Individuality in organisms

hs University WtmmiiSdUMeSmes

ndividuality in
Organisms

CHARLES MANNING CHILD

THE UNIVERSITY OF CHICAGO
SCIENCE SERIES

Editorial Committee

ELIAKIM HASTINGS MOORE, Chairman

JOHN MERLE COULTER

ROBERT ANDREWS MILLIKAN

INDIVIDUALITY IN ORGANISMS

The University of Chicago Press
Chicago, Illinois

AsPtttB

the CAMBRIDGE UNIVERSITY PRESS
LONDON AND EDINBURGH

THE MARUZEN-KABUSHIKI-KAISHA
TOKYO, OSAKA, KYOTO

KARL W. HIERSEMANN
LEIPZIG

THE BAKER & TAYLOR COMPANY
NEW YORK

c

INDIVIDUALITY IN
ORGANISMS

By

CHARLES MANNING CHILD

Of the Department of Zoology
The Uni'versity of Chicago

THE UNIVERSITY OF CHICAGO PRESS
CHICAGO, ILLINOIS

Copyright 1915 By
The University of Chicago

All Rights Reserved

Published November 1915

Composed and Printed By

The University of Chicago Press

Chicago, Illinois. U.S.A.

CONTENTS

CHAPTER PAGE

I. The Problem i

The Characteristics of the Organic Individual; Unity and
Order in the Life of the Individual; Reproduction and
Individuation; Metabolism and Protoplasm; Terminology.

II. Theories of Organic Individuality 21

Theoretical Review and Critique; A Dynamic Conception of
the Organic Individual.

III. Metabolic Gradients in Organisms 50

Susceptibility Gradients in Animals and Plants; Further
Physiological Evidence for the Existence of Metabolic Gra-
dients; Embryological Evidence for the Existence of Axial
Metabolic Gradients; Developmental Gradients in Agamic
and Experimental Reproduction; Conclusion.

IV. Physiological Dominance in the Process of Indi-
viduation S8

The Experimental Material; The Independence of the Apical
Region; Dominance and Subordination in Experimental
Reproduction; The Reconstitution of an Individual from an
Isolated Piece; Some Modifying and Limiting Factors in
Animal Reconstitution; Conclusion.

V. The Range of Dominance, Physiological Isolation,

AND Experimental Reproduction 127

Experimental Control of Spatial Relations of Parts and of the
Range of Dominance; Experimental Obliteration and
Determination of Axial Gradients and Dominance; The
Extension of Dominance during Development; Experimental
Physiological Isolation and Reproduction in Plants; The
Localization of Experimental Reproduction in Relation to
Different Axes; Conclusion.

VI. Discussion, Conclusions, and Suggestions . . .170
The Nature of Dominance; The Nature of Inhibition;
Origin of Metabolic Gradients and of Dominance; Morpho-
logical Differentiation in Relation to Metabolic Rate; The
• Fundamental Reaction System; Agamic Reproduction in
Relation to Physiological Isolation; Gametic Reproduction;
Heredity, Evolution, and Other Problems from the Dynamic
Standpoint.

PREFACE

The present book is an attempt to state, and to
present some of the evidence in favor of, a conception of
the nature of organic individuality which has gradually
developed in the mind of the writer during the course of
some fifteen years' investigation of the simpler processes
of reproduction and development in the lower animals.
In these forms organic individuality appears in rela-
tively simple terms, and it is here if anywhere that we
must look for the key to the problem of individuality in
the higher animals and man.

With the great variety of facts at hand and the
limited space available, it has often been difficult to
decide what particular points of the evidence to include
in the consideration and what to omit. To those
familiar with biological facts it will doubtless be evident
that many data from various lines of investigation have
been either barely mentioned or entirely omitted.

The attempt has been made to show in some degree
the wide range of applicability of this conception of
individuality to various biological fields, and it is per-
haps permissible to express the hope that, not only the
physiologist and botanist, but also the neurologist, the
psychologist, and the sociologist may find something
of interest in it. Chaps, i and ii are necessarily some-
what abstract and condensed and may seem to some
readers to demand too extensive a background of bio-
logical knowledge. A re-reading of these chapters
after reading chaps, iii-vi will assist in decreasing this
difficulty.

In the book Senescence and Rejuvenescence, recently
published, the writer was chiefly concerned with the

X INDIVIDUALITY IN ORGANISMS

periodic changes of the age cycle in the organic indi-
vidual as one aspect of the life cycle. The present book
deals primarily with the problem of the nature of the
unity and order in the organism, the constancy of char-
acter and course of development, the maintenance of
individuality in a changing environment, and the
processes of physiological isolation, disintegration, and
integration or individuation in reproduction. The two
books, concerned as they are with these intimately
associated aspects of the life cycle, are in many respects
complementary and together constitute a presentation of
the more important results and conclusions from the
writer's investigations and a consideration of certain
biological problems from the point of view attained.

For permission to reproduce or to make drawings
based upon figures which have appeared in other books
acknowledgments are due to publishers as follows: to
the University of Chicago Press, for Figs. i6, 17, 25-27,
47, 48 in part, reproduced from Senescence and Reju-
venescence; to Wilhelm Engelmann, for Fig. 13, drawn
after von Graff's Fig. 8, Tafel XVIII, in Monographie
der Turhellarien, I, RhabdocoeKda; to Henry Holt & Co.,
for Figs. 14 and 15, reproduced from Lillie, Development
of the Chick; to Gustav Fischer, for Fig. 86, reproduced
from Hildebrand, Die Gattung Cyclamen; to B. G.
Teubner, for Fig. 92, drawn after Fig. 84 in Goebel,
Einleitung in die experimentelle Morphologic der Pflanzen.
For certain figures drawn from botanical preparations
and for other figures not original, acknowledgments are
made in the legends. The writer is also deeply indebted
to Dr. C. J. Herrick and to Dr. W. J. G. Land, both of
the University of Chicago, for reading the manuscript
and for suggestions and criticisms.

C. M. Child

October, 1915

CHAPTER I
THE PROBLEM

The organic world appears in the form of more or less
clearly defined and limited and more or less complex
entities which for lack of a better name we call individ-
uals. The individual is not necessarily a single whole
organism; it may be a part of a cell, a single cell, or a
many-celled organ or complex part of the organism;
or, as in most plants and some of the lower animals, a
number of organisms possessing certain organs or parts
in common, and therefore remaining in organic con-
tinuity with each other, may together constitute an
individual. In at least most organic individuals a
more or less orderly series of changes in structure and
behavior which comprise the life-history occur, and in the
course of these changes the individuals may give rise to
new individuals by some sort of reproductive process.

In order to define the problem of the organic indi-
vidual, it is necessary to inquire whether any funda-
mental identity or similarity is discoverable in all
individuals and whether the changes which they undergo
are subject to any general laws which we can at present
apprehend.

THE CHARACTERISTICS OF THE ORGANIC INDIVIDUAL

The term *' individual," meaning in its etymological
sense something undivided or which cannot be divided,
is open to various objections. Division of individuals
to form new individuals is a characteristic feature of

2 INDIVIDUALITY IN ORGANISMS

living forms, and many individuals are made up of
other individuals and these in turn of others. Divisi-
bihty is as characteristic of the organic individual as
indivisibihty. Nevertheless individuahty is a very real
thing in the organic world. There may often be diffi-
culty in determining the presence or absence of indi-
viduality or the limits in space or time of particular
individuals, but such difficulties do not in the least
shake our faith in the existence of the individual. What-
ever the anatomist and the histologist tell us concerning
the constitution of the human body, of hundreds of
organs and millions of cells, it is perfectly evident that
each human being is an individual, because his behavior
proves it. And the same is true for the single cell with
its nucleus, cytoplasm, centrosomes, plastids of various
kinds, '^ mitochondria," chromosomes, chromomeres, etc.
We call the cell an individual because of its behavior.

What then are the fundamental characteristics of
this behavior ? In what does the individuality consist ?
In the first place, the organic individual is alive and
therefore consists essentially of the complex of substances
termed in general protoplasm; secondly, it is more or
less definitely limited in size; thirdly, it possesses a
more or less definite morphology, a visible form and
structure, which is associated in some way with dynamic
and primarily chemical activity; fourthly, a greater or
less degree of order, co-ordination, correlation, or har-
mony, as it is variously called, is perceptible in the char-
acter of its form and structure and in the dynamic
activities of its constituent parts. In short, the organic
individual appears to be a unity of some sort, its indi-
viduahty consists primarily in this unity, and the process

THE PROBLEM 3

of individuation is the process of integration of a mere
aggregation into such a unity, for this unity is not
simply the unity of a chance aggregation, but one of a
very particular kind and highly constant character
for each kind of individual. In all except the simplest
individuals it determines a remarkable degree of uni-
formity and constancy, both in the spatial relations of
parts and the order of their appearance in time, and also
in co-ordination or harmony of functional relation of
these parts after their development.

If this conception is correct, the fundamental problem
of the organic individual is the problem of the nature
of this unity. The first step in consideration of this
problem is to inquire whether this unity is real or appar-
ent. Conceivably it may be onlyan apparent or pas-
sive unity resulting from a pre-established harmony of
some kind between the constituent parts, a unity like
that of a house constructed from girders, stone, and
other materials, each part of which is measured and cut
beforehand according to a definite plan. The real unity
here is in the plan, not in its material reaHzation. This
conception of the individual leads necessarily to the
assumption of a creative entity of some sort which con-
trols and orders the physico-chemical organism as man
controls and orders the materials of the house which he
builds. This dualistic or ''vitalistic" conception of the
organic individual, carried to its logical conclusion, denies
the possibility of solution of the problem of organic indi-
viduality by scientific methods. Some of its special
forms will be briefly considered in another chapter.

On the other hand, the unity of the organic indi-
vidual may be an active unity resulting from interactions

4 INDIVIDUALITY IN ORGANISMS

between its parts, but such a unity may again con-
ceivably be the result of a pre-established harmony in
construction like that of a steam engine, or it may be a
unity which itself determines, constructs, and har-
monizes as a flowing stream sculptures its channel and
develops a characteristic morphological structure and
dynamic activity in mutual relation to each other.'
According to this last purely mechanistic conception the
problem of individuahty is accessible to scientific investi-
gation and may be solved by scientific methods.

While it is impossible to exclude absolutely the
dualistic alternative as long as a complete mechanistic
solution of the problem has not been reached, the
advance of scientific knowledge has resulted in demon-
strating the mechanistic character of one feature after
another of the organism and in narrowing the field
within which vitalistic assumptions are still possible.
We know that actual energetic relations do exist between
the different parts of the individual. These relations,
which are often called physiological correlation, are of
various sorts: mechanical, such as pressure or tension
between parts; transportative, consisting in the trans-
portation or exchange of substances between different
parts; transmissive or conductive, consisting of changes,
impulses, or excitations transmitted or conducted from
molecule to molecule or from particle to particle.
Physiological correlation of these different kinds un-
questionably plays a very important part in the unity of
the individual, and the only possible method of pro-
cedure to determine whether the unity consists essen-

^ Child, "The Regulatory Processes in Organisms," Jour, of
Morphol.,XXU, 191 1.

THE PROBLEM 5

tially in such correlation is the scientific method, that
of experimental analysis and synthesis of data. Only in
this way is there any possibility of determining whether
or not the mechanistic conception is adequate. My
own experiments, together with the experimental and
observational data already at hand, point the way toward
a conception of organic unity which is somewhat different
from current views, but still entirely mechanistic, and
which, as I believe, makes possible further advance
toward a solution of the problem.

UNITY AND ORDER IN THE LIFE OF THE INDIVIDUAL

Life in general consists of the life-histories of indi-
viduals. Individuals arise from other individuals,
undergo a more or less definite and orderly series of
changes known as development, usually reproduce new
individuals in a more or less orderly way, and either
undergo complete physiological disintegration into new
individuals in the process of reproduction or else finally
lose their unity by the cessation of their activity in
death.

The definitiveness and constancy, the degree of order
in the behavior of the individual as regards the morpho-
logical and physiological relations of its parts in space
and the sequence of the changes during its life, must
be considered as, to some extent at least, a criterion of
the degree of unity or individuality which it possesses.
In the simplest individuals order is scarcely apparent;
it is sometimes difficult to determine whether a particular
aggregation of protoplasmic substance is an individual in
the biological sense or merely an aggregation. Again,
in some cases a given order is local or temporary and is

6 INDIVIDUALITY IN ORGANISMS

soon succeeded by another. Each pseudopodium of an
amoeba, for example, is to some extent an individuation
of a part of the amoeba protoplasm, but it soon disap-
pears or gives way to another individuation, and so on.
Between such simple and evanescent individuals as this,
at the one extreme, and the human body with its amaz-
ingly complex structural and functional order and its
relative permanency at the other, there are of course
many intermediate conditions. In general, organic evo-
lution appears from this point of view to consist in an
increasing complexity and stability of order; in other
words, the degree of individuation, the unity of the
individual, increases in the course of evolution. The
series of changes which constitute development becomes
more and more definite, constant, and complex in ap-
pearance, and the product of these changes, the fully
formed individual, shows an increasing complexity and
stability of structure and an increasing variety and
degree of interrelation of parts. In fact, a progressive
morphological and physiological complication seems to
occur both in individual development and in evolution.
Between the unicellular organism and the adult human
being the difference appears to be almost infinite, but
the human individual is at the beginning a single cell
with much less complex visible structure than many
unicellular forms.

The process of visible structural complication which
occurs in both development and evolution is commonly
known as differentiation. Different regions of the cell,
different cells or cell groups, become different from each
other and from the original undifferentiated or so-called
embryonic condition. These differences are in general

THE PROBLEM 7

brought about by the formation and accumulation in or
about the cell of substances not present in the undiffer-
entiated cell. Differentiation is of course merely a
visible indication of differences of some sort in physio-
logical activity in different parts, although physiological
differences may exist without visible differentiation.
The physiological differences appear to consist at least
in large part of specialization in activity, that is, the
various fundamental activities of life which are all
present to some degree in the unspecialized cell or part
become more or less definitely distributed and localized
among different parts, a process often called division of
labor. Such specialization of parts is a characteristic
feature of life in all except the simplest individuals, and
even there it is probably present to some extent.

Physiological specialization and the differentiation
which may result from it occur in an orderly way, and
in fact constitute the fundamental evidence for the
orderly character of the individual. The orderly
course of specialization, and differentiation proceeds
very much as if there were underlying it a plan or scheme
characteristic for each kind of individual which is
worked out in a regular constant order, as the construc-
tion of a building according to a plan follows a regular
course. The orderly localization of parts and the
orderly sequence in their appearance with reference to
certain directions in the developing individual indicate
the existence of some sort of ordering capacity under-
lying and preceding the stages where the order becomes
structurally visible. It is evident that this underlying
order, plan, or whatever it may be that determines the
developmental and physiological order in the individual

8 INDIVIDUALITY IN ORGANISMS

is the foundation of individual unity and order, but in
attempts to solve the problem of the individual this
fact has not always been clearly recognized.

This underlying capacity for unity and order finds
its primary expression in what is commonly called
the polarity and symmetry of the individual. These
terms polarity and symmetry refer to the fact that in
the appearance and maintenance of the structural and
functional order in the individual certain geometrical
relations, characteristic for each individual, are dis-
tinguishable. These relations are commonly expressed
in terms of axes or planes. To say that an individual
possesses a polar axis or a plane of symmetry is merely a
convenient way of stating the fact that it is possible to
conceive as drawn through the individual a line or a
plane with reference to which order is perceptible.
Such a geometrical conception is an abstraction from
the fact that order is actually perceptible to a greater
or less degree in all directions. It is merely a selection
of those ideal lines or planes to which the order is most
directly, simply, or permanently related, and these then
serve as a system of co-ordinates, so to speak, to which
the order is conveniently referred.

The geometrical relations of order differ in different
individuals. In some cases the order is referable to a
system of lines passing through a common center and is
designated as radial or radiate. In other cases the
order is referable to one or a certain number of axes and
is therefore an axiate order, and it is often convenient
to refer to planes instead of axes of symmetry. In the
living individuals as they exist in nature various com-
binations of these relations occur. In the starfish, for

THE PROBLEM 9

example, a high degree of order is apparent in relation
to an axis drawn through the center of the body vertical
to the plane in which the arms extend. Centering about
this axis is an order referable to radii centering in this
axis, and again the order in each arm is referable to a
plane passed through the radius of each arm and the
central axis of the body. In a bilaterally symmetrical
individual, such as man and many animals, the order
can be referred to three axes — longitudinal, transverse,
and dorso-ventral — at right angles to each other, or
perhaps better to a longitudinal axis, and two planes,
transverse and ventro-dorsal, passed at right angles to
each other through this longitudinal axis.

These axes and planes drawn through the individual
as a whole represent merely the general plan of orderly
arrangement. Geometrical relations are also distin-
guishable in the order of various parts and organs, and
these relations do not necessarily coincide in direction
with the geometrical scheme of the whole individual
but differ from it in all conceivable ways. Evidently
the geometrical relations of order in the organic indi-
vidual, particularly where the structure is complex, are
by no means as simple as the general scheme might seem
to indicate.

The reason for making a distinction between polarity
and symmetry lies in the fact that in most axiate
individuals one axis, the polar axis, is distinguishable
as the chief or major axis of the body. In the direction
of this axis the specialization and differentiation are
most marked and the order in this direction is usually
more conspicuous or more stable and commonly appears
earlier than that in other directions. This axis is also

lO INDIVIDUALITY IN ORGANISMS

very often the chief direction of growth, so that the body
becomes elongated in this direction and the polar axis
becomes the longitudinal axis. In short, the so-called
polarity of the individual represents the direction of the
chief or major order, while the axes of symmetry repre-
sent the directions of minor orders.

The two terminal regions of the polar axis exhibit in
general distinct and characteristic differences in behavior
and structure. In most plants, in sessile animals, and/
in radially symmetrical forms generally, these two]
regions are commonly called the apical and basal regions, i
while in bilaterally symmetrical motile animals they/
are usually known as anterior and posterior. The apicall
or anterior region is primarily the region of greatest \
dynamic or metabolic activity in the individual: in the
plants it becomes the growing tip, the region of most
active primary growth, while in the animals it becomes
the most highly specialized and differentiated region of
the body, and in those forms which possess a central
nervous system, including all except the simplest animals,
the chief part of the central nervous system, the cephalic
ganglion or brain and the chief sense organs usually arise
in this region; in other words it becomes the head and
in motile forms usually precedes in locomotion.

The basal or posterior end, on the other hand, is
primarily the least active region, although in many
forms it may become secondarily a region of growth or
specialized activity because of certain changes during
the life of the individual which will be considered later.
In sessile forms it is usually the region of attachment and
may develop special organs of attachment, while in
motile forms its activity is more or less under the control

THE PROBLEM il

of the apical or anterior region. In fact it is impossible
to escape the conclusion that certain general features
common to most or all axiate individuals are similarly
related to the polar apico-basal or antero-posterior axis,
as it is variously called. As regards other axes, the
differences in relation between them and the differences
in behavior and structure in different individuals
complicate the matter, but I shall show that there are
good grounds for believing that an organic or physio-
logical axis is fundamentally the same in all cases,
whether it is an axis of polarity or symmetry of a whole
organism or of a part. These geometrical relations
serve primarily to express in a general way the fact that
spatial order of certain kinds exists in the organic indi-
vidual, but the orderly sequence of events in time is also
referable to a greater or less degree to the geometrical
scheme of the individual or part. In development the
specialization and differentiation make their appearance
and undergo their progressive changes in a more or less
definite sequence with respect to the chief axes. In
many cases the original geometrical plan of the indi-
vidual undergoes modification or gives place to a differ-
ent plan. Such changes are sometimes brought about
by conditions within the individual and sometimes
occur in response to changes in external conditions. In
general, however, it may be said that in any given kind
of individual the plan is always the same or undergoes the
same changes and is always worked out in essentially
the same way during development, provided external
conditions are the same. Under altered external con-
ditions departures from the plan may occur, and indi-
viduals result which differ more or less widely from the

12 INDIVIDUALITY IN ORGANISMS

usual form or structure. As a matter of fact, no two
individuals are exactly alike, and there is abundant
reason to believe that this is so because no two individ-
uals are or have been subjected to exactly the same
conditions.

From whatever standpoint we view the facts we
must always return to the conclusion that the unity and
order so characteristic of the life of the organic individual
are in some way or other an expression of a fundamental
ordering and determining capacity of some sort which
makes the individual what it is.

REPRODUCTION AND INDIVIDUATION

Reproduction, the formation of new individuals
from parts of those already existing, occurs in all living
forms, and the question of the origin of the new individual
and of the process by which the part becomes a new whole
individual is perhaps the most interesting aspect of the
whole problem of individuality. In the agamic or
asexual forms of reproduction, which give rise to new
complete organisms in the plants and lower animals, we
see the existing individual dividing into two or more,
sometimes in a very regular and definite manner, some-
times apparently falling apart, as it were, into frag-
ments or single cells; or it gives rise, sometimes in a
particular region, sometimes in regions manifestly
determined by chance factors, to one or more small
outgrowths, buds, which increase in size and become new
individuals, and these in some cases remain organically
connected with the parent, in others become completely
separated and independent. In many cases the char-
acter of these reproductive processes varies with the

THE PROBLEM 13

physiological condition of the individual and often also
with external conditions. In some organisms, for
example, many kinds of reproductive processes occur,
their character varying with internal and environmental
conditions.

Very generally it is possible to distinguish more or
less clearly an orderly character in these reproductive
processes; some of them are in fact orderly to a high
degree. But they differ so widely in different organisms
that attempts to discover common fundamental factors
underlying the various forms of the process have not
been very successful.

. In addition to those reproductive processes which
give rise to whole new organisms there are also those
which result in redupHcation of more or less complex
parts. The repetition of radially arranged parts, such,
for example, as tentacles in a sea-anemone, arms in a
starfish, a whorl of leaves or the parts of a flower in a
plant, and on the other hand the succession of parts
along an axis, leaves or branches along the stem of a
plant, or the segments in the body of the earthworm, are
all reproductive processes and involve processes of indi-
viduation. All such reproductive processes must be
included in any attempt at a theory of reproduction.

And finally there remains the process of sexual or
gametic reproduction in which the union of two cells,
the gametes or their nuclei, is followed by a series of
developmental changes. In most cases of gametic re-
production the two gametes are sexually differentiated
as parts of two different individuals or in different organs
of the same individual before they come together.
Moreover, they are themselves individuals, and their

14 INDIVIDUALITY IN ORGANISMS

union results in a new individuation. In the higher
animals this form of reproduction is, with very rare
exceptions, the only process which gives rise to organisms.

Apparently gametic and agamic reproduction are
very different processes, but we must at least raise the
question whether they are similar in any way, or, if
they are different, what the difference may signify.

It is in those parts of pre-existing individuals which
become new whole individuals that the process of indi-
viduation goes on before our very eyes, and it is there
that we have the opportunity to determine something of
its nature. It is by no means necessary for us to wait for
the occurrence of reproduction in nature. In many
of the simpler organisms we can bring about the occur-
rence of reproduction at will, simply by cutting out
pieces of the body and so isolating them from their
physiological relations with other parts. Such pieces
may become new organisms or parts of organisms more
or less like those from which they were taken. These
experimental reproductions constitute, as I shall show,
invaluable material for the study of the organic indi-
vidual and of the process of individuation, although their
value for this purpose has not heretofore been generally
recognized.

METABOLISM AND PROTOPLASM

The living organism consists of a substance, or more
properly a complex mixture of substances, in which the
series of chemical reactions known as metabolism
occurs. The fundamental constituents of protoplasm
occur in what is known as the colloid condition, i.e., they
do not form a true molecular solution, but exist as sus-

THE PROBLEM 15

pended particles larger than molecules in the fluid
medium, which in the case of protoplasm is water.
Living protoplasm may range in its physical condition
from a semi-fluid to a stiff jelly-like substance according
to the aggregate condition of its particles. This mix-
ture of substances, protoplasm, is the visible substratum
of the living form, and in it the changes which constitute
life occur. Changes in its aggregate condition and in
the chemical constitution of one or more of its parts
form the basis of speciaKzation and differentiation and
so of structure and form, and the energy of the organism
originates from certain of the chemical reactions which
occur in it.

MetaboHsm consists in a complex series of inter-
related chemical reactions in protoplasm. On the one
hand, nutritive substances are transformed and built
up into protoplasm or into other substances characteristic
of living organisms, and, on the other hand, portions of
the protoplasm and of these other substances are broken
down and oxidized, setting free energy, which appears in
the various activities of life. What we know of metab-
olism indicates that the oxidations are in general of
fundamental importance in the whole reaction system.
Apparently life cannot continue without them, and the
other reactions are to a greater or less extent associated
with and dependent upon them. In a given organism,
under given external conditions the rate of oxidation is
in some degree a measure of metabolic activity and of
life. Objection is sometimes made to the term ''metab-
olism" because of its vagueness. It is of course true
that we do not know all the various reactions and their
relations to each other and to other conditions, but we

i6 INDIVIDUALITY IN ORGANISMS

^do know that for a given organism metabolism is in
general a definite and characteristic system of reactions
subject to variation with change in conditions but never-
theless maintaining in the long run a certain rate and
character.

In general terms, protoplasm is the foundation of
structure and form, and metabolism, of function, in the
organism. The relation between structure and function
has been the subject of much discussion. For some the
organism possesses a certain structural organization
which arises in some way or other quite independent of
function and which makes function possible, just as a
man-made machine possesses a certain structure which
makes its function possible. Such an organism must be
constructed before it can begin to function, and hy-
potheses of this character are chiefly concerned with
the supposed method of construction. This conception
of the organism ignores the fact that it is always func-
tioning while it is alive : life is function. In no case does
the organism begin to function only after its construction
is completed; it always functions from the beginning;
it constructs itself by functioning, and the character of its
functional activity changes as its structural develop-
ment progresses. Structure and function are mutually
related. Function produces structure and structure
modifies and determines the character of function.

Here it is possible to refer only very briefly to a con-
ception of the relation between structure and function
which I have discussed more at length elsewhere.'
According to this view protoplasm and structure rep-
resent primarily those products of metabolism which

^ Child, Senescence and Rejuvenescence, 1915, pp. 26-31.

THE PROBLEM 17

are relatively stable under the ordinary physiological
conditions and in such physical condition that they can-
not escape from the organism without change. There-
fore they accumulate, and their accumulation constitutes
growth, and their differences in different parts constitute
the morphological structure of the organism. The less
stable products appear only temporarily or not at all as
structural features, for they are decomposed and elimi-
nated. These differences in stability are of course only
relative and between extremes numerous intermediate
degrees occur. Moreover, a structure which is stable
under certain conditions may, under altered conditions,
become unstable and be broken down and replaced by
other structures. In general, structural stability in-
creases both during the development of the individual
and the course of evolution. The evolutionary increase
in structural stabiUty is in fact what makes possible
the structural permanency and complexity of the higher
as compared with the lower organisms.

