
he is exposed to it. This leads to various uncomfortable and some-
times very troublesome effects, which are similar to those which
occur with infection. The temperature may rise (fever) and histamine
is liberated in the tissues. It is this which causes 'hay fever' and the
characteristic skin weals of some allergic reactions.

It also became known, when attempts were made to perform blood
transfusion, that the blood of two individuals of the same species
frequently cannot be mixed, because agglutination occurs. This led
to a number of blood groups being distinguished and the recognition
that the blood of a donor should be of the same group as that of the
receiver.

The agglutination is caused by substances analogous to antibodies
which are present in the serum. These antibodies, or agglutinins as
they are called, are naturally compatible with the corpuscles of
their own type of blood, but they cause agglutination with other
types, through a reaction which is similar to the antigen-antibody
reaction.

Four main groups of blood have been recognized which are
labelled as follows:

Corpuscles (antigens) O A B AB

Plasma antibodies (agglutinins) a(3 /3 a —

The blood group O contains antibodies to both A and B. For this

reason it will agglutinate blood of groups A, B and also AB. On the

90 Inside the Living Cell

other hand the serum of A and B is not capable of agglutinating cor-
puscles of type O, because group O corpuscles are not antigens for
the A and B antibodies.

The blood of group AB contains no antibodies, so that an indi-
vidual with this blood can receive blood from any other group with-
out harmful effects. On the other hand, if it is given to an individual
having blood of any other group, it will be agglutinated because all
these bloods contain its antibodies.

This is a very complicated situation and we may wonder how it
came about. The different blood groups are controlled by genes, so
that the blood group of each person is determined by that of his
parents in accordance with the Mendelian laws of heredity. It would
seem likely that the present blood groups are derived from quite dis-
tinct ancestral strains of the human race, but they go back to a stage
long before the development of the present races of mankind as the
distinction of blood groups is not to any great extent a matter of race.

It would be expected that if an individual inherited from his
parents two incompatible components of blood, he would not sur-
vive and that, in the course of time, genie incompatibility would be
eliminated. However, some other antigenic components of blood
have been recognized, which are also inherited by Mendelian rules.
One of these is the so-called 'Rhesus factor' which has been found
to be present in 83 per cent of the population of England and
about 85 per cent of the white population of New York, and may
lead to incompatibility between the blood of the unborn child and
that of the mother if she does not possess it and her husband does.
The human placenta is not a perfect barrier against the passage of
antibodies and in some cases antigens from the foetus may pass into
the blood stream of the mother and there give rise to the formation of
antibodies. If these find their way back into the child, they produce
a disastrous reaction (haemolytic disease of the new born).

Another example of the immunity mechanism being too efficient
has been found in attempts to graft skin from one person to another.
An individual can replace a certain amount of lost skin by growth
from the edges of the wound inwards. If the area of skin lost is very
great, this process is rarely successfully completed. The task of
making so much new skin is too much for the resources of the injured
person. It is possible to help the growth of new skin by grafting skin
from other uninjured parts of the individual's own body. This can
be taken off in thin layers which do not cause a serious injury and
they help to cover the wound and act as centres from which new
skin cells are formed. It would be a great help if skin taken from
other individuals could also be grafted. It would then be possible to

Antibodies and Immunity 91

keep skin banks for use in accidents. But it has been found, both with
human beings and animals, that although such grafts appear to be
quite happy for a few days, after this some reaction takes place and
the foreign skin is sloughed off. When a graft of a foreign skin is
tried a second time the wound rejects it from the start.

This is an immune reaction. The body reacts to the skin of
another individual of the same species by producing antibodies (a
process which takes a few days) and these react with the grafted skin
and cause it to shrivel up. When the antibodies have already been
formed in a previous grafting operation this reaction is immediate.

This is an even more highly specific response than the antibody
response to foreign proteins which was discussed above, as it dis-
tinguishes between the different individual members of one species.
Practically the only exception is the case of identical twins, which are
able to exchange skin grafts whether they are human beings or
cattle. In fact this provides a conclusive test (nearly 100 per cent true)
for identical twins.

It is an extremely remarkable phenomenon. Why should the im-
munity mechanism be capable of distinguishing between distinct
(i.e. not identical) individuals? The way in which the individual mem-
bers of a species differ is in their genetic constitution. As we have
seen, the hereditary character of an individual, as derived from the
parents, is determined by the genes he inherits. The collection of
genes which any individual receives from each parent is determined
in the crossing-over process, which, as we have seen, gives him a selec-
tion of the genes carried by the parents. It follows that, except for
identical twins (which have arisen from one fertilized egg and there-
fore have the same genetic constitution), the chances are that prac-
tically every individual human being will be genetically different
from all others. This would not be the case in inbred animal colonies,
derived from a single pair, in which after a time the same or a nearly
similar genetic constitution may be established, but human commu-
nities have not reached this condition.

These facts have been well understood for a long time. The sur-
prising and new finding is that the antibody mechanism can dis-
tinguish the differences of genetic constitution of individual mem-
bers of a species. An antibody reaction occurs whenever a foreign
gene is introduced.

Since the antibody-forming mechanism is so sensitive, we should
now ask why it does not respond to the individual's own genes or
proteins. How does it know that a particular protein is its own; or
that a piece of tissue belongs to it and is not 'foreign'? This may arise
from the fact that the power of producing an immunological response

92 Inside the Living Cell

is one which develops very late in the development of the individual.
It was known that embryos do not have the ability to make anti-
bodies. Thus it appeared that all the antigens which are present in the
embryo when the immunological apparatus begins to function pro-
duce no response and only those antigens which are introduced after-
wards can make antibodies. In 1949 F. M. Burnet^ and F. Fenner fol-
lowed this idea up with the suggestion that, if foreign substances were
introduced into the embryo during the pre-immunological stage, they
would also be tolerated, i.e. they would not be treated as 'foreign'
antigens when the antibody machinery begins to function.

This has been confirmed in laboratory experiments by Medawar
and his associates. They found that if cells from a strain cba of
brown mice are injected into a white of another strain a while the
latter are still in embryo, it will, when adult, accept skin grafts of
the former. This is a convincing demonstration that the antibody
mechanism does not respond to substances which have been present
in the embryo state. In fact, it is found that this state of 'tolerance'
also persists for a few days after birth. In the case of birds, it can be
produced by injecting the foreign material into the egg. Exactly how
the antibody-producing cells become unresponsive to the proteins
which were present in the embryo is still unknown and must wait for
further knowledge of the actual mechanism of production of anti-
bodies.

There is another way — a rather drastic one — in which individuals
can be made unresponsive to foreign proteins and grafts of other
individuals. If the individual is subjected to a sub-lethal dose of
X-rays, the antibody mechanism is disorganized and for a time the
injection of foreign proteins will be tolerated, i.e. no antibodies will
be formed. This of course has its own dangers, because it leaves the
organism defenceless against bacteria, etc, and in fact, after receiving
a substantial dose of radiation, many animals die of infection. But it
also makes it possible to introduce foreign bodies such as red mar-
row cells, which, as we have seen, will help to keep the individual
going until it has repaired its own machinery for making red blood
cells. During the interval at which the animal is being helped by
foreign cells, it is what is known as a chimera, i.e. a composite of
cells of more than one genetic origin.

Now Sir Macfarlane Burnet, om.

XII

Cancer

H

The growth of an animal organism is, as we have seen, very accurately
controlled. Every kind of cell keeps in place and in step with the
others. We might wonder what would happen if the controlling
mechanism, whatever it is, were to break down so that some cells
just went on multiplying without stopping. This is precisely what
happens with malignant cells, which form cancers. Not only do they
escape from the control of the organism, but they often infiltrate
through tissues other than the one in which they originated. They
may also find their way with the blood stream or the lymph into
many other tissues and they finally cause death, either by interfering
with the working of the body or by taking all the available nourish-
ment and so starving the rest of the body.

At the present time one person in six in Britain and one in seven
in the United States die of cancer. An enormous amount of work has
been done to try to discover why cancerous cells escape the normal
body controls; and also to find ways of killing them without destroy-
ing too many healthy cells at the same time. However, until we know
what mechanism controls cell multiplication in a healthy body, it
will be difficult and perhaps impossible to specify what has gone
wrong in a cancerous growth.

Nevertheless, a great deal of information about the oriein of cancer
(carcinogenesis) has been found, and it is probable that many of the
relevant facts are known, although it is not yet possible to fit them
all into a coherent pattern.

Numerous theories of the origin of cancer have been put forward,
all of which can produce some facts and observations which they are
able to explain. These theories fall into three types :

(1) The Mutation Theory: According to this, a cell in the body
undergoes a mutation as a result of which it becomes essentially a
different kind of cell. Of course many changes of this type will pro-
duce cells which cannot live or at least are unable to multiply. But

94 Inside the Living Cell

according to the theory, occasionally a new type of cell is formed
which can escape from the normal body controls, and thus will be
able to invade and replace other tissues of the body.

The most cogent fact in favour of this theory is the discovery that
a number of chemical substances cause cancer and these substances
have usually also been found to be mutagens, they are capable of
producing definite mutations when appUed, for example, to the sperm
of flies or to fungi.

This discovery is undoubtedly one of the great milestones in cancer
research. It was known in the nineteenth century that cancer was
common among certain types of workers, such as chimney sweeps
and cotton spinners, who used a particular kind of oil for lubricating
the spindles, and workers in the shale oil fields. However, it was not
till after 1920 that attempts were made to find the chemical sub-
stances responsible. In 1921, Block and Dreifuss of Zurich showed
that the active substance in coal tar was concentrated in the higher
boiling fractions. The first identification of active compounds was
achieved in 1932 by Kennaway, Hieger and Mayneord. It had been
found in the study of tars that the activity seemed to be associated
with a particular kind of fluorescence spectrum. This was followed
up and eventually it was discovered by Hieger that the pure hydro-
carbon substance, 1-2 benzathracene, possessed this kind of spectrum.
This led to the synthesis of a large number of similar hydrocarbons
by Cook and eventually pure hydrocarbons, which produce skin
cancers in mice and rats, were isolated from tar and also made syn-
thetically. A fair number of such hydrocarbons have since been
identified.

More recently, it has been found that many other types of chemical
compounds are capable of causing cancer — sometimes in one organ,
sometimes in another. The list of known carcinogens has become
quite extensive (see Appendix, Fig. 8). It includes certain dye
stuffs (e.g. butter yellow) and also intermediates used in the dye
industry, like naphthylamine, which were found to be responsible for
bladder cancer which has been common among workers in the dye
industry.

Most of these substances have also been found to be mutagens.
This is the case with compounds of the 'mustard* series, which have
already been mentioned as the first chemicals to be shown to have
the ability to produce mutations (see p. 63). But numerous other
types of carcinogens have since been shown to be mutagens, so that
there is at least a close connection between carcinogenicity and the
ability to cause mutations. One characteristic of mutagens is that they
are capable of reaching the genetic material in the nucleus of the

Cancer 95

cell and of reacting with it. Some of these substances react very
readily with the nucleic acid (dna) of the chromosomes; and when
such reactions occur, the normal mechanism of cell division may be
upset, and also the message carried by the genie code may also be
interfered with.

Because such reactions occur, some carcinogenic chemicals are
used extensively as anticancer agents. They interfere with cell division
and may diminish the rate of growth altogether. The destruction of
cancer cells by drugs is, however, a very difficult problem. A success-
ful anticancer agent is one which attacks cancer cells, while leaving
normal cells relatively unharmed. One of the few distinctive charac-
teristics of cancer cells is that they are multiplying rapidly, so that
it would appear that the most likely approach would be to destroy
cells while they are in the process of cell division. The most likely
anticancer agents are those which interfere with cell division and
these are also usually mutagens.

It was pointed out by Haddow that most carcinogens are also
anticancer agents. This might seem to be a paradox, but it must be
remembered that the initiation of a cancerous growth is a very rare
event. These substances more frequently cause other kinds of damage
to the living cells, so that their use as anticancer agents is quite
feasible.

Ionising radiations such as X-rays and radiations from radioactive
substances also conform with this rule. It was in fact discovered
very soon after Roentgen's discovery of X-rays that they not only
cause serious 'burns', which frequently become malignant; but they
can also, when carefully employed, be used to destroy malignant
growths. In many ways these radiations are an ideal method of deal-
ing with malignant growths near the surface of the body. The cancer
cells are more sensitive to the radiations than normal healthy cells
usually are, so that it is possible to destroy them without injuring
healthy tissue too much; and it is possible to localize the exposure to
a considerable extent. However, X-rays are not invariably successful
as a small proportion of the cancer cells may escape, especially when
secondary growths have started, and these may be a centre for new
malignant growths. For this reason, if it could be achieved, treatment
by drugs which would specifically destroy malignant cells is much
to be desired, but only limited successes have been achieved so far.
The most successful agents used so far, apart from radiations, are :

(i) compounds of the 'mustard' series, which, as already mentioned,
are mutagens and are capable of reacting with the chromosomes.

(ii) antipurines\ these are substances which are very similar to the

96 Inside the Living Cell

normal components of dna. They become incorporated in dna, but
when this has occurred, they do not function properly so that cell
division is inhibited.

The cancer cell may be vulnerable while in the act of cell division,
but once formed, it is rather tough. It is less differentiated than the
ordinary cells and lacks their special abihties. It seems that many of
the special abilities of the cells from which the cancer originated are
lost, and all that is left is a high ability to grow and multiply. The
cancer cell is in fact usually an aggressive cell which is able to push
its way into other tissues. The cancer cell is often less responsive to
the immunity mechanism than normal cells — it is often possible to
graft a cancerous growth from one animal to another of the same
species. This has enormously assisted cancer research because
tumours of a standard kind can be multiplied at will by grafting
small pieces into healthy animals.

(2) Vims Theory of Cancer. Many attempts have been made to
extract a non-living agent from tumours which is capable to giving
rise to tumours when injected into animals. In most cases, if the
extracts are free from Uving cells, no successful transmission of the
tumour has been achieved. However, Murphy and Peyton Rous in
1910 found that it was possible to transmit certain avian tumours, by
injecting cell-free extracts of the tumour cells into healthy birds.
These extracts contained the submicroscopic particles of the cyto-
plasm but they certainly did not contain living cells. It was concluded
that these extracts contain an agent or virus which causes the disease.

Another example was the Shope papilloma virus of rabbits. These
are non-malignant growths, which occasionally become malignant,
found in the rabbits of New Jersey and from them Shope succeeded
in extracting a virus, which was capable of producing growths in
healthy rabbits.

However, although many attempts have been made to do so, it
has not been possible to produce cell-free agents, which will cause the
disease, in more than a few types of cancer.

Nevertheless, some workers adhere to the theory that cancer is
essentially a virus disease. It is suggested, for example, that apparently
healthy cells harbour the virus but it is only when the cell becomes
weakened or diseased that the virus can act effectively. Or it is
suggested that the carcinogenic chemicals act by reducing the resist-
ance of the cell, so that the virus already present becomes effective.
Another version of the same theory is that the change from a normal
to a cancer cell is due to the modification of some of the cytoplasmic
particles which are normally present. It might be that the carcinogenic
substances give rise to a change in the nature of some of these par-

1 1 Bacteriophage attacking a bacterium (B. Coli). Photograph by
Dr R. W. G. Wyckoff. The picture shows two cells. The cell above
has only just begun to be attacked, while that below appears to be
disrupted

12 A later stage of the attack of Bacteriophage on B. Coli. The
picture shows a cell full of newly formed bacteriophage particles

Cancer 97

tides, which is inherited from one particle to another.

On the whole, however, it can be said that the virus theory of
cancer meets with great difficulties, except in those cases where a
virus agent can be demonstrated. In other cases it is implied either
that a dormant virus is normally present, which only becomes active
when stimulated in some way, or that one of the normal cell particles
becomes a virus when it is acted on by a carcinogenic substance.
There is no doubt that the carcinogenic substance can act on the
cell particles present in the cytoplasm, but very little is known at
present about how their behaviour is changed and whether such
changes could be transmitted from one cell to its descendants.

(3) Metabolic Theories of Cancer. It is an undoubted fact that
cancer cells behave differently from the normal cells in which they
originate. Much work has been done on these differences, but few
really characteristic effects have been found. It is found, however,
that the activity of some enzyme systems is increased and that of
others is diminished. These differences are often so great that the
cancer cell lives in an essentially different way to the normal cells.
Warburg and others have shown, for example, that the cancer cell
obtains its energy, not like most normal tissues from the oxidation of
sugars, but by a less profound type of splitting of the sugar molecules,
known as glycolysis^ Warburg has therefore proposed that the car-
cinogenic agents are in fact cell poisons which interfere with the
normal mode of life of the cell and compel it to adopt a new mode in
order to live. The change from the normal to the cancer type on
this theory is the cell's effort to continue to live when its normal
mode of life is interfered with. One result of this effort is that it is
stimulated to an abnormal rate of cell division.

There is no doubt that many cells are adaptable to changes in their
environment. It was shown by Hinshelvvood that bacteria in particular
are very good at adapting themselves to a new way of life. They can
grow in the presence of dyes which interfere with their ordinary mode
of living. There is usually a 'lag' period in which the cells are adapting
themselves to the new conditions. During this, new systems of
enzymes are formed and new methods of synthesis are found to re-
place those which have been blocked by the dye. According to the
metabolic theory, the cancer cell is an adaptation to new conditions —
a response to the loss of normal functioning. According to this view,
carcinogenic substances act by interfering with enzyme systems
which the cell normally uses. They act indeed as selective poisons
and they compel the cell, in order to continue to exist, to find an
alternative way of living.
^ See p. 47.

98 Inside the Living Cell

However, chemical poisons are not the only agents which cause
cancers. It was found by Dr Oppenheimer that if plastic substances
are embedded in the skin of an animal, a cancerous growth may be
formed. The nature of the plastic seems to be quite unimportant,
but if the plastic film is pierced with holes, no cancer may occur. It
seems, therefore, that the cancerous growth is due to interference with
the flow of oxygen and food materials and to the effort of the cells to
improvise an alternative way of living.

Cancers may also be formed by continual irritation of a tissue
or if a wound is prevented from healing. It is possible that lip cancers
caused by contact with pipe stems are due to this, but chemical
action by tobacco tar is also a possibility.

(4) Cancer due to Hormonal Disturbances. Somewhat similar in
origin are cancers due to hormonal disturbances, which are common
in older men and women. Many bodily processes, as we have seen,
are controlled by hormones and require an accurate balance of
hormonal actions, if they are to function properly. We have also
seen that the glands which produce hormones are themselves con-
trolled by other glands, such as the pituitary. In ageing individuals,
the proper balance of hormones may be upset and an abnormal
growth of glands may occur, either by overstimulation or because
the gland is making an effort to produce sufficient quantities of a
hormone which are required.

Mammary cancers are very common in certain strains of female
mice, but removal of the ovaries reduces the incidence of the mam-
mary tumours. It follows that some substance produced in the
ovaries stimulates the occurrence of mammary tumours. In the same
way, breast cancer of women is sometimes successfully treated by
removal of the ovaries; similarly tumours of the prostate gland of
males is sometimes treated by castration. It is found that much can
be done in cases in which tumours are the result of the hormonal (or
endocrine) system being out of balance by restoring the balance by
suitable hormone therapy.

It has also been possible to replace hormones by the synthetic
equivalent. In fact, one of the most successful chemical treatments
of cancer is by the synthetic oestrogens (see p. 166) used for cancers
of the breast and the prostate.

CONCLUSIONS

It will be obvious that the subject of cancer is a large and very
complex one and there are many different facets to the whole problem.
It is quite possible that cancer is not one phenomenon but many.

Cancer 99

that different types of cancer have distinct origins which do not have
a great deal in common. On the other hand, it may be that there is a
single pattern, the nature of which is still unknown, which underlies
the different manifestations; and that the different theories outlined
above, which emphasize one or other of the different features, all
express a certain aspect of whole truth.

Cancer is not like an infectious disease in which, once the agent
has been identified, measures of control or cure can often be devised,
and the natural defence mechanisms of the body can also be stimu-
lated. It is essentially a disease of organization and as such it is
probably peculiar to the higher forms of life. To understand it we
obviously need to know much more about the way in which the
normal healthy organism works. We have seen that cancers can be
produced by extraneous agents acting on the body cells. The fact
that the incidence of different types of cancer varies very considerably
from one country to another shows that it is to a considerable extent
a disease produced by environmental conditions, and as such it ought
to be possible to reduce the incidence very considerably when the
causative agents have been recognized. It is well known that human
beings are subjected to ionizing radiation from cosmic rays, which
may be expected to produce a certain incidence of cancer. In thirty
years, at sea level, the dose from this source is about one roentgen.
In some localities there is also a quite appreciable amount of radio-
activity in the ground and in drinking water, which would have to
be taken into account. As we have seen radioactive substances like
radium and radiothorium (and also strontium from atomic bombs)
are concentrated in the bone, where they may remain for long periods
and eventually cause bone cancers.

We might ask whether cancer would occur if these environmental
influences were eliminated. Do cancers arise spontaneously in an
apparently healthy organism? Since a fair number of chemical sub-
stances have been found to be carcinogenic, it may be that the
organism itself may produce carcinogenic substances, either in its
normal operations or through some errors of metabolism. The weak
carcinogenicity of one normal constituent of the body, cholesterol,
which is present in fats, has been demonstrated by Hieger. While
the function of this substance is unknown, it is rather curious that
it is related chemically to the steroid sex hormones and it might be
that it is a by-product of the formation of these substances.

It can hardly be asserted that the body has developed any natural
defence mechanisms against cancer, but the fact that the incidence
of cancer increases rapidly with age, suggests either that as the body
ages, the controlling mechanisms become weaker, so that it is easier

100 Inside the Living Cell

for cancer cells to originate or that, in young vigorous organisms the
cancer cell is unable to estabUsh itself, i.e. the controlling mechanism
is strong enough to control it. It is a curious fact that, in some
instances, cancers cannot be recognized for a long time after the
carcinogenic agent has been introduced. There is a long 'lag' period
during which the carcinogenic agent is present, but apparently has
no effect.

XIII

The Origin of Life and Photosynthesis

H

The smallest body which possesses all the characteristics of life, the
cell, is quite obviously an extremely complicated structure. At its
simplest it contains at least two or three complex and interlocking
cycles of activity, which are concerned with the breakdown of food
materials and with the formation of high-energy and other inter-
mediates which are used for the synthesis of the characteristic com-
pounds. The agents which bring about these changes are themselves
formed from the products. Apart from all this basic chemical activity,
there are also the special arrangements by which cell division occurs.

In trying to discover how systems of such complication came
into existence in the first place, it is probably hopeless to look at cells
as they exist now, because they have an enormously lengthy history
behind them. Unfortunately, in our present day world, we do not find
any intermediate steps, except perhaps viruses (which are a special
case and a product of cells) which would help to bridge the gap be-
tween the living and the non-living.

Yet all life must have originated and been elaborated on the
earth. There must have been a time when the earth was too hot for
any living things as we know them. The igneous rocks underlying the
present superficial strata bear evidence of having crystallized out
from a molten magma. The stratified rocks which overlie them bear
witness in their fossil relics to the long history of life on the earth and
many of them — the chalks, corals and limestones are actual residues
of former life on the planet. But the earliest stages are missing from
the fossil record, and it is probable that something approaching life
as we know it at present had already been evolved before any en-
during traces were left.

It might have been expected that if the original processes which
led to living structures were completely natural, they would still be
taking place and that we should find, somewhere in the world, pro-

102 Inside the Living Cell

cesses which might be thought to represent at least a step or two in
the elaboration of living forms.

It is a very curious thing that, with one or two exceptions, practi-
cally no intermediate stages between simple inorganic substances
and living things or their products are known to exist. Practically all
the organic substances on the earth are products of life. It is possible
that the hydrocarbon oils which exist very plentifully in some strata
are an exception, but this is uncertain and it is quite likely that they
are to a considerable extent themselves the products of highly de-
veloped life, like the coal deposits. This is demonstrated by the fact
that, in some cases, they contain 'optically active' compounds, i.e.
when two possible forms of a compound exist, which are mirror
images of each other, only one occurs. This is an indication of living
origin, because natural processes (except crystallization) do not
usually distinguish between the two optically active forms of one
molecule.

We do not find organic compounds being elaborated by any pro-
cess which does not involve life anywhere in the world.

We are forced to adopt one of three alternatives:

(1) Life did not originate in this world. As Arrhenius suggested,
when he was confronted with the difficulty of finding sufficient time
for the evolution of life, it might have arrived from other worlds in
the form of spores. However, this only shifts the difficulty of account-
ing for the origin of life to other places, and in any case the evidence
of cosmology suggests that the earth has existed for an appreciable
fraction of the period of existence of the whole universe.

(2) Life did originate on this world, but the conditions at the surface
of the earth have altered so greatly, that reactions leading to life no
longer occur. This view has been put forward and supported by the
Russian scientist, A. I. Oparin, in a book on The Origin of Life, which
was first published in an English translation in 1938. Oparin argues
that in the early stages of the earth's history, the surface rocks con-
tained considerable quantities of metallic carbides, which, by react-
ing with water, would form first methane or acetylene and later more
complex hydrocarbon compounds. This would provide an 'organic'
environment, which would be much more favourable to the forma-
tion of complex organic compounds than that which exists now.
There is evidence for hydrocarbons like methane in the atmospheres
of other planets. Methane, as well as ammonia, have been detected
in the atmosphere of Jupiter and methane alone in Uranus and Nep-
tune.

Consider what would happen if such an atmosphere were exposed
to intense solar radiation. The sun produces not only visible light, but

The Origin of Life and Photosynthesis 103

ultra-violet rays, as well as high speed particles (ionizing radiations),
which may well have been much more intense in the past than now.
These radiations are capable of breaking hydrocarbon molecules
into fragments called radicals, which can react with each other and
build up into long chain compounds. In the presence of ammonia a
great variety of nitrogenous compounds could also be formed in this
way. Experiments have been made by Miller in an attempt to repro-
duce these circumstances. A mixture of water vapour, carbon dioxide
and ammonia was exposed to electric discharges and it was found
that a variety of compounds including amino acids was formed. If
amino acids are formed in a concentrated condition, the effect of fur-
ther irradiation may well be to cause them to combine into long pep-
tide chains such as occur in proteins. This type of condensation can
be effected artificially. It involves (see p. 17) the elimination of water
molecules. The water can easily be removed from amino acids,
yielding compounds called amino acid anhydrides. Woodward and
Schramm have shown that these anhydrides link up spontaneously
into peptide chains.

It is of course a long way from this to the exactly organized struc-
tures of the natural proteins and we do now know how this gap could
be bridged. But it no doubt would help a great deal if we could sup-
pose that at an early stage in the earth's history a surface layer rich
in carbon compounds were present. Certainly under such conditions
very complex compounds could be formed by the action of solar
radiation. Not only could we expect complicated hydrocarbons and
organic nitrogen compounds to be formed, but phosphorus too could
become activated and attached to organic residues and it might well
be that, out of the myriad possible combinations, eventually a simple
self-reproducing system would be formed, which would increase its
amount by making use of the organic material around it and would
thus impose its own pattern on the organic environment.

We have no idea what this primitive self-reproducing system was
like or of its transformation towards living structures as we know
them. The living cell as we know it is the end of an enormous series
of developments, not the beginning. Life itself has entirely destroyed
the primitive environment from which it arose and all the inter-
mediate steps. The only possible and likely relic from this stage of the
evolution of life might be some of the petroleum deposits — but these
again are in sedimentary rocks and may have been formed by living
things at a much later date.

(3) Steps leading to the creation of life from inorganic materials are
still going on in the present world, either in some unsuspected place
or on such a small scale that they have not been recognized. We can

104 Inside the Living Cell

only say that if such circumstances exist they remain to be discovered.

We can obtain some clues about the circumstances under which
living things originated from our knowledge of their chemical com-
position.

As I mentioned above, all living things are optically active, i.e.
when two forms of a compound are possible only one is present. Thus
all the amino-acids present in proteins are of the /-configuration. It
follows that at some stage or other something must have happened
to decide that living things should use compounds with the / (left-
handed) rather than the reverse d (right-handed) configuration.

There are few natural ways which discriminate between these two
forms of a compound. As I mentioned above, one of these is crystal-
lization. Some minerals crystallize quite naturally into optically
active forms, e.g. quartz crystals are found in either one or the other
form and this is also true of other minerals. It is thus possible that
life acquired its preference for one configuration (in the case of pro-
teins, the /-form) by being associated with a particular kind of crystal.
Thus if the original germ of life started in contact with a crystal of
quartz (or clay minerals) which had optical activities, it may well be
that one of the possible forms of amino acid was made use of and,
having been established, the pattern involving this type was con-
tinued to the present day. Another way in which this could come
about has been suggested by Dr J. H. Northrop, who pointed out
that, when organic substances crystallize, they may give optically
active crystals (in equal quantities). But when pairs of crystals of
opposite sign are formed, they may easily be separated from each
other by perfectly natural agencies like the wind, so that localities
may occur where an excess exists of organic material of one confi-
guration. It must have been some such accident as this which im-
printed on living things their preference for /-amino acids. The natur-
ally occurring sugars also are found with particular configurations.

It is probable that there could be a similar family of living things
making use of the J-compounds — in which all the important com-
pounds were of the opposite configuration. There is no sign of this
family on this world, but it might exist on another planet.

It is, however, difficult to reconcile the idea of a primitive organic
environment with the amount of oxygen now in the earth's atmo-
sphere. There is in fact plenty of oxygen to burn all the carbon com-
pounds completely to carbon dioxide and water. Under the condi-
tions suggested for the elaboration of complex compounds, oxida-
tion would certainly occur. Even in the present world, few organic
substances can remain exposed to the atmosphere and sunshine and

The Origin of Life and Photosynthesis 105

water for any length of time without becoming oxidized. We could
only get over this difficulty if we suppose that the composition of the
atmosphere has also changed with time and that there is much more
oxygen now than in previous epochs. Where could oxygen have come
from? There is much oxygen and nitrogen in the sun and it is not im-
possible that atomic particles of these substances which are shot out
from the sun are captured by the earth in sufficient quantities to
change the atmosphere over long periods of time. Hydrogen is too
light to be permanently held in the earth's atmosphere.

LIFE AND ENTROPY

There is no doubt that under the influence of life there has been a
great elaboration of complex materials on the earth. Can this be ex-
plained as a purely natural process, operating under the known laws
of physics? At first sight it seems to be contrary to the tendencies
which we observe in the non-living universe. Here the direction in
in which spontaneous processes occur is always down-hill; energy
always becomes more dissipated; materials always pass from more
organized states to more mixed-up states.

This tendency was recognized by Lord Kelvin and formulated as
the law of dissipation of energy. Now better known as the second law
of thermodynamics, it can be expressed in a number of different
ways. One of them, as formulated by Willard Gibbs, makes use of the
concept of entropy. Entropy is a measure of mixed-up-ness or dis-
order, or of the degree of dissipation of energy, and the mathemati-
cians' statement of this law is that entropy is always increased in
natural processes which occur of their own accord.

The growth and elaboration of organisms seem at first to be a
direct contradiction of this law. We see in living processes an increase
in elaboration and order. It is only when the organism dies that dis-
integration occurse and disorder increases. It would thus seem that
the whole progress of life on this planet, which has led to the evolu-
tion of more and more complex structures, is contrary to the laws of
the inanimate world.

It therefore seems that life has found a way of evading the other-
wise universal tendency to dissipation and decay. Kelvin, when he
formulated the law of dissipation of energy, made a possible excep-
tion in favour of the operations of life. J. N. Lewis thought that living
things are 'cheats in the game of entropy'. However, scientific opinion
at the present day takes the view that such an exception is unneces-
sary. The entropy law only requires that on the whole energy is dis-
sipated and entropy is increased. There can be a local decrease of

106 Inside the Living Cell

entropy and increase in organization if it is compensated by a greater
increase of entropy elsewhere.

If we examine the matter more carefully, we find that life is not
independent of the great processes of the natural world. All life de-
pends directly on the great outflowing of energy from the sun, which
is a dissipative process, by which entropy is increased. Animal life is
entirely dependent on plants for food; and plants make direct use of
the energy of the sun's radiation. If the sun's rays were cut off, life
could not maintain itself for more than a very short time on this
planet.

Thus we reach the conclusion that the elaboration of complex
living structures is directly dependent on the dissipation of solar
energy. There is a local decrease of entropy in the living organism,
but it is compensated by a great increase of entropy in the dissipation
of radiation from the sun.

PHOTOSYNTHESIS

The basic process on which all life depends is thus the utilization
of solar energy for the elaboration of complex compounds. However
this originally occurred, it now takes place almost entirely in the
photosynthetic apparatus of plants and algae, which have evolved a
highly efficient apparatus for making use of the visible radiations
arriving from the sun.

The photosynthetic apparatus of green plants makes use of a green
pigment called chlorophyll, which is present in small granules called
chloroplasts, which exist in the green cells. Chlorophyll is a not re-
markably complex molecule, and its structure is largely known.
Its most distinctive feature is the presence of a magnesium atom
surrounded by four indole groups, making a kind of flat plate.^
However, chlorophyll is only capable of bringing about photosyn-
thesis when it is in the chloroplast. In recent years much has been
learnt about the structure of these bodies from electron microscope
photographs of thin sections of them. All the chloroplasts which have
been examined show a laminated structure (see PI. 13), i.e. a system
like the sheets of a book packed parallel with each other. Sometimes
the sheets or lamellae run through the whole chloroplast but in other
cases there are a number of more or less separate piles of sheets.
Each lamella is found to consist of two membranes joined at the ends
and each membrane is 65 A thick^ and the space between them is
about the same thickness. This is about the thickness of a small pro-

M A = 10-8 cm.

2 Similar to haemoglobin (p. 164) with Mg in place of Fe.

The Origin of Life and Photosynthesis

107

tein molecule (see Fig. 17). It is likely that the membranes consist of
films of protein with a fatty layer between and it has been suggested
that the chlorophyll molecules form a film between the protein and
the fatty substance.

It was shown by Hill in 1939 that the primary action which chloro-
plasts bring about in light is to split water into the two fragments or
radicals, H and OH, thus:

H2O ^ H + OH
Oxygen is eventually liberated from the OH, while the reactive hy-
drogen atoms are available for doing chemical work. They will com-
bine with any suitable hydrogen acceptor. In the plant they are used
chiefly to combine with a coenzyme (dpnh)^ which is used to transfer

♦-^st roma*4<*- granum

repeating 250-270A
period J. *

65X-i--i
30X|--_-.

0-4 0-6y"

FIG. 17. Part of section through a chloroplast showing the lamellae (Steinmann
and Sjostrand)

the hydrogen to form energy-rich compounds. It has been suggested
that the function of the membranes in the protoplast is to keep H
and OH apart from each other so that they do not recombine.

The overall reaction is to take carbon dioxide from the atmosphere
and 'reduce' it by the addition of hydrogen and the removal of oxy-
gen, so that it can be converted into carbohydrates. The course of
this reaction has been worked by Calvin and his associates by the use
of carbon dioxide labelled with radioactive carbon. They find that
the carbon dioxide enters into a complicated cycle of changes. It is
taken in at one stage of this cycle and the product is reduced to a
sugar-like compound by the dpnh, which has been formed from hy-
drogen atoms under the action of light in the protoplast.

The main result of these researches was to show that the forma-
tion of sugars and other carbohydrates is not a simple direct process,
^ Diphosphopyridine nucleotide, hydrogenated.

108 Inside the Living Cell

but one which occurs in a number of stages. This is probably an ad-
vantage from the plant's point of view because it has to make large
numbers of other compounds besides carbohydrates and the inter-
mediates of the carbon dioxide-sugar process are also available for
carrying out many other necessary reactions. It is interesting to find
that the basic process of photosynthesis is the splitting of water (H2O)
into H and OH, and the use of the reactive hydrogen atoms to bring
about reactions which result in the production of energy-rich com-
pounds, on which all animal life depends.

Since all life still depends on photosynthesis, it is impossible not
to conclude that the earliest manifestations of life made use of the
energy of radiation. As we have seen, the ionizing radiations produce
highly reactive species of molecules directly and it is quite possible
that at the beginning the process which led to living things used the
more penetrating radiations. But in the photosynthetic mechanism of
the living plant, a means has been found of using the much gentler
visible radiations of light, which are harnessed in the mechanism of
the chloroplast. This is effected by the chlorophyll molecules which
can absorb visible light and use it to split water. The chlorophyll sys-
tem acts as an energy transformer, and a very efficient one, for the
conversion of light into chemical energy.

Theoretically the energy of at least three photons of visible light^
should be required to turn one molecule of carbon dioxide into
carbohydrate. Many measurements have been made to determine
the number actually required in living cells. The results are rather
variable owing to the extremely variable circumstances which ob-
tain; but under good conditions a value of eight photons per molecule
has been observed. This indicates that the efficiency of the photo-
synthetic apparatus in converting light energy under the best condi-
tions is at least 35 per cent.

The amount of solar radiation falling on the earth is quite
enormous. For example, in the United States, on the average 1,000
calories reach each square foot of land during the daytime every
minute.^ For an acre of land this equivalent to the output of an elec-
trical station generating 2,600,000 kilowatts! About half of this is
made up of heat rays not available for photosynthesis. The total
amount of energy which could actually be used for photosynthesis
under the best possible conditions is thus about J X 35 per cent X
1,000 = 175 calories per sq foot per minute or 3-85 X 10^ calories

^ The photon is hr where h is the Planck constant and v the frequency of the
light, hv varies from 40 to 70 kcals. per molecule according to the wave-
length of the visible light.
2 This is enough to heat 1 litre of water 1 "C.