' If the organic individual Is a physico-chemical entity
of this kind the foundation of its unity and orderly
character must be present somewhere and somehow in
this metabolic-protoplasmic system. Definite relations
in both space and time must exist among the reactions
occurring in the protoplasm, and the problem of indi-
viduality resolves itself into the problem of the nature,
origin, and maintenance of these relations. It is with
the problem in this form that this book is chiefly con-
cerned.

TERMINOLOGY

In order to avoid confusion and for the sake of con-
venience and brevity it is necessary to fix upon and

i8 INDIVIDUALITY IN ORGANISMS

define certain terms to be used. The individual which
forms the starting-point of a developmental, reproductive,
or life-history I shall call the primary individual. This
primary individual may give rise by reproduction to
secondary individuals, or, by the individuation of certain
organs within itself, to partial or organ-individuals.
When such secondary, partial, or organ-individuals
continue to constitute parts of the unity of the primary
individual it is the dominant individual and they are
subordinate individuals. The segments of the earth-
worm body or the leaves of a plant are such subordinate
individuals. When the primary and secondary indi-
viduals each constitute a more or less distinct unity
though still organically connected they are co-ordinate
individuals. In many trees and in some branching
colonial animals various branches approach or attain the
condition of co-ordinate individuals. Between strictly
co-ordinate and the extremes of dominant and sub-
ordinate individuals there are of course various inter-
mediate degrees. A comnion or general individuality
resulting from the physiological combination of a
number of more or less co-ordinate individuals, either
similar or of different kinds, is a composite individual.
Strictly speaking, all organisms except perhaps some of
the simplest unicellular or monoplastic forms are to
some extent composite individuals for different cells,
and even different parts of a cell may possess a physio-
logical unity and order of their own, but since the
following chapters are chiefly concerned with the larger,
more general, features of organic individuality rather
than with its more minute details, the term will be used
primarily for the more extreme cases in which a number

THE PROBLEM 19

of morphologically and physiologically well-defined and
usually multicellular individuals make up a relatively
persistent composite individual. Most plants and the
so-called colonial animal forms are good examples.
The individuals which make up a composite individual
are constituent individuals. These may be either parts
of a cell, different cells, or cell groups composing
organs.

As regards the various axes of the axiate individual,
uniformity of designation is also highly desirable. The
polar, longitudinal, apico-basal, or antero-posterior
axis, as it is variously called, represents the primary
or major order when such an order is present in the in-
dividual. In cases where the axes of the individual
arise de novo and are not simply carried over from pre-
existing individuals, this axis is apparently the first to
arise and other axes arise in relation to it. It is often
convenient, therefore, to call this axis the major axis of
the individual and the other axes minor axes.

With reference to particular axes, we are accus-
tomed to distinguish position and direction according
to the general plan of the individual, the relation of
certain axes to others, the characteristic position,
behavior, or direction of movement of the organism.
The following terms are commonly used for this pur-
pose: apical and basal, distal and proximal, anterior and
posterior, peripheral and central, median and lateral,
dorsal and ventral, besides various others which refer
to particular regions, such as cephalic and caudal, oral
and aboral, etc. All these terms are useful in particular
cases, but greater uniformity and simplicity are desir-
able for purposes of general consideration.

20 INDIVIDUALITY IN ORGANISMS

As following chapters will show, there is reason for
believing that what we call an axis in the organism
represents the general course and direction of a gradient
in rate of metabolic reactions, the rate of reaction being
highest at one end, or in a certain region, and decreasing
from this point in the direction in which we conceive
the axis to extend. Moreover, the physiological and
structural order along any axis is definitely related to this
gradient. If all organic axes are fundamentally meta-
bolic gradients we may call the region of highest rate
in any axis the apical region or end, the region of lowest
rate the hasal region or end, while other intermediate
regions may be distinguished as more or less apical or
basal, and opposite directions in the axis as respectively
apical and basal directions. From this point of view
apical and basal regions of radially symmetrical whole
organisms are merely the apical and basal regions of the
major axis of such organisms and so the most conspicu-
ous or most widely separated apical and basal regions
of the body, but not fundamentally different in their
dynamic significance from the corresponding regions of
other axes. In the following pages it will often be con-
venient to use these terms, "apical" and ''basal" in
this general way for bilaterally as well as for radially
symmetrical forms.

CHAPTER II

THEORIES OF ORGANIC INDIVIDUALITY

Having formulated the problem, it is necessary to
inquire what progress has already been made toward
its solution. The first section of this chapter is a very
summary consideration of this question. Since the
experimental and observational data upon which my
own conclusions are based are so varied and their rela-
tions to the problem in many cases so complex, the
inductive method of procedure is impossible within the
limits of the present book. It has seemed necessary,
therefore, to state my conclusions briefly in categorical
form as a working hypothesis before attempting to review
and interpret the various lines of evidence. This I
have attempted to do in the second section of the chapter.

THEORETICAL REVIEW AND CRITIQUE

The organic individual has very often been compared
to a human society or state with orderly division of
labor and correlation among its component parts.
The fundamental feature of the human state, that
which distinguishes it from a mere aggregation of
human beings and makes it an individual, is some kind
and degree of law and order, of co-ordination and control
of the activities of its constituent units; in short, some
degree and kind of government. If the organism is a
cell-state or organ-state some degree and kind of govern-
ment must exist in it, but in making such comparisons
biologists have often ignored or failed to recognize

22 INDIVIDUALITY IN ORGANISMS

the importance of this fundamental point. There has
been much discussion of ''formative substances" and
their distribution and role, and the magic word ''organi-
zation" has served as the all-sufhcient answer to many
questions, while the fundamental problem of unity and
order involved in the origin and action of the so-called
formative substances and in the nature of organization
has too often been completely neglected.

Various theories of the organism, which may be called
corpuscular theories, have been advanced and have met
with more or less general acceptance. Among these
Weismann's germ-plasm hypothesis is most famihar and
has played the most important role in biological thought.
These theories postulate in one form or another a multi-
tude of specific material entities, each of which represents
in some way some characteristic of the organism. The
organism as we know it is the product of their combined
and harmonious activity. Examination of these theories
shows that these hypothetical entities, gemmules,
determinants, physiological units, pangenes, specific
accumulators, or whatever we prefer to call them, are
themselves endowed, ex hypothesi, with the essential
characteristics of individuals and that the organism as
a whole is merely a composite of their orderly activities.
Neither the problem of the individuality of the hypo-
thetical units nor that of their orderly combination and
unification in the organism receives any adequate con-
sideration in those theories. They merely translate
the problem into hypothetical terms which are beyond
the reach of scientific method. The combination of
these units into the individual is assumed to occur as the
facts demand, and although the problem of the control

THEORIES OF INDIVIDUALITY 23

and ordering of millions of such units through all the
changes involved in the development of a complex organ-
ism, say the human being, is one which staggers human
intelligence, it is practically ignored. Even some of
our present-day speculations which attempt to assign
actual topographic positions in the chromosomes to par-
ticular factors in heredity ignore completely the prob-
lem of the ordering and control of these factors which is
involved in their assumptions. In fact, if we subject
this group of theories to logical analysis we soon reach
the point where it is necessary to assume the existence
of something very like a superhuman intelHgence as the
underlying principle in all of them. They leave the
essential problem unsolved, but their implications are
anthropomorphic and teleological.

DuaHstic or ^'vitalistic" theories of the individual
recognize the real problem more or less clearly, but
assume the existence of a non-mechanistic ordering and
controlling principle. Before the development of the
experimental method in biology the doctrine of vital
force as something peculiar to the organism and funda-
mentally different from the forces acting in the inorganic
world was very generally accepted, but as evidence for
the validity of physico-chemical laws in the activities
of living things accumulated, the hypothesis of vital
force was discarded by most biologists. Within recent
years, however, various attempts have been made to
show the inadequacy of mechanistic conceptions of life.
Driesch, at present the chief exponent of this line of
thought, has postulated the existence of a controlling
and ordering principle which he calls entelechy, following
Aristotle. Entelechy is independent of and superior

24 INDIVIDUALITY IN ORGANISMS

to physico-chemical laws, and controls and orders the
physico-chemical factors in the organism to a definite
end or purpose. It constructs the organism as a
man constructs a machine. In many respects it
resembles human intelligence, but seems to be far
superior to it. Other neo-vitalistic theories are more
or less similar in their general conceptions, but differ
in detail.

In certain respects these theories constitute a real
advance over the corpuscular theories, for they recog-
nize and state more or less clearly, instead of ignoring,
the essential problem. For the present, however,
most of us find little intellectual satisfaction in the solu-
tion which they offer, and they are either frankly specu-
lative or involve unwarranted or premature assumptions,
and, like the corpuscular theories, they place the prob-
lem beyond the bounds of science.

Various attempts at solution or progress toward
solution of the problem of organic individuality have
been made along physico-chemical lines. The evident
unity and order, the individuality of the inorganic
crystal, together with the discovery of the existence of
fluid crystals, have led to comparisons of the organism
with the crystal and so to hypotheses which postulate
an essentially crystalline character for organic unity
and order. According to these hypotheses the laws
underlying this unity and order are essentially those
governing the aggregation and arrangement of mole-
cules. The construction of the orderly framework of
the organism is the expression of such laws, and its
activities represent the chemical changes which go on
in this framework.

THEORIES OF INDIVIDUALITY 25

These hypotheses are open to various objections.
The crystal consists primarily of Hke molecules though
under certain conditions some crystals may show differ-
ences in constitution at the two poles resulting from the
presence of other substances besides the primary con-
stituent of the crystal. The organism, on the other
hand, is a complex of many different kinds of molecules,
some of which are undergoing breakdown and being
built up anew during life, and, moreover, there is no
optical or other evidence that protoplasm in general is
fundamentally crystalline in structure. The unity of
the crystal is a static unity, a unity of form and arrange-
ment, and disappears or is replaced by another unity
when chemical change occurs, while the unity of the
organic individual is a dynamic unity dependent pri-
marily for its existence on chemical activity and dis-
appearing when such activity ceases. To beheve that
metaboHsm results from structure and *' organization"
as the activity of the man-made machine results from
its structure is to ignore the fact that metabolism is the
formative agent in the organism. Undoubtedly crystals
or crystalloid individuals are present in at least many
organisms, but their individuahty is quite distinct from
that of the organism.

Some biologists, while not admitting that the organ-
ism is fundamentally crystalline, assume that its con-
stituent molecules possess unknown properties which
determine its unity and order. These hypotheses are
open to the same objections as the crystal hypotheses.
All such hypotheses in fact proceed on the assumption
that a certain more or less complex ''organization"
is necessary as a starting-point; the machine must

26 INDIVIDUALITY IN ORGANISMS

somehow be constructed before it can run. Actually,
however, the organism runs throughout its construction
from the condition of amorphous protoplasm to that
of a complex anatomical system.

Modern investigation of the chemistry of the organ-
ism has demonstrated that the chemical correlations, as
they are commonly called, which exist between its parts
are most various and complex and often highly specific
in character. Certain parts produce substances which
are essential to the normal activity or structure of other
parts, and the statement is frequently made that every
organ in the body is an organ of chemical correlation,
which means merely that it produces something which
plays a role in making other parts what they are.

On the basis of these facts the hypothesis has been
advanced, and is at present widely current, that the
unity and order in the organism consist primarily in such
chemical correlations. These chemical correlations de-
pend upon the production and transportation within the
organism of more or less specific substances, and it is
evident that parts more or less specifically different
must be present in order to produce such substances.
These hypotheses provide no solution of the real prob-
lem of individuality, for they all involve the assumption
of an underlying order or "organization" which makes
orderly chemical correlation possible. To return to the
analogy between the organism and the state, exchange
between human beings arises from the existence of differ-
ent individuals with different needs. In order that the
exchange may be orderly and specific in character some
degree of unity and order must exist in the activities of
the parties to the exchange. This order may result

THEORIES OF INDIVIDUALITY 27

from the authority of one person and its transmission to
others, or from that of consensus of opinion, but in either
case it is not the act of exchange nor its character which
determines this order but the order that detei mines the
exchange and its character. The orderly union of
human beings to form an individuahty which shows the
most various degrees of individuation from the family
through the clan and tribe, etc., to the highly developed
modern state is based primarily on authority of some
kind and its transmission, not upon the material relation
of the production and transportation of substances.
When this union of men exists, no matter how primitive
its character, the substances which it receives in ex-
change may play a very important part in determining
the character and course of its further development. If
the unity of the organic individual is in any way com-
parable with that of these composite social individuals,
it is evident that it must originate in some ordering or
controlling factor which makes possible the existence
and orderly and definite arrangement of specific parts.
These two types of relation^ — authority or dominance
of some sort and its transmission to subordinate parts
and the production and transportation of substances —
represent the two kinds of relation possible between
persons, organs, cells or parts of a cell, so far as direct
mechanical relations of contact, pressure, or tension
are not concerned. The unity of the social individual
evidently depends primarily upon the transmissive
rather than the transportative kind of relation. If
the organic individual is in any way comparable to it
we might reasonably expect to find the same thing
true there.

28 INDIVIDUALITY IN ORGANISMS

Various biological theories have concerned them-
selves primarily with that particular aspect of unity and
order which appears in the geometrical relations of parts.
These are commonly known as theories of polarity and
symmetry, but since polarity and symmetry are funda-
mental features of organic individuality, these theories
must be regarded as theories of the organic individual.
It is unnecessary to discuss these theories particularly,
for they fall into the same groups as those already con-
sidered, and are open to the same objections. They
either assume the existence of some kind of structural
order or '^ organization," physical or chemical, or some
sort of pre-existent plan or pre-established harmony,
or they ignore or fail to recognize the real problem and
postulate migrations or distributions of formative sub-
stances, differences in tension, permeability, or other
properties, as if such factors could behave in an orderly
and constant way without a constant underlying order
of some sort.

Some few biologists have attempted to deny the
existence of individuality in the sense of a definite
determining and controlling unity and order. The
basis of such denials is usually the fact that organisms
behave differently under different external conditions,
while the more important fact that a definite unity and
order exists in these different reactions is completely
overlooked.

This brief consideration of the various lines of
biological thought concerned with the problem of the
individual is sufficient to show that the problem is by no
means solved. The remainder of the present book is an
attempt to make some progress toward a solution of the

THEORIES OF INDIVIDUALITY 29

problem along somewhat different lines from those
already considered. My own investigations in this
field, extending over some fifteen years, together with
the facts already at hand, as I see them, have forced
me to the conclusion that the organic individual is
fundamentally neither a structural system, whether
physical or ''vitaHstic" in character, nor a system of
chemical reactions, but rather a system of relations
between a physical substratum or structure and chemical
reactions. These relations, I believe, constitute the
fundamental problem of life, so far as it is a biological
problem, and as one aspect of it the problem of biological
individuality. This is the point of view which underlies
the conception of the individual presented in the follow-
ing pages. Since the relations between protoplasmic
substratum and chemical reactions, whatever their
physical or chemical character in particular cases, are
essentially dynamic, I have called it a dynamic con-
ception.

A DYNAMIC CONCEPTION OF THE ORGANIC INDIVIDUAL

The foundation of unity and order in the organic
individual is the transmission of dynamic change,
''stimulus," "excitation," from one point to another in
the protoplasm. In the course of such transmission the
transmitted change undergoes a decrement in intensity
or energy so that finally at a greater or less distance
from its point of origin it becomes inappreciable or
ineffective. In the simplest case such a transmitted
change originates in a region of high metabolic rate, and
transmission occurs to regions of lower rate. The region
of high metabolic rate results in the final analysis from

30

INDIVIDUALITY IN ORGANISMS

the action of factors external to the mass acted upon,
whether part of a cell or a cell mass. A simple schematic
consideration will serve to make these points clear.
Let us assume a spherical mass of living protoplasm
(Fig. i) which is morphologically and physiologically
homogeneous except as regards the essential features of
protoplasm or cells. Such a mass, whether consisting
of a single or of many cells, possesses no axis, is undiffer-

FiG. I. — Diagram illustrating the origin of a single axial gradient in
protoplasm: a, the point of action of the external factor.

entiated, and is not a physiological individual. Now let
us suppose some external factor which increases meta-
boHc rate, a ^'stimulus," to act on this mass in the region
a of its surface. The first result of such action is an
increase in the rate of metabolic or of certain metabolic
reactions in the region a. This is followed by a spread-
ing or irradiation of a dynamic change, either over the
surface of the mass or through it from the region a.

THEORIES OF INDIVIDUALITY 31

This change is fundamentally a transmission, not a
transportation, for it consists in the passage of a certain
energetic change and not in the bodily transportation of
substance.^ Such a process of transmission may be
compared to the spreading of waves in a pond from the
point where a stone is thrown into the water, although
it probably does not always or necessarily consist of a
series of rhythmical changes like the water waves.

The question of the nature of the transmitted or con-
ducted excitation has been the subject of much investi-
gation and discussion, and many different attempts to
answer it have been made. Recent investigation indi-
cates, however, that whatever its exact nature, it
involves an increase in metabolic activity. It seems
in fact to be a wave of increased chemical activity
spreading from the point of origin much as a wave spreads
in a pond. The question of the relation of the electrical
and chemical changes observed in the transmission of
excitation in protoplasm does not concern us here.
The fact of transmission and the increase in metabolic
activity in connection with it are the important points
for the present purpose.

The transmission of excitations is one of the char-
acteristic features of living protoplasm, and undoubtedly
occurs to a greater or less extent in all protoplasm. In
its simplest form it is perhaps little more than a spread-
ing or irradiation to a greater or less distance of the
change produced at the point of origin, but in its most

^ It should perhaps be noted that from the standpoint of current
physico-chemical theory transmission itself may be regarded as
molecular, atomic, ionic, or electronic transportation. Nevertheless,
the differences between such transportation and the transportation
in mass of substances is sufficient to warrant the distinction made.

32 INDIVIDUALITY IN ORGANISMS

highly specialized form, the nerve impulse, it probably
differs more or less widely from the initial change.

The second point of importance in connection with
such transmission is the existence of a decrement in
intensity or energy of the change in the course of its
transmission. Apparently a part of its energy is used
in overcoming a resistance or inertia or in producing
other changes which play no part in further transmission.
The existence of this decrement, which may be called
the transmission-decrement, determines that at a greater
or less distance from its point of origin the transmitted
change becomes inappreciable or ineffective, and trans-
mission does not proceed farther. In Fig. i the intensity
of excitation or the amount of increase in metabolic
activity is indicated diagram.matically for different
distances from the point of origin in a by the bands of
different width concentric at a. The limit of effective-
ness of transmission depends on the intensity or energy
of the original change produced at a and, secondly, upon
the character of the protoplasm. The higher the con-
ductivity of the surrounding protoplasm — in other words,
the less its resistance or the greater its sensitiveness to
the transmitted change — the greater the distance to
which the change will be transmitted before becoming
ineffective, and vice versa. In the existence of this
transmission-decrement the resemblance to the trans-
mission of waves in water and to various other forms of
physical transmission, such as electrical transmission, is
also apparent. A decrement in velocity of transmission
accompanies decrement in intensity, at least in certain
forms of transmission, but is not of primary importance
in the present consideration.

THEORIES OF INDIVIDUALITY 33

If the external factor acts only momentarily at a, the
increase in rate of reaction at a is usually only momentary
or of short duration, and a sooner or later returns to or
approaches its original condition, perhaps in some cases
with a gradually disappearing rhythm of increase and
decrease in rate. The transmitted change consists in
this case of a wave or a series of successively decreas-
ing waves of change.

It is probable that even the occurrence and passage
of such momentary changes as these in a substratum so
sensitive and so intimately associated with the reactions
as protoplasm produce changes which persist for a longer
or shorter time after the metabolic change has dis-
appeared, but such changes are usually slight or inap-
preciable. If, however, the external factor continues
to act on a for a sufficiently long time, or if it acts
intermittently with sufficient and not too great fre-
quency or intensity, it produces sooner or later more or
less permanent changes in the protoplasm, which are
most marked in the region a and decrease with the
transmission-decrement. The exact nature of these
changes is not certainly known, but their effect is to
increase the reactive capacity, to alter the protoplasm
so that in the absence of external stimuli, or with a
given intensity of external stimulus, a rate or intensity
of chemical reaction exists higher than the rate under
similar conditions before the change. In the terms
generally employed, the irritabiHty of the protoplasm is
increased.

Since this change is greatest in the region a, Fig. i,
where the excitation is greatest, and decreases with
increasing distance from this region, the result of

34 INDIVIDUALITY IN ORGANISMS

continued or frequently repeated excitation is the estab-
lishment of a gradient in the condition of the protoplasm
which constitutes a more or less permanent material
substratum for a persistent metabolic gradient inde-
pendent of the local external stimulus. In short the
effect of the local action of an external factor on
protoplasm may sooner or later result in the estab-
lishment of a metabolic gradient, or the material basis
for such a gradient, which persists for a longer or shorter
time after the external factor has ceased to act. As a
matter of fact, such gradients, once estabhshed, often
persist throughout the life of the individual. These
gradients may be directly visible in a graded structure of
the protoplasm as well as in differences in rate of reaction,
or they may appear only or chiefly in the differences in
rate, according to the nature of the protoplasm. There
is considerable evidence to show that when once estab-
lished to a certain degree they tend to persist and even to
become more marked, because the rate and extent of
further changes in the protoplasm at different levels
of the gradient are determined by the differences in rate
of reaction at these different levels. As a rapidly flow-
ing stream quickly removes from its channel obstacles
which a slowly flowing stream removes only slowly or
not at all, so the changes in protoplasm which make a
higher rate of reaction possible are more rapid and more
extensive with a high than with a low rate of reaction.
If these considerations are correct, and there are,
as will appear, many facts to support them, it is evi-
dent that a persistent metabohc gradient associated
with a material gradient in the protoplasmic sub-
stratum may arise as the result of the local or differential

THEORIES OF INDIVIDUALITY 35

action of an external factor on a morphologically and
physiologically homogeneous living mass.

The formation of metabolic gradients in another way
is possible, at least in single cells. If, for example, inac-
tive substances of different weight from the active pro-
toplasm are present in the cell, and if the position of
the cell with respect to the force of gravity remains
unchanged for a sufficiently long time, the inactive
substances and active protoplasm may be more or less
definitely localized in different parts of the cell and so a
metabolic gradient may result. Again, it is conceivable
that continued intake of nutrition at some particular
point of the cell surface might load that portion of the
cell with inactive reserve substances and so give rise
to a gradient. To what extent the origin of metabolic
gradients is due to such factors as this is still a question.
In the frog's egg gravity undoubtedly contributes to in-
tensify the existing gradient by bringing about a further
separation of the heavier yolk granules and the lighter
protoplasm, but it is not responsible for the origin of
the gradient. Unquestionably the primary factor in
the origin of these persistent metabolic-protoplasmic
gradients is in most cases at least a reaction-gradient,
and the persistent or permanent gradient in the proto-
plasmic substratum is secondary.

Such metabolic gradients are, I believe, the simplest
expression of physiological unity and order in living
protoplasm, and at the same time they are the simplest
and primary form of the organic axes of so-called
polarity and symmetry and the starting-point of the
mysterious ^'organization." They are factors in deter-
mining the direction of growth and differentiation and

36 INDIVIDUALITY IN ORGANISMS

so are the basis of the geometrical space relations and
the sequences in time which arise during the develop-
ment of the individual. They may then be called
axial gradients. The region of highest rate in such a
gradient is the apical, the region of lowest rate the basal
region of the axis which represents the general direction
and course of the gradient.

Other factors besides actual rate of metabolic reac-
tions are doubtless concerned in the formation and
establishment of these gradients in protoplasm, but
these are associated either with the rate of reaction or
its change, or with the character of the protoplasm in
which the reaction occurs and have to do rather with
particular cases than with the gradient in general. The
intensity of reaction, for example, is probably such a
factor. A sudden or very rapid increase in rate on
excitation is probably more effective in producing trans-
mitted changes than a gradual increase, and it is also
probable that in protoplasm with a high reaction-
intensity excitations are transmitted to greater distances
than where the reaction-intensity is low. Excitation
and transmission are undoubtedly also correlated with
the physiological stability and physico-chemical con-
stitution of the protoplasm. Such factors as these
may play a part in determining length, slope, or other
characteristics of the gradient, but the primary factor
in its production appears to be rate of reaction.

In a metabolic gradient a relation of dominance
and subordination exists between the level of highest
and the levels of lower metabolic rate. A brief con-
sideration will show that this relation is a simple and
necessary result of the differences in rate of reaction,

THEORIES OF INDIVIDUALITY 37

In the first place, the apical region (a, Fig. i) is the chief
factor in determining the rate of reaction at other levels,
for in the varying conditions of a natural environment
it responds more rapidly or with a higher rate of reaction
than other levels of the gradient to external exciting
conditions, and it is also more sensitive and may react to
conditions which produce no reaction at lower levels
of the gradient. With every such increase of metabolic
rate in response to external exciting factors a gradually
decreasing wave of change spreads from this region of
highest rate, as in the original excitation which gave
rise to the gradient, though the intensity, velocity, and
limit of effectiveness of the transmitted change may be
much greater than in the original transmission.

This change transmitted from the apical region a
plays the chief part in determining the metabolic con-
dition at other levels, because a is the region of highest
metabolic rate and the changes transmitted from it are
more intense than those from other levels and because the
establishment of the protoplasmic gradient makes con-
duction in this direction more effective than in any other.
Consequently the region a dominates or controls other
regions within a certain distance and to a greater or
less degree by influencing, through the changes trans-
mitted from it, their metabolic rate. Dominance or
control of one part over another in the organism is
fundamentally, I believe, a matter of difference in
metaboHc rate, the region of higher rate being dominant.

If, after such a gradient is established, some other
region, such as h, Fig. 2, undergoes excitation at the
same time as a and by an external factor of the same
intensity as that acting at a, the response will be

38 INDIVIDUALITY IN ORGANISMS

less rapid and less intense than that of a, and, as
indicated in Fig. 2^ the transmitted change will be
weaker, perhaps less rapid, and its limit of effectiveness
less than that arising from a. The influence of the
region b must then be less than that of a in determining
the metabolic rate in other regions and a remains the
dominant region.

Fig. 2, — Diagram illustrating origin of major and minor gradients in
a simple case: a, apical region of major gradient; h, apical region of
minor gradient.

If, however, the region h or any other region is suffi-
ciently intensely or sufficiently often locally excited
independently of a, a persistent gradient may arise with
relation to h without destroying that related to a. In
such a case two dominant regions, a and h, exist, but a
may still dominate h to a greater or less extent unless
the reactive capacity or irritability of h becomes equal

THEORIES OF INDIVIDUALITY 39

to that of a. Every other point in the mass, so far as it
is within the limit of effectiveness of both a and b, will be
subordinate to both to a greater or less degree, and its
metabolic condition will be the resultant of its position
in the two gradients. Such an organism possesses not
only a polar axis or gradient, but an axis or gradient of
symmetry as well, and in the same way other gradients
and other relations of dominance and subordination
may arise. Obviously it is possible for various gradients
to exist simultaneously in a living mass, and their rela-
tions may be very different in different cases, as are the
relations between the different axes in organisms. Inter-
ference between different gradients in opposite or nearly
opposite directions, or obliteration of one gradient by
another of higher rate of reaction, undoubtedly occurs, as
following chapters will show, but their relations need
not be considered here.