The Origin of Life and Photosynthesis 109

per acre per day of nine hours' sunshine. This would be sufficient to
produce about one ton of carbohydrate material from carbon
dioxide, on the basis of the best efficiency which is reached in labora-
tory experiments. Actual agricultural operations do not approach
this degree of efficiency. The average yield of agricultural produc-
tion is not more than two tons per acre per year,^ which is clearly
very much less than the optimum obtainable from a plant under ideal
conditions. There are of course many reasons why oiily a small frac-
tion of the photosynthetic energy of sunshine is actually utilized in
practice, but it is clear that in sunshine, particularly in the tropics, we
have a source of energy and of synthesis far exceeding all other
sources, such as coal, oil and atomic energy, and the photosynthetic
apparatus of green cells is remarkably well adapted to its utilization
to make a host of compounds.

^ The data given here are taken from an article by F. Daniels in Holloender's
Radiation Biology, Vol. III.

XIV

Specialized Cells: Muscles^ Nerves and Sense

Organs

H

It is difficult to convey any idea of the extraordinary variety of cells
in the animal organism. Besides those already mentioned there are,
for example, cells which lay down the bones of the skeleton in the
young animal; the cells which produce hair and nails, which in many
cases continue to function for the whole of life; and the mammary
cells of the female which, after the birth of young, produce large
quantities of milk. The latter are a veritable factory of proteins, fats
and other constituents. Here is a description by Professor H. D. Kay
of the operation of the milk-producing cells of the cow. 'The day's
work of one of these cubical cells entails the following cycle of opera-
tions. It begins as a rather squat cell with the nucleus in the middle.
Granules or globules, some of which stain with fat soluble dyes, then
begin to appear in the part of the cell nearest the alveolar space. The
cell begins to increase in length and size, the nucleus remaining close
to the basement membrane. The secretory products soon fill the
whole of one end of the tall distended cell. These products, and pos-
sibly a small part of the cytoplasm of the cell itself, are now extruded
into the lumen as milk, following which discharge the cell returns to
its original squat shape. This whole process is repeated several
times. . . . During the twenty-four hours in an actively lactating cow
there may be up to four or five, or even more cycles of operation. . . .*
However, perhaps the most interesting cells are those which give
the animal the power of movement and those which enable him to
become aware of the nature of his environment. These cells, which
form muscles and sense organs and nerves are so important in the
higher manifestations of life, that we must look at them in some de-
tail. They have to be taken together because, as a rule, the operation
of one involves the functioning of at least one of the other two.

Specialized Cells: Muscles, Nerves and Sense Organs 111

MUSCLE

The simplest organs which produce motion are the flagella or
streamers attached to some bacteria (Fig. 18). These seem to be
threads or filaments of protein. They are capable of swaying from
side to side in a kind of wave motion and this propels the bacterium
through the water. Spermatozoa have a similar organ in their tails
which enables them to swim through the vaginal fluid until they reach
the ovum. These tails contain the enzymes which are required to pro-
duce the energy they need from the breakdown of sugar — they are in
fact miniature muscles. However, it is likely that they start their
journey with a supply of 'fuel', i.e. sugar, and when this is used up
they are unable to go any further.

The muscles of the higher animals are large masses of cells which

FIG. 18. A bacterium with flagellae (B. proteus vulg., after Weibull)

have the power of contracting. They are often attached to the bones
of the skeleton by tendons and are able to exert considerable forces.

Muscle is clearly a very complicated organ and it has been studied
from several points of view. These concern themselves with different
parts of the whole question; they are all necessary and a complete ex-
planation of muscle action must cover the three main problems to
which an answer is required. These are:

(1) The mechanical arrangements whereby contraction occurs;

(2) the biochemical reactions which provide the energy;

(3) the 'activation process', usually stimulated by nerve signals,
which causes the muscle to contract.

All these have been exhaustively studied and at different times the
emphasis has been put mainly on one or the other. For a long period
(1920-1940) much of the interest was in the biochemical reactions
which provided the energy of muscular contraction and these have
been fairly fully worked out. It has been established that the energy-

112 Inside the Living Cell

providing mechanism is not greatly different to that of other cells
(see p. 47) and that the energy produced by the oxidation of sugar
becomes available to the muscle in the form of the adenosine triphos-
phate (atp) — another example of how the biochemical mechanisms
are closely related to each other, since the same basic energy yielding
cycle is used both for protein and nucleic acid synthesis and for giving
the energy required by the muscles.

The chemical composition of muscle has also been studied for a
long time. Kiihne in 1864 extracted a soluble protein called myosin.
Nearly eighty years later, Straub in the laboratory of Szent-Gyorgyi
found another component in muscle which he called actin. The com-
pound of actin and myosin (actomyosin) had marked contractile pro-
perties, and as myosin itself was found to be a powerful agent for
decomposing atp and as the latter caused the contraction of syn-
thetic actomyosin threads, it appeared that the most important agents
in the muscle mechanism had been isolated.

t" — ^ — 1 / h*^ r^'~^

Myosin y*

filament jctin r ^^A^ H S- filament

filament

FIG. 19. Structure of striated muscle fibres (Huxley)

More recently attention has been given to the structure of the
fibres in the intact muscle. The existence of striations or banding in
many muscle fibres had been observed by microscopists during the
nineteenth century. The striations consist of alternate zones having
higher and lower refractive index and were sometimes called dark
and light bands. The spacing of the bands is about 2-5/i, i.e. between
1/100 and 1/1000 of a millimetre. Their existence has been confirmed
by electron microscope pictures of thin sections of muscle (see
Plate 14). Numerous attempts have been made to identify the sub-
stances present in the bands by dissolving out the different com-
ponents separately. Finally, Dr H. E. Huxley and Miss Hanson on
one hand and Drs A. F. Huxley and R. Niedergerke both proposed
in 1954, on the bases of their observations, the picture shown in
Fig. 19.

The continuous threads running through the muscles are actin fila-
ments, which are held together at intervals by a kind of spacing plate
(the Z line). The myosin filaments run side by side with these, but

13 A cross section of a chloroplast from Aspidistra elatior. (Repro-
duced by permission from Steinmann and Sjostrand, Experimental
Cell Research, 8, 15, 1955.)

. . • • >."'♦; ^ • >-• . • » . . ..'..•■»>; •.»."•. ' • - . • • • . ■ ■ . • • *»^ . < .. • • , •

•• -M*-,*^

* • • • .' 4 • " . *

K- •:•;•?••; :-:--;^ :;'-'V:;:/;C^C>^:^:-v •;;^^^^;v-^v>•^;^^^^ ■:::•:•:••;;:•:•>.*•: -/••:••:•

' ^•..*. ;v» • •/.^ .* • ,'■ • ' *.*.*,♦• '• •' • ' * . ■ •■••*'•.*.**• ./ '•..'•■••. .'....••.•■*••*-•.•- . •

14 Electron micrographs of muscle fibrils. The upper picture is a
section across the main direction of the fibril. The larger circles are
the myosin filaments and the small points are the actin filaments. The
lower picture is a longitudinal section which shows the various bands
described in the text. These photographs are reproduced by per-
mission of Dr H. E. Huxley

!

Specialized Cells: Muscles, Nerves and Sense Organs 113

they do not occupy the whole space between the Z lines. When the
muscle shortens, the two kinds of filaments of actin and myosin re-
spectively, slide past each other. The actin filaments do not them-
selves shorten when the muscle contracts, but there is in the middle
region between two Z lines a filament which is very contractible. It
thus holds two rather rigid actin rods together and permits them to
slide through the spaces between the myosin rods.

According to this structure, which is now fairly well supported by

I MYOSIN ~}

Position I

I MYOSIN I

TrLnnnnnJinj

1 ACTIN I

Position 11

I MYOSIN ~J

Position III

FIG. 20. Mechanism of contraction
in muscle fibres

numerous observations, the contraction of muscle is due to the sliding
of myosin and actin fibres alongside each other. Much less is known
about this aspect of the mechanism. It is certain that atp must be in-
volved in this process as the source of energy; but the way in which
it operates is uncertain, although various possible processes have
been suggested. It is likely that the two filaments move with respect
to each other, by attachments between them which are broken in one
way and remade in another. Thus the one filament may move along

H

114 Inside the Living Cell

the other by a centipede kind of motion (Fig. 20). The atp would be
involved either in the making or the breaking of attachments and the
energy required would be provided in some way by the energy rich
bonds of the atp. Fig. 20 is a rather fanciful picture of such a process.
The final question we must ask is how the muscle is activated, i.e.
what causes it to begin to contract. As a general rule, as we shall see,
muscle action is excited or initiated by electrical disturbances arriving
down a nerve. This excitation causes the movement of inorganic ions
across the membrane in which the muscle fibres are enclosed. There
is a special region of the muscle fibre surface which is in contact with
the nerve and it is here that the processes leading to excitement take
place. It was shown by Sir Henry Dale and Prof G. L. Brown that,
when the nerve signal arrives in this region, the substance acetyl-
choline is liberated. It was also found that an enzyme capable of de-
stroying acetylcholine is also present; so that the acetylcholine
formed only lasts for a short time. The idea that the actual process of
muscle excitation is brought about by acetylcholine has been con-
firmed by two facts: (1) substances which inhibit acetylclioliiie action
interfere with the excitation of the muscle, (2) acetylcholine, when
applied artificially, does give rise to muscle contractions. However,
very little is known about how the electrical disturbance arriving
along the muscle causes the release of acetylcholine, and not a great
deal is known of the changes which acetylcholine brings about, ex-
cept that it causes marked changes in the ease with which sodium and
potassium ions can penetrate the muscle membrane.

SENSE ORGANS

Sensitiveness to the environment is probably a fundamental pro-
perty of living cells. Even the most primitive forms of life respond to
some degree to heat or cold, to light and to chemical substances. Sen-
sitiveness to light is probably a part of the original character of life,
since as we have seen, the original living complex must have been
photosensitive and able to use light energy. Sensitiveness to chemical
compounds must also be a very primitive feature of life, since many
chemicals upset the delicate balance of the life processes and, in fact,
many products of enzyme action are themselves inhibitors and pre-
vent enzymes working properly. Organisms often cannot live unless
the substances they produce are removed. They also need a fresh
supply of the substances used for food and can often detect them at
appreciable distances, e.g. the whelk can detect the compounds
formed by decaying animal matter at a considerable distance.

Many primitive animals are light sensitive to some extent over the

Specialized Cells: Muscles, Nerves and Sense Organs 115

whole surface. Others, such as starfishes, have specially sensitive eye
spots, and illumination brings about movement either towards or
away from the light. ^

Out of these primitive responses, living things have developed
during the ages the most astonishing sense organs. In a world in which
the competition for food was severe, the possession of even slightly
more effective means of finding it must have had great survival value.
It is not surprising that the struggle for existence has produced a
steady improvement until the beautiful arrangements possessed by
the higher animals at the present time were elaborated. Special cells
have been developed which have in many cases a most extraordinary
sensitiveness to particular kinds of stimulus. The olfactory cells of
the nose of some animals is capable of detecting extremely small
quantities of some chemical compounds. It is well known that dogs
can follow a human being by smell the day after he has gone along
the track. It has been demonstrated that in the nose of the dog a single
molecule of a fatty acid must be capable of stimulating the smell
organ.^

Insects also have an extraordinary smell ability. Female gypsy
moths can, it is said, attract the male from distances up to two miles
by means of an odour. The male will respond to the excised scent
gland of the female or even to a piece of blotting paper which has
been touched by the scent gland but if the antennae of the males
are removed, they are quite unresponsive. Parasitic wasps can locate
their larval hosts, even when buried several centimetres in wood.

The human eye is a most fantastic example of fitness and efficiency.
It is incredibly complicated. The retina of each eye contains about
120 million sensitive elements in the form of rods and about 6 million
cones. About a million nerve fibres take their responses to stimula-
tion into the brain. It is obvious that many of these sensitive elements
must share a single nerve fibre.

The purpose of this arrangement is to provide both night and day
vision. The rods, which provide for night vision, are extremely sensi-
tive and many of them share a single nerve fibre. This means that the
nerve fibre has a large number of terminations, which are spread out
so as to catch all the available light. It has been shown by Dr M. H.
Pirenne that the sensitiveness of the eye approaches the maximum
possible, which could be reached if every quantum of light impinging
on the retina produced a sensation. Careful measurements by Pirenne

1 N. Millott, 'Animal Photosensitivity', Endeavour. XVI, 19, 1957.

2 One molecule of the fatty acid, when applied to about 10* sense cells, can
be detected. The chance of two molecules reaching one cell in the smell organ
under these circumstances is remote.

116 Inside the Living Cell

have shown that at the Hmit of detection, not more than a very few
quanta are actually absorbed in the rods in order to produce a
detectable sensation of light. Because one nerve serves an appreciable
area, this kind of vision is not very sharp.

The cones which sometimes have an optic nerve to themselves and
sometimes share an optic nerve in groups of two or three, provide
for a sharply discriminating kind of vision under conditions of good
illumination. They are also sensitive to different kinds of light, as
they are responsible for colour vision.

What does light do when it reaches the retina? It is absorbed by
a pigment called visual purple or rhodopsin, which can easily be
extracted from the eyes of animals and has been much studied in
recent years by R. A. Morton in Liverpool and by G. Wald in
Harvard. It has been found to be a protein combined with a special
light absorbing molecule. The latter is a near relative of Vitamin A —
thus explaining why deficiency of Vitamin A leads to night blindness.
Different animals make use of slightly different derivatives of Vitamin
A; beef rhodopsin uses Vitamin Ai and fish rhodopsin the slightly
different Vitamin Ao. These molecules all have a row of alternating
single and double bonds, which give them their light absorbing
property. Molecules of this kind have one electron for each carbon
atom, called a ;r -electron, which is in a special state. Instead of being
anchored to one carbon atom as the others are, it is free to wander
up and down the carbon chain. The effect of the absorption of light
is to activate the --electrons, so that they take up the energy of the
light absorbed and acquire a state of high energy in which they are
able to escape from the light absorbing molecule into the protein
it is attached to. This results in the bleaching of the dye. Dr Wald
has described this change as follows : 'When rhodopsin is exposed to
light in the retina, two major changes occur, the carotinoid (the dye
molecule) is cleaved from the protein and is degraded through orange
intermediates, first to orange retinene 1, and then to colourless

Rhodopsin (or visual purple)

slov^ly
in dark

Vitamin Ai-f protein -^ Retinene + protein

Retinene is the 'aldehyde' of Vitamin A

Specialized Cells: Muscles, Nerves and Sense Organs 1 17

Vitamin Ai. The retina can also resynthesize rhodopsin in two ways,
first from retinene, and relatively slowly from Vitamin Ai.'

What happens when the protein of rhodopsin receives the liberated
electron is not clearly known, but the final result is to stimulate
the optic nerve, so that electrical disturbances pass down it to the
brain. This can be demonstrated by experiments, in which an elec-
trode is placed in contact with the optic nerve of the frog. Whenever
light falls on the retina an electrical disturbance passes down the
nerve, which can be detected by means of the electrode and if an
amplifier is put into the circuit with a loud speaker, the result of
the passage of the electric impulse is a click in the loud speaker.
It appears from these experiments that the frog responds to changes
in the intensity of light, i.e. when the light is turned on or off, but
steady illumination produces no further response. However in man
and probably other mammals steady illumination causes a con-
tinuous stimulation of the nerves.

NERVES

This brings us to the links between the sensory organs and the
muscles. Usually this is via the brain — a sort of clearing house for
the information coming from the senses; but this will require several
chapters by itself. For the moment we will look at the nerves them-
selves, white threads of many different sizes and lengths; sometimes
in bundles, sometimes singly, which traverse the body like a telephone
system.

For many years scientists have wondered how these messages are
transmitted. Even Newton speculated on this and thought that it
was a purely mechanical transmission of a mechanical impulse. To
Descartes what was transmitted was 'like a very subtle wind . . . con-
tinually mounting in great abundance from the heart to the brain,
flows from there through the nerves to the muscles, and gives motion
to the members'.

According to this the nerves were just tubes which permitted the
flow of 'animal spirits'. It was L. Galvani who showed that the flow
was electrical, and analagous to the electric discharges obtainable
from static electric machines which were common at this time. He
showed that frog's muscles could be made to contract by electric
currents applied to the nerve of the spinal column. 'Animal spirits'
thus became 'animal electricity' which was supposed to be secreted
by the brain and travelled along the nerves.

Many years of research have confirmed the electrical nature of
the impulses which pass down the nerve fibres. This can easily be

118

Inside the Living Cell

demonstrated by putting electrodes on two parts of an exposed nerve
and connecting them to an electrometer. These impulses can even
be detected outside the body using the sensitive electronic amplifiers
now available, yet it has been found that the analogy with electric
currents in metallic conductors is not at all close. The nerve fibres
themselves are rather compHcated structures. When examined in the
living state, or recently excised, they consist of long cylindrical
threads, usually 2-20 fx in diameter, although much larger ones are
sometimes found. Every nerve has grown out of a nerve cell and as
a kind of extension of it (Plate 15). Each fibre of a typical nerve con-

Fat

Molecules

Protein

\ / Protein

A single

lipo-protein

layer

Core or axon

Myelin sheath
(concentric layers
of lipo-protein)

FIG. 21. A nerve with myelin sheath

sists of a *core* or axon, which is enclosed in a membrane. In the
case of 'myelinated' nerves, the core is covered by a glistening sheath
of a white substance called myelin which has been shown by electron
microscope pictures to consist of a large number of concentric layers
of material. This material is a combination of protein with a fatty
substance, which together constitute a 'lipo-protein'. It has been
found that the molecules of the fat are enclosed between thin layers
of protein and the long hydrocarbon chains of the fatty substance
are arranged with their lengths pointing outwards from the centre
of the fibre (see Fig. 21). Inside this myelin sheath is a soft jeUy-

Specialized Cells: Muscles, Nerves and Sense Organs 1 19

like substance consisting of proteins and inorganic salts which is
in fact continuous with the cytoplasm of the nerve cell, and contains
mitochondria and other particles necessary to maintain the life of
the cell (Plate 15).

During recent years many experiments have been made, especially
by A. L. Hodgkin and A. F. Huxley, using the giant nerve fibres
of the squid, which are so large (0-5 mm. or more in diameter) that it
is possible to put electrodes under the axon jelly and so detect the
changes which occur when the nerve impulse passes. These fibres
are not myelinated, but they do possess a lipo-protein covering, which
plays a very important part in the action. The composition of the
fluid inside the fibre is very different to that of the blood plasma in
which is usually immersed (see Fig. 22). In fact, while the blood
plasma contains about twenty times as much sodium (Na) as potas-
sium (K) the interior of the fibre contains ten times as much potassium
as sodium. There is also a marked difference between the concentra-
tions of chloride (CI) ions inside and outside.

— — — — — _«.''
Axoplasm I

Na K CI I

49 410 40 \

440 22 560

FIG. 22. Distribution of inorganic
ions inside and outside the nerve fibre

These differences persist in an unexcited fibre because although
potassium can pass freely through the membrane, sodium cannot
and there is also an efficient mechanism (a sodium pump) for re-
moving sodium from inside the membrane when its concentration
is too high.