The physiological dominance of one part over another
is certainly not a constant, unchanging relation, but
depends upon the metabolic rate in the dominant apical
region and the conductivity of other regions. The meta-
boHc rate in the dominant region is also not constant,
but must fluctuate with changes in external conditions.
With a slight rise in metabolic rate in the dominant
region its influence on other regions is slight and does
not extend far, but when the increase is great the degree
of dominance is greater and extends to a greater distance.
Certainly in the primitive individual these relations must
be regarded as constantly undergoing change in degree
and extent, though under the usual conditions they must
also show a general average. So for each level there will
be a general average of the effect of the transmitted

40 INDIVIDUALITY IN ORGANISMS

change upon it, and the gradient will be further intensi-
fied by the fact that slight transmitted changes do not
reach the more remote parts at aU while they do affect
the parts nearer the dominant region. It is this general
average which determines the more conspicuous and
lasting effects at different levels.

The continued existence of a metaboHc gradient of
this kind undoubtedly determines an increase in the
conductivity of the protoplasm for the transmitted
excitation. Many facts indicate that within certain
limits the occurrence and repetition of transmission
increase the conductivity, and in all animals except the
simplest a nervous system which possesses a very high
degree of conductivity develops in relation to the primary
gradients. In most organisms there is therefore an
extension of dominance during development, the trans-
mitted changes become effective through a greater dis-
tance, and their limit of effectiveness, which of course
determines the range of dominance, becomes farther and
farther removed from the point of origin . This extension
of dominance, however, is itself limited by the changes
known as senescence, which become evident in a general
decrease in reaction rate. These relations of parts,
dependent in the final analysis on differences in meta-
bolic rate, constitute, as I beHeve, the foundation of
unity and order in the organic individual, the starting-
point of physiological individuation.

If this conclusion is correct, the organic individual,
as a living entity possessing some degree of physiological
— not merely physical — unity and order, consists in its
simplest forms of one or more gradients in part of a cell,
a cell, or a cell mass of specific physico-chemical consti-

THEORIES OF INDIVIDUALITY 41

tution. The process of individuation is the process of
establishment of the gradient or gradients as a more
or less persistent condition, and the degree of individua-
tion depends upon the permanency of the gradient, the
metabolic rate in the dominant region, the conductivity
of the protoplasm, and probably on other factors as well.

From this point of view the assumption of a
mysterious, self-determined organization in the proto-
plasm, the cell or the cell mass as the basis of physio-
logical individuality becomes entirely unnecessary. The
origin of physiological individuality is to be found, not
in living protoplasm alone, but in the relations between
living protoplasm and the external world. In view of
the fact that the organic individual after its formation
is far from independent of its environment, it is difficult
to see why we should assume that it is independent and
self-determining in its origin.

It must not be supposed, however, that every new
individual originates in the manner described above.
When the axial gradient is once established in a cell or
an organism, it may simply persist through the process
of cell division or other forms of reproduction so that
the unity and order of the new individual represent
simply the unity and order of the parent or a part of it.
In such cases the basis of individuality is inherited from
the parent. In nature we fi.nd both possibilities realized :
physiological individuality may arise de novo through
the relation between living protoplasm and its environ-
ment, or it may be inherited from previously existing
individuals. To put it more concretely, an axial gradient
cannot arise in the first instance independently of con-
ditions external to the mass of protoplasm concerned,

42 INDIVIDUALITY IN ORGANISMS

but, once established, it may persist through many
generations.

The question at once arises whether a quantitative
gradient, such as has been described, constitutes an
adequate basis for the physiological specialization and
structural differentiation which arise in relation to the
axes of the individual and in the higher organisms
become very complex. Organs showing very definite
qualitative differences in chemical constitution and
metabolism and great differences in functional activity
develop in the organism. Qualitative specific differ-
ences of some sort are commonly believed to be necessary
as a starting-point for such complexity, hence the usual
theoretical assumption of some sort of underlying
organization as the basis of organic individuaHty.
Some of the facts bearing upon this question will be
considered in later chapters; here attention may be
called to three points: first, it is a familiar fact of
chemistry that purely quantitative differences may bring
about the formation of qualitatively different products
from the same reacting substances, and in a complex
physico-chemical system, such as living protoplasm, the
possibilities for the origin of qualitative from quantita-
tive differences is very much greater than in the simple
chemical reaction in the test tube ; secondly, it is by no
means clear what is quantitative and what is quahtative
in organic structure and form, or in metabolism, for
many structural differences which are ordinarily con-
sidered as qualitative prove on analysis to depend on
quantitative differences in certain constituents of the
complex; and, thirdly, morphological differences usually
regarded as qualitative can unquestionably be produced

THEORIES OF INDIVIDUALITY 43

and controlled experimentally by metabolic changes
which are primarily quantitative. The morphology of
the channel of a rapidly flowing stream is very different
from that of a stream which flows slowly, and there can
be little doubt that in the organism substances which are
decomposed and transformed or eliminated with a high
rate of reaction remain -and accumulate in the protoplasm
and may form characteristic morphological features when
the rate of reaction is low.

In this connection the question must be raised
whether the transmitted change is always of the same
sort and produces the same efifect in a protoplasm of
given constitution. It is impossible at present to give a
definite answer to this question, but there seems to be
no positive evidence to show that the qualitative char-
acter of the effect is determined by the character of the
transmitted change, although it has often been assumed
that this is the case. It is very probable that the
chemical or physico-chemical character of the trans-
mitted change differs more or less widely in plants and
animals, and in embryonic protoplasm as compared with
the fully developed medullated nerve, but the efifect in
each case seems to be primarily excitatory and quanti-
tative. It seems even possible that in passing through
different tissues the character of the transmitted change
»may differ more or less according to the constitution
of the tissues, but its efifect may still remain essentially
quantitative.

If it should be demonstrated that the same proto-
plasm may transmit different kinds of excitations, then
of course dififerent processes of morphogenesis and
differentiation might be determined by the specific

44 INDIVIDUALITY IN ORGANISMS

character of the transmitted changes affecting different
regions. The demonstration of such relations would
of course complicate our conception of the course of
development, but would not necessarily alter our views
concerning the fundamental principles of individuation.
The problem of the nature of transmitted changes in
protoplasm has been the subject of much experiment
and discussion and is still by no means solved, but our
knowledge concerning them is sufficient to permit us to
formulate a working hypothesis of the organic individual
in terms of these transmitted changes rather than in
terms of transported chemical substances.

As soon as local differences in chemical constitution
of the protoplasm arise, whether they result from differ-
ences in metabolic rate or from differences in character
of the transmitted change, the relations commonly called
chemical correlation, consisting in the production and
transportation of different specific substances, begin to
play a part, and from this point on these chemical rela-
tions are factors of great importance in determining the
character of the different parts, until in the adult stage
of the highest forms, man and the other mammals, the
complexity of chemical correlation is bewildering, as
the work of recent years on hormones and internal
secretions has clearly demonstrated. From the point of
view developed here chemical correlation is, however,
a secondary factor, for the underlying order which
determines the orderly character of chemical correlation
consists in the quantitative gradients which arise in the
living mass.

Since a transmission-decrement in energy or in
intensity of the transmitted change exists, the change

THEORIES OF INDIVIDUALITY 45

is effective only within a certain limit of distance which
we may call its range, and since physiological dominance
depends upon the transmitted change it is similarly
limited in range. If physiological individuation depends
upon dominance of this sort associated with the meta-
bolic gradient determined by transmitted changes, the
range of dominance must determine a physiological
limit of size, which the individual cannot exceed without
the physiological isolation of some part from the domi-
nance which previously determined the individuality.
As already pointed out, the range of the transmitted
change and so the range of dominance varies with the rate
of reaction in the dominant region and with the conduc-
tivity of the protoplasm; therefore the physiological size
limit of the individual must vary with the same factors.
Reproduction in its simplest asexual forms results
from the physiological isolation^ of parts of the indi-
vidual body in consequence of their coming to lie beyond
the physiological limit of size. Such physiological
isolation may result from: first, increase in size of the
body of the individual by continued growth until some
part of it is brought beyond the range of dominance;
secondly, decrease in the range of dominance and limit
of size by decrease in the rate of reaction in the dominant
region ; thirdly, decrease in the conductivity of the proto-
plasm for the transmitted changes; fourthly, the direct
local action of some external factor on a subordinate
part, increasing its rate of reaction to a sufficient de-
gree to make it more or less independent of or insus-
ceptible to the effects transmitted from the dominant

^ Child, "Die physiologische Isolation von Teilen des Organismus,"
Vortrage und Aufsdtze iiher Entwickelungsmechanik, H, XI, 191 1.

46 INDIVIDUALITY IN ORGANISMS

region. This change I have called decrease in receptiv-
ity of the subordinate part for the transmitted change.
The effect of physiological isolation of a part is
essentially the same as that of physical isolation. In
the lower organisms where its physiological and morpho-
logical characteristics as a part are less stable than in
the higher forms and it is able to respond to the altered
conditions accompanying- physiological isolation, it
loses more or less completely its character as a part
because the conditions which determined and maintained
its specialization no longer act. Consequently it under-
goes dedifferentiation to a greater or less degree and so
approaches or returns to the undifferentiated or embry-
onic condition, and is then capable, if differences in meta-
bolic rate in the direction of the original gradient or
gradients still exist in it, or if conditions determine the
origin of new gradients in it, of development into a
new individual. I have shown that development and
differentiation are in general accompanied by a decrease
in metabolic rate which constitutes physiological senes-
cence and that the dedifferentiation of isolated parts
brings about rejuvenescence varying in degree with the
degree of dedifferentiation.^ New individuals formed
from physiologically or physically isolated parts of pre-
existing individuals may therefore be physiologically
younger than the individuals from which they arose and
so be capable of repeating the developmental history
and process of senescence. Asexual reproduction in
general results from such physiological isolation of
parts and their dedifferentiation and redifferentiation
into individuals. In the higher animals physiological

^ ChM, Senescence and Rejuvenescence, 1915; particularly chaps, ii,
iv, V, vi, viii, x, xv.

THEORIES OF INDIVIDUALITY 47

isolation of parts probably does not occur except occa-
sionally in embryonic stages, for with the evolution and
development of the nervous system in the individual the
transmission-decrement decreases and the effective range
of transmission therefore increases until in the nerves
of mammals the transmission-decrement is inappreciable
under natural conditions in the lengths of nerve fiber
available for experiment. In these forms the physio-
logical limit of size of the individual determined by the
range of dominance is very great and is never attained
by the individual because growth is limited by the
progress of differentiation in the course of development.
In such organisms, then, physiological isolation does not
occur except occasionally in embryonic stages before the
nervous system has developed or under special condi-
tions which limit the range of dominance or decrease
the receptivity of subordinate parts.

Moreover, in the higher animals the degree and
stability of specialization of parts of the body is so great
that in most cases they do not respond to physiological
or physical isolation by reproduction, but either die or
remain largely unchanged. For these reasons asexual
reproduction among the higher animals is rare and is
limited to early developmental stages. Sexual or
gametic reproduction which results from the union of the
two gametes or sex cells, which are usually speciaKzed
and differentiated as egg and sperm, is somewhat more
complex than asexual reproduction, but I have already
endeavored to show that there is a fundamental physio-
logical similarity in the two processes,^ and I shall con-
sider the question briefly in a later chapter.

^ Child, Senescence and Rejuvenescence, 1915, chaps, vi, x, xiii, xiv, xv.

48 INDIVIDUALITY IN ORGANISMS

This, then, is in brief the dynamic conception of the
organic individual which has grown out of years of
experimental investigation, observation, and analysis of
facts already at hand. Its distinctive feature is the
interpretation of physiological unity and order in terms
of differences in rate of reaction and of transmitted
changes, instead of in terms of a hypothetical organiza-
tion and of transportation of chemical substances. Ac-
cording to this conception the central nervous system in
its relation to other parts is merely the final expression of
relation which is the foundation and starting-point of
organic individuation. This conception provides a
working hypothesis based on a great variety of evidence
and readily accessible to experimental and analytic
investigation, and while it is manifestly far from being
a complete solution of the problem of organic individual-
ity, I believe that it throws some Hght on various
characteristics of the organism the nature and sig-
nificance of which have heretofore remained obscure.

It is perhaps necessary to point out that this dynamic
individuality is not the only kind of individuahty which
exists in the organic world. Physical individuals of
crystalline or crystalloid character, and perhaps physico-
chemical individuals of other sorts exist in organisms.
It is not with these, however, that we are concerned, but
with that sort of individuality which is distinctive of
the living organism, which determines harmonious
development and functional unity throughout the con-
tinuous dynamic change which constitutes life. Where
this organic individuahty makes its first appearance it
is impossible to say. The cell or protoplast in its
simplest terms usually shows some degree of such indi-

THEORIES OF INDIVIDUALITY 49

viduation, but it is probable that some real or apparent
individuations which arise temporarily or are persistent
in the cell approach more nearly the inorganic than
the organic kind. Nevertheless, wherever a region of
high metaboKc rate arises in protoplasm, there some
degree of organic individuation arises, at least for the
time being, provided relations already existing do not
interfere with or inhibit the establishment of a metabolic
gradient.

According to the dynamic conception organic indi-
viduality results in the final analysis from the relations
between living protoplasm and the world external to it.
If we accept this view we should expect to find morpho-
logical structure and differentiation making their first
appearance in the superficial regions of the protoplasmic
mass. These are in more direct relation with the
external world and therefore more irritable and with
the establishment of a region of high metabolic rate a
metabolic gradient must arise much more rapidly in the
superficial than in other regions. The facts agree well
with this view, for the first indications of individuation
in the organism are very generally superficial and in
many of the simpler forms, such as the infusoria among
animals, orderly morphological differentiation is always
limited to the superficial regions. The nervous system
is also superficial in origin. In the plant cells also the
superficial portions of the cytoplasm generally show a
higher degree of stability than other regions and are
apparently chiefly concerned in whatever morphological
protoplasmic differentiation occurs. If organic indi-
viduality is self-determined there is no apparent reason
for its appearance as a superficial phenomenon.

CHAPTER III
METABOLIC GRADIENTS IN ORGANISMS

If metabolic gradients are of such fundamental impor-
tance in the organic individual it should be possible to
discover various proofs or indications of their existence.
This chapter is a survey of some of the experimental
and observational evidence for the existence of metaboHc
gradients.

SUSCEPTIBILITY GRADIENTS IN ANIMALS AND PLANTS

The resistance or susceptibility of living protoplasm
to various poisons can be used, with certain precautions
and within certain limits, as an index of its metabolic
condition. This method, which may be called the sus-
ceptibility method, makes it possible, particularly in
early stages of development and in small, simple animals,
to compare the susceptibilities and so to obtain a general
idea of the differences in metaboHc activity of differ-
ent regions of the body of a single organism. Many
different substances may be used as reagents for deter-
mining susceptibility, such, for example, as the alcohols,
ethers, and other narcotics, and acids and alkalies.
Various products of metabolism, among them carbon
dioxide, and certain conditions, such as lack of oxygen,
serve the same purpose. But the cyanides, which are
powerful poisons, are in many respects the most satis-
factory reagents, and they have been used in most of
my experiments.

50

METABOLIC GRADIENTS 51

The relation between metabolic activity and sus-
ceptibility to these substances is primarily quantitative,
the degree of susceptibility depending upon the rate or
intensity of metabolism or of certain fundamental
metaboHc reactions. In aqueous concentrations of a
given reagent which kill within a few hours, the sus-
ceptibihty varies directly with the general metabolic
rate; the higher the rate of metaboHc activity, the
sooner does death occur. In very low concentrations,
however, to which the organism is able to acclimate or
accustom itself to some extent, we find the relation
reversed. The higher the metabolic rate, the greater the
degree of acclimation and therefore the less the sus-
ceptibiHty and the later the occurrence of death. These
two methods of comparing susceptibiHties I have called
the direct and the acclimation method.

The question how these various substances act upon
the living organism, whether they enter directly into the
chemical reactions or whether they change the physical
condition of the protoplasm or certain of its constituents
in such a way that the reactions cannot continue, has
long been and is still the subject of discussion, but cannot
be considered here. Whatever the nature of their
action, there can be no doubt concerning the general
relation between susceptibility to them and metaboHc
condition, although under certain conditions the rela-
tion may be masked or altered by certain incidental
factors.

For the direct form of the method, which is the
simplest and most widely appHcable, the procedure con-
sists in the immersion of the animals to be examined,
either singly or in lots, in a concentration of cyanide

52 INDIVIDUALITY IN ORGANISMS

or other reagent used, which has been previously deter-
mined as a concentration which will kill the animals in
the course of a few hours under the given conditions
of temperature, etc. In many of the lower animals
death is followed at once or in a few moments by a
visible disintegration and complete loss of structure
and form of the part concerned, and in such cases the
progress of death can be directly observed. In other
cases other means of determining the death-point may
be employed or the animals may be removed from the
reagent at definite intervals and the progress of death,
and so the susceptibility, determined by observing
whether and to what extent recovery occurs in each
case. When the method is used in this way regions of
high metabolic rate die earlier than those of low rate.

In the indirect or acclimation form of the method we
find that the degree of acclimation varies with meta-
bolic rate. With this form of the method regions of
high metabolic rate are least susceptible in the long run
because they become acclimated more readily, while
regions of lower metabolic rate undergo less accHmation
and so are inhibited to a greater degree and may even
die. The susceptibility gradients observed with these
two modifications of the method are themselves opposite
in direction, but are different expressions of the same
metabolic gradient.^

Several species of the flatworm Planaria constituted
the material for my first observations on susceptibility
gradients. The results obtained were so definite and

^ For more extended discussions of this method see Child, Senes-
cence and Rejuvenescence, 1915, chap, iii; also "Studies on the Dynamics
of Morphogenesis and Inheritance in Experimental Reproduction, V,"
Jaur. of Exp. ZooL, XIV, 1913.

METABOLIC GRADIENTS 53

striking in character that the desirability of comparative
study of different forms at once became evident. Up
to the present time some fifty species of animals from
various groups have been examined by means of the
susceptibility method, either in the adult or embryonic
stages or in both, in the attempt to determine to what
extent regional differences or gradients in metabolic
condition with respect to the axial or any other directions
in the body are characteristic features of the animal
organism/

In each form examined a more or less distinct and
regular gradient in susceptibility has been observed in
the direction of the major axis of the body and in many
cases gradients in the direction of the minor axes and of
the axes of various organs and parts as well.''

^ The forms examined include twelve species of ciliate infusoria
among the protozoa, the post-embryonic or adult stages of the fresh-
water hydra, and three species of hydroids among coelentrates; one
ctenophore, eleven species of turbellaria, and certain larval stages of one
trematode among the flatworms. Dr. L, H. Hyman, working under my
direction, has examined in the same way nine species of oligochete anne-
lids and one polychete. Susceptibility studies have been made upon the
eggs and embryonic or larval stages of the following forms: starfish,
sea-urchin, the polychete annelids Nereis, Chaetopterus, Arenicola,
Hydroides among the invertebrates, and two species of fishes and the
salamander and frog among the vertebrates.

2 The data concerning susceptibility gradients, so far as they have
been published, will be found in the following papers: Child, "Studies
on the Dynamics of Morphogenesis and Inheritance in Experimental
Reproduction, I-V, VII, VIII," Jour, of Exp. ZooL, X, XI, XIII, XIV,
XVI, XVII, 1911-14; ''Studies, etc., VI," Archiv fur Entwickelungs-
mechanik, XXXVII, 1913; "Certain Dynamic Factors in Experimental
Reproduction and Their Significance for the Problems of Reproduction
and Development," Archiv fiir Entivickelimgsmechanik, XXXV, 1913;
" Susceptibility Gradients in Animals," Science, XXXIX, No. 993, 1914;
"The Axial Gradient in Ciliate Infusoria," Biol. Bull., XXVI, 1914;
"Axial Gradients in the Early Development of the Starfish," Amer.
Jour, of Physiol., XXXWU,igis.

54 INDIVIDUALITY IN ORGANISMS

In organisms or parts with a radial structure gradients
in susceptibility may commonly appear in the direction
of the radial axis, and in those animals and developmental
stages where the outer body surface consists of active
living cells and is not covered by a heavy cuticle or
exoskeleton a susceptibility gradient from the surface
inward has been frequently observed.

In the simpler multicellular animals and in those
unicellular organisms which possess definite permanent
axes, the susceptibiHty gradients along the main body
axes often persist from the beginning of development
throughout life without essential change, but in many
cases they undergo various changes during the course
of development : they may disappear and new gradients
arise with advancing differentiation and the appearance
of new organs, or they may undergo reversal in direction
in some or most of the tissues of the body. In all cases,
however, so far as observed, such changes occur in a
definite and orderly way, so that the relation between the
original and the final condition is essentially constant
and characteristic for a given species. In spite of the
developmental alterations, it is true, as far as observa-
tions go at present, that for each of the main axes of the
body a definite susceptibiHty gradient exists, at least
during the earlier stages following the appearance of the
axis, and a definite relation exists between the direction
of the gradient from high to low susceptibiHty along a
given axis and the course of development and differen-
tiation and the functional correlation of different parts
with reference to the same axis.

The following figures will serve to show something
of the definiteness of the gradient along the apico-basal

METABOLIC GRADIENTS

55

axis in single cells. Figs. 3-7 show the course^oLdeath
and disintegration along the axis in Stentor coeruleus,
one of the common infusoria. Fig. 3 represents the
normal animal in extended condition, showing the

Figs. 3-7. — Axial susceptibility gradient of Stentor in cyanide:
Fig. 3, intact animal; Fig. 4, beginning of disintegration; Figs. 5-7*
successive stages of disintegration.

flattened peristome at the free apical end with its spiral
of large cilia, the shorter ciUa over other parts, the longi-
tudinal striations or fibrillae, and the elongated basal
region with organ of attachment.

56 INDIVIDUALITY IN ORGANISMS

In cyanide the body undergoes some contraction,
death begins at the apical end (Fig. 4) and is accom-
panied by the instantaneous loss of all movement and
disintegration of structure in the part concerned, and the
protoplasm swells and spreads out in the water, as indi-
cated by the dotted outline in Fig. 4. Other parts
remain intact and the cilia continue to vibrate. From
the apical region death and disintegration proceed along
the body as shown in Figs. 5-7, the line of demarkation
between the dead and disintegrated and the living
portions remaining distinct at all times until the progress
of death ends at the basal end of the body. The rate
of progress of death over the whole body may vary from
a few seconds to five or ten minutes, according to con-
centration of cyanide used, temperature, and other con-
ditions. Deviations from this course are very rare and
are probably the result of local stimulations of one part
or another of the body.

In Fig. 8 the beginning of death and disintegration
in the unfertilized starfish egg is shown. The region of
the egg where disintegration begins is that region where
the nucleus lies nearest the surface. When the egg
develops this region gives rise to the apical end of the
embryo and larva. From this region disintegration
proceeds through the egg along the axis determined by
the eccentric position of the nucleus (Fig. 9), and this
axis corresponds with the major axis of the embryo and
larva. The same susceptibiHty gradient also appears
in embryonic and early larval stages. In these cases the
death gradient does not indicate the presence of more
than one axis. In many forms other axes are also
indicated by the course of death. In the embryo of the

METABOLIC GRADIENTS 57

frog, for example, which is bilaterally symmetrical and
in which three axes, the major or longitudinal axis and the
minor transverse and dorso-ventral axes, are distinguish-
able in the arrangement of parts, disintegration begins
first of all at the anterior end and proceeds posteriorly,
and at any level of the body it begins in the median
dorsal region and proceeds laterally and ventrally.

The susceptibility gradients in particular organs or
parts of the body also show a relation to the axes of these
parts. In the elongated tentacles of hydra and various

Figs. 8, 9.— ^Axial susceptibility gradient of starfish egg in cyanide

sea-anemones, for example, death begins at the tip and
proceeds toward the base, and in nerves, so far as exam-
ined, a susceptibility gradient exists and death proceeds
in the direction of conduction.

Many other examples might be cited to show the
relation between the progress of death over the body
and the axes with reference to which an order in the
course of development, the arrangement of parts, or the
behavior of the organism can be distinguished. For the
present, however, it must suffice to say that the results

58 INDIVIDUALITY IN ORGANISMS

of experimentation along this line have demonstrated
beyond a doubt the existence of such gradients as a
general feature of the constitution of the animal body.

Such susceptibiHty gradients may be demonstrated,
not only by the course of death over the body, but by
the different degrees of retardation or inhibition of
growth and development at different levels under the
same experimental conditions. I have described such
retardation or inhibition gradients as observed in the
flatworm Planaria,^ and in the development of the sea-
urchin I have found it possible to alter and control to
a high degree the form and proportions of the larva
through the differences in susceptibility along the axes
to various reagents. Such gradients are also very
clearly evident in many cases described by various
authors of the effect of external conditions of various
kinds on development. The abnormal forms produced
in such experiments almost invariably indicate the exist-
ence of axial differences in susceptibiUty. The gradient
which appears in such cases is usually the acclima-
tion gradient, the regions of highest metabolic rate
being least susceptible and so least affected, but if
the external factor acts with sufficient intensity or if
acclimation does not occur, the differences in suscepti-
bility are parallel with the metaboHc gradient itself. In
the embryo of the frog, which has been much used for
experiments of this sort, various experimental conditions
may retard or inhibit developmental processes in the

' Child, "Studies on the Dynamics of Morphogenesis and Inherit-
ance in Experimental Reproduction, IV^, Certain Dynamic Factors in
the Regulatory Morphogenesis of Planaria dorotocephala in Relation
to the Axial Gradient," Jour, of Exp. ZooL, XIII, 191 2.

METABOLIC GRADIENTS 59

posterior region of the body while in the anterior region
development proceeds more or less normally. In such
cases the posterior regions, which possess a lower meta-
bolic rate than anterior regions, do not acclimate to the
conditions as readily as the latter and are therefore
retarded or inhibited to a greater extent in their develop-
ment. Such embryos produce certain characteristic
forms of monsters, more or less completely normal
anteriorly and increasingly abnormal in the posterior
direction. Where acclimation does not play a part the
anterior regions of the embryo may be most, the posterior
least, affected and another type of monsters results. In
many of these monstrous forms the symmetry gradients
as well as the major gradient appear more or less clearly.

In fact the field of teratogeny, the experimental pro-
duction of monstrous or abnormal forms, contains a
large amount of evidence for the existence of suscepti-
bihty gradients, but neither the relation between sus-
ceptibility and metabolic rate nor the existence of the
metabolic gradients has been recognized by the investi-
gators in this field. There is no doubt that further
experiments directly concerned with the problem of
susceptibility and metaboHc gradients will afford even
more definite and positive results.

These gradients in susceptibihty indicate the ex-
istence in the animal organism of more or less definite
metabolic gradients essentially quantitative in nature.
In other words, we find a definite order in the gradation
of rate or intensity of general metabolic activity in
directions coinciding with those in which an orderly
sequence of events and arrangement of parts or an
orderly behavior of the organism in other respects are

6o INDIVIDUALITY IN ORGANISMS

distinguishable. Alteration or even reversal of certain
gradients during development in some cases makes it
necessary to distinguish between the primary gradients,
existing at the beginning or in the early stages of devel-
opment, and the secondary gradients, which arise by
alteration of the primary.

The primary relations between the most conspicuous
metabolic gradients and the chief axes of the individual
is briefly as follows. The major axis is represented by a
gradient in which the apical region is always primarily
the region of highest, and the basal, that of the lowest,
rate of reaction. Stated in different terms, the region
of highest metabolic rate in this gradient always gives
rise in development to the apical region or head of the
animal, the region of lowest rate to the basal or posterior
end. In radial gradients the region of highest rate may
be either peripheral or central according to the character
of the radius. In bilaterally symmetrical animals the
relations differ in different cases. In at least most
bilaterally symmetrical invertebrates the median ventral
region is primarily the apical region of the minor body
axes, and from this region gradients of decreasing rate
extend laterally and. dorsally . In the vertebrates, on the
other hand, the median dorsal region is primarily the
apical region, and gradients of decreasing rate extend
laterally and centrally. The fact must be emphasized
that these are the general and primary relations and that
they may be altered in various, but always orderly and
definite, ways during the development of the individual.

These facts indicate very clearly that the chief axes
of the animal body are represented dynamically by
metabolic gradients and that each organ or part arises

METABOLIC GRADIENTS 6l

in a relation to one or more of these gradients which is
definite and characteristic for each kind of organism.
The relation of the central nervous system to these
gradients is highly significant. The apical portion of
the central nervous system, the cephalic ganglion or
brain, always arises in the region of highest metabolic
rate in the whole body, the apical region of the major
axis, and such portions of the central nervous system
as appear in other parts of the body, e.g., the longitudinal
ganglionic nerve cords of various invertebrates and
the spinal cord of vertebrates, always arise in the regions
of highest rate in the minor axial gradients. In the
bilateral invertebrates this is the median ventral, in
the vertebrates the median dorsal, region. In short,
it may be said that where a central nervous system is
present it is the organ characteristic of the apical, i.e.,
the dominant, region in each of the chief axial metabohc
gradients. The functional dominance of the central
nervous system in the later life of the animal is then
simply a more highly specialized expression of the
primary relation of dominance and subordination
existing at the beginning of individuation between
regions of high and those of lower metabohc rate.

As regards plants, I have as yet examined only some
fifteen species of marine algae, but in all of these the
apical region of each axis shows the highest suscepti-
bility to the higher concentrations of cyanides and the
susceptibility decreases very markedly in the basal direc-
tion. In these plants there is no such disintegration at
death as in the lower animals, although in the more
transparent forms the breaking up and coagulation of
the protoplasm can be observed inside the cell. By first

62 INDIVIDUALITY IN ORGANISMS

staining the plants with neutral red and then killing
with cyanide or some other reagent the susceptibility
gradient can be made visible, for as the cells die the
red of the stain at first becomes deeper because of in-
creasing acidity, then changes to yellow as the alkali of
the solution enters, and finally all color disappears.

FURTHER PHYSIOLOGICAL EVIDENCE FOR THE EXISTENCE
OF METABOLIC GRADIENTS

The susceptibiHty gradients do not constitute the
only experimental evidence for the existence of meta-
bolic gradients in the organism. Estimations of carbon-
dioxide production by means of the Tashiro biometer,^
which were made by Dr. Tashiro at my request, have
confirmed the results obtained by the susceptibiHty
method in all cases subjected to this test. The gradient
in carbon-dioxide production is similar to the gradient
in metabolic rate indicated by the differences in sus-
ceptibiHty. On the other hand, in the case of certain
nerves I have been able to confirm Tashiro 's recent
discovery of a gradient in carbon-dioxide production in
the direction of conduction of the impulse along the
fiber by the demonstration of a gradient in susceptibiHty
in the same direction, and have found a similar sus-
ceptibility gradient in certain other nerves for which
carbon-dioxide production has not been determined.
The gradient in the production of carbon dioxide indi-
cates the existence of a gradient in the rate or intensity
of the respiratory processes, the oxidations, the region

^ Tashiro, "A New Method and Apparatus for the Estimation of
Exceedingly Minute Quantities of Carbon Dioxide," Am. Jour, of
Physiol., XXXII, 1913.

METABOLIC GRADIENTS 63

of highest carbon-dioxide production being the region of
highest respiratory rate. Since the oxidations are un-
questionably reactions of fundamental importance in
the metabolic reaction system, the estimations of
carbon-dioxide production lead to the same conclusions
concerning the existence of metabolic gradients as do
the results obtained by the susceptibility method.

So far as technical and other sources of error can be
eliminated, the rate of oxygen consumption of different
parts of the body may be used like the rate of carbon-
dioxide production as a measure of respiratory activity.
The use of this method in animal physiology has been
such that the data, while of great value for various other
purposes, have in most cases no bearing upon the problem
of metabolic gradients. In the plants, however, the
rate of both oxygen consumption and carbon-dioxide
production have been found to differ in different parts
in such a way as to indicate very clearly the existence in
the plant-body of metabolic gradients. The growing
bud, for example, respires at a higher rate than the
full-grown stem or leaf.

Differences in electrical potential indicating differ-
ences of some kind in chemical or physical activity are
known to occur very generally in different parts of
both animal and plant organisms and even in different
parts of the same organ or cell. The presence of these
electrical differences gives no clue to the exact nature
of the physical or chemical differences which produce
them, but it is becoming more and more evident that in
both animals and plants they are to a large extent
associated with differences in metabolic activity. So
far as this is the case, we should expect in general that

64 INDIVIDUALITY IN ORGANISMS

parts with a higher respiratory rate would appear by the
usual methods as electro-negative to regions of lower
rate.

Some twelve years ago Mathews' observed a differ-^
ence in electrical potential along the main axis of certain
simple animals, the hydroids, the parts nearer the apical
end being electro-negative to those nearer the basal end.
In these forms the susceptibiHty method indicates that
the metabolic rate decreases from the apical toward the
basal end; that is, in the same direction as the decrease
in electro-negativity. Probably a similar electrical
gradient exists in nerves, although in the nerves of the
higher animals the change is undoubtedly very slight
within the physiological limits of length. As regards
the plants also various data on the differences of electric
potential suggest the existence of metabolic gradients,
although the fact that the observations were made with
other objects in view often leaves the evidence incon-
clusive as regards the matter of gradients.

In the early stages of development of the starfish I
have been able to make the axial metabolic gradient
directly visible to the eye by differential staining in the
living animal,'' the stain in this case consisting of a
colored precipitate formed within the cells by the oxida-
tion of certain substances added to the water. The rate
of formation of this precipitate in different cells differs
with the amount or activity of enzymes or other con-
ditions which influence the rate of oxidation. In those

^ A. P. Mathews, "Electrical Polarity in the Hydroids," Am.
Jour, of Physiol., VIII, 1903.

* Child, "Axial Gradients in the Early Development of the Starfish,"
Am. Jour, of Physiol, XXXVII, 1915.

METABOLIC GRADIENTS 65

cells where the rate of oxidation is highest the precipitate
is formed most rapidly and vice versa. In the starfish
embryos and early larvae the precipitate appears first in
the cells of the apical region, and a very definite color
gradient along the main axis arises in living animals,
while in animals which have been killed before staining no
gradient appears. This method is undoubtedly capable
of wide application.

These various methods and results indicate the
possibilities of demonstrating the existence of the meta-
bolic gradients in organisms by biochemical and physio-
logical methods. Unquestionably future investigation
will give us much more accurate and extensive data than
we possess at present.

EMBRYOLOGICAL EVIDENCE FOR THE EXISTENCE OF
AXIAL METABOLIC GRADIENTS

Gradients in rate of cell division, size of cells, con-
dition or amount of protoplasm in the cells, rate of
growth, and rate and sequence of differentiation are
very characteristic features of both animal and plant
development. Such gradients are definitely related to
the axes of the individual or its parts, and are evidently
expressions of axial metabolic gradients. While the
existence of such gradients indicates the existence of
gradients in activity of some sort, the various kinds
of gradients are not all necessarily present where meta-
bolic gradients exist. In some cases the visible gradient
may be a gradient in rate of growth or in protoplasmic
constitution; in still others a gradient in sequence of
differentiation, etc., and sometimes metabohc gradients
exist without any structural indications of their presence.

66 INDIVIDUALITY IN ORGANISMS

At best these various kinds of gradients are merely
general indications of differences in metabolic rate,
and undoubtedly in many cases the visible differences
along an axis represent something more than differences
in metabolic rate. The important point is that visible
indications of graded differences in metabolic rate occur
so generally in definite relations to the chief axes of the
body.

In the animal egg a gradient in the distribution of
the yolk is often visible before development begins, and
in such cases that part of the egg which gives rise to the
apical region of the embryo contains less yolk than the
basal region. Associated with this gradient in most
cases we find differences in the size of cells appearing
in very early embryonic stages. In the egg of the frog,
which is an excellent example of this sort of egg, the
yolk gradient is very distinct, and the early develop-
mental stages show a gradient in the same direction in
the rate of cell division and the size of the cells formed
(Figs. lo, ii). The yolk gradient and the associated
gradient in cell division differ widely in different kinds of
eggs: in some cases only the apical region of the egg
divides at all, other parts serving as a source of nutrition
which is gradually used up during development. At
the other extreme are cases in which no yolk gradient is
distinguishable and differences in division rate and size
of cells do not become evident until later stages.

In all cases developmental gradients of some sort
appear sooner or later as expressions of the metabolic
axial gradients and usually become more distinct as
morphological development proceeds. The so-called
law of antero-posterior development is a partial recogni-

METABOLIC GRADIENTS

67

tion of this fact. This *'law" is merely a statement
of the observed fact that in the development of the
animal from the egg organs first become morphologically
visible in that region which becomes the anterior or
apical end, and from this region morphogenesis pro-
ceeds posteriorly or basally in a regular, orderly manner.
In short, a gradient in morphogenesis exists along the
major axis of the body, the apical end preceding. In
addition to this major gradient more or less definite
morphogenic gradients appear in relation not only to

Figs. 10, 11. — Two stages of cleavage of frog's egg, showing axial
gradient in cell size resulting from gradient in rate of division.

the minor axes of the whole body, but also in relation to
the axes of particular organs or parts. In fact the law
of antero-posterior development is merely a statement
for the major axis of the more general law of axial
developmental gradients.

Embryonic stages of a flatworm among the inverte-
brates and the chick among the vertebrates will serve
to show these developmental gradients. Fig. 12 is a
diagrammatic outline of the adult stage of a small
bilaterally symmetrical flatworm, showing ''brain,"

68

INDIVIDUALITY IN ORGANISMS

pharnyx, and alimentary tract; Fig. 13 is a longitudinal
section, almost in the median plane, of an embryo of the
same species. The anterior end is toward the left.

«*

^-.i^

13

Figs. 12, 13. — Axial developmental gradients in flatworm, Plagio-
stotnum girardi: Fig. 12, outline of adult worm, showing eyes, cephalic
ganglia, pharynx, and alimentary tract (after von Graff); Fig. 13,
longitudinal section near median plane of embryo, head at left, showing
the apico-basal or longitudinal and ventro-dorsal gradients in rate of
development (from Bresslau).

The organs of the anterior end, the brain and pharnyx,
consist of numerous cells, and the morphological arrange-
ment is already apparent, while the whole postpharyn-

METABOLIC GRADIENTS 69

geal region, which in the adult is by far the larger part of
the body, is very short and consists of but few cells.
This major gradient is very distinct, but the ventro-
dorsal gradient is also evident. The section shows
that multiplication of cells and structural development
are proceeding chiefly in the ventral region, while the
dorsal region consists of relatively few cells. Examina-
tion of transverse sections of embryos would show the
transverse gradients: we should find that the develop-
ment was proceeding more rapidly in the median ventral
region than in the lateral regions. The transverse
and the ventro-dorsal gradients are in reality different
components of the same gradient. The fact is that a
developmental gradient extends laterally and dorsally
from the median ventral region. In such a bilaterally
symmetrical animal there are then two chief develop-
mental gradients, a major, from the anterior region
posteriorly, and a minor, from the median ventral, or in
some cases most of the ventral region, laterally and
dorsally. In other bilaterally symmetrical inverte-
brates relations are in general similar. In Fig. 2 (p. 38)
the relations in a simple case of this sort are diagram-
matically indicated.

In the vertebrates the longitudinal gradient is
similar to that in the invertebrates, but instead of a
ventro-latero-dorsal gradient, as in the invertebrates,
the gradient is dorso-latero- ventral in direction. Fig. 14
represents an early stage of the chick embryo in which
the head is just becoming morphologically distinct, but
other organs are not yet formed, while in Fig. 15, a
later embryonic stage, the head region is advanced
in development, and differentiation of the body is

70

INDIVIDUALITY IN ORGANISMS

progressing posteriorly, the successive formation of the
somites or segments being a conspicuous feature of this
progress. Fig. i6 is a transverse section of an early

Figs. 14, 15. — Surface views of two early stages in embryonic
development of chick, showing progress of development in basal direction
from the head-region (upper end) and laterally from the median region;
s, somites (from F. R. Lillie).

stage before distinct organs have begun to form. At
this time cells are separating from the outer layer of
the body in what will later become the median dorsal

METABOLIC GRADIENTS

71

region, and passing inward to form the mesoderm.
Most of the region of the embryo behind the head in

(^1

Figs. 16, 17. — Transverse sections of chick embryo at different
levels, to show developmental gradients.

Fig. 14 and the extreme posterior region of the embryo
in Fig. 15 are at about this stage of development.

72 INDIVIDUALITY IN ORGANISMS

Fig. 17 is a transverse section at a stage of development
corresponding to that attained at the level of the sixth
somite of the embryo in Fig. 15. At this stage the
embryonic nervous system is present in the form of a
tube open dorsally, and differentiation has progressed
both laterally and ventrally from the median dorsal
region. In the other vertebrates, including the mam-
mals, the developmental gradients are similar.

Differences in rate of growth constitute another
feature of these developmental gradients, but the rela-
tion between the axial metaboUc gradient and rate of
growth is not simple, for the period of highest growth
rate occurs at different times in different parts accord-
ing to the time of their formation, and it may happen
at certain stages of development that the rate of growth
at the apical end of a metaboHc gradient is lower than at
the basal, because the region at the apical end began
its growth first, has grown at a more rapid rate, and is
therefore completing its growth earlier than the region
at the basal end. Nevertheless, so far as it is possible
to compare corresponding stages in the development of
different parts, along an axial gradient, differences in
rate of growth corresponding to the gradient do appear.
The head-region, for example, at the stage of highest
growth rate grows more rapidly than the posterior
region of the body at its stage of highest rate, and
similar relations exist with reference to other gradients.

In the egg of the plant as well as in that of the animal
developmental gradients usually appear in early stages.
In the eggs of many of the lower plants the first division
is transverse, the two cells thus formed representing
apical and basal regions of the plant, and in most of the

METABOLIC GRADIENTS 73

plant groups a more or less definite relation exists
between the directions of the early divisions and the
major axis of the embryo. In these cases a more or less
distinct gradient in division rate, cell size, and cellular
constitution usually appears either at the beginning of
development or in early stages. On the one hand, this
gradient shows a definite relation to the position of the
egg with respect to surrounding parts of the parent or-
ganism, and, on the other, the region of smallest size and
most rapid division of the cells and most abundant and
deeply staining protoplasm is the region of highest rate
of reaction and becomes the apical region of the embryo.
Fig. 1 8 shows this gradient in the embryo of a moss, the
uppermost cell in the figure representing the apical region
of the embryo.

In most of the higher plants only a portion of the egg
takes part in the formation of the embryo, the remain-
der forming a suspensor, a stalk on which the embryo
is carried. Fig. 19 shows the cellular gradient in the
early developmental stage known as the proembryo
of Ginkgo, a gymnosperm related to the conifers. The
embryo proper arises later from the small-celled tissue
in the lower part of the developing egg. Some of the
cycads also show a very definite gradient of this sort.
In the angiosperms, the higher seed plants, where the
egg is attached to the wall of the embryo-sac, the embryo
arises from its free apical end.

A characteristic feature of the plant individual in all
except the simplest forms is the growing or vegetative
tip. This growing tip is the region of most active
nuclear division and growth and with rare exceptions
forms the free end of the individual and gives rise to

74

INDIVIDUALITY IN ORGANISMS

other parts of the plant body. In the complex higher
plant, stems, branches, buds, roots, and various other
parts possess a growing tip, at least during earlier stages,
and each such part is to a certain extent an individual.

Figs. i8, 19. — Axial developmental gradients in embryonic stages
of plants: Fig. 18, embryo of moss, apical cell at upper end (from prepara-
tion loaned by W. J. G. Land); Fig. 19, proembryo of gymnosperm
{Ginkgo); apical region of plant arises from lower end (from Lyon).

In most of the lower plants a single cell forms the apex
or center of the growing tip, and it may be larger than
other cells \Yith a gradient of decreasing size extending
from it, as in the stem of the alga in Fig. 20, but during
the course of plant evolution the apical cell gradually

METABOLIC GRADIENTS

75

gives place to an apical region, consisting of several or
many cells, and in the course of this change the apical
cell itself becomes relatively smaller, and a gradient of
increasing size extends from the apical region (Figs. i8,
36). The gradients in size in different forms depend

Fig, 20. — Axial gradient in cell size in alga Cladostephiis (from
Pringsheim) .

upon the relation between frequency of division and
growth in size of the apical cell, and this relation shows a
characteristic range in each form. Even where the
whole plant body is a single multinucleate cell, the
apical regions of stem and branches are undoubtedly
physiologically growing tips. In the higher plants the

76

INDIVIDUALITY IN ORGANISMS

growing tip consists of several or many cells. Figs. 21
and 22, longitudinal sections through the growing tips of
a stem and a root respectively, show the gradients in cell

Figs. 21, 22. — Axial developmental gradients in growing tips of
seed plants: Fig. 21, stem-tip of Hippuris; Fig. 22, root-tip of Trades-
cantia (from preparations loaned by Department of Botany, University
of Chicago).

size and protoplasmic condition which extend from the
growing tips. In the stem-tip these gradients extend to
a much greater distance than in the root- tip and Fig. 21
shows only a fraction of them.

METABOLIC GRADIENTS 77

In the development of the plant the growing tip is
the first part of the individual to become distinguishable,
and from it other parts arise. In the moss embryo in
Fig. 18 the growing tip is already present as the upper-
most cell and other cells have arisen from it in an orderly
way. In the higher plants the growing tip is not usually
localized until later stages. In Gingko, for example, the
growing tip of the plant is not yet distinguishable at
the stage of Fig. 19, although the small-celled region is
the growing tip of the whole proembryo and in this the
growing tip of the plant-stem later appears. In certain
algae the major axial gradient in the egg is apparently
determined by external factors, such as light, but in
most plants this gradient is determined by the relation of
the egg to the parent body, the growing tip of the plant-
stem arising from the apical region of this gradient.

The vegetative stages of certain liverworts and the
sexual generation of various ferns show a high degree of
bilateral symmetry and often consist, at least during the
earlier stages of their growth, of single elongated flattened
individuals (Figs. 23, 24) with a growing tip, a, at one
end, often with a thickened longitudinal midrib and with
root-like outgrowths on the ventral surface, the surface
facing the substratum as the plant grows. In many
cases these individuals undergo division by branching or
by the formation of buds on the surface in later stages.
In these plants, as in bilaterally symmetrical animals,
three axes — longitudinal, transverse, and dorso-ventral —
are distinguishable; in other words, order is apparent in
three directions. Various indications of gradients in
activity appear in the same directions. As regards the
major axis, the rate of cell division and growth is highest

78

INDIVIDUALITY IN ORGANISMS

in the apical region and decreases basally; as the plant
grows older, death may even begin at the basal end and
proceed apically while the apical end is still growing
actively. Evidences of a transverse gradient in activity
appear in a decrease in growth toward the lateral margins
and in many forms in a decrease in thickness of the
body in the same direction. In the direction of the

Figs. 23, 24. — Bilaterally symmetrical prothallia of liverwort,
Marchantia (dorsal view), and a fern (ventral view).

dorso-ventral axis which is determined by the action of
light and perhaps other external factors, the differences
in metabolic activity are indicated by the outgrowth of
root-like structures and the sexual organs, and in some
forms of scales or leaf-like structures on the ventral
surface, and also in some forms by the greater density
of cellular structure in the ventral region.

METABOLIC GRADIENTS 79

DEVELOPMENTAL GRADIENTS IN AGAMIC AND
EXPERIMENTAL REPRODUCTION

Among the lower animals and most plants new indi-
viduals arise, not only by the process of gametic or sexual
reproduction, but by various agamic or asexual pro-
cesses, such as division, budding, etc. These processes
vary greatly in different forms and even in the same
individual under different conditions, but their essential
feature is the formation of a new individual from a part
of a pre-existing individual, a process which usually in-
volves more or less dedifferentiation and redifferentiation
in a new direction. Although these agamic reproductive
processes differ more or less widely from embryonic
development, the metabolic gradients characteristic
of the individual either persist from the original indi-
vidual or arise anew in each case, and developmental
gradients of some sort appear in relation to them.

In the formation among animals of new individuals
by budding, as, for example, in the hydroid, Pennaria
(Figs. 25-27), the hydranth becomes distinguishable
first, the stem later, and closer examination shows that
apical regions of the hydranth are somewhat in advance
of basal. In Figs. 26 and 27, for example, the apical
tentacles are more advanced in development than the
basal.

In the flatworm, Sknostomum, division occurs after
the body attains a certain length, the first visible indica-
tion of the new individual being the appearance of a new
head-region (Fig. 28) at a certain distance from the
original head. This new head-region acquires control
of parts posterior to it and finally separates as a new
animal. By continued division before separation of

8o

INDIVIDUALITY IH ORGANISMS

each new individual thus formed chains of from eight
to sixteen individuals or zooids, as they are usually
called, in various stages of development may result

Figs. 25-27. — Pennaria tiarella: Fig. 25, h, h', h", Figs. 26 and
27, stages of development of hydranth; m, medusa bud.

(Fig. 29). Many other cases of division among animals
are essentially similar.

In many of the lower animals agamic reproduction
can be induced experimentally by isolating pieces. In

METABOLIC GRADIENTS

8l

/^

A

H

the flatworm Planaria (Fig. 30) a piece such as a or &,
or almost any other piece, cut from
the body will develop into a whole
animal of small size by the forma-
tion of a new head at one end and a
new tail at the other and a trans-
formation and redifferentiation of
the internal organs of the piece into
those of a whole animal as indi-
cated in Figs. 31-33. In the out-
growth of the new tissue at the
two cut surfaces the axial gradients
appear as gradients in rate of
growth. Fig. 31 shows that the
outgrowth of new tissue is more
rapid at the apical than at the
basal end of the piece and more
rapid in the median than in the
lateral region of each cut surface,
and Fig. 34, a side view of the
piece, shows more rapid outgrowth
at each end in the ventral than in
the dorsal region. In this case the
axial gradients in the piece persist
from the parent individual, and the
head arises at the apical end of
the piece, the tail at the basal end.
In other cases of experimental
reproduction from isolated pieces
the axial gradients appear either
in the same or in some other way
according to the kind of individual and the conditions.

Figs. 28, 29. — Asexual
reproduction in flat-
worm, Stenostomum:
Fig. 28, stage of two
zooids; Fig. 29, chain of
several zooids.

82

INDIVIDUALITY IN ORGANISMS

In agamic reproduction in plants each new individual
arises as a localized region of growth and the growing
tip is the first region to become clearly defined. New

v-^f

JO

A

33

34

Figs. 30-34. — Planar ia doroto-
cephala: Fig. 30, structure of ali-
mentary tract and arrangement of
central nervous system; a, h, two
regions indicating pieces for reconsti-
tution; Figs. 31-33, stages of reconsti-
tution; Fig. 34, side view of early
stage.

buds, new roots, and other parts arise in this way in
nature and under experimental conditions. The small
outgrowths along the sides of the growing stem-tip in

METABOLIC GRADIENTS 83

Fig. 21 (p. 76) are stages in the formation of leaves and
the developmental gradients appear to some extent in
them.

In many plants new ''adventitious" individuals
arise, either in nature or under experimental conditions,
from cells already differentiated as part of an individual.
In the liverwort, Metzgeria, new individuals may arise
either by division of the growing tip resulting in bifurca-
tion of the flat body, as shown in Fig. 35, a, a, or after
injury to, or removal of, the growing tip by a renewal of
division and growth in differentiated cells. Fig. 36
shows the cellular structure of the growing tip in a
well-developed individual and Fig. 37 the early stage
of a new individual formed from a differentiated
cell. In both figures the gradient in cell size is clearly
evident.

Among the higher seed plants, as well as among lower
forms, the ''adventitious" formation of new individuals
from differentiated cells occurs, as for example in the
begonias, where buds capable of producing new plants
arise under certain experimental and natural conditions
from the epidermal cells of leaves. The epidermal
cells which take part in the formation of such a bud
lose their differentiated, vacuolated condition, become
filled with protoplasm, like embryonic cells, and divide
rapidly. Fig. 38 is a surface view of the formation of
such a bud involving several epidermal cells, but centered
chiefly in parts of four cells, and Fig. 39 is a longitudinal
section through a bud formed from two cells. The
double contours in Fig. 38 show the thickened cellulose
walls of the original epidermal cells, the single contours
within them the cells formed by their repeated division,

84

INDIVIDUALITY IN ORGANISMS

and the shading indicates in a general way the disap-
pearance of the vacuoles and the filling of the cells with

a,

a

Figs. 35-37. — Metzgeria, a liverwort: Fig. 35, portion of pro-
thallium, showing midrib and apical regions, a, a; Fig. 36, cell structure
of growing tip, showing apical cell, a, and gradient in cell size; Fig. 37,
cell structure of an adventitious bud, showing apical cell, a, and gradient
in cell size (Figs. 36, and 37 from Goebel).

protoplasm. A gradient in cell size and protoplasmic
condition appears in both cases, in Fig. 38 from the
center to the periphery of the region involved and in

METABOLIC GRADIENTS

8S

Fig. 39 from the upper part at the free surface of the
leaf downward. These gradients are evidently the

Figs. 38-41. — Origin of adventitious buds in seed plants: Fig. 38,
surface view and Fig. 39, section of bud arising from differentiated epi-
dermal cells of leaf of Begonia (from Kegel); Figs. 40, 41, development
of bud in callus (from Simon).

visible expression of gradients in metabolic activity, the
smallest, most protoplasmic cells indicating the region of
most intense activity, and it is from this most active

86 INDIVIDUALITY IN ORGANISMS

region that the apical vegetative tip of the new plant
individual develops.