Because of the inequalities in the amounts of the sodium, potas-
sium and chloride (which are present as electrically charged ions)
inside and outside the fibre, there is a difference of electrical potential
across the fibre membrane and as a result the outside has a positive
charge and the inside a negative charge. (Fig. 22.)

When a nerve impulse passes down the fibre, the permeability of
the membrane to sodium ions is momentarily increased. As they
are at a higher concentration outside, they immediately pass through
into the interior, and as a consequence the negative charge on the
inner surface is reduced and it may be reversed. (Fig. 23.)

When the impulse has passed, the membrane properties are re-

120 Inside the Living Cell

stored. The excess of sodium ions which has reached the inside of the
fibre is pumped out again and some potassium ions also escape so
that the original condition of the charges on the membrane is restored.
The nervous impulse is thus a transitory disturbance in the electri-
cal condition of the nerve membrane which travels along the nerve.
A single disturbance of this kind takes as a rule a few thousandths of
a second to pass any point on the fibre. Its speed down the fibre is

Na-" K +

+• + -(- + +

\

+ 4- +

-I- ■+ -I- -I- -M

DIRECTION OF IMPULSE

-h-f-h-hH I-H--4-

FiG. 23. How the electric impulse passes down the
nerve fibre

comparatively low, depending on the diameter of the fibre (about
twenty metres a second in the giant squid nerves). This speed is of
course very much less than the normal speed of electric conduction
processes (which may approach the speed of Hght). It is determined
by the rates of the various processes by which the impulse is trans-
mitted.

Much remains to be learnt about the nervous impulse, and about
how it is started and how it acts when it reaches its destination in the
muscle. It seems a rather unpromising mechanism for precise control
of muscle action. We could hardly have anticipated from what we
know of its mechanism that such a process should have been capable
of such an extraordinary degree of elaboration so that it eventually
became the basis of mentality.

XV

77?^ Brain and What It Does

H

Now we come to one of the largest collections of cells in the animal
body, the brain. In man. it consists of a mass of nerve cells or
neurones, at least 12.000 million of them, connected together by
very numerous branching nerve-like threads, of which the largest are
called axons (see Plate 15). Into it go most of the nerves coming
from the sense organs and out of it come many of the nerves which
control the muscles. We could say that the brain is like a telephone
exchange in which the proper circuits are connected together, but
this does not help very much in explaining how the information
provided by the senses is organized and made use of, and how
complicated muscle actions are controlled.

We can easily recognize a number of different ways by which
muscles can be put into action. The simplest way is the involuntary
reflex action in which a sensation brings about an automatic in-
voluntary response, like the contraction of the iris when a bright
light is shone before the eye, or the sudden withdrawal of the hand
if a needle is pushed into it. These reflex actions often scarcely
involve the brain at all; they occur in animals in which at least the
higher parts of the brain have been removed. Similar to these are
responses which control automatic muscular rhythms, like the
beating of the heart, where one stage of the rhythmic process, when
it is completed automatically, initiates the next. Other processes,
like breathing, are semi-automatic. They normally look after them-
selves, but we can interfere with them if we want to.

Something is known about the ways in which such automatic or
semi-automatic actions are controlled. The muscles themselves are
provided with sensory nerves which send out messages indicating
their state of contraction. This was discovered by Sir Charles Bell
as long ago as 1826. In a famous paper, which he presented to the

122 Inside the Living Cell

Royal Society, he said :

'We are sensible of the most minute changes of muscular exer-
tion, by which we know the position of the body and limbs, when
there is no other means of knowledge open to us. If a rope-dancer
measures her steps by the eye, yet on the other hand a blind man can
balance his body. In study, walking and running, every effort of the
voluntary power, which gives motion to the body, is directed by a
sense of the condition of the muscles and without this sense they
could not regulate their actions.'

He showed that the muscles are provided with sensory nerves,
which send out messages indicating their state of contraction and in
other ways also indicate what have been the results of the muscle
contractions. These messages reach the brain so that 'between the
brain and the muscles there is a circle of nerves; one nerve conveys
the influence from the brain to the muscle, and this gives the sense of
the condition of the muscle to the brain'. (Fig. 24.)

This method of control is quite common in machines, which are
often provided with governors or speed controllers, arranged so that
the output of the machine controls the power which is supplied to it,
i.e. the input. A good example is the governor of a steam engine.

^tor^nerve

FIG. 24. The nervous circle

which closes a valve when the pressure exceeds a critical value.

The flow of water through a pipe can be kept constant by making
the outflowing water partly close a valve in the inflow when the out-
flow speed is too great. Electric motors can easily be arranged with
speed controls which cut down the operating current when the speed
is too high. The general principle is known as 'feedback', i.e. some of
the output is used to control the input.

The same principle is used in controlling the muscles. The im-
pulses received from the sensory nerves stimulate or inhibit the
impulses sent out by the motor nerves, and this controls whether
the muscle will be stimulated to contract or not stimulated. If the
sensory nerve is cut the muscle is as completely paralysed as if the
motor nerve is cut. In this way the muscle is kept in a constant state
of tension which is required to do the work it has to do.

The Brain and What It Does 123

Similar and probably much simpler involuntary responses to
stimuli are quite common even in very primitive kinds of animals.
Some organisms will move towards the light, others will move into
shaded regions. For example, some hydrozoa, like Gonionemus,
prefer to be in the shaded parts of an illuminated tank; others, such
as Englena viridis, seek the lighted parts. The mollusc, Pholas, in
which the whole surface is said to be light sensitive, retracts its
syphon in response to light, while in Ciona intestinalis, light causes
a closure of the syphon apertures. In sea urchins the spines react to
variations in light, e.g. in Diadema antillarum, spines are moved
vigorously towards regions shaded by objects.^

Worms like Branchiomma have light-sensitive tentacles, and the
passage of a shadow over these tentacles causes it to withdraw into
its tube. Starfish often have light-sensitive patches (or primitive eye-
spots) on the tips of the rays.

Many organisms react to the presence of chemical substances,
like oxygen. These are all automatic responses to stimulation which
act directly on the organism's means of propulsion.

But in all the more developed forms of animal life, something
more than simple reflex action is required. The animal must not only
receive sense impressions; in order to make use of them, it must be
able to make some sort of an interpretation of them. The interpreta-
tion is based on experience, i.e. on the results of previous actions in
similar situations. The animal learns from experience, and he could
not learn if he did not remember.

This is an entirely new thing in the world of life; a totally different
mode of response to stimulation and one which has turned out to be
enormously more effective. It is, in fact, the first beginnings of
mentality since the animal learns something about his surroundings
and uses, or tries to make use of, that knowledge.

Very many experiments have been made of this learning process.
Professor J. Z. Young has experimented with octopuses. They were
offered either a crab or a crab with a white plate behind it. The
animal lived among some bricks at one end of a tank and the crab
was lowered by a thread at the other end. As soon as the moving
crab appears, the octopus swims or walks across the tank and throws
itself over the crab, gathers it up and returns with it to its home.
When the crab with the white plate was presented, an electric shock
was applied as soon as the octopus had seized the crab. This caused
it to go back to its home quite quickly and it very soon learnt to
associate this unpleasant experience with the white plate so that,

1 N. Millott, Endeavour, XVI, 19 (1957).

124 Inside the Living Cell

when the crab and plate are presented, it merely 'leans forward from
its home and watches the situation'.^

The most famous experiments were, however, those of Pavlov. If a
hungry dog is shown food, its mouth begins to water. It is obvious
that the sensation given by the sight and smell of the food in some
way causes the salivary glands to excrete. This is a simple reflex
action, and the salivation is an involuntary response. If a bell is rung
before the food is shown, the animal will associate the sound of the
bell with food and the sound of the bell will, for a time, cause his
mouth to water. This is a conditioned reflex. It is obvious that connec-
tions have been established in the brain between messages arriving
from the eyes or nose and the ear, in such a way that a message
received by the ear stimulates a response which is normally produced
by ear and nose messages.

The conditioned reflex was regarded by Dr J. B. Watson and the
behaviourist school of psychology as a sufficient explanation of
nearly the whole of animal behaviour. They regarded all responses
of the individual animal to environment as either direct (like the knee
jerk) or somewhat modified by recoflections of previous situations
which might cause the simplest response to be delayed or inhibited;
or 'conditioned' by circumstances indirectly connected with them
(like the sound of Pavlov's bell).

But in fact the 'conditioned reflex' is itself a very complicated
phenomenon and the existence of many of the profounder abilities
of the brain are implied in it. As I said, it obviously requires the
existence of memory of past experiences and the abiUty to interpret
present experiences in the light of past, and therefore the ability to
learn from experience.

These must be discussed in detail; but in the first place it will be
helpful to have a clearer idea of the actual organization in the brain,
and some notion of how it handles the messages it receives from the
senses.

BRAIN ACTIVITY

The brain of a large animal, as I have said, consists of an enormous
mass of neurones. Some distinct parts can be demonstrated; the cere-
bellum, which is largely concerned with the more 'animal' functions
such as balance and the posture of the body, and the cerebrum,
which is covered by the cerebral cortex, which reaches its greatest
development in man, a great convoluted membrane, about a hundred

^ B. B. Boycott and J, Z. Young. S.E.B. Symposium on Physiological
Mechanisms of Animal Behaviour (1950), p. 432.

The Brain and What It Does 125

neurones deep, the so-called grey matter of the brain (to distinguish
it from the underlying white matter). The cerebrum in man contains
9,000 million cells, out of the 12,000 million present in the whole
brain.

Our knowledge of what goes on in the brain is rather meagre.
There is undoubtedly a great deal of electrical activity going on.
As we have seen, it is possible, if a nerve fibre is laid bare, to detect
the passage of an electrical impulse or wave by placing electrodes at
two different points along it.

A , i ' ^ ■ ^ i » *- — » A -J* k * ^ * » —

iM^V/^Ai^VM'

FIG. 25. Electroencephalograph patterns of brain activity recorded through
the skulls of human beings (the regular marks below the tracings are time
signals at 1 second intervals).

A. A normal subject. The eyes were closed between the two arrows on the
time line

B. A subject who has had a brain injury. The sharp downward spikes are
very characteristic of certain kinds of injury

C. A normal subject (the oscillations are less magnified vertically than in A)

D. Same subject as C, in sleep induced by a drug

(Reproduced by courtesy of Dr W. F. Floyd, Middlesex Hospital Medical
School)

These electrical impulses can also be detected outside the brain
by electrodes placed on the skin at different points. With a delicate
electronic amplifier called an electro-encephalograph, it is then pos-
sible to detect oscillations or surges of electric action within the brain
(Fig. 25). These surges must occur on a rather large scale, i.e. to
produce an effect outside the skull many of the electric circuits of the
brain must be oscillating in unison. So far it has been difficult to
connect these oscillations very closely with the mental state of the

126 Inside the Living Cell

individual being examined, except that in epileptics sharp spikes are
observed. They may only be a kind of 'carrier wave' on which the
actual messages are superimposed. We are like strangers outside a
great power station; we hear faintly the hum of activity going on
inside it, but we do not know what it means. We do know that the
oscillations are larger and more clearly defined when we are awake
and giving attention, and give way to a long, slow rhythm when we
are asleep.

More detailed information has been obtained by experiments with
animals. By means of electrodes placed on the optic nerves of a
frog, we can detect the electric impulses coming from the excited
nerve cells in the retina. They can be traced a little way into the brain
itself and then are lost among the innumerable fibres which join the
neurones.

It is also possible to locate the region of the cortex which the
signals finally reach, by putting electrodes into the exposed brains
of patients and also by electro-encephalograph records taken on the
outside of the skull. It is found that the impulses produced by the
retina are spread over a great area of the brain. The picture on the
retina is enormously magnified and it would appear that millions of
brain cells are concerned with the vision of one instant.

The brain of an animal, and especially human beings, is thus
primarily an instrument for co-ordinating large numbers of separate
impressions. The pattern of the impressions means much more than
the separate bits of which it is made up.

When we look at a half-tone picture in a newspaper, we no doubt
see the separate dots, but we are only interested in the pattern of
light and shade which they make. The brain has a remarkable power
of finding the pattern in millions of diverse sense impressions and
there must be something basic and inherent in its structure which
permits it to do this If it is more concerned with the pattern of the
diverse sense impressions than with the details of the individual
impressions, it must follow that the interrelations between the
brain cells which receive the impressions are more important than
the actual impression made on each one.

If we look at a square, it is the fact that it is a particular kind of
geometrical figure that we seize upon. We are indifferent to its size.
The actual size of the image on our retina depends on our distance
from the object. The projection of the square in our brain thus
depends on our distance from it — yet we have no difficulty in recog-
nizing it as the same square. It is clear that in interpreting very
simple experiences, the brain has to make very complicated adjust-
ments. However, you can only find a pattern in your sensory

The Brain and What It Does 127

impressions by recognizing something you have seen before. You
could not recognize objects as squares unless you had previously
seen and made a note of this feature. So picking out the features of a
pattern and recognizing them involves memory and also to some
extent learning about them. So we see that we do not spontaneously
interpret our sense impressions. The use we make depends on our
memory of similar experiences and the meanings we have found in
them.

MEMORY

There is every reason to think that the stimulation of brain cells
leaves a permanent trace of some kind. This is not very surprising,
as all living cells are to some extent influenced permanently by
external agencies acting on them. The state of a cell at any one
moment depends on what has happened before and everyone who
has worked even with simple organisms such as bacteria knows how
hard it is to reproduce a particular condition exactly. This amounts
to saying that all living cells have some kind of a memory of what
has happened to them, so that it is not surprising to find, perhaps
to a much more highly developed degree, that brain cells are perma-
nently influenced by the sensory images they received. But the exact
nature of the memory record, whether it is proved by changes in
molecules or structures, or whether permanently circulating currents
of electricity are set up, is hardly known.

The memory record made by the brain must, however, have one
characteristic if it is to be of any use. It must be available for inspec-
tion. It is no use having a gramophone record unless we can play it.
So our memory impressions wifl be useless unless they can at least
be used for recognition. There must be some way of comparing the
sensation of the moment, with what is remembered of previous
sensations. Exactly how this is done is not known. We might think
that the sense impressions circulate among the cefls until they find
something with which they correspond — holes into which they fit.
Alternatively it is possible that the memory record is able to send
out again, perhaps rather weakly, a copy or repetition of the elec-
trical impulses which caused it — so that we could have a direct
comparison between the fresh impulses arriving from the senses and
the fainter images coming from the memory record.

DEALING WITH SENSE IMPRESSIONS

It will be obvious from this that the reception of a sense impression
in the brain is not an isolated act, but something which is part of a

128 Inside the Living Cell

continuing process — in fact of something which has been going on
since birth. The new sense impressions just take their place as a
continuation of a long series and the interpretation which is found
in them depends on what has been learnt about the previous
experiences.

This means that the memory record is not a static thing — some-
thing finished and done with. It is like a message on a long tape, on
which fresh signals are appearing all the time. But there is a vital
difference from a tape message. The messages which arrive in the
brain are co-ordinated and compared with previous part of the
record. We recognize this feature or that feature — in fact the meaning
of the sense impression is determined entirely by this comparison
with previous experiences and by the interpretations which we are
able to make as a result.

Our experience at any one moment is thus a combination of the
sense images of the moment with memories of the past, and it is
difficult to separate the one from the other. This characteristic, i.e.
that sense impressions are interpreted in terms of the meaning
which has been found in previous experiences, seems to be a
fundamental ability of the human brain and probably also to a less
extent of that of the higher animals. The relative importance of that
part coming from the sense and that coming from the memory may
vary very greatly. But something of this kind seems to be a necessity
where behaviour is to be determined both by the present situation
and by what has been learnt from past experiences.

So far we have been thinking of the impressions reaching the
senses in a rather simple way. We have actually tried to isolate rather
simple sense impressions and see how they are dealt with. But in
practice we are receiving all the time a multitude of sense impres-
sions, some important and some unimportant. How do we manage
to sort them out?

It is clear that, out of the multitudinous sense impressions that
pour in on us, we are able to produce a consistent picture, which
includes the most important elements of the situation which need
concern us. This picture necessarily involves discrimination between
the important and unimportant; it necessarily concerns itself with
the broad outlines and not with every detail of the sense impressions.
It is to a great extent built up by recognizing features we are familiar
with. Thus one glance at an object is sufficient for us to recognize it
as a table. We do not have to study it carefully and try to make up our
minds whether it is really a camel or an ant-hill.

So we see that our sense impressions are not used at first hand;
it is the interpretations which we are able to make of them which are

#•'

15 A nerve cell of the mammalian spinal chord and the associated
nerve fibre. This is not a photograph but a diagrammatic representa-
tion which shows the principal features. The little black 'worms' are
mitochondria and the grey ones are microsomes. The open circles
are fat-bearing particles. (From an article by J. Z. Young, Endeavour,
15, 5, 1956, reproduced by permission.)

^^l^

t

't^

16 Nerve cells in the visual cortex of the brain of a cat. (Reproduced
by permission of Dr D. A. ShoU and Methuen and Co. from The
Cerebral Cortex)

f -? , -if

*!^S.

The Brain and What It Does 129

useful. It follows from this that besides the regions in the brain which
are able to register sensations, there must be those which are capable
of finding significant patterns m the sensations as they are received,
which are concerned with the meaning and interpretation of the
sense pictures of the moment. It is quite likely that at this stage
much of the detail is lost. Also the actual size and orientation of the
object becomes of less importance. We recognize an object as the
same at different distances, although the area it occupies on the retina
may be quite different. We recognize a particular shape such as a
square whatever its size. This means that the recognition occurs at a
level where the actual size is unimportant, only the pattern is sig-
nificant. We have very little idea how this is effected in the brain.

It might be natural to expect that this process of finding meanings
takes place in the cerebral cortex. It is surprising, however, how
much damage can be done to the brain without apparently impairing
its functions. A hundred years ago a quarryman in Vermont, Phineas
Gage, had a crowbar driven through his skull, causing an ugly wound
in both of the frontal lobes of the brain. By good luck the wound
healed and the man was able to return to work. His intelUgence and
ability were not seriously impaired and his memory was good;
however, he was said to be less balanced and more easily excited to
anger.

An operation known as prefrontal lobotomy or leucotomy, in
which many of the nerve connections between the frontal lobes are
severed, has been performed on a considerable number of people
with mental derangements. It appears to have a genuine effect on the
tendency of the patient to worry, but it does not radically affect his
memory.

From this it would seem that a considerable number of nerve
connections in the brain can be broken without seriously affecting
the memory or the general ability of the person, although it may be
that some of the more general abilities, which we call judgment and
character, may be lost.

A possible reason for this is that the total number of possible
paths between the nerve cells is so enormous that only a small
proportion of them is actually used. When some paths are broken,
alternative ones are available. Another conceivable explanation is
that the number of nerve cells is very much greater than is actually
required, or at least greater than the number made use of. There is
little doubt that in some people there is a great deal more mental
activity than in others. Is it possible that many people only use a
small part of the brain they have?