In many woody plants the cut end of a stem or
branch develops a mass of wound tissue, the caUus, and
in this callus new buds arise independently of other
parts of the plant and become connected with them
secondarily. In all such cases the differentiation of the
vascular bundles which connect the new buds with the
old parts proceeds from the buds. Fig. 40 shows an
early stage of bud-formation in the poplar at the periph-
ery of a mass of callus on the cut end of a stem, and
Fig. 41, a later stage in which vascular connection with
other parts has been estabHshed. In such cases the
appearance of the new bud is the first step in the forma-
tion of the new individual; it is followed by the appear-
ance of a gradient in growth and differentiation from the
bud toward other parts.

In isolated pieces of plants the formation of new
growing tips or the outgrowth of resting buds occurs
in certain more or less definite portions with relation
to the axes. The removal of the chief growing tip of a
stem results in outgrowth or altered growth of the
uppermost buds or branches. When these are removed
those lower down react, and so on. Evidently a gradient
in the capacity to respond or in the rate of response to
the altered conditions exists along the major axis, and
those buds or branches which react first dominate those
below them and prevent them from reacting in the
same way.

In isolated pieces of the bilaterally symmetrical
liverworts, such as Marchantia (Fig. 23, p. 78), the
position of the new buds evidently represents the region

METABOLIC GRADIENTS 87

of highest metabolic rate in the piece as a resultant
of the three axial gradients (see Figs. 99-102, p. 167),
and the formation of new individuals in these regions
inhibits their formation elsewhere, although practically
every cell of the plant-body is capable under proper
conditions of giving rise to a new individual.

CONCLUSION

All the various lines of evidence considered agree in
showing that axial gradients in the dynamic processes
are characteristic features of organisms and that a
definite relation exists in each individual between the
direction of the gradient in any axis and the physiological
and structural order which arises along that axis. In
the major axis the region of highest rate in the metabolic
gradient becomes the apical or anterior region of the
individual, and in the minor axes also the regions of
highest rate in the gradients represent particular features
of the order in each case. Along any axis particular
parts apparently represent particular levels in the
gradients. The variety, extent, and agreement of the
evidence is all the more interesting in view of the fact
that such gradients have not heretofore been recognized
as characteristic features of organic constitution.

CHAPTER IV

PHYSIOLOGICAL DOMINANCE IN THE PROCESS
OF INDIVIDUATION

According to the theory outhned in chap, ii, the
organic individual is fundamentally a dynamic relation
of dominance and subordination, associated with and
resulting from the establishment of a metabolic gradient
or gradients. In the present chapter some of the evi-
dence for the existence of dominance in the process of
individuation is considered.

This evidence is obtained primarily from the experi-
mental reproductions, because only here is it possible to
analyze and control the process of individuation to any
considerable degree. The egg is usually a more or less
highly specialized individual at the time embryonic
development begins, and the earlier stages of its individ-
uation commonly occur in such relations to the parent
body that they are not readily accessible to experimental
investigation. Nevertheless, the evidence indicates
very clearly that the process of organic individuation is
fundamentally the same in the egg and embryo and in
experimental reproduction.

The evidence presented here concerns primarily the
major axis, because the facts are simpler and more com-
plete with respect to this axis. Experimental isolation
of pieces with reference to the minor axes is usually
complicated by the presence of the major gradient, and
the order along the major axis is often such that parts
necessary for continued life are absent from various

PHYSIOLOGICAL DOMINANCE 89

regions of the minor axes. For these reasons the experi-
mental investigation of dominance and subordination
in relation to the minor axes is variously compHcated
and limited in different cases. Nevertheless, the funda-
mental similarity of the different directions of order in
the individual is indicated by various lines of evidence,
and there are no grounds for hesitation in extending to
the minor axes general conclusions reached concerning
the major axes.

THE EXPERIMENTAL MATERIAL

Reproduction can be induced experimentally in the
plants and many of the lower animals by the isolation
of pieces and in various other ways. These experimental
reproductions, when properly controlled and analyzed,
constitute invaluable material for study of the problem
of the individual, for it is often possible to increase or
decrease dominance and so to extend or decrease its
range, to alter the conductivity of protoplasm, to
determine the elimination of old and the establishment
of new metabolic gradients, and in these and other
ways to control the process of individuation to some
extent, and to determine the results of such control.

Most plants and many of the lower animals give
rise to new individuals by division, budding, and other
agamic processes, and the new individuals thus formed
often remain organically connected and give rise to a
composite individual, such as a tree among plants or a
hydroid colony among animals. In such reproductions
definite and orderly space or distance relations are
observable, which themselves suggest the existence of a
limited range of dominance, The occurrence of division

90

INDIVIDUALITY IN ORGANISMS

when a certain size or length is
attained, or the appearance of
buds at a certain distance from
the chief growing tip in plants, are
cases in point. In many cases
experimental control and alteration
of these relations throws a flood of
Hght upon the problem of their
nature. It is with material and
experiments of this sort that the
present chapter is largely
concerned.

I have shown elsewhere that
the process of progressive develop-
ment and differentiation in the
individual is accompanied by a
decrease in the metabohc rate
determined by the accumulation
of relatively inactive constituents
in the protoplasm. These changes,
which constitute
senescence, may
"^ eud in death if
they go far enough.
On the other hand,
any change which
brings about the
removal of such
previously accu-
mulated material
makes possible an
acceleration in metabolic rate, and such changes con-

FiGS. 42, 43. — Tuhularia: Fig. 42, a single
individual; Fig. 43, asexual reproduction
from tip of stolon.

PHYSIOLOGICAL DOMINANCE 91

stitute rejuvenescence. The facts indicate that all
reproductive processes bring about rejuvenescence to
some degree, and it is certain that the new indi-
viduals which arise by division or budding from other
individuals or from experimentally isolated pieces are
to some extent physiologically younger than the parent
individual from which they arose. ^ Rejuvenescence in
such cases results from the loss of the differentiation as
a part in that portion concerned in the reproductive
process, and with the new individuation a new process
of senescence begins.

Among the lower animals which have served as
material for the study of regeneration or regulation
two forms have been used to a large extent in my own
experiments and must be briefly described here. The
hydroid Tubularia in its simple unbranched form as
a single individual (Fig. 42) consists of hydranth, stem,
and stolon, the hydranth forming the apical end of the
stem and bearing two sets of tentacles, reproductive
organs between them, and a mouth at its apical end.
The stem grows vertically from the surface of attach-
ment, and the stolon adheres to the surface, forming an
organ of attachment, and elongates by growth at its
tip. Stem and stolon are covered by a horny cuticle,
the perisarc. The apical end of the metabolic gradient
of the major axis is the apical region of the hydranth, and
from this region the rate decreases basally through the
hydranth. In the stem the metabolic rate is lower than
in the hydranth, and there is a slight decrease in rate in
the basal direction, but at the growing tip of the stolon
there is a short, slight gradient in the opposite direction,

^ Child, Senescence and Rejuvenescence, 1915.

92 INDIVIDUALITY IN ORGANISMS

The primary form of asexual reproduction in Tubu-
laria is represented in Fig. 43. When the stem and
stolon together attain a certain length, which varies with
the metabolic condition of the animal but under favor-
able conditions may be five to eight centimeters, the
stolon turns away from the substratum and gives rise to a
hydranth; then a stem forms and elongates below this
hydranth, and a new stolon arises from the base of this
stem. This process of reproduction itself suggests that
the tip of the stolon is subordinate to the original
hydranth until it attains a certain distance from it and
then is able to produce a new hydranth, and experi-
ments show that this is true. If the original stem
elongates still further new hydranths may arise along
the stolon and at the base of the stem, as these regions
become physiologically isolated.

In Corymorpha, a form related to Tubularia, the
hydranth is much larger, the stem naked except near
the base and reaching a length of ten to twelve centi-
meters, and instead of a stolon the basal end is imbed-
ded in sand and bears delicate root-like outgrowths as
holdfasts (see Figs. 74, 78, pp. 143, 145).

Planaria dorotocephala (Fig. 30, p. 82), a ilatworm
and one of a number of species much used in experiment,
is a much more highly differentiated, bilaterally sym-
metrical form, with distinct head and ^' brain" and two
ventral nerve cords, and with definite, though rather
diffuse, alimentary and excretory organs. Sexual organs
appear in this form only under certain conditions. This,
as well as various other species of the group, undergoes
fission after it attains a certain variable size, the separa-
tion usually occurring at about the level ff in Fig. 44.

PHYSIOLOGICAL DOMINANCE

93

The separated posterior portion becomes a new animal
while the anterior portion develops
a new posterior end, and fission is
sooner or later repeated. There is
no morphological indication of a
second individual or zooid in the
posterior region of the body, but
one or more such individuals are
indicated by the metabolic gradient
of the major axis and by various
other physiological differences.
The apical region of this gradient
is the head of the animal, and from
the head the metabolic rate de-
creases to the level where separa-
tion occurs in fission; there a
sudden rise in rate occurs, and then
again a downward gradient toward
the posterior end. The region
where the rate rises suddenly
represents the apical end of the
second individual and the down-
ward gradient following is the
gradient of the major axis of this
zooid. In the shorter animals
only one of these zooids is present,
but as the length increases the
basal body region may show two,
three, or more of these distinct
gradients. Represented graphi-
cally the metabolic gradient in such
an animal is like the curve in Fig. 45 ; a is the head-region,

Fig, 44. — Planaria
dorotocephala , outline,
indicating several zooids
in basal region; ff, usual
level of fission.

94 INDIVIDUALITY IN ORGANISMS

the long slope the body of the anterior chief zooid, which
forms most of the body of the worm, h represents the
apical end of the second zooid, c that of a third, etc.
These zooids are the result of successive physiological
isolations of the basal region as the animal grows in
length. First a single zooid is formed at the basal end,
but the range of dominance is short in this undeveloped
individual, and as growth proceeds its basal
region soon becomes physiologically isolated,
and a second zooid arises, and so on. While
the degree of physiological isolation is not

Anterior Posterior

Fig. 45. — Graphic representation of major axial gradients in a

Planaria with several zooids: a, head of animal; h, c, apical regions
of secondary zooids.

sufficient to permit the development of the new indi-
vidual to proceed very far, some degree of rejuve-
nescence in the part does occur and its metabolic rate
rises slightly, and with each successive isolation there is
a further increase in rate, so that in each successive
zooid the gradient is at a level somewhat higher than
that of the preceding.

PHYSIOLOGICAL DOMINANCE

95

The act of fission in this animal consists of an inde-
pendent motor reaction of the posterior zooid or group.
When the worm is creeping quietly,
the posterior zooid or the zooid
group suddenly attaches itself to the
surface on which the animal is
creeping, while the whole anterior
individual endeavors to advance
and the body in front of the attached
region becomes greatly stretched
(Fig. 46) and finally ruptures. The
occurrence of fission can often be
controlled experimentally in a way
that shows the variable range of
dominance very clearly. If an ani-
mal is very slightly stimulated, e.g.,
by a shght jarring of the aquarium,
the posterior zooid will often attach
itself, and fission will occur, while
with stronger stimulation the animal
is able to control this region and it
does not become attached but ad-
vances with the rest of the body.
Evidently when the animal is only
moderately active the posterior re-
gion is physiologically isolated, but
when it is intensely active the range
of dominance of the anterior indi-
vidual extends to this posterior
region and determines its subordina-
tion in behavior. Similarly, in very old animals which
have been prevented from undergoing fission by keeping

Fig. 46. — Planaria
dorotocephala in the act
of fission.

g6 INDIVIDUALITY IN ORGANISMS

them on a layer of vaseline or other surface to which they
cannot attach themselves, the tissues are often so tough
that rupture does not readily occur, and the anterior
individual struggles more and more violently to free
itself from the hindrance which is preventing its advance.
In these animals such struggles often terminate in the
complete subordination of the posterior zooid: it is not
torn loose from its attachment, but lets go its hold
and no longer reacts independently. Later, when the
anterior individual has become more quiet, the same
procedure may occur again. Evidently as the activity
of the anterior individual increases the range of domi-
nance increases, and, if fission does not occur at once, the
posterior zooid may finally be brought under control.
Moreover, one of the simplest ways of inducing fission
in this species is to cut off the head of the anterior indi-
vidual. Such animals creep about even in the absence
of the head, but under these conditions the posterior
zooid is more completely physiologically isolated and
separation soon occurs if the tissues are not too tough. ^
Experiments to be described below will show other
ways in which the existence of dominance can be demon-
strated and its range varied and controlled in these and
other animals and in many plants.

THE INDEPENDENCE OF THE APICAL REGION

The apical region of the organic individual is, to a
large extent, independent and is capable of developing,

* For a more extended consideration of the process of fission and the
various indications of the presence of the posterior zooids see Child,
"Physiological Isolation of Parts and Fission in Planaria, "Archiv
filr Entwickelungsmechanik, XXX, 11. Teil, 1910; "Studies on the
Dynamics of Morphogenesis, etc., Ill," Jour, of Exper. ZooL, XI, 191 1;
"Studies, etc., VI," Archiv fur Entwickelungsmechanik y XXXV, 1913.

PHYSIOLOGICAL DOMINANCE

97

at least to an advanced stage, in the complete absence
of other parts of the body. This independence is very
evident in Tuhularia and Planaria. Pieces one or two

Fig. 47. — Reconstitution of single and biaxial apical structures from
short pieces of stem of Tuhularia, to illustrate independence of apical
region.

milHrneters in length cut from the stem of Tuhularia
usually develop into hydranths with a very short stem
or partial hydranths with more or less of the basal region
absent (Fig. 47). The result depends on the condition

98 INDIVIDUALITY IN ORGANISMS

of the animal, the length of the piece, and the level of
the stem from which it is taken. The shorter the piece
from a given level of the stem the more completely is its
development limited to apical parts, as Fig. 47 shows.
The shortest pieces give rise to nothing but the apical
ends of the hydranths, with mouths and the apical row
of tentacles. In no case do such pieces produce basal
parts of the hydranth without apical parts. Where
anything is missing it is always the more basal region,
either stem or more or less of the basal hydranth region.
The results in such pieces constitute, I believe, a demon-
stration that the apical region of the individual arises
first and other regions are determined later, as far as the
length of the piece permits.

The development of hydranths or apical portions of
hydranths may occur at one or both ends of such short
pieces as indicated in Fig. 47. This difference arises
according as the original metaboHc gradient in the stem
is more or less marked. In such short pieces of the
stem the difference in metabolic rate at the two ends of
the piece is but shght in any case. If, however, the rate
at the apical end of the piece is enough higher than that
at the basal end, development at the apical end pro-
ceeds more rapidly than at the basal end, the apical
end is dominant, and the piece produces a single hydranth
or part. But if the gradient is very sHght in the piece
the two ends react at the' same rate, and since the
presence of the wound at each end brings about an
increase in metabolic rate at each end, equal or nearly
equal gradients in opposite directions arise and hydranths
or apical parts arise at both ends with their axes opposed.
Often, even in such cases, the original gradient appears

PHYSIOLOGICAL DOMINANCE 99

in the smaller size or more incomplete condition of the
structure formed at the basal end of the piece. In Fig.
47 one case near the bottom of the figure is shown in
which one end is a hydranth with both sets of tentacles,
the other a partial hydranth with only the apical set and
the reproductive organs.^

In Planaria the development of short pieces is
essentially similar. Short pieces from various levels of
the body may undergo complete transformation into
single or double heads without the formation of other

Fig. 48. — Reconstitution of single and biaxial apical structures from
short pieces of Planaria, to illustrate independence of apical region.

parts of the body or with more or less of the anterior body-
region (Fig. 48) . When a single head arises, it is at the
anterior end of the piece. The conditions determining
the development of these heads are the same as those in
Tuhularia. As in Tubularia also, the original gradient
may appear to some extent in the more rapid and more
complete development, larger size, and dominance in
motor activity of the head at the original anterior end
of the piece, as in the case at the right of Fig. 48.

^ For more extended consideration see Child, "Analysis of Form-
Regulation in Tubularia. V, Regulation in Short Pieces," Archiv fiir
Entwickekmgsmechanik, XXIV, 1907; "Die physiologische Isolation
von Teilen des Organismus," Vortrdge und Aufsatze iiher Entwickekmgs-
mechanik, H, XI, 191 1, 101-19. Further references are given in these
papers.

lOO INDIVIDUALITY IN ORGANISMS

These double apical regions and heads have been
observed by many investigators in various animals and
have commonly been called axial heteromorphoses,
because the apical structure at the basal end of the piece
was regarded as something which was out of place and
abnormal. This, however, is not actually the case, for
the development of these double or biaxial structures is,
as I have shown, subject to exactly the same laws as the
development of the usual single individual, only in these
pieces the conditions are such that the original gradient
is almost absent, and the increased activity at the basal
end may establish a new gradient in the reverse direction,
although some indication of the original gradient may
remain in the smaller size or less complete development
of the part at the basal end. In these short pieces, in
fact, the original polarity is almost obliterated and the
estabUshment of a new reversed polarity in relation
to the basal cut end is possible. At each end the
relation between the metabolic gradient and the devel-
opment of an apical structure is exactly the same as
in any other case of development. The apical region
arises at the apical end of the gradient and the devel-
opment of other parts follows as far as the gradient
extends from each end, or in the case of single struc-
tures as far as the length of the piece permits. By
means of the susceptibility method I have been able
to demonstrate these relations between the metaboHc
gradients and the single or double development of such
pieces.

The development of biaxial or multiple apical struc-
tures from pieces has been observed in various other
animals, and, while their relations to the metabolic

PHYSIOLOGICAL DOMINANCE ioi

gradients have not been determined, their character and
the conditions of their development indicate that when-
ever an apical structure arises it represents the apical
region of a metabolic gradient.

In the plants also conditions are apparently similar.
The apical region of a plant individual may arise inde-
pendently of other parts, and if it becomes structurally
connected with them later the connection develops
progressively from the new apical region toward other
parts and not in the opposite direction. The formation
of buds on the leaves of begonia and in wound callus,
described above (pp. 83-86), are cases in point, and
many other sirnilar cases might be cited. The develop-
mental gradients in such cases indicate that the new
apical structure or part represents the apical region of a
metabolic gradient.

These conclusions concerning the independence of the
apical region and its relation to the metabolic gradient,
which are based upon experimental demonstration for
certain cases and highly convincing evidence for others,
are in full agreement with the facts of embryonic develop-
ment. There also, so far as experimental evidence has
been obtained, the apical region of the individual is the
apical region of a metabolic gradient, and precedence
of the apical region in development and the develop-
mental gradients in the direction of the major axis
indicate that this relation is general. I believe
we are justified in concluding that in this respect
development of the organic individual is always and
everywhere the same. Further evidence in support
of this conclusion will be presented in the following
pages.

I02 INDIVIDUALITY IN ORGANISMS

DOMINANCE AND SUBORDINATION IN EXPERIMENTAL
REPRODUCTION

The existence of a relation of dominance and sub-
ordination along the major axis is shown by the fact
that, while the apical region is independent, other levels
of the body can develop only in organic connection with
more apical levels or with the apical region itself. In
Tubularia and related forms stolons arise only in relation
to stems or hydranths and stems, stem regions appear
only in relation to higher levels in the gradient, etc.
Stolons may grow out from stems in the absence of
hydranths, and under certain conditions when the meta-
bolic gradient is slight stolons may even arise at both
ends of a piece of stem, but no case has ever been
observed of the development of a stolon independently of
other more apical levels.

This relation is also very evident in Planaria.^
The reconstitutional development of pieces from the
middle and posterior regions of the anterior individual,
such as a and b in Fig. 49, ranges according to the
physiological condition of the animal and with experi-
mental conditions from a normal complete animal like
Fig. 50 through various intermediate forms, of which the
anophthalmic is shown in Fig. 51, to headless forms, like
Figs. 52 and 53. The headless forms produce all parts
of the body basal to the level which they represent, but
never give rise to any part characteristic of more apical
levels. The reason why they do not produce heads
will appear in the following section. Thus, headless

» Child, " Studies on the Dynamics of Morphogenesis, etc., I,"
Jour, of Exp. Zobl., X, 191 1 ; II, ibid., XI, 191 1 ; " Experimental Control
of Morphogenesis in the Regulation of Planaria,'' Biol. Bull., XX, 1911.

PHYSIOLOGICAL DOMINANCE

103

forms from the level b
of Fig. 49 give rise to
new tails and to all parts
below their own level
(Fig. 53)^ but never
produce a mouth and
pharynx, while headless
forms from the level a
or any level apical to it
give rise to mouth and
pharynx as well as to
postpharyngeal regions
(Fig. 52), but never to
regions representing
more apical levels than
themselves.

If, however, a head
of any sort, even a rudi-
mentary, anophthalmic
head, like that of Fig.
51, with no eyes and
small, very incompletely
developed, cephalic
gangha, arises on a piece
from the level b, then
the regions of the piece
adjoining the new head
give rise to the parts
representing all levels
between the head and
the level which the piece
b occupied in the original

Figs. 49-53, — Planaria doroio-
cephala: Fig. 49, outline indicating
regions a and h from which pieces are
taken; Figs. 50-53, different results
of reconstitution, depending on pres-
ence or absence of a new head-region.

individual. In other words,

I04 INDIVIDUALITY IN ORGANISMS

the development of parts apical to the original level of
the piece takes place only in relation to the development
of a new apical end, while the development of parts basal
to the original level of the piece is determined by the
piece itself, even in the absence of a head. All the facts
indicate that the same relation exists in other animals.
It has already been pointed out (pp. 83-86) that
when new growing tips arise in wound callus or from
differentiated cells of plants, growth and differentiation
proceed from these, not toward them. Plant stems,
lateral branches, and leaves are subordinate parts or
individuals of the plant and develop only under the
dominance of growing tips. The root of the higher
plant is likewise a subordinate individual. It possesses
a growing tip and between this growing tip and other
parts of the root individual the same relations of domi-
nance and subordination exist as between the stem-tip
and other levels of the stem, but the root as a whole
develops only in subordination to some part of the plant,
a stem-tip, a stem, a branch, a bud, a leaf, or some part
of a root system already present. The same is true for
the root-like structures, the rhizoids of the lower plants.
The roots and rhizoids of the plant have apparently
much the same relation to the organism as a whole as
do the stolons of Tubularia and related forms. They
are individuals, each with an axial gradient and a
dominant region of their own, but they are specialized
individuals, and arise from the basal region of the
major axis of the individual which controls their forma-
tion, whether it is a single bud or branch, a leaf, or the
whole stem of a composite plant individual. It is prob-
able that these subordinate individuals really represent

PHYSIOLOGICAL DOMINANCE 105

partially inhibited gradients (see pp. 178-81). Certain
external conditions, such as moisture and darkness,
favor the development of roots, but do not determine
their origin. It is commonly stated by botanists that
roots may arise on any or almost any part of a plant
where external conditions permit their development
or where the need for them exists. This is true in a
sense, because most plants are composite individuals,
and when one of the constituent individuals of the plant,
such as a bud, branch, or leaf, is sufficiently isolated
from an existing root system, or under certain external
conditions, that individual may develop a root or roots.
Physiologically or physically isolated parts of a plant may
undergo transformation into stem-tips without relation
to other parts and the stem-tips determine the formation
of other parts, but even though various parts of plants
may give rise to roots in the absence of stem-tips, in no
case does any other isolated part of a plant undergo
transformation into roots alone. Moreover, in develop-
ment in nature roots and rhizoids in general arise only
after the primary apical region has been determined.
They are, in short, subordinate to the indivi(Jual as a
whole, but, like leaves and various other plant ''organs,"
possess a certain degree of individuation of their own.
The question of the nature of the correlative influence of
the root system upon other parts of the plant is one of
considerable interest and is touched upon in chap, v
(pp. 159-63)-

THE RECONSTITUTION OF AN INDIVIDUAL FROM AN
ISOLATED PIECE

In the case of Planaria dorotocephala it has been
possible to analyze the process of reconstitution to some

io6 INDIVIDUALITY IN ORGANISMS

extent and so to control it experimentally in various
ways, and my experiments have led to certain conclusions
concerning the nature of reconstitution. A part of the
evidence on which these conclusions are based has
already appeared in various papers/ but some of it is still
unpubHshed. Here only some of the more important
points and the conclusions are briefly presented. The
results of the reconstitution of pieces in Planaria doro-
tocephala differ widely in different cases. I have found
it convenient to distinguish five different forms: the
normal (Fig. 50, p. 103), an individual in all respects
like the type of the species; teratophthalmic (Fig. 54,
A, B),vi\ which the eyes show various degrees of fusion,
inequality, or other departures from the usual condition,
but the head as a whole shows the usual form; terato-
morphic (Fig. 55, ^, ^), usually with a single eye in the
median line and the cephalic sensory lobes more or less
approximated or completely fused at the front of the
head instead of in a lateral position; anophthalmic
(Fig. 56, A, B), with an outgrowth more or less like a
head and containing a small gangHonic mass, sometimes
with cephaUc lobes fused at the front, but without eyes;
headless (Figs. 52, 53, p. 103), in which the cut end
merely heals without outgrowth of new tissue. Differ-
ent degrees of development of the cephaUc gangHa

^ Child, "Studies on the Dynamics of Morphogenesis, etc., I,"
Jour, of Exp. ZooL, X, 1911; II, ibid., XI, 1911; IV, ibid., XIII, 1912;
VII, ibid., XVI, 1914; VIII, ibid., XVII, 1914. See also Child, "Experi-
mental Control of Morphogenesis in the Regulation of Planaria,"
Biol. Bull., XX, 191 1 ; "Certain Dynamic Factors in Experimental
Reproduction and Their Significance for the Problems of Repro-
duction and Development," Archiv fiir Entwickelungsmechanik, XXXV,
1913.

PHYSIOLOGICAL DOMINANCE

107

correspond to these different types of head^ and are
undoubtedly the fundamental factors in determining
general form and localization of the parts in the head.

J «

('Mi:

cic

(jno

:m)

C10

TV

54^

:P4) CMJ

^j^r

54^

55^

55 B

56^

56^

Figs. 54-56. — DifEerent results of reconstitution in Planaria
dorotocephala: Fig. 54^, teratophthalmic animal; Fig. 54^, different
forms of eyes in teratophthalmic animals; Fig. 55^!, B, teratomorphic
forms; Fig. 56^, B, anophthalmia forms.