XVI

Actions and How They Are Performed

H

We have been discussing how the brain deals with the messages it
receives from the senses, and how it reduces them to a useful form
by finding significant patterns in them. We have now to go a step
further and see how these patterns or pictures are translated into
actions, which enable the animal to satisfy its needs and to continue
to live — actions which are often extremely complicated.

There seem to be two chief ways by which these actions are
stimulated and controlled — and both ways have been developed in
animals to a quite extraordinary degree.

INSTINCT

First, there is the instinctive response. The animal is in a situation
which it has not met before, but nevertheless it is able to perform
very complex actions, the purpose of which cannot be known to it.
Under these circumstances the ability to perform these actions must
be innate, i.e. the animal is bom with it.

Chickens can run about soon after being hatched and will peck at
any small, bright object; a calf or newly-born rabbit is soon able to
suck from its mother. Birds can fly, when their wing feathers have
developed, with little or no practice. These are all complicated
muscular actions. They involve bringing great numbers of muscles
into action, each in the right order and at the right time. The signals
which stimulate these muscles must reach their destination at just the
right times. These signals must be dispatched by the brain and it
follows that in such cases the pattern of nervous stimulation which
produces the action must be present in the animal's brain at birth
only requires a suitable stimulus to cause it to function. Thus no

Actions and How They Are Performed 131

doubt it is the feel of the mother's breast, or any similar object,
which initiates the sucking. Similarly, young birds will gape, i.e. open
their mouths for food, whenever a small object approaches the nest.
The red-backed shrike has an innate tendency to impale its food on
sharp thorns.

In all these cases the patterns of action are all present in the brain
— imprinted, as it were, among the nerve connections and only
requiring the appropriate stimulus to bring them into action.

This method has reached its greatest development among the
insects, which carry out extraordinarily complex actions in their life
cycles, obviously without any 'understanding' of their object. Fabre
and other students of insect life have recorded many remarkable
examples. For example, the hunting wasp, Ammophila, lays its eggs
on the top of a caterpillar, which is alive and has merely been
paralysed by injecting a nerve poison into the nerve centre of each of
its segments. It is then placed in a hole in the ground which was
previously prepared, which is then sealed and the caterpillar remains
alive but immobile, ready to feed the larva when it hatches out.

Another wasp, Eumenes, constructs a clay urn, the lower part of
which is filled with live caterpillars. Suspended from the top by a
thread and out of reach of the caterpillars, is a single egg. When the
egg hatches out the larva hangs on the thread and is able to eat the
nearest caterpillars without danger. When it gets bigger it can descend
from its thread and consume the rest.

The insect performs a quite extraordinary series of actions without
any knowledge of what it is doing. It has been found that each series
of this kind is a sequence. One step forward cannot be taken until
the previous one is completed. Every step, when complete, provides
the stimulus for the next. The series of actions can, as a rule, be
only carried out in one order.

Students of instinct, such as Tinbergen, Lorenz and Bierens de
Haan, have recognized that, to initiate an instinctive action, a par-
ticular sensory stimulus is required, which 'releases' the series of
actions which follow. What it really does must be to 'release' a
sequence of signals which pass along the nerves to the muscles and
so cause the necessary muscular movements. The ability to make
these nerve signals is obviously imprinted on the nervous system,
waiting only for the proper sense stimulus to bring it into action.

Tinbergen has shown that the releaser is easily imitated and that
it has to have certain features while others are unimportant.

There is some doubt as to how much latitude is possible in per-
forming the actions. In many cases an insect is quite at a loss if the
sequence of actions is interfered with. For example, if the clay cage

132 Inside the Living Cell

of the Eumenes is broken, the wasp may mend it, but it will not
place the egg inside. Its power of modifying the action to suit the
circumstances is very limited; but cases in which the action can be
varied have been noted.

LEARNING TO ACT

Although instinct is very remarkable, it obviously has great dis-
advantages as a basis of animal life. It does not, as a rule, offer the
possibility of learning from previous experiences and therefore it is
at a loss when presented with a new set of circumstances.

The 'intelligent' animals have developed a very different method
of finding actions which are appropriate to the circumstances — the
method of learning by trial and error. One feature of these actions
is that they are first of all performed very badly. The movements
of a young mammal are at first disorganized and it only gradually
learns how to use its limbs effectively. It will try to repeat actions
which produce satisfying results, and after many repetitions they
become easy. This means that the nerve connections which enable
them to be carried out are not initially present. They become estab-
lished by being used, but when established the action can be repeated
with very little trouble. Thus when a young kitten has ieamt' to
run, all the muscle actions occur in the correct sequence, without
any attention being given to the details. Animals thus acquire a
whole repertory of muscle sequences which they are able to perform —
and accurately — in order to bring about desired actions.

To be useful these actions have to be controlled by the information
which the sense impressions give the animal about his surroundings.
We have seen that from this information he constructs a 'picture'
of the surroundings — a picture which, as we have already seen, is
not just a momentary photograph but an interpretation based on
previous knowledge and experience. A way in which muscular
actions were related to the 'picture' which an animal or person makes
of his surroundings was suggested by Dr K. J. N. Craik, a Cambridge
psychologist, who died in 1945. He suggested how human beings
(and possibly animals) may make use of the 'model' or 'picture' of
their surroundings in planning actions. The suggestion was that the
brain keeps on trying out possible actions within the 'model' or
'picture' of the outside world, which it has constructed. It attempts
to forecast the results of possible actions within the model and when
one is found which appears to give a satisfactory result, the action
may follow. For example, a man using a cricket ball must predict
the result of his action. This prediction is made within the picture he

Actions and How They Are Performed 133

has in his mind of his surroundings. Similarly, when a cat jumps
on a mouse, the action is planned within the model or picture which
tlie cat has made of his surroundings. As a result a whole complex
matrix of muscular action, which is brought into play, is planned as
a coherent whole.

This idea suggests also how the matrix of nervous signals which
will be required to bring about the necessary muscular movements is
assembled. Trying out' an action involves making all the nervous
connections which will be required if the action is effected. Many
of the connections will already exist because they belong to the group
of connections which bring about a muscular movement which has
been learnt. The animal only selects from those it has learnt a group
of movements of which it can predict the result. It makes its selec-
tion within the model or picture of the circumstances surrounding it.

According to this, there is a very close relation between the '

organization of the sense impressions and the organization (or \

learning) of muscle movements. The two modes of organization have '<

in fact been achieved together. The first task in the development of '

an infant is to make this link between his organization of sense im- ;

pressions and his learning to perform muscle movements. In his i

first few months he gazes intently at bright objects and tries to touch I

them, feel them, suck them and so on. He is also trying to perform
satisfying actions — such as reaching an object he sees, or sucking •

it and he is thus at the same time learning how to perform muscular |

movements.

It is probably very important that the construction of the picture \

of the world shall go on concurrently with the learning of muscular !

movements. The child in fact is learning to make muscular move- |

ments within his picture of the surroundings and the movements are !

helping him to construct his picture. The plan, as it emerges, is in
fact a plan of possible actions. Our picture of the world is a strictly !

utilitarian one. \

This method of dealing with experiences is obviously only possible i

in a long infancy, when the young animal is sheltered, fed and pro- i

tected. The young animal is not 'teachable' initially — he learns for
himself how to perform action by trial and error. 1

The advantages of such a method of organizing actions are very j

great. It permits of a great number of factors being taken into account I

in determining one action; it allows of a very close correspondence i

between the action and the circumstances, i.e. it permits of accurate
prediction of the result of the action. It allows the animal to learn ]

from his experiences. All this gives animals a degree of effectiveness '

which no simple system of conditioned reflexes could have. It also

134 Inside the Living Cell

leaves possible a degree of freedom and spontaneity, because in the
last resort they are able to and, in fact, must make a choice between
the various possible actions which are open to them.

It must be recognized that there is always an element of spon-
taneity in an animal's response to its sensations, sometimes more
prominent and sometimes less prominent. It occurs even in very
simple organisms and is very deeply rooted in Nature. It existed, in-
deed, before the development of brains and nerves. The amoeba
pushes out its pseudopodia in an apparently aimless way; its proto-
plasm moves first one way and then another without apparent rhyme
or reason. The Paramecium oscillates endlessly in the water in its
search for food. Such movements are perhaps a consequence or re-
flection of the Brownian movements of small particles as they are
buffeted about by the ceaseless and chaotic thermal motions of matter.

It is a long way from this to even the most irregular movements of
animals, but it seems a reasonable inference that undirected move-
ments come first and are reduced to order. The animal does not so
much have to initiate movements as to impose order and control on
the aimless movements of its limbs. It has to co-ordinate many in-
coherent movements into purposeful and effective actions.

Craik's suggestion provides a possible way in which this is
achieved. It is a method of finding solutions, by trial and error, of the
problems set by the senses. The brain acts as a kind of super-
computer in solving these problems. Those movements which it fore-
sess will probably be inadequate are rejected. The control narrows
down the range of actions to those which seem to satisfy the require-
ments of the situation.

However, it is important to realize that an element of spontaneity
remains, and is necessary because, otherwise, there would be no
actions from which the most suitable could be selected. This spon-
taneity is perhaps a relic of the undirected movements of the young
animal, of its playfulness when it is learning to co-ordinate its muscles
with its sense impressions. A kitten uses its muscles in a frankly ex-
perimental way. The movements are at first spontaneous and un-
directed, and it learns only gradually to co-ordinate them so as to pro-
duce effective actions within its 'picture' of the surroundings. But
even in the adult animal, there remains an element of play or ad-
venture— call it what you will — which is necessary if the animal is
to continue to live successfully. It may be very dangerous to lose this
element of initiative, since an animal which can perform actions
which are perfectly adapted to a particular environment may be un-
able to cope with changed conditions. The animals which have sur-
vived the changes caused by human activities are those which were

Actions and How They Are Performed 135

adaptable enough to live and prosper under changed conditions.
Thus the sparrow survives, but the fen buzzard is practically lost.
Even now animals are discovered to be coping with new conditions,
e.g. the tomtit has taken to breaking the metal foils which cover milk
bottles; swallows have discovered the advantages of making use of
the eaves of houses to build their nests. It would appear that even the
most rigid instinctive actions must have involved initiative at one
time. The instinctive action, like the laying of eggs in a living cater-
pillar, must originally have been a random impulse, which was suc-
cessful and became imprinted on the inherited nervous pattern. We
have to conclude that random impulses are often useful, but the
balance between initiative and routine is a very delicate one. Too
much initiative may be as destructive as too little. The animal is
often poised on a razor-edge between the perils of deviating too much
from a regular habit and too little.

FREE WILL

We can now ask whether animals, and human beings too, have any
real power of making a choice between the various possible actions
which they may be called upon to perform. In the last resort, do they
have any free will in making a choice among the various possible
actions — or, to put it another way, what is it which determines the
choice which they do finally make? We have seen that the power of
choosing varies very greatly. Some actions are almost automatic re-
sponses to the sensory stimulus; others are influenced to a greater or
less degree by learning from similar experiences. In these cases the
action is not a direct response to the sensation; it is obvious that it is
modified by the memory records of earlier experiences, so that the
action is the result not only of the present sensations, but also of what
has happened in the past. The present sensations are therefore modi-
fied by taking their place and being interpreted in terms of the whole
picture which the animal makes of his world. The action is deter-
mined to some extent by the whole previous life of the animal, and
by what he has learnt about the world. Under these circumstances
can we say that the animal really makes, or is capable of making a
choice? Is not its action really predetermined — even if it is modified
by the experiences of the past? Is it not still an inevitable consequence
of all the influences, past or present, which are acting on it?

The answer to this seems to be that if an action is determined not
only by the immediate sensations — by what an animal sees, hears and
feels — but also by its past experience and what it has learnt through-
out its life, there may be an element of uncertainty. If the action is

136 Inside the Living Cell

brought about by a process involving past experiences, which are
combined in various ways by trial and error or by the accident that
sometimes one recollection and sometimes another may be pre-
dominant, we may expect that the action which emerges from a given
situation may not be always the same, i.e. will not be completely pre-
determined.

We can therefore allow an animal some ability to 'choose' in its
actions. The extent of its power of choosing will depend on its ability
to distinguish between the various possibilities and the range of its
knowledge. If its experience and learning are very limited, the power
of choice will obviously be limited too. If its experience is extensive
and it has learnt a great deal about the world and about the conse-
quences of its actions, the effectiveness of its choice may be consider-
able.

In such a case, the action is determined not only by the present cir-
cumstances, and also by old experiences acting in a delayed way, but
also by what has happened to these experiences in the animal's brain,
i.e. what it has learnt from them, how far it has succeeded in com-
bining them and extracting meaning from them — in putting two and
two together. In other words, the action is determined to some extent
by the personality of the animal, i.e. the way in which it differs from
other animals, either in its original set up or in its history.

So we see that we cannot give a very simple answer to the question
of whether an animal has freedom of choice between possible actions.
From one point of view it might be considered that it has not, be-
cause, even if actions are determined by very complex influences,
arising from the present and past, it is still determined by the sum
total of all the effects present in the animal's brain.

Yet this is not a very useful way of describing the animal's action.
It is more useful to regard the animal's brain as essentially an organ
for co-ordinating all its actions; of bringing its whole life and exper-
ience to bear on each distinct action. The life of the animal under
these conditions is not made up of isolated acts; it is a continuous
activity in which past and present are always blended. The degree to
which this is achieved of course varies enormously; but in the higher
animals where actions are controlled very largely by learning, we are
certainly justified in regarding this complex amalgam of past ex-
periences and learnt abilities as providing 'personality', i.e. each
animal is unique and therefore its responses contain at least a degree
of idiosyncrasy. We can conclude that we are justified in allowing a
degree of freedom of choice to the extent that the animal is capable
of distinguishing the different possibilities.

XVII

Using Tools and Symbols
H

Everything which has been said so far applies, at least to some extent,
to the higher animals as well as to man. They have memory and
recognize things; their acts are determined partly by their immediate
sense impressions and partly by what they remember. They are
capable of learning how to perform complicated actions; they appear
to choose between possible actions and to base their choice to some
extent on an estimate of the results which are likely to follow. We
must ask, what is it in addition to all this which distinguishes a human
being from an animal? Have we left out anything in our description
which would be distinctive of human life?

There can be no doubt that two of the essential characteristics of
human life are the extensive use of tools and of language. Both of
these occur only to a very rudimentary extent in animals — although
we could not say that they do not occur at all. For this reason we
might perhaps be justified in thinking that human abilities are de-
velopments from abilities already existing in the animals. Let us con-
sider then what such developments involve.

The changes necessary to transform apes into men have been listed
by Dr R. K. J. Hayes^ as follows:

(1) the hands are remodelled and reformed so as to enable them
to use tools and weapons;

(2) the pelvis is developed to form a bone ring, which can carry
the trunk upright;

(3) the vocal organs are refined so as to make them capable of pro-
ducing articulated sounds of varying pitch;

(4) the brain undergoes an all-round development, with an im-
provement in memory and foresight and becomes capable of
abstraction and symbolism:

^ R. K. J. Hayes in A.A.A.S. 1954.

138 Inside the Living Cell

(5) new inborn tendencies are acquired to indulge in types of play
which give the opportunity of developing muscular skills and
a close correlation between muscular action and sense im-
pressions.
This is a rather formidable list of 'improvements'. Their chief effect
is to permit the accurate use of tools and the development of
language.

Tools are external objects which are used as extensions of the limbs
so as to increase their reach and performance. They are controlled by
the brain in exactly the same manner as the limbs themselves. When
a violinist plays, the muscles which operate the instrument are con-
trolled by means of the information reaching the brain from the
sense of touch and hearing. The nervous circle goes from the muscles
to the instrument and re-enters the brain through the senses. The
accurate use of tools thus depends on using the sense data to control
precisely the muscular actions which are producing the movements
of the tool. This is obviously not a new faculty, but only a more pre-
cise use of abilities similar to those which higher animals possess.

In a sense the human voice can also be regarded as a tool, which is
controlled in exactly similar ways, i.e. I listen to the sound of my
voice and modify it according to what I hear. The muscles which pro-
duce the voice are thus controlled through the sense of hearing.

It must have been discovered by early man that he could make
sounds which influenced actions and so bring about desired results.
Sounds could be produced which were either ingratiating or frighten-
ing or merely served as a signal to draw attention to something.

The voice thus took its place, like the limbs, as a means of bringing
about actions. Just as human beings planned their muscular actions
within their pictures of the world, so they also produced sounds as
part of their reaction to the picture provided by the senses. In such
sounds we see the origin of language. Certain sounds became asso-
ciated with particular situations and thus acquired a meaning and
thus, in time, a 'language' of sounds was built up. From this point of
view the important thing about the language is that initially it is a
muscular act — a series of useful and effective muscular responses to
a given situation.

From this point of view it can be seen that the organization of
language is perhaps a development of an ability which was already
present in the animal brain, which already provided a means of org-
anizing from the sense picture something of a very different nature,
viz. appropriate muscular actions. It is not a very big step from this
to the setting up of a mechanism for producing appropriate sounds
and associating these sounds with situations. The sounds are very

Using Tools and Symbols 139

different in nature from the circumstances elucidating them. The
word tree does not in any way resemble a tree; nor does the muscular
effort required to produce this sound have any obvious connection.
The significant thing is that the association of the word with the ob-
ject is effected with great ease.

Language thus began as an association of sounds and of related
muscular acts with certain elements of the sense picture. The human
brain, as it has developed, provides an enormous extension of this
type of connection.

The consequences have been remarkable and have led to the de-
velopment of a kind of living which is quite different to anything
found in the rest of the animal kingdom.

In the first place the association of different sounds with different
kinds of objects leads to descriptive language. The sense impressions
are replaced by something quite different, i.e. words. The words then
offer an alternative representation of the world — a representation
which is clearly not the same as the original sense picture, but one
which can take its place. The prime characteristic of the human brain
is the great ease and precision with which this transition from direct
sense impressions to words, used as symbols, can take place.

Let us think, very briefly, of the consequences of the translation of
the actual world into a substitute world of words. In the first place it
provides the possibility of communication between one human being
and another — which is only possible to a very limited extent in
animals.

When I see a tree, the mental picture I have belongs to me alone. I
cannot communicate what I see to any other person directly, but only
by using symbols like tree, green, tall, elm, which call up similar im-
pressions from his own experience. In this way, the actual world is
replaced by a world of symbols, i.e. words which take the place of the
actual world. This has had the most profound consequences for
human life. It helps the discovery of the nature of the actual world,
since words are themselves an analysis of the world, and putting to-
gether words is to perform operations, not in the actual world, but in
the substitute mental world of symbols. The exploration of the actual
world has been enormously assisted by making use of the symbolic
substitute world — and in fact it would be impossible to get very far
in any other way.

Secondly, words are clearly useless unless they are heard and under-
stood by somebody, so that the new world of words is not a private
world, but is essentially shared with other people. In this way human
knowledge came into existence as something in which individuals
can share, but which they cannot possess exclusively by themselves.

140 Inside the Living Cell

Human knowledge is thus quite different from the instinctive
knowledge, if we could call it knowledge, of insects or from that of
animals, who clearly acquire a 'practical' working knowledge of the
world they live in, which is at least sufficient for them to live. Human
beings have this 'practical' knowledge; but over and above it is the
knowledge derived from the alternative world of symbols. As a rule
the human being does not discover these symbols for himself — ^he
learns them from others and he learns for himself the correspon-
dences between the symbol and his own observations. It is often very
difficult to distinguish direct interpretations of experience and those
which are derived from the shared symbols.

In this way human Hfe has become necessarily and essentially a
communal life. The human being is like a radio station — being both
a transmitter and a receiver. It is tuned in, as it were, to all or many
of the other stations and is continually receiving and emitting mes-
sages. In fact the interpretations a person makes of his sense exper-
iences are very much influenced by his symbolic picture, i.e. by the
shared knowledge he has learnt from other people.

So we see that human beings, through their great ability to replace
the actual world by a symbolic world, which goes alongside the real
world and interacts with it all the time, have found a quite new way
of living. The world of symbols is obviously not static. It is interact-
ing with the real world all the time. Every new experience has to be
fitted into the symbol world; and in the same way every act is deter-
mined, not only by the actual world, but by all sorts of considera-
tions which come from the world of symbols.

The two worlds, the world of direct sense impressions and the
world of symbols, go on side by side and the one helps to interpret
the other. From the world of words and other symbols — the accumu-
lated knowledge which we have acquired — we are helped to interpret
what we see. What we do is obviously determined by the symbolic
outlook to a considerable extent.

It is a characteristic of human life that it is not static — like the life
of the bee or the life of animals, which necessarily go on in much the
same way for many generations. For one thing, human knowledge
is not stationary — the symbols which human beings use to interpret
life to themselves develop or at least change from time to time. This
is because human beings, although they can learn the meaning of
symbols, have to interpret their own experiences for themselves.
They must always be engaged in interpreting their sense impressions
and in finding symbolic interpretations which are satisfactory to
themselves and can be communicated to others. They naturally use
accepted symbols a good deal, but because the act of interpretation

Using Tools and Symbols 141

is a personal one, the interpretations rarely remain unchanged for
long and successive generations may find new ways of looking at
things, which will lead to new ways of doing things. In other words
the search for meanings in which human beings are engaged, leads to
invention and discovery.

As a result of this the human way of living is dynamic. As I have
already pointed out, it would be wrong to regard the animal way of
life as completely static; but the human way has a totally different
tempo of change.

To sum up, the peculiar ability of the mass of cells which make
up the human brain is not only to make a picture or model of the world
from the information it obtains from the senses but to construct an
alternative substitute world of symbols, which is a shared superstruc-
ture of human life. Everything human depends on this ability of
human beings to live at one and the same time on these two levels —
the level of direct experience and the level of symbols. The direct
experience belongs to the individual alone; the symbolic level is
necessarily shared because the symbols of which it is made and the
ways in which they are combined are a communal possession.

xvni

Is the Brain a Calculating Machine?

I have discussed at some length the various abihties of the remark-
able mass of cells which forms the human brain because it has fre-
quently been claimed in recent years that suitable models for brain
action are to be found in machines which really parallel only a very
small part of the whole performance. There is no doubt that many of
the simpler types of response of brains can be copied by machines.
The latter can be provided with sense organs, which detect and re-
spond to sounds and light and even to other physical agencies, such
as radio waves, for which we have no sense organ; they can also be
provided with a memory, since data can be put into the machine (often
in the form of punched cards) and stored away for future use; with
the power of recognition, in that the data supplied can be compared
with previously established standards (e.g. a slot machine, which can
recognize the proper coins and reject others; and finally with the
power of prediction, as when a gun predictor follows an aeroplane
and causes a gun to deliver a shell, not where the aeroplane was, but
where it will be when the shell arrives. Machines can also be provided
with controllers (feed-back) which maintain constant conditions, e.g.
of temperature or pressure or speed, and in this way resemble the
organism which maintains itself in the narrow range of conditions in
which it can function.

This is an impressive list and could no doubt be extended. It might
suggest that the analogy between brains and machines is a very close
one. Attempts to develop these analogies have been encouraged very
much by the fact that the mode of operation of modern electronic
computors is not unlike what goes on in the brain, which, as we have
seen, consists of an enormous number of neurones, connected to-
gether into a very complex network by thread-like connections
(Fig. 26). The nerve fibres entering the grey matter of the brain are

Is the Brain a Calculating Machine?

143

connected with the cells by small swellings or knobs which are called
synapses. This junction has the special property that it will not let
signals pass until they reach a certain strength. The synapse thus gives
an 'all or none' response; if the signal is strong enough it passes; be-
low a certain strength nothing happens.

One nerve may be joined in this way with many nerve cells, and
each nerve cell has synaptic contacts with many different nerves. This
provides an enormous number of pathways between the nerve cells.

FIG. 26. A mammalian motor neurone, showing
various sizes of synoptic knobs, several of which
are often attached to one nerve fibre. (Adapted
from J. C. Eccles, The N europhysiological Basis of
Mind. [Oxford Press])

In order to excite a nerve cell, a definite number of signals must reach
it within a definite period through these synapses. When it is excited,
it fires a 'volley', i.e. a complete signal, through the axon going from
it. This 'volley' is independent of the nature of the stimulus, provided

144

Inside the Living Cell

that it is sufficient. Ttie axon usually branches before it reaches other
nerve cells, so that the excitation may reach numerous other cells.

In the cerebral cortex it is difficult to recognize definite points of
contact between the nerve cells. The latter often have fine thread-like
appendages called dendrites and the axons from other nerve cells
often split into many branches (Fig. 27). It is often seen that the
branches of the coming-in axon runs parallel with the branching arms
of the dendrites for a distance and stimulation passes from one fibre
to the other in this region and thus into a new nerve cell.

When a neurone has fired a Volley' there is a refractory period.

R enlorhin.

FIG. 27. Typical neurones and their branching connections, found in different
layers of the cerebral cortex of an ape. (From Dr Lorente de No's article in
Fulton's Physiology of the Nervous System. [Oxford University Press])

which lasts for about a thousandth of a second, during which it can-
not be stimulated again. The neurone can also be inhibited, i.e. a sig-
nal from another neurone will bring it into a state in which it cannot
fire.

The neurone thus resembles a relay which repeats a sufficiently
strong signal and which remains 'dead' for the 'refractory period'
after being fired. To a considerable extent this kind of behaviour can
be reproduced by arrangements of thermionic valves. They act as
relays and can be arranged so that they will respond only to signals
above a certain strength. They also have a recovery period after
'firing' and can be prevented from firing at all.

Is the Brain a Calculating Machine? 145

A great deal of effort has been devoted to attempts to build up
models, using these principles, which will possess at least some of the
abilities of brains. In the first place, can we find a basis for memory?
A good many years ago Dr Lorente de No pointed out that a num-
ber of neurones arranged in a circle would act as a permanently re-
verberating circuit. A pattern of impulses fed into the circuit at any
point will continue to circulate indefinitely. It is only necessary that
the neurones should be able to provide sufficient energy and that the
ring should be sufficiently long so that the first neurone is in a recep-
tive state when the message arrives back. Patterns of impulses could
thus be stored in the brain in a permanent or semi-permanent form.
Drs McCulloch and Pitts have investigated mathematically the pro-
perties of such networks of neurones and suggested that they provide
a possible basis of memory since the patterns of impulses will con-
tinue to circulate long after the immediate occasion which produced
them has passed.

Although there is no doubt that innumerable circuits exist in the
brain, it is doubtful if this suggestion offers a satisfactory theory of
memory. Lashley has extended the idea somewhat. If the brain con-
tains innumerable circuits, how does a circulating impulse know how
to stay on its ov/n circuit and to ignore many others? Lashley has sug-
gested that a pattern of excitation spreads out from the neurones
originally stimulated. The activity which starts at one point in the
cortex may spread out and give rise to radiating waves of activity.
When many points are stimulated simultaneously, the different waves
of activity will interact with each other like the waves formed when a
number of stones are thrown into a pond, giving rise to interference
patterns and to a pattern of standing waves. This would perhaps give
a basis for understanding how general characteristics (called *univer-
sals') are recognized, irrespective of their size and position. The inter-
ference patterns finally produced would be similar, and would not
depend on such details.

An alternative proposal was made by Drs McCulloch and Pitts.
They suggested that the sensory impulses are received on several
layers of neurones, which are, as it were, tuned to different scales of
magnitude. Each of these is brought, in turn, into a receptive condi-
tion by a 'scanning' mechanism. The signals sent on to the rever-
berating circuits are thus the sum or average of those produced by the
original pattern, when measured on the different scales, so that what
matters is the essential character of the pattern rather than its size
or position.

Hebb has gone still further and suggested that the conducting cir-
cuits between neurones are themselves modified by the passage of

K

146 Inside the Living Cell

impulses, so that the patterns which are frequently excited tend to
become permanent. Hebb thinks that the change is to be regarded
as a kind of growth process, as a result of which the terminations of
the axons increase in size as the result of the passage of impulses. This
means that although a short-lived memory might be maintained by
a reverberating circuit of neurones, if stimulation is frequently re-
peated, it will become very much easier to produce the stimulated
pattern. The memory will then be, as it were, 'built in' to the cortical
structure and will not be destroyed by electric shocks and other
disturbances.

These proposals are obviously very sketchy and incomplete and
readers will have no difficulty in concluding that no really adequate
models of brain action exist — except for perhaps the very simplest
types of action.

The existence of memory obviously requires that patterns of some
sort based on the original sense stimuli are stored somewhere in the
brain. The form in which this storage is effected is not really known.
It is not necessary to suppose that the original impulses go on circu-
lating in the brain for ever. What is required is a mechanism which
will store them up in such a form that they can be repeated. This
could occur, as Hebb suggested, by structural changes in the con-
nections between the neurones, as a result of which a particular pat-
tern of stimulation is easily re-established. Another possibility is that
the impulses are stored in the neurones themselves, e.g. when a series
of impulses reaches a neurone, it may bring about permanent changes
in its structure of such a kind that the neurone will be capable of
emitting the same series of impulses at a future time. Thus the
neurone may not only be a receptor of signals and a relay; but a re-
peater unit. It will store up patterns of impulses and repeat them at a
later stage. However such a process has not been demonstrated.

As I have explained, the memory mechanism is quite fundamental
to brain action. Without memory there can be no recognition and no
learning. But it is also probable that the memory is not a simple
memory of the actual original sensations but of the interpretation
which is made of them. It is the meanings which are remembered,
rather than the sensations themselves. The situation is therefore a
very complex one and it is difficult to separate one aspect from
another. But the very least we can say is that behind the neurones in
which the immediate sense impressions are received, there must be
others in which they are compared and interpreted in terms of the
memory record of previous experiences. That is, the immediate sen-
sations find their places in the continuing record of the structure of
experience which has been gradually built up during all our lives.

Is the Brain a Calculating Machine? 147

It is this feature which gives continuity to our lives and to that of
all animals with a developed brain. We must therefore picture the
brain, not merely as a kind of telephone exchange for linking senses
with muscles, but as something which stores up to some extent every-
thing which has been experienced and unites it with the present. It
is essentially an organ of continuity and the very least that we can
say about its mechanism is that it must provide a means by which the
present can take its place within the record of the past. A calculating
machine which resembled a brain would thus have to be permanentiy
modified by every operation it performed. When presented with a
sequence of numbers, it would have to recognize in them features like
previous sequences, and perform operations which had produced suc-
cessful results before. It would be rather boring to press this any
further; but I think it will be evident that the gap between the brain
and the machine is very great. Indeed the real work of the machine is
still being done by the operators who set it up.

We must however enquire a little further about what goes on in
real brains. We have found that patterns of stimulation arrive from
the senses in the receptor areas, and from these patterns of excitation
spread out through innumerable circuits to other regions. In these
regions they meet and are compared with patterns arising from pre-
vious experiences. How this is done we hardly know, as we do not
know how the memory record is stored, and how it is made available
for comparison. But it is evident that the original sensations are dealt
with at more than one level and finally we come to the level of aware-
ness and consciousness, at which the interpretation which has been
made is finally judged and action is set in motion or, possibly, no
action is decided upon.

What is this level of consciousness? It seems as if there is something
in the brain which is capable of 'looking at' the images as they are
. formed at lower levels, which may either come from the actual sen-
sory impressions or from the memory record. We might, as an
analogy, think of the lower levels as like a television set which re-
ceives impulses and produces a picture from them. To produce a pic-
ture is, however, not to be conscious of it; and here we come to per-
haps the greatest mystery and the most difficult feature in the whole
range of living phenomena.

For a scientist, consciousness and awareness are subjective
phenomena which are not capable of instrumental verification. We
are aware of what we experience ourselves, e.g. the sensation of green
when looking at a green field. We have absolutely no way of telling
what sensations other people are aware of, except from what they
tell us about their experiences. The sensation of green, which I am

148 Inside the Living Cell

aware of, is thus a subjective phenomena which cannot be measured
by scientific instruments in the same way as a nerve impulse. For
this reason many scientists think that the subjective aspect of sensa-
tions should not be considered in attempting a scientific account of
the brain.

I think, however, that such subjective feelings and 'sensations'
are a legitimate study of science. While it is true that we are only
directly aware of our own sensations, yet we do not doubt that other
people have similar ones, as their behaviour only makes sense if we
assume, for example, that they see and hear and feel substantially
what we see and hear and feel ourselves. So it will be necessary to
bring sensations into the scientific picture somehow, although at
present we have practically no idea as to how it can be done. There
is very little doubt that the 'sensation' is connected with what goes on
in the nerve fibres because, as Dr Penfield has shown, the electrical
stimulation of exposed parts of the brain causes sensations, e.g.
coloured lights and feelings, like a tingling in the extremities and so
on. In fact it is to some extent possible to locate the cells which are
concerned with vision or feeling by experiments of this kind; but the
fact that stimulating a particular group of cells with electric currents
produces a sensation of green does not prove that the sensation
actually arises there. It may well be that the pattern of excitation
which is started in these cells spreads to other parts of the brain.

In recent years some remarkable discoveries about this memory
record have also been made by Dr Penfield. In the course of operations
on the exposed brain, which are performed on epileptic patients with
the object of discovering and eradicating the region in which the
epileptic seizure originates, the cortex is stimulated electrically by
means of electrodes placed at different points. The operation is per-
formed without anaesthesia and the patient is able to describe his
experiences. In some cases the effect of stimulation was to cause
memories of long-forgotten events to return into the consciousness.
For example, Dr Penfield states^ that M.M. heard 'a mother calling
her little boy' when point 1 1 on the first temporal convolution was
stimulated. When it was repeated at once, she heard the same thing.
When repeated twice at the same point, she heard it each time, and
she recognized she was near her childhood home.

At point 12 nearby, on the same convolution, stimulation caused
her to hear a man's voice and a woman's voice 'down the river some-
where' and she saw the river. It was at a place 'I was visiting', she said,
'when I was a child'.

Three minutes later, while the electrode was held in place 13, she

1 Proc. Nat. Acad. Sciences (U.S.), 44, 59 (1958).

Is the Brain a Calculating Machine? 149

exclaimed that she heard voices late at night and that she saw the 'big
wagons they used to haul the animals (of a circus) in'.

Eleven minutes later, the original point 11 was stimulated again.
She no longer heard the mother calling her little boy. Instead she
heard 'the voices of people calling from building to building'.

Later still, when a coated electrode was inserted at 17 so as to
stimulate the first temporal convolution deep down in the fissure of
Sylvius, she said, 'I had ... a familiar memory, in an ofl&ce somewhere.
I could see the desks. I was there, and someone was calling to me, a
man leaning on a desk with a pencil in his hand'.

Dr Penfield concludes that there is in the brain a permanent record
of the stream of consciousness, which is preserved in amazing detail.
The detail cannot be recalled to the memory by voluntary effort. All
that can be recalled as a rule are rather vague generalizations and
summaries of the previous experiences. Yet these are sufficient for
recognition, for example, of persons or scenes one has seen before.

/»■'

XDC

Ageing and Death

H

We have followed as far as we can the extraordinary developments of
the life of cells. We must now turn to another question and ask why,
if their organization is so perfect and well adapted to their functions,
should they not continue to live, failing accidents, at least as long as
they are provided with the necessary nutriments and with a suitable
environment?

One could perhaps draw a distinction between unicellular organ-
isms and multicellular organisms. It has often been suggested that the
former are effectively immortal, since they undergo repeated sub-
division and, given enough food material, they need never die. How-
ever, the distinction between unicellular and multicellular organisms
in this respect is probably more apparent than real. A colony of cells
will go on increasing its numbers by cell division until it has used up
the food materials in its particular environment. No colony has the
whole world to expand into, if for no other reason than that, sooner or
later, it will come into competition with other organisms. When the
available food has been used up, the majority of the cells must die,
and new cells can be formed only so far as fresh food becomes avail-
able. It often happens also that a colony of cells destroys its environ-
ment by producing waste products, e.g. acids, in which it cannot grow.
Many unicellular organisms developed ways of escaping from this
situation, e.g. by producing spores, which may be able to establish a
new colony elsewhere. But in this case the colony is in a similar situa-
tion as that of multicellular organisms, which die after forming new
individuals from special cells like the sperm and egg. The colony of
single cells behaves very similarly when it has exhausted its habitat.
It dies, but a few cells or spores may escape and found a new colony
elsewhere.

Ageing and Death 151

However, this analogy is rather superficial. The multicellular
animal is usually mobile and can get away from its waste products.
It does not have to die because it has either used up the nutriments
around it, or because it has fouled its environment. Are we to sup-
pose that it dies because it has fouled its 'internal environment', i.e.
that waste products gradually accumulate inside the organism, which
clog the efficient working of the different organs and in the end cause
death?

It is certain that in many organisms ageing processes do occur.
Even with animals kept in good surroundings with adequate food and
protected from other predatory organisms the organism as a whole
gets less efficient. The bodily processes slow down, the muscles be-
come less effective, the bones often become brittle, many tissues
undergo degenerative changes — e.g. tendons become less elastic, and
growth processes, e.g. growth of hair and healing of wounds, slow
down, with the result that the repair of damages caused by wear and
tear becomes less effective. Finally, some vital organ ceases to func-
tion and the organism as a whole ceases to live.

It has been very difficult to discover why organisms age in this way
and what determines the average length of life. In the natural state
ageing processes are rarely encountered because death usually occurs
owing to some mischance while the animal is still in its prime. Yet
there is no doubt that animals which are carefully maintained, live
for various characteristic average periods. It has been suggested, as
I mentioned above, that organs become clogged by waste products,
or by substances of a poisonous nature which they are unable to
get rid of. This may be so, although it has not been clearly demon-
strated. In a more general sense it can be supposed that cells will only
function efficiently for a limited period. Membranes may break
down or become clogged and synthetic mechanisms may begin to
work incorrectly. Accidental damage may occur in the cell which it
is unable to repair.