As a matter of fact, these different forms are a more or
less arbitrary grouping of what is actually a graded series
of forms from the normal head at one extreme to the

^ See Child and McKie, "The Central Nervous System in Tera-
tophthalmic and Teratomorphic Forms of Planaria dorotocephala,"
Biol. Bull., XXII, 191 1.

io8

INDIVIDUALITY IN ORGANISMS

0 C

a

headless form at the other. I have determined experi-
mentally that these different forms
represent different degrees of retarda-
tion or inhibition of the process of
head formation. Their formation can
be controlled experimentally in a
great variety of ways. For example,
the percentage of pieces producing
heads, which we may call the head-
frequency, is less in shorter than in
longer pieces, in pieces from more
basal than in those from more apical
levels of the body; less in pieces from
young than in pieces from old ani-
mals, in pieces from starved than in
pieces from well-fed animals, in pieces
which are kept quiet than in those
forced to move about.

The effect on head-frequency of
substances which decrease metabolic
rate, such as dilute solutions of
cyanides and narcotics, is of great
interest, for it is definite and modi-
fiable experimentally, but not uni-
form. In series of pieces of equal
length, a, &, c. Fig. 57, taken from
animals of the same size and as
nearly as possible the same physio-
logical condition, the head-frequency
under natural conditions is highest in
the a-pieces which represent the most

apical region below the head of the anterior zooid, in

Fig. 57. — Outline
of Planaria doroto-
cephala, indicating
regions, a, b, c, from
which pieces are
taken.

PHYSIOLOGICAL DOMINANCE 109

the ^-pieces it is lower, and in the ^-pieces lowest of
all. If such a series of pieces is placed for a few hours
after cutting in a low concentration of cyanide, alcohol,
etc., the head-frequency in the a-pieces is considerably
lower than in water, that in the Z)-pieces slightly lower
or about the same as in water, while that of the c-pieces
is higher than in water. This result is characteristic,
but the actual percentages can be altered by differences
in concentration of the reagents, tem-
perature, and many other factors.

Although at first glance these re-
sults appear hopelessly confusing,
they depend upon a very simple rela-
tion between that region of the piece
which gives rise to the head and other Fig. 58.— Dia-
parts. In an isolated piece of the grammatic outline of

. . \ 1^, piece of Planaria to

planarian body (Fig. 58) the head niustrate relations of
arises from the cells of the region x, new apical region, x,
which are more directly affected by ^^^^ ^asal region,

- Ill . 1 2, and old body

the wound and undergo rapid j-ggion
dedifferentiation and rejuvenescence
and so attain a higher metabolic rate than cells farther
away from the cut surface and begin soon after
section to divide and grow rapidly. If these cells give
rise to a head, the region y undergoes more or less
transformation to form the body of the new individual.
I have found that the head-frequency varies directly
with the metabohc rate in x, the head-forming region,
and inversely with the metabolic rate in the region y.
This relation may be stated in the formula, head-
frequency = ^^^ . This means that the higher the meta-
bolic rate in x, the more likely the piece is to give rise

no INDIVIDUALITY IN ORGANISMS

to a head, and vice versa, and it also means that the
higher the metabolic rate in the region y^ the less likely
the piece is to give rise to a head. If this relation is
altered by an increase of rate % relatively to rate y^ head-
frequency is increased; if by an increase in rate y rela-
tively to rate x^ head-frequency is decreased. On this
basis all the experimental effects of different physio-
logical and external conditions on head-formation can
be readily accounted for, and it has even been possible
in many cases to predict the results of various experi-
ments.

Some of the facts on which this conclusion is based
are as follows: By means of the susceptibility method I
have demonstrated that the act of section always in-
creases metabolic rate, particularly in the part basal
to the cut. This condition of stimulation continues in
the pieces for several hours after cutting and only gradu-
ally disappears.^ The more basal the level of the piece
in the original body, the more its metabolic rate is
increased by section. In the cases of pieces a, ft, c in
Fig. 57 the metaboHc rate during the first few hours
after section is higher in h than in a and higher in c than
in h, although before section the rate decreased from
a to c. This difference in stimulation of pieces from
different levels results from the different degrees of
subordination. The region c is subordinate to all more
apical regions and is much more dependent upon
impulses coming from these regions than is the region a,
which is subordinate only to the head. When the chief
paths of conduction in the nervous system are cut they

^ Child, "Studies on the Dynamics of Morphogenesis. VII,"
Jour, of Exp. Zo'ol., XVI, 1914.

PHYSIOLOGICAL DOMINANCE iii

are stimulated ; consequently the more basal the level of
a piece the more its rate is increased by section. I
have also found that the shorter a piece, the higher the
metabolic rate after section. Long pieces are stimulated
but little, except at the ends, chiefly the apical, but in
short pieces the rate increases greatly.

Another simple experiment^ shows that under ordi-
nary conditions it is determined within three to six
hours after section whether or not a head will develop
on a piece. This is during the period of stimulation of
the piece, and when we compare head-frequencies and
metabolic rates during the period of stimulation follow-
ing section, we see that the higher the metabolic rate
in the piece as a whole, i.e., the region y, Fig. 58, the less
likely a head is to develop, and vice versa. The head-
frequency is lower in more basal pieces such as c than
in more apical pieces like a, and in shorter than in longer
pieces, because the metabolic rate in the region y is higher
at the time of determination of the course of develop-
ment. Pieces from young or starved animals also have
a higher metabolic rate'' and a lower head-frequency
than similar pieces from old or well-fed animals.

These facts may seem to involve a paradox, but
their interpretation is actually simple. The two regions
X and y (Fig. 58) of the piece behave differently after
section. The cells of x are so extremely affected by
the presence of the wound and the altered conditions
that they rapidly dedifferentiate and begin to divide
and grow, and so approach or attain an embryonic

^ Child, "Studies on the Dynamics of Morphogenesis. VIII,"
Jour, oj Exp. Zool., XVII, 1914.

' Child, Senescence and Rejuvenescence, 19 15, pp. 155-63.

112 INDIVIDUALITY IN ORGANISMS

condition. The cells of y^ however, merely undergo a tem-
porary increase in metabolic rate. The region x is a
small group of cells undergoing dedifferentiation, while y
represents a considerable portion of a fully developed
individual with established relations of parts and spe-
cialized nerves which are much more efficient than
embryonic protoplasm as conducting paths. The region
X originally has a higher metabolic rate than y^ because
it represents a more apical level in the gradient, and its
rate rises still farther as it begins to dedifferentiate.
The facts of experiment indicate that in order to produce
a new head rate x must not merely be higher but much
higher than rate y. The relation between x and y
is evidently this: if rate x is sufficiently above rate y, x
develops independently of y into a head and dominates
y, while otherwise y dominates ic to a greater or less ex-
tent and so retards or inhibits head-formation, and the
various forms between normal and headless condition
are produced.

This relation ^^ can be altered in various ways:
by means of dilute narcotics it is possible according to
the method of use either to decrease the stimulation
in y resulting from section or to delay the reaction in x
until after the increased rate in y has largely or wholly
disappeared, or finally the relation may be altered by the
more rapid and more complete acclimation of the young
cells at X as compared with the older cells of y (see
pp. 51, 52). In pieces such as c. Fig. 57, where under
ordinary conditions x and y are, so to speak, evenly
matched in the struggle for dominance and the head-
frequency is low, all these methods increase the head-
frequency, because the relative increase in rate of x as

PHYSIOLOGICAL DOMINANCE 113

compared with y overbalances the absolute decrease in
rate produced by the narcotic. In pieces like a, where y
is only slightly stimulated by section and rate x is so
much higher than rate y that the head-frequency is
very high, the effect of narcotics is to decrease head-
frequency, because in such cases dominance is not
reversed and only the direct inhibiting effect of the
narcotic on the region x appears.

Head-frequency may be increased in all pieces by
inducing them to move about.' The apical end, of
course, precedes in such movement, the cells of the region
X are subjected to more excitation than in a piece which
is not moving, and the higher metabolic rate of x results
in increased head-frequency.

In Planaria maculata and certain other species the
degree of subordination of basal regions of the body is not
as great as in P. dorotocephala; consequently the increase
in metabolic rate after section in pieces from this region is
less than in P. dorotocephala, and in these species the
head-frequency of such pieces is almost or quite as great
as that in more apical pieces. Various other differences
in the reconstitutional process in different species of
planarians only serve to confirm the conclusions reached
in the case of P. dorotocephala.

These and many other facts have forced me to the
conclusion that the head which appears in the recon-
stitution of a piece is not physiologically a part of the
piece and is not formed by the piece, but develops, so to
speak, in spite of it. Only when the metabolic rate
of the cells at x is high enough to make them essentially

^ Child, "Experimental Control of Morphogenesis in the Regulation
of Planaria,^' Biol. Bull., XX, 191 1.

114 INDIVIDUALITY IN ORGANISMS

independent of y do they begin the development of a
new individual by the formation of a head. The
formation of a head at the end of a piece is then exactly
the same process as the transformation of short pieces
into heads when no other part of the body is present
(pp. 96-101). The new head arises independently of
other parts and dominates them. The influence of
other parts on head-formation is merely inhibitory or
negative, while the influence of head-formation on other
parts is determinative or positive. The process of
development of the cephalic ganglia in the head formed
on a piece also indicates the independence of the head.
The ganglia arise in the new tissue independently of the
parts of the nervous system in the old tissue of the piece
and become connected with these parts only secondarily.^
This fact suggests that head-formation actually depends
upon the establishment of a metabolic gradient in the
region x with its apical region near the free end and
decreasing in rate toward y. If this occurs, a head forms,
but if rate y is high enough in relation to rate %^ head-
formation is inhibited or retarded b)r the interference
between two gradients in opposite directions. Inhibition
or retardation of head-formation consists then in the
interference of one metabolic gradient with another in
the opposite direction or the obliteration of the one by
the other.

The new basal end of the piece develops from a group
of cells 2, Fig. 58, which react to the wound at the basal
end by more or less dedifferentiation and growth. This

^ S. Flexner, "The Regeneration of the Nervous System of Planaria
torva," etc., Jour, of MorphoL, XIV, 1898; Child and McKie, "The
Central Nervous System in Teratophthalmic and Teratomorphic Forms
of Planaria dorotocephala," Biol. Bull., XXII, 191 1.

PHYSIOLOGICAL DOMINANCE 115

reaction is less rapid than that of x (see Fig. 31), because
they represent a lower level in the gradient and their
relation to the region y is different from that of x. The
rate of development and completeness of the new basal
end varies directly with the metabolic rate in y; any
conditions which decrease the metabolic rate in y
decrease the development of the basal end, and vice
versa. We may say then that tail-frequency =^|^ but
that under the usual conditions, when a gradient is
already present in the piece, rate z is so low that it
becomes negligible, and the formula becomes tail-
frequency = rate y. This holds true as long as a new
zooid does not arise in this basal region. If a new zooid
does arise there in consequence of physiological isolation,
as is often the case in headless pieces, then the lower
the rate in y, the more rapid the development of this
posterior zooid. In headless pieces the large size of the
posterior outgrowth (cf. Figs. 52, 53, with Fig. 50, p. 103)
is due to the fact that this region is not physiologically
the basal end of the individual but a second individual.^
The development of the basal region is then depend-
ent upon the presence and influence of more apical
regions, while the development of the head occurs
independently of other parts, so far as it is not inhibited
by them. The relation between the major axial gradient
and these differences of behavior in different regions is
evident. The process of reconstitution of a new indi-
vidual from a headless piece in Planaria is a process of
development beginning at two different levels, first, at
the apical end of the piece with the formation of a

' Child, ''Studies on the Dynamics of Morphogenesis. Ill," Jour,
of Exp. Zool., XI, 1911.

Ii6 INDIVIDUALITY IN ORGANISMS

new head, and, secondly, at the basal end with the
formation of a new tail. The new apical region as the
region of highest metabolic rate determines the estab-
lishment of a new major axial gradient, which has the
same direction as the original gradient but possesses a
higher rate, and in consequence of these changes the
parts of the piece below the new apical region undergo
more or less structural change into parts characteristic
of more apical levels, until sooner or later a stable con-
dition of the gradient is attained, and this determines
the completion of reconstruction.

It can scarcely be doubted that the process of recon-
stitution of pieces into new individuals is fundamentally
the same in all animals, though it may differ widely in
details, with the kind and physiological condition of the
individual or piece and the nature of the external con-
ditions under which reconstitution occurs. Moreover,
it is essentially the same process as reconstitution in
plants, in that it consists in the development of a new
individual beginning with the apical end. The chief
difference is that in animals the development of the new
individual is usually closely associated with the cut
surface or surfaces, while in plants the reaction of the cells
at the cut surface usually does not at once cover it with
more or less embryonic rapidly growing cells, as it does in
animals, and, since the plant is usually a composite
individual^ other apical regions already present become
dominant, or new apical regions arise in other parts before
a new apical region develops at the cut surface. In
some cases, where only a small part of the apical region is
removed, a new growing tip develops from the cut
surface, and in such cases the formation of the new grow-

PHYSIOLOGICAL DOMINANCE 117

ing tip is, I believe, essentially the same process as the
formation of the new head in Planaria. In cases where
wound callus develops, new growing tips may arise in
that. In the formation of a new growing tip in callus
tissue (pp. 85-86) and its later connection with other
parts of the plant we have again a process very similar
to the formation of a head in a piece of Planaria, and the
development under its dominance of other parts, so far
as they are not already present. In both cases the new
apical region is not determined by other parts but
develops independently of them, and its later relations
to them are determined by its own dominance.

SOME MODIFYING AND LIMITING FACTORS IN ANIMAL
RECONSTITUTION

The development of double or biaxial apical regions
from short pieces has been discussed above (pp. 98, 99).
In some cases biaxial basal regions
instead of apical regions arise from
pieces. Pieces of the stems of certain
hydroids sometimes produce stolons
at both ends, biaxial tails have been p^^, 59.— Experi-
observed in short pieces of Planaria mentally determined
by Morgan, and I have been able to reconstitution of

, , . 11. biaxial basal ends in

produce them experimentally m some ^^^^^ ^^ Planaria.
cases (Fig. 59) by altering the relations
of metabolic rate between the regions x and y (Fig. 58)
with the aid of narcotics. In the earthworm and related
forms various investigators have observed the develop-
ment of tails at both ends of pieces from the more basal
regions of the body. My own experiments indicate that
when the development of the new tissue at a cut end of

ii8 INDIVIDUALITY IN ORGANISMS

a piece is completely dominated by the piece it gives
rise to a basal structure. Such dominance means simply
that the old tissue has a high enough metabolic rate to
determine the direction of the gradient in the new tissue.
In Planaria the development of tails at both ends of a
short piece is apparently due simply to the fact that the
metabolic rate in the piece is high enough so that the
new tissue does not become dominant at either end but
develops under the control of the old tissue. Dr. Hyman
has found that the conditions determining the formation
of double tails in Lumbriculus seem to be essentially the
same as in Planaria, though the factors which produce
them are somewhat different. She has been able to
determine experimentally to some extent the produc-
tion of heads instead of tails in such pieces by methods
similar to those which I have employed for altering
head-frequency in Planaria. She has also observed the
development of structures intermediate between head
and tail, or rather inhibited, rudimentary cephalic ends,
in which certain caudal characteristics appear later.
These are apparently cases in which the new tissue was
at first to some extent independent but later became
subordinated to the old.^

The absence of any outgrowth at the apical end of a
piece, as in the headless forms of Planaria (Figs. 52, 53),
occurs when head-formation is completely inhibited,
but the degree of dominance is not sufficient to deter-
mine development as a tail. In such cases local con-
ditions at the cut apparently determine the result and
the wound simply heals. In some other forms the wound

^ I am indebted to Dr. Hyman for permission to use these unpub-
lished data.

PHYSIOLOGICAL DOMINANCE 119

reaction involves more growth than in Planaria, and in
such cases considerable outgrowths may sometimes arise
which are neither heads nor tails, but cell masses of
indeterminate character which gradually differentiate
in relation to adjoining parts and may finally show both
apical and basal characteristics.

In many of the flatworms and various other forms
only an apical cut surface above a certain level of the
body gives rise to a head, while tails may arise from cut
surfaces at any level basal to the head of the parent
body. In some of these cases the level where head-
formation ceases lies a considerable distance from the
cephalic ganglia, while in other cases head-formation
does not occur when the cephalic ganglia are removed;
but when parts of the head are removed leaving a por-
tion of the cephalic ganglia intact — sometimes half or
more, sometimes only a small part, is necessary — such
parts develop again. In the headless pieces there may be
more or less outgrowth at the apical end of the piece,
but it is indeterminate in character. Some authors
have maintained that in such cases the cephalic ganglia
or the more apical regions of the longitudinal nerve
cords exercise a specific formative influence of some
sort and so determine the development of a new head,
but there is no real evidence in favor of this view.
Probably the head fails to develop in such cases either
because the cells reacting to the wound do not attain
a high enough metabolic rate to become independent
of other parts and their development into a head is
therefore inhibited, as in the headless pieces of Planaria,
or because these cells do not dedifferentiate to a suffi-
cient extent to be capable of giving rise to a new cephalic

I20 INDIVIDUALITY IN ORGANISMS

ganglion and so to a head, while they may still be able
under the dominance of other parts to produce basal
regions of the body. The development of the apical
region or of the apical part of the central nervous system,
which in all except the lowest animals is the primary and
dominant part of the apical region, is a self-determining
process, independent of other parts, while the develop-
ment of other parts is determined by their relations
to dominant regions. It is highly probable therefore
that a more complete loss of differentiation is necessary
as a condition for head-formation than for the develop-
ment of other parts. As a matter of fact, we find that
as the capacity for reconstitution becomes limited by
increasing differentiation the capacity for head-formation
disappears first of all. Many animals in which recon-
stitution of new heads does not occur are still able to
reproduce all subordinate parts, and with further limita-
tion it is the more subordinate parts, such as legs and
other appendages or caudal regions, which the body is
capable of reproducing.

This limitation is more or less progressive from lower
to higher forms, until in the higher vertebrates the
capacity for reconstitution under any known con-
ditions is limited practically to tissue regeneration.
The primary limiting factor is unquestionably the
increasing physiological stability of the protoplasmic
substratum, in consequence of which the capacity for
dedifferentiation and rejuvenescence, at least under
ordinary conditions, is more and more narrowly
limited.^

^ Child, Senescence and Rejuvenescence, 1915, pp. 35, 39> 4i~43> 53»
194, 267, 304, 463-65.

PHYSIOLOGICAL DOMINANCE 121

The embryonic stages of different animals differ
widely as regards their capacity for reconstitution.
In the sea-urchin and starfish isolated cells or groups
of cells of the developing embryo down to a cer-
tain limit may give rise to complete larvae of small
size, while in other forms, such as the annelids and
mollusks, isolated parts of the embryo show little or
no reconstitutional change, but remain alive for a time
and continue to differentiate as they do when they
remain asi parts of an intact embryo. From the failure
of the isolated parts to undergo reconstitution the con-
clusion has been drawn that they are independent of
each other in the intact embryo, and that development
in these organisms is a sort of mosaic made up of inde-
pendent parts with some sort of pre-established harmony
between them. If this view is correct, there is no rela-
tion of dominance and subordination in these stages
of development. The failure of isolated parts to under-
go reconstitution does not, however, demonstrate the
absence of dominance but merely the ineffectiveness
of isolation. The absence or limitation of embryonic
reconstitution in certain forms is apparently due, like
the increasing limitation of reconstitutional capacity
in higher animals, to the higher specialization of the parts
of the egg and embryo in these forms. There is good
reason to believe that in such eggs the condition in em-
bryonic stages is the result of differentiation dependent
upon dominance and subordination of parts in the earlier
life of the egg, and that specialization has gone beyond
the stage where it can be greatly altered by isolation.
Development proceeds in isolated parts as far as it has
been determined by past relations with other parts or as

122 INDIVIDUALITY IN ORGANISMS

far as nutritive or other conditions permit, and then
ceases. There can be little doubt that relations of
dominance and subordination exist during embryonic
stages, and that these are factors in determining what
occurs in later stages. According to this view, the
difference between these eggs and those in which a high
degree of embryonic reconstitution occurs is primarily
a difference in the stability or fixity of the effects of
previously established metabolic gradients. At the one
extreme are eggs in which axial differences at the be-
ginning of embryonic development are probably largely
or wholly differences in metabolic rate, at the other,
those in which specialization and differentiation of parts
have gone far beyond this condition. The egg, in short,
is an individual, and some eggs are more highly special-
ized individuals than others.

The proportional relations of parts in reconstitution,
of which much has been made by Driesch, Morgan, and
others, are obviously, so far as they exist, dependent
upon metabolic relations between the parts. On a short
piece of Planaria, for example, a smaller head usually
develops than on a long piece. This fact has often been
regarded as in some way associated with the fact that
the shorter piece will produce a smaller animal than the
longer and that the size of the new head foreshadows
the size of the animal. As a matter of fact, the size
of the head formed by pieces of the same size may
differ widely in different cases and can be controlled
experimentally to a very large extent by controlling
metaboHc conditions. The higher the metabolic rate in
the region x, Fig. 58, in relation to that of the region y,
the larger the head, and vice versa. The size of the

PHYSIOLOGICAL DOMINANCE 123

head in relation to other parts is determined primarily
by its ability to grow at their expense. In a shorter
piece there is less material available for such growth
than in a longer piece, consequently a smaller head
develops. Essentially the same relation exists as
regards other parts. Where an excess of nutritive
material is available the relation is not necessarily very
different, for each part uses nutrition instead of the
substance of other parts according to its metaboHc
activity, i.e., according to its position in the axial
gradients, so that in this case also the chief factors in
determining the proportions of parts characteristic of
each form are the metabolic relations between them.
In the early stages of development in nature the simple
quantitative gradation in size from the apical toward
the basal region appears, but as specialization occurs
and the differences in metabolic rate at different levels
bring about changes in metabolic character the size rela-
tions must of course become more complex.

The return or approach to the characteristic form of
the species which very commonly takes place in the
reconstitution of pieces has been regarded by Morgan
and others as largely a matter of the physical rearrange-
ment of the substance of the piece. That changes in
shape may be brought about in soft-bodied forms like
the flatworms by mechanical conditions connected
with motor and other functional activities of the ani-
mals, I have shown. ^ Wherever such factors play a

^ Child, "Studies on Regulation. IV," Jour, of Exp. ZooL, I, 1904;
VII, ibid., II, 1905; "Studies on Regulation. IX, X," Arch, fiir
Entwickelungsmechanik , XX, 1905; "The Regulatory Change of Shape
in Planaria dorotocephala,^' Biol. Bull., XVI, 1909.

124 INDIVIDUALITY IN ORGANISMS

part in determining the characteristic shape of the
animal they undoubtedly play a part in determining
the approach to this shape in pieces undergoing recon-
stitution, but in cases where they are not primarily con-
cerned the metabolic relations are unquestionably the
primary factors in determining shape and proportions
of the whole and parts.

In most adult animals and embryonic stages which
are capable of any considerable degree of reconstitutional
reproduction, a limit of size of isolated pieces seems to
exist below which reconstitution becomes incomplete or
fails to occur. In Planaria, for example, with decrease
in size of piece head-frequency falls to zero, but with still
further decrease in size head-formation begins to occur
again and head-frequency rises. These changes are
simply due to changes in the relation StU (see pp. 109-
10). With decreasing size of the piece, y is more and
more highly stimulated by section until in pieces below
a certain size heads do not develop at all, but when
the piece becomes very small y practically disappears,
for the whole piece becomes involved in the direct
wound reaction and so corresponds to the region x
or such a region in relation to both cut ends. In such
pieces there is nothing to inhibit or retard head-
formation except the simultaneous development of a
head at the opposite end (see pp. 98-101), and in
such cases the effect is mutual and results merely in
retardation.

Here then the completeness or incompleteness of
reconstitution in relation to size of piece is wholly a
matter of quantitative metabolic relations. There is
no minimal size of piece which represents the ''organi-

PHYSIOLOGICAL DOMINANCE 125

zation" of the species reduced to its lowest terms. The
minimal size can be altered widely even now by con-
trolling conditions, and I have no doubt that if we are
ever able to isolate single cells and to provide proper
nutritive and other conditions for them we shall find that
in many of the lower animals such cells are capable of
giving rise to new individuals, as they undoubtedly are
in many plants.

Most investigators have regarded the minimal size
of pieces undergoing reconstitution as something abso-
lute and have failed entirely to note that it differs with
the physiological condition of the animal, the region of
the body, and the various external conditions which
affect metaboUc rate. To determine the smallest piece
of animal capable of reconstitution under given con-
ditions is merely to determine one special case out of an
indefinite number of possible cases.

CONCLUSION

The experimental evidence demonstrates, first, the
essential independence of the apical region in both
plants and animals, and, secondly, determination and
control by this apical region of the developmental
processes at other levels of the major axis of the indi-
vidual. The reconstitution of pieces into new individ-
uals is fundamentally the same process as embryonic
development, and the same relation of dominance and
subordination exists in both. The different results of
reconstitution in pieces of different size, from differ-
ent levels, in different physiological conditions, under
different environmental conditions, etc., depend pri-
marily upon relations of dominance and subordination,

126 INDIVIDUALITY IN ORGANISMS

determined by the relations of metabolic rate in
different parts. In the higher animals other factors,
such as the stability of the differentiated cellular
substratum, may contribute to limit reconstitutional
capacity.

CHAPTER V

THE RANGE OF DOMINANCE, PHYSIOLOGICAL

ISOLATION, AND EXPERIMENTAL

REPRODUCTION

If the conception of physiological dominance which
is presented in chap, ii is correct, the existence of a
transmission-decrement in the impulses, stimuli, or excita-
tions which are the effective agents in dominance must
determine a certain range of dominance and therefore
a physiological size limit or limit of length for each axis,
which cannot be exceeded without physiological isola-
tion of the part that lies beyond the range of domi-
nance. Moreover, the limit of dominance in a given case
must vary with the metabolic rate in the dominant
region and the conductivity along the path of trans-
mission. Its effectiveness upon a subordinate part may
also depend upon the receptivity of the part to the
transmitted excitations, and this may be determined
by local conditions to which the part is subjected. If
the characteristic gradients are present or arise in a
physiologically isolated part, such a part may become
a new complete individual, if it is not so highly special-
ized or differentiated as to be incapable of reacting to the
altered conditions by dedifferentiation and redevelop-
ment. Some of the evidence bearing upon these aspects
of the problem of dominance is considered in this
chapter.

127

128

INDIVIDUALITY IN ORGANISMS

EXPERIMENTAL CONTROL OF SPATIAL RELATIONS OF
PARTS AND OF THE RANGE OF DOMINANCE

The dimensions and distance relations of parts along
an axis can be altered by altering the metaboHc rate in
the dominant region or throughout the organism and
so increasing or decreasing the length of the gradient.