On the other hand, it seems to be true that tissues which are capable
of regenerating themselves by cell division will continue to function,
possibly indefinitely. Cell cultures continue to grow (by cell division)
in artificial media for many generations, long after the animal from
which the cells were obtained has died. Sea urchins live indefinitely,
because all their cells undergo continuous replacement.

This would seem to indicate that the cell has a limited life time and
will die unless it is able to regenerate itself by cell division. This does
not explain why animals have such very different life spans. Different
mammals are extraordinarily similar in their general construction.
They have very similar organs which function in similar ways and

152 Inside the Living Cell

produce similar substances and apparently are constructed of similar
kinds of cells. The pancreas of different mammals produce insulins
and enzymes which are almost interchangeable. Yet, nothwithstand-
ing their close similarity, the life times of such cells within the dif-
ferent mammals vary enormously. This suggests that the determining
factor is not a characteristic of the individual cells, but something
which is determined by the organism as a whole.

We are still ignorant of what this factor is which determines the
life span of animals. There are some suggestive facts. Frequently the
life span is proportional to the size of the animal. This may be con-
nected with the fact that the life span is proportional to the time taken
by the animal to reach maturity, i.e. to reach its full size with all its
organs functioning. It has even been shown that if the attainment of
maturity is artificially delayed, e.g. by keeping the immature animal
very short of food, the life span is thereby increased. Some fishes
which apparently continue to grow indefinitely also live for very
long periods. This suggests that ageing begins when growth ceases.
But why should the ageing process take so much longer with a large
animal like an elephant than with mice or rabbits? It would appear
that the ageing process is slower with a larger number of cells than
with a smaller collection. This may perhaps indicate that the cells
of an organ help each other and in a large organ there is a greater
possibility of the wear and tear, which must occur in some cells, being
made up by others.

However, we must not suppose that increase of life span is neces-
sarily desirable. If there were no death, then there could not be any
new individuals either. Unless there were a continual stream of new
individuals, there could not be any evolution. Other things being
equal there will be a greater possibility of evolutionary changes if the
time from one generation to the next is as short as possible. The mini-
mum time is that required for the individual to come to maturity and
to produce and launch the next generation. From an evolutionary
point of view, any longer life-time is wasteful; but of course in order
that each generation shall be able to perform its duties, there has to
be some margin of viability and under favourable conditions,
animals will usually be able to live for a time after their useful repro-
ductive life has finished. From this, it would seem that the life span is
determined biologically by the time taken for the animal to reach
full maturity and produce offspring. The larger and more complex
the organism, the greater is the time required for this development.
Beyond this it may well be that there is an evolutionary advantage in
individuals not living too long and that this has led to the develop-
ment of factors in the organisms which result in ageing processes

Ageing and Death 153

occurring at the end of the reproductive period.

If this is correct, it would seem that the life span is determined by
the interplay of two effects — the necessity of living long enough to
start off the new generation and, having performed this task, the fact
that a further life time is unnecessary and, in many respects, harm-
ful to the well-being and development of the species. It is quite pos-
sible that mechanisms exist in organisms which bring about this
limitation of the life period, when the biologically useful period is
over, but we do not know what these mechanisms are.

We can conclude that most organisms have not only a mechanism
for reproduction and growth, but also an ageing mechanism which
automatically brings life to a gradual end when the useful period
is over. We must regard this ageing mechanism as 'built in' to the
cells as an essential feature of their construction, a kind of bio-
logical clock with a time scale which is characteristic of each species.
We do not know where the clock is located. It has often been thought
that ageing is brought about by the failure of one or other of the
endocrine glands and attempts have been made to produce rejuvena-
tion by grafting glands or injecting glandular extracts, e.g. from the
testicles of juvenile animals. These treatments have not had more
than a very limited success. Although they are capable of supple-
menting to some extent the operation of failing glands, they do not
prevent the ageing process as a whole.

It might be said that these considerations do not apply to mankind,
which is no longer subject to evolutionary change. While it is true that
over the historic period no definite evolutionary trends seem to have
been noted, we cannot be certain that evolution is not actually taking
place or that selection of certain types is not actively occurring. How-
ever, in human communities, factors other than simple reproduction
and survival assume a much greater importance than in animal com-
munities. It can be argued that survival of the community de-
pends on qualities such as 'wisdom', i.e. the correct interpretation of
experience, in which age is a definite advantage. It can be argued and
was argued for example by Bernard Shaw that human beings usually
die or become senile at an age when their experience would be of
great value to the community and also before they have made use of
their full potentiality. However, Shaw's picture of life among the
ancients is not a very attractive one and not many people would re-
gard it as particularly desirable to live to 150 just to sit and contem-
plate.

Life in an aged population would really be very static, unless
human nature changed a good deal. Even now most of the initiative
and new ideas come from the comparatively young. It would seem

154 Inside the Living Cell

to be better to have a continuous stream of new and vigorous persons
than to extend the individual life period considerably. If the average
life span were, say, 120 or 150 years, the whole tempo of life would be
different. Education would continue up to the age of 30 or 40; full
maturity from 40 to 100 or 120 and a gradual decline into old age from
the age of 100 upwards. It is not at all certain that this would produce
any improvement on the present conditions. It would greatly reduce
the tempo of change in human Hfe. There would not be the same urge
to get things done. Young men would have to wait very much longer
for their chances. Many of the eminent people who died during the
last fifty years would still be alive and probably, if their health were
reasonably good, occupying the same posts which they held in about
1910 to 1920. The result would be that in the end the longer life span
would produce exactly the same problems and difficulties as the
present one. Indeed it seems likely that the present span of life is rea-
sonable in relation to and perhaps determined by the speed of human
life processes. The normal span is already long enough for individuals
to develop their potentialities to a reasonable extent and to give a
sufficient balance between initiative and experience, which is required
if human society is to be neither stagnant nor juvenile. Of course, it is
likely that under existing condhions a great many people do not suc-
ceed in developing their full possibilities, but this is a fault of the org-
anization of society, which does not offer sufficient opportunities for
development, rather than of the time available. The people who have
not developed a satisfying life or have not found out how to use their
full abilities during their first sixty years are unlikely to do much
better with another thirty or forty.

One other possibility may be mentioned — that one factor which
limits the possible development of human beings and therefore their
valuable life, is the quantity of brain available. There may be a limit
to the number of new impressions which a brain can effectively uti-
lize. It seems to be a general experience that the ability to entertain
new ideas and absorb new experiences diminishes with advancing
age. New experiences have to be assimilated into the accumulated
patterns of experience that have been developing throughout life.
Older people often find it difficult to adapt their thinking to new con-
ditions and to the new ideas of a new generation. This is because their
thinking is organized around conceptions which are no longer valid —
their brains in fact contain great organizations of experience which
are no longer of any value, because the circumstances they were
based on have passed away. The structure of their brains does not
permit them to cast away all this now useless lumber and start again.
The only way in fact in which they could start again is by being re-

Ageing and Death 155

born and starting with the brain of a newborn infant and learning
everything afresh in the new environment. This is exactly what occurs
under the present conditions of human life.

*■ '

XX

Life in the Universe

N

We started with simple atoms and molecules — the building bricks of
all the complex patterns which have been discovered in living things.
We then worked our way upwards through proteins and enzymes to
the larger aggregations, such as nucleoproteins, which occur in
chromosomes and viruses and in the cytoplasm of cells. The living
cell, taken as a whole, represents a much higher level of complication
still, since many of these structures must be present in every living
cell in a precisely organized manner. It is only at this stage that we
reach a level which is recognizably living, in the sense that everything
necessary for continuing life in a natural environment is present.

It is probable that the smallest visible cell contains about a quarter
of a million protein molecules of many kinds and larger cells many
more. Taking the average protein molecule as containing about
20,000 atoms, we see that the smallest independently living units con-
tain something like five thousand million (5,000,000,000) atoms,
united into molecules of great complexity and all the molecules org-
anized into a single functioning whole.

There are as many or more cells in the human body than atoms in
a single cell, so that the cell comes midway in the scale of complexity
between atoms and man. If we were to take another step of the same
order of magnitude, we should have to include a great deal of the
visible universe! Man is therefore in a midway position between the
minute atoms of which the universe is constructed and the almost
unimaginable whole. It is clear from this that human beings and
other large animals are of colossal dimensions on an atomic scale and
hardly insignficant even when judged on a cosmic scale (see Fig. 28).

In a broad sense we have been tracing the great landmarks in the
ascent of life. They are four in number, although no doubt many
intermediate gradations could be recognized. The first stage, the com-

Life in the Universe

157

plete elaboration of the basic chemistry of life, is already achieved in
the simple free-Hving cells and, very broadly speaking, it undergoes
no further change. The second stage, the organization of numerous
cells into large organisms, involves further steps, in which there is an
increase in complexity of the same order of magnitude as that in the
elaboration of the cells from simple molecules. But the problems of

Whole of the visible universe

t

I r Man

Higher organisms < Higher animals

I Large plants

Cells

Nucleoproteins

Proteins, starches
Nucleic acids

Peptides and other
intermediates

t
Amino acids, sugars and other
simple organic compounds

Red blood cells
Bacteria
Protozoa
Amoebae
Animal cells

Genes, chromosomes
Viruses

Cytoplasmic particles
Bacteriophages
Chloroplasts

Enzymes
Toxins
Antibodies
Starches, cellulose
DNA

Hormones
Complex sugars
Small proteins

Number of

atoms
1079

1025—1028

1010-1015

10'- 1 09

104-106

102-104

101-102

FIG. 28. Hierarchy of natural structures

organization are different. They involve the processes of differentia-
tion, specialization and growth, which are involved in uniting vast
numbers of cells into a single organism. These processes are very
little understood.

158 Inside the Living Cell

The third stage occurred when the organism became a mobile
animal and acquired muscles to move about, sense organs to tell it
about its environment and a brain to join them together. The animal
in this way became aware of the world it lived in, at least to a limited
extent.

The fourth great stage in the ascent of life, which must be regarded
as still in its infancy, is the attainment by man of the power to replace
the actual world, as it is experienced, by a world of symbolic equiva-
lents, which can be manipulated in the mind and rearranged. As I
have pointed out, this permits the storing of experiences, so that it be-
comes possible to have knowledge which is not based on the limited
experience of each individual. As a result of this, human knowledge
exists and grows independently of the individuals who make some
use of it. It possesses a dynamism of its own. The knowledge and the
symbolisms in which it is expressed and the ways of thinking and
acting to which these give rise, give human societies their peculiar
character. The society becomes in fact a superorganism, because the
individual cannot live to more than a very partial extent without it
and the way he Uves is largely determined, not by his own simple re-
actions to his experience, but by the knowledge and ways of living
which the superorganism has arrived at.

There are of course other superorganisms besides the human ones,
e.g. those created by the social insects, but so far as we know, the
latter are held together by instinct. The human superorganisms are
characterized by shared knowledge, which, as we have seen, is
achieved by the human ability to turn experiences into symbolic
equivalents which can be passed from person to person. The super-
structure of communal knowledge provides a kind of scaffolding with-
in which the life of every individual is lived. It is not a static scaffold-
ing, but one which changes continuously. For this reason the life of
an individual in Western Europe in the twentieth century for example,
is quite different to that of a similar individual a hundred years
earlier, although his mental capacities may be similar. Not only does
his mind operate on different data, but it operates in different ways
which are determined by the very different symbolic environment in
which he lives. The individual life is an interaction between the float-
ing climate of ideas and his own personal reaction to them.

The shared mass of ideas, which do not belong to any one person
exclusively, but are the possession of a whole community, or a large
part of it, form what may be regarded as a collective mind, which
possesses a kind of life of its own. It develops according to its own
laws or inner necessities. This has been clearly recognized in the
many histories which have been written about the development of

Life in the Universe 159

this or that aspect of human thought. For example, we can recognize
clearly the lines of development of western music or science through
many generations of different people.

In conclusion, readers may ask what human meaning can be ex-
tracted from the knowledge which has been gained of the cell, its
construction and its achievements. Does it tell us anything about the
meaning of life? In the first place, it should be explained that science
does not concern itself with ultimate meanings; it is only concerned
with what it finds out about things as they exist and the relations be-
tween them. It does ask how they came to be in their present state,
but it does not ask what they exist for or what is the object of it aU.
Many scientists deny that there is any sense in such questions. Yet,
although science does not provide answers, human beings persistently
look for them and have always wanted to round off their useful know-
ledge by some conception of the meaning of it all. It is, in fact, a neces-
sity of the human mind to have some sort of a unifying conception
which will hold together all the fragments of knowledge which have
been acquired and which is capable of giving meaning and perspec-
tive to human life as it progresses from one generation to another.^ So
from a human point of view, we are justified in asking whether we
can find any meaning in the extraordinary pageant of events which
has been disclosed. Is it all just an accident — as some scientists have
suggested — the result of an extremely improbable event which
brought together the atoms, forming the first self-reproducing sys-
tem; so giving rise to the first germ of life? Or, as has been suggested
here, is it not inevitable that such complex systems will be formed
whenever suitable conditions occur, i.e. water, radiation and a variety
of inorganic substances? Whichever may be the correct view of the
origin of life, there can be no doubt that in living systems, as they
have evolved and as we find them today, we have chemical systems
of a very complicated kind, which form a single family of living
things. Are we then justified in regarding life, in all its variety, as
merely an exhibition of an immensely complicated chemistry?

I think we cannot avoid concluding that this is correct as far as it
goes, but it is not the whole truth. We can say in this, as in other
fields, that the whole is greater than the parts. We may be able to
break down the organism into its cells, and the cells into the interlock-
ing component cycles of activity, yet the functioning cell is more than
the sum of the chemical processes of which it is made up and the
organism is more than the sum of the cells of which it is composed.

This can be illustrated by a simpler example. If we combine a

1 1 cannot justify this statement here, but readers who are interested will find
a discussion in my book Science and Human Life (Pergamon Press).

160 Inside the Living Cell

number of atoms of carbon, hydrogen, nitrogen and oxygen together
in a particular way, we obtain the vivid blue dye, methylene blue. We
could not have suspected from what we knew of these atoms that,
when combined in this way, they would exhibit this property. Never-
theless, once we have the dye, we may be able to account for its pro-
perties in terms of the atoms and their mode of combination. We can,
for example, account for the colour as due to the oscillation of elec-
trons in a particular cyclic molecular framework, and this can be
'explained' in terms of the electronic structure of the atoms them-
selves. If necessary we may and we frequently must add to our de-
scription of the atoms in order to enable us to account for methylene
blue and other substances in terms of them, but we could hardly have
predicted methylene blue (or other dyes) if we were completely ig-
norant of its existence and behaviour.

We can well believe that the ability to produce such dyes is implicit
in the atoms of carbon, nitrogen, etc, but it lies dormant until such
compounds are actually made. We can say that a new property, the
vivid blue colour, emerges in a particular combination. While the pos-
sibility is implicit in the atoms, it is essentially a property of the
whole of the combination and not of its separate parts.

In just the same way we see new kinds of behaviour emerging at the
different levels of life, which could hardly have been predicted if only
the simpler systems which are made use of were known.

The new level of organization can be analysed into its component
mechanisms, and the new organization is implicit in the components,
but nevertheless when it has been achieved, something new has
appeared, which is more than the sum of the separate mechanisms of
which it is made up.

From this point of view, we see, as Bergson did in his concept of
emergent evolution, that in the course of evolution there has not only
been an increase of complexity of the parts, but also the emergence
of new properties, which although they are potentially present in the
simpler systems, do not really exist until they are actually produced
and when they are achieved are essentially more than the isolated
parts.

Therefore, although we are entitled to see in life a mechanical pro-
cess, a consequence of the natural combining possibilities of atoms
when supplied with energy, leading to the formation of self-per-
petuating compounds which can make use of solar energy and impose
their pattern on the environment and thus give rise to ever more and
more complex organizations, yet we must also see this as providing a
basis for the emergence of new 'wholes' with types of behaviour
which we should not have suspected, although they were nevertheless

Life in the Universe 1 6 1

present potentially all the time.

Judging by the uniformity of Nature as we see it here, life must
have appeared in many places in the Universe, wherever the neces-
sary conditions — water, light and simple compounds — exist. We
must think of living things, at various levels of complexity, as part
and parcel of the Universe of light, heat and material atoms. We ought
not to think of living organizations merely as complex mechanisms;
they are more than this. They exist in their own right, as emergent
creations of the 'elan vital' or 'life force', call it what you will. Men-
tality, whatever it is — we know very little at present — also cannot be
regarded as an outsider, but as implicit from the start, waiting to be
realized. We can see the Universe, comprising both matter and life,
as one process; though in extent, duration and complexity it is beyond
our comprehension. We must believe one thing about it, although we
cannot prove it, but it is a reasonable conclusion from what we do
know, that in some sense the end was implied in the beginning.

A few years ago it was fashionable to think of life as a remote
and improbable accident in a life-less Universe. Life was thought of, as
Jeans said,^ as 'an unimportant by-product' in 'a Universe which was
clearly not designed for life, and which, to all appearances, is either
totally indifferent or definitely hostile to it'. It seemed, he said,
'incredible that the Universe can have been designed primarily to pro-
duce life like our own; had it been so, surely we might have expected
to find a better proportion between the magnitude of the mechanism
and the amount of the product'.

However, although the size of the mechanism is clearly fantastic,
we do not know the amount of the 'product'. It could be argued just
as easily that the colossal dimensions of the Universe ensure that the
conditions (energy, stability, environment) required for the develop-
ment of life will occur in many places. To argue that life is an
improbable accident is no more cogent than to suggest that the
Universe itself is improbable and unlikely. Perhaps it is, but it does
exist and we can only accept it. In the same way it is only reasonable,
although our knowledge is so Hmited, to accept life, and also
mentality, as something which is normal and even inevitable.

It is just as likely that the Universe was designed primarily to
produce life like our own, as the reverse. That men do exist, with all
their extraordinary abilities and potentialities, is the primary fact of
human life and it is better to put that and all that is implied by it in
the forefront of our picture of the world, than to suggest that it is all
an improbable accident.

^ The Mysterious Universe (Cambridge University Press).

162

Inside the Living Cell

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APPENDIX I

Riboflavin

O
II

HN NH

HC— CH
I I

CH3 O

I ' II

CHoOHCCHOH-ONH-CHjCHjCOOH

I
CH3

Pantothenic acid

NH

o

COOH

N

CONH.

p-Aminobenzoic
acid

Nicotinarr.ido

C-NH2 X
I II I I

CH3 C. CH C^ /

\ CH3 C-CH2CH2OH

HjC CH{CH2)4COOH

Biotin Aneurln (Thiamine)

Figure 1 . Some of the vitamins — substances essential to life.

CH

HC^ \-CONH2
I! I

Gcid amide u^ q

Nicotinic

d-Ribose

HC

I
HCOH

I O

HCOH

HC
I

N=CH-N

I II

C=C— C— NH,

I I

N N Adenine

I CH
HC

I
HCOH

I 6 d-Ribose

HCOH

OH HC-
I

CHo— O — P — O— P-O-CH2

II II

O o

Phosphoric acid

(2 mols.)

Figure 2. Structual formula of co-enzyme I. ATP consists of the
right-hand side (adenine and J-ribose) combined with three mols

of phosphoric acid.

164

Inside the Living Cell

CH,

CH

•♦Acid Group /

NH

I 1

d-Ribooo- ' phospho-group^ Basic Group ',

Part of protein molecule

Figure 3. The riboflavin phosphate molecule showing how it
combines with the protein part of the enzyme.

NH2

COOH

SO2NH2

p-Aminobenzoic Sulfanilamide
acid

Figure 4. A vitamin and its antagonist.

c c c

/ \ / \ / \
c c c c

\ I I /

C_N N— C

/ \ / \

C Fe C

\ / \ /

C— N N— C

/ \ I \

c c c c

\ / \ / \ /

c c c

The central haem part
of haemoglobin

I I I

c c c

/ \ / \ / \

■c c c c

\ I \ /

C— N N— C

\ / \

Co C

/ \ /

C— N N— C

/I I \

c c c c

\ / \ / \ /

c c c

I I I

The central part of
Vitamin BI2

Figure 5. The central parts of the molecules of haemoglobin
and vitamin B12.

Appendix I

ADENINE

THYMINE

£^-

165

GUANINE

CYTOSINE

#»-"

Figure 6. The bases present in nucleic acid (dna) showing how
they are bonded together by hydrogen bonds.

166

Inside the Living Cell

HO

-<^ /^^"^ ^CH2-CH-COQH

NHo

HO.
HO

CH,OH

I 1

I. Thyroxin

^

CH-CH,-NH-CH.

OH

!I. Adrenalin

OH

r

Me
Me

CO

^

o^^y\/

111. Compound E or
Cortisone

CH

HoAyV

IV, Estradiol
(Female sex hormone)

Figure 7. Some of the hormones.

.OH

CH

V. Testosterone
(Mo/e sex fiormonoj

Polycydic hydrocarbons which occur in tars, such as benzpyrene.
Certain dyestuffs, such as 'butter yellow', and other azo-dyes.

'Mustard gas' S(CH2.CH2.C1)2 and analogous compounds,
such as nitrogen 'mustards', e.g., CH3N(CH2.CH2.C1)2.

/S-Naphthylamine and aminophenol, chemicals used in the dye-
stuffs industry. The latter is formed naturally in the human
bladder in some abnormal conditions and gives rise to bladder

cancer.

Plastic films of various materials, embedded under the skin.
Ionizing radiations from X-ray sets or from radioactive

substances.

Fig. 8. Some carcinogenic agents.

APPENDIX II

Selected References and
Books for Further Reading

CHAPTERS I— III

Further information will be found in modern text books of biochemistry.

PROTEINS AND ENZYMES

Northrop, J. H., Kunitz, M. and Herriott, R. M., Crystalline Enzymes

(Columbia Univ. Press, 2nd Edition, 1946).
Springall, H. D., The Structural Chemistry of Proteins (Butterworth, 1954).
Dixon, M. and Webb, E.C., Enzymes (Longmans Green, 1958).
Neurath, H. and Bailey, K., The Proteins (Acad. Press Inc., New York,

1954). (The standard work on proteins.)
Kendrew J. C. and others. Nature, 181, 662, 1958.

CHAPTERS IV — V

For a good summary of recent work, see: "»-

Crick, F. H. C, Mechanisms of Biological Replication (S.E.B. Symposia,

Vol. XII, Cambridge Univ. Press, 1958).
Other references:

Watson, J. D. and Crick, F. H. C, Nature, 111, 11>1, 1953.
Crick, F. H. C, Griffith, J. S. and Orgel, L. E., Proc. Nat. Acad. Sciences

(U.S.A.), 43, 416, 1957.
McElroy, W. D. and Glass, B. (ed.), A Symposium on the Chemical Basis of

Heredity (Johns Hopkins Press, Baltimore, 1957).
Brachet, J,, Biochemical Cytology (Acad. Press Inc., New York, 1957).
Engstrom, A. and Finean, J. B., Biological Ultrastructure (Acad. Press,

New York, 1957).
Loftfield, R. B., The biosynthesis of protein in Progress in Biophysics and

Biophysical Chemistry, Vol. 8 (Pergamon Press, London, 1957).
Palade, G. E., J. Biochem. Biophys. CytoL, 2, No. 4, suppl., 85 (1956)

(concerning structures in cell cytoplasm).
Davidson, J. N., The Biochemistry of the Nucleic Acids (Methuen's

Monographs on Biochemical Subjects, 2nd edition, 1953).
Taylor, J. H., Woods P. S., and Hughes W. L., Proc. Nat. Acad. Sciences

(U.S.A.), 43, 122, 1957.

CHAPTER VI

Griffith, F., /. Hygiene (Lond.), 27, 1 13 (1928).

Dawson, M. H. and Sia, R. H. P., /. exp. Med., 54, 681 (1931).

AUaway, J. L., ibid., 55, 91 (1932).

Avery, O. T., MacLeod, C. M. and McCarty, M., ibid., 79, 137 (1944).

168 Inside the Living Cell

Benoit, J., Leroy, P., Vendrely, C. and Vendrely, R., C. R, Acad. Set.

(Paris), 244, 2320 (1957); 245, 448 (1957).
Hotchkiss, R. D., Cold Spring Harbor Symp. Quant. Biol, 16, 457 (1951).
Haldane, J. B. S., The Biochemistry of Genetics (Allen and Unwin, London,

1954).
Swanson, G. P., Cytology and Cytogenetics {VxQn\.\ce-V{2i\\, 1957).

CHAPTER VII

The Hazards to Man of Nuclear and Allied Radiations (Medical Research

Council) H.M.S.O., London (1956).
Report of the Committee on Pathological Effects of Atomic Radiation

(Nat. Acad. Sciences — N.R.C., Washington, 1956).
Bacq, Z. M. and Alexander, P., Fundamentals of Radiobiology

(Butterworth, 1955).
Radiation Biology (ed. by A. HoUaender for National Research Council)

(McGraw-Hill, 1954.)

CHAPTER VIII

Fraser, D., et. ai, Proc. Nat. Acad. Sci., 43, 939 (1957).
Fraenkel-Conrat, H. L., /. Amer. Chem. Soc, 78, 1882 (1957); idem.,

Biochim. Biophys. Acta, 24, 540 (1957); idem., Proc. Nat. Acad. Sci., 41,

690 (1955).
Gierer, A. and Schramm, G., Nature, 111, 702 (1956); Z. Naturforsch.,

llB, 138 (1956).
Bawden, F. C, Plant Viruses and Virus Diseases (Chronica Botanica Co.,

1956).
Jacob F. and Wollman E. L., in A Symposium on the Chemical Basis of

Heredity, ed. W. D. McElroy and B. Glass (Johns Hopkins Press,

Baltimore, 1957).
Stanley, W. M., Science, 81, 644 (1935).
Hershey, A. D. and Chase, M., /. Gen. Physiol, 36, 39 (1952).
Benzer, S., Proc. Nat. Acad. Sci., 41, 344 (1955).

CHAPTERS IX — X

Raven, C. P., Outline of Developmental Physiology (Pergamon Press, 1954).
Waddington, C. H., The Strategy of the Genes (Allen and Unwin, 1957).
Brachet, J., Chemical Embryology (Interscience, New York, 1950).
Booth, L. G. and L. J., Energetics of Development (Columbia Univ. Press,
1954).

CHAPTER XI

Medawar, P. B., The Uniqueness of the Individual (Methuen, London,

1957).
Burnet, Sir F. Macfarlane, Enzyme, Antigen and Virus (Cambridge Univ.

Press, 1956).
Boyd, W., Fundamentals of Immunology (Interscience, New York, 1956).

Appendix II 169

CHAPTER XII

Hieger, I., One in Six (Wingate, 1955).

Huxley, Sir Julian, Biological Aspects of Cancer (Allen and Unwin, 1958).

CHAPTER XIII

Bernal, J. D., The Physical Basis of Life (Routledge, 1951).

Oparin, A. I., The Origin of Life on the Earth, 3rd ed. (Oliver and Boyd,

1957).
Terrien, J., Truffaut, G. and Carles, J. Light, Vegetation and Chlorophyll

(Hutchinson, 1957).
Hill, R. and Whittingham, C. P., Photosynthesis (Methuen's Biochemical

Monographs, 1955).

CHAPTER XIV

MUSCLE theories:

See Huxley, A. F., Muscle Structure and Theories of Contraction. In

'Progress in Biophysics and Biophysical Chemistry', Vol. VII (Pergamon

Press, 1957).
Huxley, A. F. and Niedergerke, R., Nature, 173, 971 (1^54).
Huxley, H. E. and Hanson, J., Nature, 173, 973 (1954).
vision:

Pirenne, M. H., Vision and the Eye (Pilot Press, 1948).
Pieron, H., The Sensations: their function, processes and mechanism (trans.

M. H. Pirenne and B. C. Abbott) (Muller, 1952).
nerves:

Eccles, J. C, The Neurophysiological Basis of Mind (Oxford, 1953).
Fernandez-Moran, H., The Submicroscopic Structure of Nerve Fibres. In

'Progress in Biophysics and Biophysical Chemistry', Vol. IV, p. 122

(Pergamon Press, 1954).

chapter XV
Sholl, D. A., The Cerebral Cortex (Methuen, 1957).
Adrian, E. D. (Lord Adrian), Physical Background of Perception (Oxford

Univ. Press, 1947).
Sherrington, Sir C, Man on his Nature (Cambridge Univ. Press, 1957).

chapter XVI

Craik, K. J. W., The Nature of Explanation (Cambridge Univ. Press, 1943).
Tinbergen, N., The Study of Instinct (Oxford Univ. Press, 1951).
Bierens de Haan, J. A., Animal Psychology (Hutchinson, 1949).
Physiological Mechanisms in Animal Behaviour. (Symposia of the Society

for Experimental Biology, No. 4; Cambridge Univ. Press, 1950).
Schiller, H. {ed.). Instinctive Behaviour (Int. Univ. Press, New York,

1957). (A collection of the classic papers.)

170 Inside the Living Cell

CHAPTER XVUI

Hebb, D. O., The Organisation of Behaviour (John Wiley, 1949).
Sluckin, W., Minds and Machines (Pelican Books, 1954).

Wiener, N., Cybernetics (Wiley, 1948). j
Walter, W. Grey, The Living Brain (Duckworth, 1953).

Ashley, W. R., Design for a Brain (Chapman and Hall, 1952). j

von Neumann, J., 77?^ Computer and the Brain (Yale Univ. Press, 1957). !

Lashley, K. S., In Search of the Engram. In 'Physiological Mechanisms of '

Animal Behaviour (Cambridge Univ. Press, 1950). ,

CHAPTER XIX ;

Medawar, P. B., The Uniqueness of the Individual (Methuen, London, 1957). |

Comfort, A., The Biology of Senescence (Routledge and Kegan Paul, 1956). |

Kapp, W. B. and Bourne, G. H., The Biology of Ageing (Symposia of ,i

Institute of Biology, No. 6, London, 1957). 3

INDEX

ACTH, 84

Adaptation, 97

Ageing, 150

Allergy, 89

Amino acids, 17

Amino acids, activation of, 51

Anaemia, sickle cell, 66

Aneurin, 30

Antibiotics, 34

Antibodies, 86

Antigens, 87

Antivitamins, 33

Atomic radiation, effects of, 58

Arrhenius, 5., 102

ATP, 46

Auerbach, C, 64

Aureomycin, 34

Auxins, 78

Axons, 118

Bacterial flagellae. 111
Bacteriophage, 70
Beadle, G. N., 64
Benoit, /., 55
Benzer, S., 73
Berg son, H., 160
Biotin, 30, 65
Bladder cancer, 94
Blood groups, 89
Brain activity, 125
Brains, mechanical, 142
Brain, oscillations in, 125
Brown, Sir G. L., 114
Burnet, Sir F. M., 92

Calculating machines, 142

Calvin, M., Wl

Cancer, 93

Cancer of bone, effect of radiations,

60
Carcinogenic agents, 64
Carcinogenic hydrocarbons, 94
Cerebral cortex, 144

Chain, E. B., 34

Chlorocruorin, 35

Chlorophyll, 35, 106

Chloromycetin, 34

Chloroplasts, 106

Cholesterol, 99

Chromosomes, 38

Chromosome breaks, 62

Chimera, 92

Citric Acid cycle, 46

Claude, A., 4S

Cobalt deficiency, 37

Co-enzymes, 30

Collective mind, 158

Cook, J. W., 94

Consciousness, 147

Copper, effect of plant growth, 36

Cori, C. F., and G., 85

Cortisone, 83

Crick, F.H. C, 42, 51

Craik,K.J. N., 132

Cytoplasm, 48

Dale, Sir Henry, 114
Death, 150
Delbruck, M., 11
Descartes, R., 117
Diabetes, 21, 81
Differentiation, 76
DNA, 42

Ducks, transformation of, 55
Dodds, Sir Charles, 84

Endocrine glands, 84
Energy sources, 46
Entropy, 105
Enzymes, and genes, 66
Enzymes, 23
Elford, W. J., 67
Evolution, 155
Evolution of man, 137
Eye, 115

172

Inside the Living Cell

Feedback, 122
Fertilization, 40
Fildes, Sir P., 33
Fleming, Sir Ambrose, 33
Florey, Sir Howard, 34
Folic acid, 33
Fraenkel-Conrat, H. L., 71
Freewill, 135
Fungi, 33, 65

Galvani, L., 117
Genes, 38, 54
Genes and enzymes, 66
Germ cells, 39
Ghiberellic acid, 78
Gibbs, Willard, 105
Goitre, 81
Globulins, 88
Growth hormones, 78

Haddow, A., 95
Hayes, R.J.K., 137
Haemoglobin, 35
Haemolytic disease, 90
Hebb, D. O., 146
Hench, P. S., S3
Hieger, I., 94
Hill, R.,\01
Hinshehvood, Sir C, 91
Hiroshima, 61
Histamine, 89
Haworth, Sir N., 29
Hodgkin, A. L., 119
Hofmeister, 18
Hopkins, Sir F. G., 28
Hormones, 80
Hormones, in cancer, 98
Huxley, A. F, 112, 119
Huxley, H. E., 112
Hydrocarbons, carcinogenic, 94
Hydrocarbons, natural, 103

Immunity, 86
Ingram, V. M., 66
Instinct, 131
Insulin, 19, 82, 85
Ionizations, 58

Jenner, Edward, 86

Kay, H. D., 110
Kelvin, Lord, 105
Kendall, E. S., 83
Kendrew, J. C, 21
Kennaway, Sir E., 94
Kogl, F, 78
Krebs cycle, 46
Krebs, Sir H. A., 46
Kunitz, M., 24

Lactation, 110

Lajtha, L. G., 45

Landsteiner, 88

Language, 139

Lashley, K. S., 145

Lethal does of radiation, 59

Learning, 123, 133

Leukemia, caused by atomic bombs,

61
Letre, H., 46
Lewis, J. N., 105
Li, C.-H., 84
Lippman, F., 51

Life span, effect of radiations, 60
Life span, 152
Longevity, 153
Lurea, S. E., 73

McJlwain, H.,3\

Man, origin of, 137

Mayneord, W. V., 94

Medawar, P. B., 92

Memory, 127, 148

Memory, basis, 146

Metabolism of cell, 46

Metals, 35

Metamorphosis, 79

Micro-organisms, food require-
ments, 31

Micro-organisms, mutations of, 64

Micro-organisms, nutritional re-
quirements, 65

Microsomes, 49

Milk formation, 110

Millott, N., 123

Index

173

Mitochondria, 48

Models of outside world, 132

Molds, 31, 64

Morton, R. A., 116

Miiller, H. T., 61

Muscle, 111

Mutation rate, effect of radiations

on, 62
Mutations, 54, 61

Mutations, produced by X-rays, 58
Mutations of micro-organisms, 64
Mutations, produced by chemicals,

63
Mutation theory of cancer, 93
Myelin sheath, 1 1 8
Myoglobin, 21
Myosin, 112

Nerve impulse, 119
Nervous circle, 122
Nerves, 117
Neurones, 143
Neurospara, 64
Newton, Sir Isaac, 1 1 7
Niedergerke, R., 112
Northrop, J. H., 24, 104
Nucleic acids, 42
Nucleus, 38
Nucleolus, 49
Oestrogens, 83
Oestrogens, in cancer, 98
Oparin^ A. L, 102
Optical activity, 104
Organizing factors, 77
Origin of life, 102

PAB, 33

Pancreas, 81

PAS, 33

Pavlov, I. P., 124

Pellagra, 29

Pen field. Sir W., \A%

Penicillin, 33

Peptide chains, 17

Photosynthesis, 106

Photosynthesis, efficiency of, 108

Pituitary gland, 84

Pirenne, M. H., 115
Pneumococcus, transformation of,

54
Proteins, 16
Protein synthesis, 50

Radicals, 59
Reduction process, 39
Reflex action, 123
Reflex, conditioned, 124
Replication of dna, 44
Respiratory proteins, 35
Retina, 115
Rhesus factor, 90
Rhodopsin, 116
Riboflavin, 30
Rickets, 29
Roentgen, W. K.,95
Roentgen (unit), 58
rna, 50

RNA, of virus, 71
Rous, Peyton, 96
Russell, W.L.,6\

Salivary gland chromosomes, 56

Sanger, F., \9

Schramm, G., 71

Sensations, 148

Sense organs, 114

Sex hormones, 83

Shaw, G.B., 153

Sheep disease, 36

Shope virus, 96

Skin grafts, 90

Smith, Lester, 37

Solar radiation, 108

Stanley, W. M., 67

Streptomycin, 34

Strontium, radioactive, in bone, 60

Sulphanilamides, 33

Sumner, J. B., 24

Superorganisms, 158

Suprarenals, 82

Svedberg, The, 18

Symbols, 140

Synapse, 143

Szent-Gyorgyi, A., 29, 112

174

Template code, 5 1
Thyroid gland, 80
Thyroxin, 81, 84
Tinbergen, N., 131
Tolerance, 92
Tools, 138
Transforming factors, 54

Vaccines, 87

Virus, 67

Virus particles, 19, 68

Virus, sizes of 19

Virus theory of cancer, 96

Vitamins, 28

Vitamin A, in vision, 116

Inside the Living Cell

Vitamin B12, 37

Warburg, O. H., 97
Wald, G., 116
Watson, J. B., 24
Watson, J. D., 42
Weiss, P., 77
Wigglesworth, V. B., 78
Wyckoff, R. W. G., 70

Xanthine oxidase, 37
X-rays, carcinogenicity of, 61
X-rays, effects of, 58

Young, J.Z., 124

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