Ill III

a

III W

60

r

I e a 81

ill II

62

61

Figs. 60-62. — Different lengths of hydranth primordium in recon-
stitution of pieces of Tubularia: Fig. 60, length at medium metabolic
rate; a, b, c, d, the four regions of the primordium; Fig. 61, length at
high metabolic rate; Fig. 62, length at low metabolic rate.

In Tubularia the reconstitutional development of a
hydranth is a transformation inside the perisarc of the
terminal region of the piece into a hydranth without
outgrowth of new tissue from the cut surface. In the
early stages of this process (Figs. 60-62) the two rows
of tentacles arise as two series of longitudinal ridges
(Fig. 60, b, d), usually distinguishable from other parts

THE RANGE OF DOMINANCE 129

by accumulations of red pigment. Various facts, some
of which have been mentioned above (pp. 79, 96-99),
show that the parts of the hydranth are determined
from the apical end in the basal direction. The point
of present interest in this process is the length of stem
concerned in the formation of the new hydranth and
the length of each of the four distinguishable regions,
a, b, c, d, of the developing hydranth. In pieces of like
physiological condition kept under the same external
environment these lengths show a high degree of con-
stancy, but they can readily be altered by altering the
metabolic rate in the pieces. Fig. 60 shows the length
and proportions of the early stage of a hydranth develop-
ing with a medium metabolic rate. Fig. 61, with a high
rate, and Fig. 62, with a very low rate. Evidently the
higher the metaboHc rate the greater the distance from
the end of the stem and from each other at which the
two rows of tentacles arise. The relative lengths of
the different parts also change with metabolic rate,
that of the region a increasing and that of the region
d decreasing with increasing metabolic rate, and vice
versa. ^

The position, size, and time of appearance of hy-
dranths and the relation of hydranths to other parts in
the reconstitution of isolated pieces of Tubularia and
related forms have been repeatedly investigated, but,
although the facts are very definite, the various authors

^ Child, "An Analysis of Form Regulation in Tubularia. II,
Differences in Proportion in the Primordia," Archiv fur Entwickelungs-
mechanik, XXIII, 1907. In this paper I showed that such differences in
proportion appeared in hydranths from different levels and ends of the
stem, but it is now known that these differences in level really represent
differences in metabolic rate.

I30 INDIVIDUALITY IN ORGANISMS

have failed to reach any very satisfactory general inter-
pretation of them. Driesch, who has used Tubularia to
a large extent as experimental material, even maintains
that they cannot be interpreted on a physico-chemical
basis. As a matter of fact, however, not only do the
facts fall readily into line with the dynamic conception of
the individual which I have outlined, but many of them
constitute valuable evidence for that conception.

I have found that previously existing metabolic
gradients in the stem of Tubularia are rapidly obliterated
and new gradients readily arise when metabolic con-
ditions change. This is due to the fact that the proto-
plasmic substratum is not very stable, and, except in
the hydranth, there is little structural differentiation
in relation to the metabolic gradient. Wherever the
stem of Tubularia is cut across, and even in many cases
where section is not complete, a metabolic gradient
arises in connection with the stimulation of the wound
and the open end exposed to sea-water and the oxygen
contained in it. The region of highest rate in this
gradient is at the cut end, and the gradient extends a
greater or less distance from the cut, according to the
physiological condition of the stem and the direction and
metabolic rate of the pre-existing gradient in the region
concerned. If the metabolic gradient resulting from
stimulation at the cut end is in the same direction as the
pre-existing gradient, then of course there is merely an
augmentation of the gradient, but if two gradients are
in opposite directions, as they are at the basal end of
a piece, they tend to neutralize, obHterate, or inhibit
each other, and the one which has the higher metaboHc
rate sooner or later obHterates the other. The evi-

THE RANGE OF DOMINANCE 131

dence indicates that when such a gradient is sufficiently
marked, that is to say, when the metaboKc rate in its
apical region is sufficiently high, and when the inhibiting
or obKterating influence of a gradient in the opposite
direction is not too great, a hydranth develops. The
formation of a stolon, on the other hand, apparently
represents a gradient which is partially inhibited or
obliterated, or, in other words, partially dominated by a
gradient in the opposite direction, but in addition to this
relation a relatively high metabolic rate in the piece
or individual as a whole is also apparently necessary
for stolon-formation. The stem represents the lower
levels of a simple uninhibited gradient, and its formation
always occurs under the dominance of a hydranth or
other region of higher metabolic rate.

It is also important for an understanding of the facts
to note that in general the metabolic rate of these animals
decreases when they are transferred from natural to
laboratory conditions, and the hydranths which develop
in the laboratory possess a lower metabolic rate than
those in nature; consequently the range of dominance
is less and physiological isolation occurs at shorter
distances from the dominant region than in animals in
nature. Moreover, the development of a new hydranth
at the cut end of a piece of stem is, I believe, a process
essentially similar to the development of a head on a
piece of Planaria (pp. 105-14). The new hydranth
region is independent of other parts and becomes
dominant over them, but during the early stages of its
development this dominance is less complete, because
the changes in the protoplasm of the stem in accordance
with the new metaboHc conditions require some time;

132 INDIVIDUALITY IN ORGANISMS

therefore removal of the original hydranth favors physio-
logical isolation of basal regions of the piece.

In Corymorpha the metabolic relations and the rela-
tions of the various parts of the body to the metabolic
gradients are essentially the same as in Tubularia, and
the demonstration of the metabolic gradients by means
of the susceptibility method in Corymorpha, where most
of the stem is naked, is not open to the objection which
might be raised in the case of Tubularia, where all parts
of the stem except the cut end are covered by the horny
perisarc, viz., that the reagent penetrates the tissues only
or chiefly from the cut end and so produces a death
gradient which is merely a gradient of penetration and
does not represent metabolic conditions.

Some of the facts and their interpretations in terms
of metabolic gradients and physiological dominance
are briefly as follows:^ In pieces of Tubularia stem
eight or ten millimeters or more in length and with a
cut surface at each end reconstitution usually results
first in the development of a hydranth at the apical end
of the piece and later of a second smaller hydranth at the
basal end (Fig. 63). Occasionally pieces from vigorous
animals which evidently possess a high metabolic
rate produce an apical hydranth and a stolon at the
basal end (Fig. 64), but before it attains any great

' I have described and discussed these facts in the following papers:
Child, "An Analysis of Form Regulation in Tubularia. I," Archiv fur
Entwickelungsmechanik, XXIII, 1907; IV and V, ibid., XXIV, 1907;
"Die physiologische Isolation von Teilen des Organismus," Vortrage
und Aufsdtze Uber Entwickelungsmechanik, H, XI, 191 1, 96-119. The
discovery since these papers were written of the existence of metabolic
gradients and their relation to physiological dominance affords a definite
basis for most of the earlier conclusions and interpretations.

THE RANGE OF DOMINANCE

133

length this stolon gives rise to a hydranth at its tip.
This is a process of reproduction like that occurring in
nature (Fig. 43, p. 90), and differs from it only in that
the distance of the second hydranth
from the first is less in the pieces
than in the animal under natural
conditions. This difference indi-
cates that, as might be expected,
the range of dominance of the
apical region is less in the experi-
mental piece than in the whole
animal in nature.

In most pieces, however, the
dominance of the apical region is
insufficient to inhibit the establish-
ment of a well-marked new gradient
in relation to the cut basal end of
the. piece, and so the formation of
a hydranth usually occurs at this
end also, as in Fig. 63. The
development of this hydranth is
usually delayed, as compared with
that of the apical hydranth, be-
cause the establishment of the new
gradient is more or less retarded
by the gradient already existing in
the original direction, and the
shorter the piece the greater the
delay, because in shorter pieces the dominance of the
apical region is more complete, or, in other words, the
gradient from the apical region is more marked at the
basal end and therefore inhibits or retards to a greater

63

64

Figs. 63, 64. — Recon-
stitution of longer pieces
of Tubularia: Fig. 63,
usual result of recon-
stitution with hydranth
at basal end; Fig. 64,
reconstitution with
stolon at basal' end.

134 INDIVIDUALITY IN ORGANISMS

extent than in longer pieces the establishment of a new
gradient in the opposite direction. In pieces more than
eight or ten millimeters long, however, the local condi-
tions -at the basal end usually determine the result
sooner or later, and the new gradient is established and a
hydranth develops here.

In pieces between eight or ten and two or three milli-
meters in length neither hydranth nor any other out-
growth arises at the basal end in most cases. In these
shorter pieces the dominance of the apical region is
sufficient to inhibit the new gradient at the basal end
to a sufficient degree to prevent hydranth formation,
and the general metabolic rate in these as in most
other experimental pieces is not high enough for stolon-
formation to occur.

In the very short pieces described in chap, iv
(pp. 96-99) the difference in metabolic rate between
the two ends of the piece dependent upon the original
gradient is so slight that in many cases the local condi-
tions at the two ends become the determining factors,
and hydranths begin to form simultaneously or nearly
so at both ends, the portion of each hydranth formed
depending on the length of the piece. If the original
gradient in the piece is sufficient to determine the more
rapid reaction at the apical end this becomes dominant
and a single, instead of a double, structure arises.

These are the chief facts of reconstitution in Tubu-
laria under ordinary conditions and their interpretation
in terms of metabolic gradients and dominance. It is
possible, however, to obtain more positive evidence in
support of these interpretations by controlling and
altering the experimental conditions. By diluting the

THE RANGE OF DOMINANCE 135

sea-water to a certain extent the metabolic rate in pieces
is increased, and under these conditions pieces which
in normal sea-water produce only hydranths at their
basal as well as apical ends produce in a large percentage
of the cases stolons which later develop hydranths at their
tips/ The hydranths in such pieces are longer and
larger than in normal sea-water.

When a piece is cut with a fully developed active
hydranth at its apical end, no hydranth appears at the
basal end until the metabolic rate of the apical hydranth
decreases or its death occurs, which in Tubularia is
usually within a few days at most. In Corymorpha
relations are similar. Evidently, then, a full-grown,
active, apical hydranth inhibits the development of a
basal hydranth in a piece, but a hydranth beginning to
develop at the apical end is usually only able to retard
to some extent the development of the basal hydranth.
The dominance of the full-grown hydranth is more
effective than that of the early stages of hydranth
development.

Various investigators have observed that when the
development of the hydranth at the apical end of a piece
is inhibited by inclosing this end in paraffin or sticking
it in the sand the development of the hydranth at the
basal end is accelerated, and it has been found that in
such cases the basal hydranth is longer and larger than
when the apical hydranth is not inhibited. Evidently
the inhibition of development at the apical end decreases
dominance, and the establishment of the new gradient
and so the development of a hydranth at the basal end

^ Child, ''An Analysis of Form Regulation in Tubularia. I,"
Archiv fur Entwickelungsmechanik, XXIII, 1907.

136 INDIVIDUALITY IN ORGANISMS

is accelerated. The same result may be attained by
compressing, sharply bending, or partially crushing the
stem at some point between the two ends. In such cases
the influence of the dominant apical region is prevented
from reaching the basal end, which is- therefore physio-
logically isolated and the establishment of the new
gradient but little retarded. Often also the develop-
ment of the basal hydranth can be accelerated by cutting
partly through the stem, so that only a slender organic
connection between the two ends remains. In these and
various other ways the controlHng influence of the apical
region can be demonstrated.

Neither the inhibition of development of the basal
hydranth by paraffining the basal end or sticking it in
sand nor the partial crushing or bending of the stem at a
certain level influences the development at the apical
end except in very short pieces. In these, inhibition of
either end may accelerate the development of the other,
and a single instead of a double structure may result.
These experiments show that in the longer pieces
dominance extends chiefly in the direction of the original
gradient, and we find correspondingly that the new
gradient which arises at the basal end does not extend
very far from that end. If, however, inhibition of the
apical end be continued for a longer time, the gradient
at the basal end extends farther from that end.

The length of the hydranths formed in very short
pieces is often, though not always, less than in longer
pieces, particularly in pieces from the more basal regions
of the stem. Driesch has made much of this point as
an indication that an adaptation of the length of the
hydranth to the length of the piece takes place in order

THE RANGE OF DOMINANCE 137

that a stem as well as a hydranth may be formed.
According to Driesch this adaptation is not determined
physico-chemically, but by the principle which he
calls entelechy and which as he beheves controls develop-
ment. Unfortunately for Driesch's view this ''adapta-
tion" does not occur in all cases, and is very incomplete,
for, as I have pointed out (pp. 96-99),' these short
pieces often give rise to hydranths or apical parts of
hydranths without stems or basal parts. The experi-
mental evidence indicates that the shorter hydranths in
short pieces are merely hydranths which are partially
inhibited by other regions of the piece, just as the head of
Planaria may be partially inhibited by other regions of
the piece. As in Planaria, short pieces, particularly those
from the more basal regions of the body, are more stimu-
lated by section, and their metabohc rate is therefore
higher throughout than that of longer or more apical
pieces. Under these conditions the gradient arising at
the cut end is much less effective in determining the devel-
opment of a new structure, the hydranth, than it is when
the general metabohc rate is lower. Figuratively speak-
ing the new gradient is partially obliterated by the gen-
eral high metabolic rate in the piece. Consequently its
length is less and the length of the hydranth determined
by it is correspondingly less than in longer pieces, and
development is also retarded. A piece of given length
may produce a single short hydranth and stem, or a
longer hydranth without stem, or biaxial hydranths, or
apical portions, and all these differences in behavior are
determined by simple differences in the gradient relations.

^ See also Child, "An Analysis of Form Regulation in Tuhularia,
Regulation in Short Pieces," Archiv fur Entwickelungsmechanik, XXIV,
1907.

138 INDIVIDUALITY IN ORGANISMS

In Planaria also the positions and space relations
of parts along an axis and the range of dominance
can be altered and controlled by means of conditions
which alter metabolic rate.^ At ordinary room tempera-
tures in well-aerated water the isolated postpharyngeal
region of Planaria (Fig. 65) forms a new individual Hke
that in Fig. 66. The new mouth and pharynx form
near the middle of the piece at a certain distance from
the new head, and the region in front of the pharynx
undergoes the internal changes which make it over into
the prepharyngeal region of the new individual. If, how-
ever, the rate of metaboHsm in such a piece is decreased
by means of dilute narcotics, by the presence of carbon
dioxide and metabolic products in the water, or by other
means, the head develops slowly, is small and usually
abnormal, and the lower the metabolic rate during
development the nearer to the head the mouth and
pharynx arise and the less the length of the new pharyn-
geal region. Fig. 67 shows the effect of a slight decrease.
Fig. 68 of a greater, and Fig. 69 of a still greater decrease
in metabolic rate during reconstitution. The length
of the region undergoing reconstitutional change is less
in Fig. 67 than in Fig. 66, still less in Fig. 68, and in
Fig. 69 practically no changes occur below the level of
the very rudimentary head.

Reconstitution of similar pieces with a very high
metaboUc rate (at high temperature) results in forms
like Fig. 70, in which the pharynx and mouth arise at a

^ Child, "Physiological Isolation of Parts and Fission in Planar ia,^^
Archiv fiir Entwickelungsmechanik, XXX (Festband fiir Roux), II. Teil,
1910; "Studies on the Dynamics of Morphogenesis, etc. Ill," Jour,
of Exp. Zool., XI, 191 1.

THE RANGE OF DOMINANCE

139

greater distance from the head and the prepharyngeal
region is longer than in Fig. 66.

u

Figs. 65-70. — Space relations of parts in reconstitution of Planaria
dorotocephala under different metabolic conditions: Fig. 65, outline
indicating level of section; Fig. 66, reconstitution under standard
laboratory conditions; Figs. 67-69, different ranges of dominance and
space relations of new parts in reconstitution with low metabolic rate
in different concentrations of narcotics; Fig. 70, reconstitution with
high metabolic rate at high temperature.

I40 INDIVIDUALITY IN ORGANISMS

The metabolic gradient associated with the new
head shows a corresponding decrease and increase in
length in such pieces. The influence of the new head-
region extends to a greater or less distance according as
its metabolic rate is high or low, and the position of the
various organs is altered correspondingly, or, as in the
extreme case of Fig. 69, no new organs are formed except
the head.

When the metabolic rate is high, as in Figs. 66 and 70,
dominance extends nearly or quite to the basal end of
the piece, though short zooids may be present as more
or less distinct gradients (see pp. 92-94) at the basal
end. Before section most of this region of the body
consisted of one or more zooids, but the development of a
head nearer to these zooids than the original head has
brought about the obhteration of the gradients which
represented them, except perhaps in the extreme basal
region, and after reconstitution a single gradient extends
over at least most of the length of the piece. When the
metabolic rate is lower, as in Figs. 67 and 68, a short
individual develops from the apical region of the piece,
but most of the broader portion is not physiologically a
part of this individual. This is very evident in the
behavior of these forms, for, when creeping about, they
are unable to control and co-ordinate this region to any
great extent; and simply drag it about like a dead mass.
As long as they remain in the narcotic they are not active
enough to undergo fission, but if they are returned to
water, fission may occur after a few days, although the
range of dominance gradually extends, and more and
more of the length of the piece comes under the control of
the head.

THE RANGE OF DOMINANCE 141

The different types of head in Planaria (see pp. 106-
14) represent, as I have pointed out, different degrees
of inhibition of head-formation, and, even after develop-
ment is completed, possess different metabolic rates,
as susceptibility determinations show. The metabolic
rate is highest in the normal head, slightly lower in the
teratophthalmic, and still lower in the teratomorphic and
anophthalmic forms. In connection with these differ-
ences in the heads it is of interest to note that when the
different forms are fed and grow, the length which they
attain before fission varies in general with the form and
metabolic rate of the head. Under ordinary conditions
normal animals usually become twelve or fifteen milH-
meters long before undergoing fission, teratophthalmic
forms usually slightly less, teratomorphic forms from
eight to ten milHmeters, anophthalmic, from six to eight
or less, according to the degree of development of
the head-region, while headless forms rarely become
more than five or six millimeters long before dividing
and often divide at a length of only three or four
milHmeters. These differences indicate very clearly
the difference in range of dominance associated with
the differences in metabolic rate in the dominant
region.

There are many ways of inducing advance in develop-
ment of the basal zooids and the occurrence of fission in
Planaria y of which the simplest is the removal of the
head of the animal. This decreases the degree and range
of dominance to such an extent that fission almost
invariably occurs within a few days. By removal of new
heads as fast as they develop fission may be induced even
in animals much shorter than those which usually

142 INDIVIDUALITY IN ORGANISMS

undergo fission.' These and various other methods all
serve merely to increase the degree of physiological
isolation of the basal region by decreasing the degree
and range of dominance.

EXPERIMENTAL OBLITERATION AND DETERMINATION OF
AXIAL GRADIENTS AND DOMINANCE

In the case of the hydroid Corymorpha (see pp. 92, 132)
the original gradient can readily be obliterated and the
establishment of new gradients determined by experi-
mental conditions. Reconstitution in pieces four or
five millimeters or more in length from the naked region
of the stem in sea-water under the usual laboratory con-
ditions is like that in most of the longer pieces of Tuhu-
laria stem (see Fig. 63, p. 133). A hydranth develops
at the apical end of the piece, and later a second smaller
hydranth appears at the basal end. The metaboHc
conditions are also similar to those in Tubularia, and
reconstitution can be altered and controlled in much the
same way in both forms. If, however, such pieces of
Corymorpha are placed after cutting in 2-2J per cent
alcohol in sea-water the cut ends heal, but hydranths do
not develop. In the course of a few days the pieces
become shorter and more rounded, decrease in size, and
lose the characteristic structure of the Corymorpha
stem. The changes in shape are indicated in Figs. 71
and 72. On removal to water after several days in
alcohol a new hydranth begins to develop on the upper
side of the piece (Fig. 73), then a stem arises below it, and

^ Child, "Physiological Isolation of Parts and Fission in Planaria,"
Archiv fiir Entwickehingsmechanik, XXX (Festband fiir Roux), II, Teil,
19 10.

THE RANGE OF DOMINANCE

143

basal structures develop on the lower side of the piece in
contact with the underlying surface, and gradually the
piece is transformed into a new small individual (Fig. 74) .
In most cases the old outline of the piece is still pre-
served by a thin layer of hardened slime secreted by the
piece while in alcohol. This is indicated by the dotted
line in Fig. 74. Susceptibility determinations show

Figs. 71-74. — Experimental establishment of a new major axis in a
piece of Corymorpha: Fig. 71, the piece after section; Fig. 72, after
reduction in alcohol; Fig. 73, appearance of new hydranth on upper
side after return to water; Fig. 74, fully developed new individual;
dotted lines indicate old outline of piece preserved by slime.

that in alcohol the original axial gradient disappears,
and that when the pieces are returned to water a new
gradient arises in the direction in which the new axis
develops. Since the pieces adhere to the surface soon
after being placed in alcohol, it is possible to keep them
in the same position throughout the experiment and so
to be certain of the original direction of the major axis
and gradient, even though they become hemispherical

144 INDIVIDUALITY IN ORGANISMS

or nearly spherical in form. In most cases, however,
there is no difficulty as regards this point, because the
longest diameter of the pieces coincides in direction with
the original axis. A comparison of the direction of the
new axis which arises after return to water with that of
the original axis shows that the former is at right angles
with the latter. The new hydranth develops without
relation to either of the cut ends from the uppermost
region of the piece as it lies in the aquarium, and this
region was originally its lateral surface. In these cases
the alcohol not only inhibits the increase in metabohc
rate in relation to the terminal cut surfaces, which
determines the development of hydranths at the two
ends, but decreases the rate throughout the piece. In
this way it obliterates the original gradient and domi-
nance to such a degree that when the metabolic rate
rises again on return to water the original axial relations
do not reappear, but a new gradient and a new dominance
arise in relation to the external conditions to which the
piece is subjected, and the axis of the new individual
coincides in direction with the new gradient. In all
cases, so far as my experiments go, the new hydranth
arises from the uppermost part of the piece, no matter
what region of the piece in its original condition this
part represents.

When short pieces, which have already produced
biaxial hydranths (Fig. 75), are used for this experi-
ment, the changes are very similar to those described
for longer pieces. In alcohol the tentacles and the
apical regions of the two hydranths die and disintegrate,
but the more basal portions gradually lose their hydranth
structure and the pieces become small rounded masses

THE RANGE OF DOMINANCE 145

in which no structure is externally visible (Fig. 76).
After return to water a new hydranth arises, as in the
longer pieces, on the uppermost part (Fig. 77), which
represents one side of the basal region of the previously
existing hydranths, and the piece undergoes transforma-
tion into a new small individual (Fig. 78). In this
case the two opposed metaboHc gradients which were
present at the beginning of the experiment were
completely obliterated and a new single gradient
arises at right angles, or, if the pieces are not kept

78

Figs. 75-78. — Experimental establishment of a new major axis
in a piece of Corymorpha which has already formed a biaxial structure:
Fig. 75, the biaxial hydranths developed from the piece; Fig. 76, the
same piece after reduction in alcohol; Fig. 77, appearance of new
hydranth after return to water; Fig. 78, fully developed new individual.

in the same position throughout, in any relation to
the original gradients as determined by the external
conditions.

My experiments along this line were interrupted
and no opportunity to continue them has as yet arisen.
I believe, however, that the new metaboHc gradient in
these pieces is primarily determined by the difference in
oxygen supply between the free upper surface and the
surface in contact, the region of highest rate represent-
ing the region of greatest oxygen supply; but further
experiment is necessary to determine positively whether

146 INDIVIDUALITY IN ORGANISMS

this or some other factor in the environmental con-
ditions is the essential one. The important point is
that a new metaboUc gradient, major axis, or polarity
is in these cases determined by external conditions,
and that morphogenesis occurs with reference to this
gradient.

In the case of a sea-anemone, Harenactis (Fig. 79),
obliteration of the original gradient is accomplished in a
somewhat different way.^ The bodies of these animals
are tubular, with partial longitudinal partitions, the
mesenteries. When the rather bulky mesenteries are
not removed, pieces cut from the body close by gradual
contraction at each end, the wounds heal, and a new
disc and tentacles develop at the apical, and a new
*'foot'' at the basal end. If, however, rather short
pieces are taken {a, h, Fig. 79) and the mesenteries are
largely cut away from the interior of the body, the
pieces close up and heal as indicated in the longitudinal
section (Fig. 80), because there is no mass of internal
tissue to prevent the two ends meeting when the piece
contracts. In such pieces the apical cut surface of the
body wall unites with the basal about the whole circum-
ference, and the result is a ring or doughnut-shaped
structure which makes an attempt to orient its body as it
does in nature by revolving about a circular axis like a
vortex ring until the region of union of the two ends
lies on its upper or outer surface.

At this region of union more or less new tissue arises,
particularly if the cut surfaces are irregular and do not

^ Child, "Factors of Form Regulation in Harenactis aUenuata, I, II,
III," Jour, of Exp. ZooL, VI, VII, 1909; "Further Experiments on
Adventitious Reproduction and Polarity in Harenactis," Biol. Bull., XX,
1910.

THE RANGE OF DOMINANCE

147

Figs. 79-83. — Reconstitution in "rings" from sea-anemone,
Harenactis attenuata: Fig. 79, longitudinal sectional outline of animal,
indicating regions, a,h, from which pieces are taken; Fig. 80, diagram-
matic longitudinal section through a "ring," showing method of closure
by union of apical and basal cut surfaces of body wall; Figs. 81, 82,
tentacle groups arising from the region of union of cut surfaces; Fig. 83,
a perfect animal developed on a ring.

148 INDIVIDUALITY IN ORGANISMS

unite smoothly, and from this new tissue all gradations
from single tentacles, through groups of tentacles of
various sorts up to complete small anemones (Figs.
81-83) arise.

The various tentacle groups in Figs. 81 and 82 and
the individual in Fig. 83 are made up of cells which are
descended from both apical and basal ends of the piece
and a more or less definite new individuation occurs
in these cells. There can be little doubt that in these
cases the origin of these various degrees of individuation
is associated with the growth of new tissue at the line of
union between the cut surfaces. The metaboHc rate in
this tissue is higher than in the other regions of the piece,
and if it is enough higher the new tissue becomes inde-
pendent and produces a new apical region, or some part
of it, according to conditions. Wherever, about the
circumference, growth of new tissue is most rapid and
extensive, there the new individual is most likely to arise.
Often it is possible to determine beforehand the region
of the circumference where such tentacle groups or
individuals shall arise, by making the outUne of one or
both cut surfaces irregular at some point or making a
number of small cuts near together in them. In such
regions there is more growth of new tissue and a new
gradient and new individual are more likely to arise.

As regards the minor axes, it is of great interest to
note the wide range of variations which occurs. Many
bilaterally as well as radially symmetrical and asym-
metrical forms appear among the tentacle-groups, and
it is evident that the symmetry of the groups is in many
cases related to the line of union and not to any pre-
existing symmetry of the parent animal. In these rings

THE RANGE OF DOMINANCE 149

we see new individuals being localized and developing
where it is impossible to conceive of any internal local-
izing and determining factors other than quantitative
metabolic conditions.

In the case of Planaria I have been able to increase
the frequency of biaxial heads (see Fig. 48, p. 99) in very
short pieces by partially narcotizing the animals before
cutting and keeping the pieces in a dilute solution of a
narcotic, e.g., chloretone, for a day or two before allow-
ing them to develop. Under such conditions the meta-
bolic rate in the pieces is of course decreased, and so
dominance in the direction of the original gradient is
still further decreased. Consequently, when the pieces
are returned to water and allowed to develop, the con-
ditions are even more favorable for the establishment of
the reversed gradient at the basal end, and biaxial
structures develop in a larger percentage of cases than
when the pieces are not narcotized. The effect of the
narcotic is simply to aid in decreasing the dominance of
the original apical region of the piece and so to increase
the probabihty of the establishment of an effective
reversed gradient and dominance at the basal end.
This experiment has not as yet been attempted with
Tubularia, but will no doubt be successful with proper
technique.

THE EXTENSION OF DOMINANCE DURING DEVELOPMENT

That the range of dominance undergoes extension
during development is evident from many facts. In the
young Planaria, for example, a second zooid arises at
the posterior end of the body when the animal is less than
five millimeters in length, i.e., the range of dominance

I50 INDIVIDUALITY IN ORGANISMS

at this stage of development is only three or four milli-
meters/ In the adult animal, however, the range of
dominance as indicated by the length of the first zooid
may be ten or twelve millimeters or even more under
certain conditions. Evidently with advancing differen-
tiation of the nervous system the conductivity has
increased, and so the transmission-decrement has be-
come less and the range of transmission greater.

In Stenostomum also the more advanced the devel-
opment of a zooid, the greater the distance from its
head-region at which the head-region of a new zooid
is determined, as will appear by reference to Fig. 29
(p. 81). Other animal forms which undergo agamic
reproduction show similar relations, and it is also
probable that the increasing capacity for co-ordination
and control of parts with advancing development, so
far as it depends on the nervous system, results to
some extent from the increase in efficiency of trans-
mission, though various other factors may also be
concerned.

In plants also similar relations appear. In the dif-
ferentiated part of the plant stem the range of domi-
nance of a bud or a growing tip over others is very much
greater than in the embryonic region of the growing tip,
but their later development is inhibited by the growing
tip as a whole, even though further growth has greatly
increased the distance between them. The dominance
of the growing tip as a whole has a much greater range
in the differentiated parts of the plant than the domi-
nance of its apical region over much nearer parts in the

^ Child, "Studies on the Dynamics of Morphogenesis. Ill,"
Jour, oj Exp. ZooL, XI, 1911.

THE RANGE OF DOMINANCE 151

embryonic or slightly differentiated tissue of the grow-
ing tip itself.

In the higher animals the extension of dominance is
evidently very much greater than in the lower forms.
In the medullated nerve fibers of the higher vertebrates
the transmission-decrement is so slight that some authors
have denied its existence. Various lines of experiment
have indicated, however, that a transmission- decrement
does exist even in vertebrate nerves (see pp. 173-75).
Tashiro has shown that a gradient in carbon-dioxide
production exists in nerve fibers, and I have observed a
distinct susceptibility gradient in certain nerves. The
nerve is essentially a specialized protoplasm which
conducts with less decrement and therefore to greater
distances than other kinds of protoplasm, and the
central nervous system arises in those regions of the
body where the transmitted changes primarily originate.

The extension of dominance during the development
of the higher animals is so great that the range of domi-
nance is undoubtedly very much greater than the size
of the individual. In these forms individual size is
limited, not by the range of dominance, but by the
decrease in metabolic rate which accompanies the pro-
gressive differentiation, and so limits growth. Only
in early stages of development, or in the lower organ-
isms, where nerves are either absent or not very good
conductors, does the size of the individual equal the
range of dominance.

EXPERIMENTAL PHYSIOLOGICAL ISOLATION AND
REPRODUCTION IN PLANTS

The course of development in the single plant
individual suggests the dominance of the growing tip

152

INDIVIDUALITY IN ORGANISMS

of the stem, but physiological isolation of parts and
reproduction of new individuals afford the only means
of demonstrating experimentally the existence of domi-
nance and its varying range. From among the accu-
mulated data concerning what the botanists commonly
call correlation, a few simple, well-known experiments

are briefly described to
show how readily physio-
logical isolation and repro-
duction may be brought
about in plants.

The young seedling of
a leguminous plant (pea,
bean) possesses the general
form indicated diagram-
matically in Fig. 84. The
further normal develop-
ment of the stem consists
primarily in its elongation
and the development of
leaves by the activity of
the growing tip at its
apical end, but if this
growing tip is removed a
new growing tip, or in some
cases more than one, arises
from the axillary region of each cotyledon, as indicated
in Fig. 85. These axillary shoots very rarely appear
when the original growing tip is present and active, but
their development results regularly from its removal.
If both of the shoots grow at about the same rate they
may both continue to develop and so give rise to two

Figs. 84, 85. — Diagrammatic
outlines of leguminous seedlings,
illustrating effect of removal of
growing tip: Fig. 84, uninjured
seedling; Fig. 85, development of
shoots from axils of cotyledons
after removal of stem-tip.

THE RANGE OF DOMINANCE 153

stems, each of the same character as the single stem in
normal plants, but if one grows more rapidly the growth
of the other is usually soon inhibited and only the one
continues to develop. If, instead of removing the
primary growing tip, we inhibit its metabolic activity
in any way without killing it or injuring it otherwise, the
result is the same as if it were removed. Inclosure of
the primary growing tip in plaster of paris of in an
atmosphere of hydrogen accompHshes this result without
injury, for it is capable of resuming growth after removal
of the plaster or return to air. If the primary tip is
inhibited in this way until the axillary shoots have
appeared and is then allowed to resume its activity, the
growth of the axillary shoots is in turn inhibited and
the primary stem continues its development, unless the
axillary shoots have attained a length two or three times
as great as that of the main stem before the inhibition of
the primary tip is removed. In that case the further
growth of the primary tip may be almost entirely
inhibited by the axillary shoots, and it may even die,
while they, or one of them, as the case may be, continue
development. Many modifications of the experiment
are possible at different stages of development and in
different plants. In stems with lateral buds, such as the
willow, if the apical growing tip is removed the upper-
most lateral bud or buds will develop and their develop-
ment inhibits the development of those lower down;
if we remove them or prevent their development by
inclosing them in plaster, the buds next below will
develop, and so on.

In many plants removal or inhibition of all the
growing stem-tips present results in the formation of

154 INDIVIDUALITY IN ORGANISMS

so-called ''adventitious" buds, which may arise from
differentiated cells, as in the case of the begonia (Figs.
38, 39), and may be scattered irregularly over various
parts of the plant according to the conditions of the
experiment. Often the presence of a single one of the
original buds is sufficient to inhibit the formation of
these adventitious buds. The appearance of adventi-
tious buds on plants in nature is usually due to the
weakening of existing growing tips through advancing
age or injury of some sort.

Such adventitious buds very often arise in large
numbers simultaneously without any regular arrange-
ment with reference to each other. The absence of
definite space relations in such cases is undoubtedly
due to the fact that they arise simultaneously, or nearly
so. Various cells here and there which happen to have
a slightly higher metabolic rate than others begin to
develop into new buds at about the same time; conse-
quently none of the buds is dominant over the others.
If, however, one of the adventitious buds gets a start
beyond the others in any way, it inhibits the further
development and may even bring about the death of
others within a certain distance of it. Moreover, where
a gradient is present in the part on which the buds ap-
pear, so that one or more buds appear first in a certain
region — the region of highest metabolic rate in the part —
they inhibit the growth of others within a certain dis-
tance or throughout the part.

In various conifers the dominance of the growing tip
of the main stem appears in a somewhat different form.
In these trees, as long as the growing tip of the main
stem is present and active, lateral branches arise radially

THE RANGE OF DOMINANCE 155

around the main stem and grow outward from the trunk,
and the branches of the second order arise in most cases
more or less bilaterally on them. Removal of the main
growing tip is followed by the bending upward of one or
more of the uppermost lateral branches, further growth
in the vertical direction, and radial instead of bilateral
outgrowth of new branches. Here one or more of the
lateral branches nearest the upper end of the stem
react to the absence of the main growing tip by changing
direction and form of growth to that characteristic of the
original tip. If this branch is removed, branches farther
down the trunk react in the same way.

According to most authorities, dominance of one
part over another is effective only or chiefly in one
direction along the stem, namely, from the apical end
downward. Buds or growing tips at or nearer the
apical end are capable of inhibiting buds farther down
the stem, but the latter are not capable or are less
capable of inhibiting the former. In recent experi-
mentation,^ however, it has been demonstrated that
these relations may be reversed, and that if shoots lower
down are allowed to grow for a long enough time and to a
large enough size, while buds higher up are inhibited
by artificial means, the lower shoots sooner or later
acquire the ability to inhibit the higher ones after the
removal of the artificial inhibition. This is what
might be expected if inhibition depends on the relations
of metabolic gradients. Under ordinary conditions
the upper levels of the stem represent higher levels in
the gradient and therefore inhibit or obHterate gradients

* W. Mogk, " Untersuchungen iiber Korrelationen von Knospen
und Sprossen," Archiv fur Entwickelungsmechanik, XXXVIII, 19 14.

156 INDIVIDUALITY IN ORGANISMS

lower down more readily than these with their lower
rate are able to reverse the whole estabhshed proto-
plasmic gradient higher up. If, however, a new gradient
at a lower level becomes established while the dominant
region above is inhibited, it is conceivable that it may in
time, by its gradual extension in the stem, obhterate
more or less completely, or perhaps reverse, the original
gradient and so dominate regions higher up, at least to
some extent. This is apparently the case in the seedling
mentioned above (p. 153) when the axillary shoots are
allowed to grow long enough while the main growing
shoot is inhibited. Under such conditions they are
apparently able to inhibit what was originally the domi-
nant region of the whole plant.

-It is often possible to isolate a part of the plant from
the dominance of the growing tip merely by cutting the
vascular bundles connecting the two parts. The devel-
opment of buds on the leaves of certain plants may be
induced by severing the chief vein or veins of the leaf,
other tissues remaining intact. In such cases buds
appear peripheral to the cut, usually near the veins,
but in some plants on the leaf margins.

The inhibiting influence is not confined to the grow-
ing tips of stems, for it has been shown that a leaf plays
apart in inhibiting the growth of the bud in its axil.
Removal of the leaf or inhibition of its activity may
bring about outgrowth of the bud, if the inhibition
from other souces is not too complete. In certain cases
it has been shown that one part of a leaf may inhibit
other parts. In Cyclamen persicum, for example, the
young seedling (Fig. 86) possesses at first only a single
leaf, one of the cotyledons. Removal or inhibition

THE RANGE OF DOMINANCE

157

by inclosure in plaster of the distal part of the blade of
this leaf before its growth is completed is followed by the
development of a new leaf surface from each side of the
basal portion, as in Fig. 87. When the whole blade of
the leaf is cut off or inhibited, the margins of the petiole
just below the level of the cut give rise to a separate new
leaf on each side (Fig. 88). Here the basal portion of
the leaf and the distal region of the petiole margin

Figs. 86-88, — Dominance and physiological isolation in leaf of
Cyclamen persicum: Fig. 86, intact seedling (from Hildebrand); Fig. 87,
development of new leaf blade from each side of leaf base after removal
of more apical portion; Fig. 88, development of new leaf from each side
of petiole margin after removal of whole leaf (from Goebel).

evidently possess the capacity to develop as a leaf, but
are prevented from doing so as long as the original leaf
or its distal portion is present or active.

Attention has been called to the fact that roots,
wherever they appear on the plant, are apparently
subordinate, specialized individuals and originate in
definite relations to parts which represent regions or
levels physiologically less remote than the root-tip
from a stem-tip or bud (see pp. 104, 105). Most plants

158

INDIVIDUALITY IN ORGANISMS

with roots possess, however, not a single root, but a
root system which is a composite individual, each root
representing a single constituent individual. In such a
root system relations of dominance and subordination
similar to those in stem systems exist. The formation of
each new root represents a reproduction and the estab-
lishment of a new root individual. In plants possessing
a single main root with lateral roots arising from it (Fig.
84) this relation appears very clearly. As the main root
grows in length directly downward, lateral roots arise

Figs. 89-91. — Effects of removal or inhibition of main root-tip on
direction of growth of lateral roots (from Bruck).

successively at a certain distance from its growing tip
and grow obliquely downward or almost horizontally.
Experiments with seedlings show that if the growing tip
of the main root is cut off, new lateral roots arise in
larger numbers or nearer the end of the main root, and
one or more of these nearest the cut end grows more
nearly in the vertical direction downward than when the
main growing tip is present (Figs. 89, 90), the behavior
differing somewhat according to the level of the cut.
Apparently in these seedlings the lateral roots which

THE RANGE OF DOMINANCE 159

have already developed do not change their direction
of growth when the chief growing tip is cut off; only
those which develop after the operation react, but they
or some of them develop as main instead of lateral roots
and later themselves give rise to lateral roots. If the
outgrowth of new roots near the cut surface is inhibited
after the removal of the main growing tip by inclosing
this region of the main root in plaster, roots which arise
above the inhibited region may react by growing more
directly downward, provided they are not too far away
from the cut surface (Fig. 91). The lateral roots which
react in this way to the absence of the main growing
tip resemble more or less closely the main root in their
later development. When the growing tips of all roots
are cut off, adventitious roots arise, usually in large
numbers and without any definite order, on the parts
remaining. Evidently the relation between the con-
stituent parts of the root system is a relation of domi-
nance and subordination like that in the stem system.

The root system as a whole seems to exert an inhibit-
ing influence on the development of roots in other parts
of the plant. When the whole root system is removed
or its metabolic activity inhibited, new roots usually
develop from the basal region of the stem if external
conditions permit their growth there; if not, they may
appear higher up on the stem. The propagation of
plants by cuttings depends on this ability to produce
roots on the stem in the absence of the root system. In
an experiment described by Goebel and represented
diagrammatically in Fig. 92, a bean seedHng was placed
in nutritive solution, b, which was kept at low tempera-
ture, whereby the activity of the root system was largely

i6o

INDIVIDUALITY IN ORGANISMS

inhibited. A part of the stem was then surrounded with
water, a, at ordinary temperature to provide the mois-
ture necessary for the growth of roots, and roots arose

on this region. Submer-
ging part of the stem in
water in this way does not
result in the development
of roots when the original
root system is active. By
inclosing a region of the
stem in a chamber con-
taining ether vapor, and
thus anesthetizing but not
kilhng it, McCallum was
able to induce the forma-
tion of roots above the
anesthetized region, as in-
dicated in Fig. 93. In this
experiment the original
root system was present
and uninjured, but the re-
gion above the anesthe-
tized level was apparently
cut off from its influence,
and, the moisture being
sufficient, new roots ap-
peared near the basal end.
These experiments with roots seem to indicate that
not only does a relation of dominance and subordination
exist between the different parts of a root system, but
that the root system as a whole dominates the stem to a
certain extent, so far as the production of roots is con-

FiGS. 92, 93. — Diagrammatic
figures illustrating experiments on
root production on the stems of
seedlings; only lower parts of
plants shown: Fig. 92, formation
of roots on stem at a when this
region is kept moist after inhibi-
tion of original root system, b, by
low temperature (after Goebel);
Fig. 93, formation of roots above
a region of stem inclosed in
narcotic atmosphere (after
McCallum's description).

THE RANGE OF DOMINANCE . i6i

cerned. If this dominance and the dominance of the
stem-tip both result from metabolic gradients, then
there must be in plants possessing roots two metabolic
gradients in opposite directions, the apical region of one
being in the stem-tip or tips, that of the other in the
root-tip or tips.

Two gradients in opposite directions along the same
axis cannot exist at the same time without interfering
with and partially obliterating each other unless they
have different paths of transmission or are of different
metabolic character. Concerning the possibility of the
simultaneous transmission of different metabolic changes
in different directions in the same protoplasm we know
nothing, and our knowledge of conducting paths in the
plant does not go far beyond the fact that some part of
the vascular bundles seems to transmit some kind of
change better than other tissues.

It is possible, however, that the influence of the root
system on the stem as a whole may be different in
character from the dominance of the main root-tip on
lateral roots. This possibility is suggested by the fact
that the range of dominance within the root system is
rather short, even where the tissues are differentiated,
while the apparent dominance of the root system as a
whole over the stem and other parts of the plant is
apparently unHmited in range or without relation to
distance. The root system takes up water and nutri-
tive salts and these are transported to other parts of
the plant. It is conceivable that the inhibiting influ-
ence of the root system on the formation of roots in other
parts of the plant may be rather a transportative than a
transmissive correlation, and that the other parts give

i62 INDIVIDUALITY IN ORGANISMS

rise to roots when this transportation falls below a
certain minimum or when they are isolated from it in any
way. This alternative is more nearly in accord with the
views of most botanists, and it seems at present more
satisfactory than the assumption of two opposed and
overlapping gradients. If, however, this relation
between root system and other parts is transportative
rather than transmissive, McCallum's experiment de-
scribed above of bringing about physiological isolation of
the upper levels of the stem from the root system by
local anesthesia seems to indicate that the transportation
is not a simple physical process but is dependent in
some way and to some extent upon the metabolic
activity of living cells.

If we accept this alternative and admit at the same
time the primary dominance of the stem-tip or tips and
the secondary dominance within the root system of the
root-tip or tips we must regard the root system as a sub-
ordinate specialized constituent individual of the com-
posite plant individual. The root, like the leaf, is
primarily determined by relations to other parts of the
plant, but requires certain external conditions for its
development and differentiation. Like the leaf also, the
root or root system shows a certain degree of second-
ary individuation among its parts.

The formation of roots is the reaction of a plant
individual to a certain relation between internal and
external conditions, and this relation may apparently
be brought about either by the inhibition of activity in,
or absence of, the original root system, or in many cases
by changes in the external conditions, such as decrease
in light and increase in, moisture, even though the

THE RANGE OF DOMINANCE 163

original root system is present. The root of the plant,
like the basal end of the animal body, is the morpho-
logical expression of the performance of a certain func-
tional activity primarily subordinate to and dependent
upon the activities of other parts. Without the activi-
ties of parts representing higher levels in the primary
gradient, root formation does not occur, but when it has
occurred the products of the special metabolic activity
of roots transported to other parts affect the metabolic
processes there and so inhibit more or less effectively the
formation of roots there.

From this point of view the apparent dominance of
the root system over other parts of the plant with respect
to root formation is not a feature of the primary and
fundamental relation of dominance and subordination
in the individual, but rather a secondary relation — trans-
portative rather than transmissive — unlike the primary
relation, and resulting from local differentiation which
is itself associated with and dependent upon the primary
relation.

THE LOCALIZATION OF EXPERIMENTAL REPRODUCTION IN
RELATION TO DIFFERENT AXES

It is often possible to alter the localization of the
new dominant region in the reconstitution of an isolated
piece by altering the gradient relations of the piece. A
few examples from the flatworm, Planaria, among
the animals and the liverwort, Marchantia, among the
plants will illustrate the point.

It has been pointed out (pp. 80, 81) that the out-
growth of new tissue on a piece of Planaria isolated by
transverse planes of section is most rapid in the median

164

INDIVIDUALITY IN ORGANISMS

ventral region of the apical end, this region represent-
ing the region of highest metaboHc rate or irritabiHty
resultant from the three main axial gradients. By alter-
ing the shape of the piece in relation to the axial gradients
it is possible to alter the position of this outgrowth and
so the position of the new head. In a piece cut very
obliquely {ahcd, Fig. 94), the head develops as in Fig. 95,
and the side of the head which arises from the more

98

Figs. 94-98. — Localization of head-formation in the reconstitution
of pieces of Plandria as resultant of apico-basal and transverse axial
gradients: Fig. 94, diagrammatic outline of part of body of Planaria,
indicating shapes of pieces; Fig. 95, asymmetrical position of head in
reconstitution of piece, abed; Fig. 96, reconstitution of piece, aehd;
Fig. 97, reconstitution of piece, aegi; Fig. 98, reconstitution of piece, afi.

apical level of the piece is likely to develop somewhat
more rapidly than the other side. This asymmetry
of position and development is due largely to the fact
that one side of the cut surface represents a higher level
in the major axial gradient than the other and so reacts
more rapidly. When the cut surface is oblique, the
major gradient becomes a factor in determining the
position of most rapid dedifferentiation, division, and
new development of ceils, and this determines the

THE RANGE OF DOMINANCE 165

position of the new head. In a piece aehd, Fig. 94,
the head develops, as shown in Fig. 96, on the apical cut
surface, but in a shorter piece aegi, Fig. 94, the head is
likely to appear at an angle to the apical and median cut
surfaces, as in Fig. 97. This condition results when the
metabolic rate of the cells on the median cut surface is as
high as that of the cells on the apical cut surface, so that
both take an equal part in giving rise to the new head.
In pieces like afi, Fig. 94, the head oiten develops nearly
or quite in the direction of the transverse axis (Fig. 98).
In such pieces there is little difference in metabolic rate
between apical and basal cut surfaces, and the cuts are
not sufficiently oblique so that the higher level in the
major gradient of the lateral as compared with the
median region of the cut surface overbalances its lower
level in the transverse gradient. Consequently the
median regions of both cut surfaces represent the region
of highest rate or irritability in such a piece and therefore
become the head-forming region. For these and many
other experimental modifications of the position of the
head in reconstitution no satisfactory general basis of
interpretation has heretofore been discovered, but I
know of no case which cannot be very simply accounted
for in terms of axial metabolic gradients.

In the bilaterally symmetrical liverwort Marchantia
(Fig. 23, p. 78), the gradient-relations are apparently very
similar to those in Planaria. In these plants practically
every cell of the body is capable of giving rise to a new
plant, but in pieces without the growing tip new growing
tips originate in definite relations to the axes, and their
presence inhibits the formation of others. In general,
on transverse cut surfaces new individuals arise, like

1 66 INDIVIDUALITY IN ORGANISMS

the head in Planaria, in or near the median ventral
region of the apical end of the piece just basal to the
cut surface (Fig. 99). When the piece is taken from the
lateral margin of the plant body and does not contain
the median region, individuals usually arise near the
apical end and ventrally on the most nearly median
region of the piece (Fig. 100). In pieces with oblique
instead of transverse apical cut surfaces the position
of the new individual varies according as the piece
contains part of the midrib or not, according to the
obliquity of the plane of the cut, and probably also
according to the region of the body. Where the piece
does not include the midrib the new individual usually
arises ventrally near the most apical region of the piece,
the major gradient being the chief factor in determining
its position. Thus in Fig. loi the new plant appears
near the lateral margin, undoubtedly because the meta-
bolic level is higher here than elsewhere. The con-
ditions here are apparently much like those which
determine the asymmetrical position of the new head in
Planaria in Fig. 95. In pieces which contain a part of
the midrib this is usually the chief factor in determining
the position of the new head. The piece in Fig. 102,
for example, is cut from one side of the body and includes
part of the midrib at the basal end of the oblique cut, and
the new bud arises here. The influence of the midrib
in localization in this form depends on the fact that the
cells in this region retain their capacity for growth and
division much longer than the cells of the lateral regions,
and so they represent a relatively high metabolic level
and bear much the same relation to the transverse
gradient that the apical growing tip does to the major

THE RANGE OF DOMINANCE

167

gradient. Because of the relatively high metabolic level
of these cells along the midrib this region plays a more
important part in the localization of reproduction than
the median region in Planaria. In fact, the experi-
mental evidence seems to indicate that the chief differ-
ence in axial relations between Marchantia and Planaria
is the higher metabolic level of the apical region of the
transverse gradient, the median region of the body.

Figs. 99-102. — Localization of new individual a*s resultant of differ-
ent axial gradients in pieces of liverwort, Marchantia: Fig. 99, usual
position in median ventral region near apical end of piece; Figs. 100-
102, different positions of new individual apparently determined by the
different relations of the axial gradients according to shape of piece
and region represented (from Vochting) .

With advancing age the region of the midrib undergoes
gradual differentiation and so loses to a greater or less
extent its high metabolic rate.

These experiments and many others which cannot be
discussed here are highly significant in that they indi-
cate the essential identity in character of the different
axes of the physiological individual. In fact, I believe
they constitute evidence of the greatest importance

168 INDIVIDUALITY IN ORGANISMS

for the fundamentally quantitative character of at least
the main axes of the body, for if the different axes are
qualitatively different, I cannot conceive how the
position of a new head or growing tip on an isolated
piece can be determined in one case chiefly by the major
axis, in another as a resultant of two or more axes, and
in a third by one of the minor axes. If, however, all
axes are fundamentally gradients in metabolic rate, the
facts are very simply accounted for, as I have tried to
show. The major axis is the major axis, not because
its nature is fundamentally different from that of other
axes, but because it arises first or because its apical region
has the highest metabolic rate of any part of the body,
and the minor axes are minor axes because they arise
later or their apical regions have a lower rate. When
the major gradient is in any way obliterated to a suffi-
cient degree one of the minor gradients may act in
exactly the same way as, though often more slowly
than, the major gradient where it is present. This
is true, of course, only for forms and stages in
which the fundamental quantitative character of the
axes has not been too greatly altered by progressive
differentiation.

The fact that a plant bud may be inhibited by the
main growing tip, by another bud, by a growing leaf, or
by a lateral branch also indicates that there is nothing
specifically different in these different inhibitions and so
suggests that these different plant axes act in essentially
a quantitative way in dominating other parts. One
may be substituted for the other without altering the
character of the effect.

THE RANGE OF DOMINANCE 169

CONCLUSION

It is possible to control and alter experimentally
the spatial relations of parts in the individual by altering
the length of the metabolic gradient and so the range
of dominance. Parts of the individual may come to lie
beyond the range of dominance in consequence of
increase in size of the whole, of decrease in range and
degree of dominance by decrease in the metabolic rate
in the dominant region, of decrease in conductivity of
the paths of correlation, and of the direct local action
of external factors which increase the independence of
subordinate parts. Parts thus physiologically isolated
may reproduce new individuals if the essential axial
gradients exist, or arise in them. In many of the lower
organisms the original axis or axes may be experi-
mentally obliterated and a new axis and dominance
established in relation to external conditions which
determine differences in metabolic rate in different parts
of the mass. In general, the range of dominance
increases during the development of the individual
because the conductivity of the protoplasm increases,
and special conducting paths develop as the morpho-
logical expression of the fundamental correlative con-
ditions in the individual. The essentially quantitative
character of different axes of the individual is indicated
by the fact that one axis may be experimentally substi-
tuted for another in determining the localization of a new
individuation.

CHAPTER VI

DISCUSSION, CONCLUSIONS, AND SUGGESTIONS

THE NATURE OF DOMINANCE

It has been assumed thus far that dominance depends
on a transmitted change, or excitation, rather than on
the transportation of substance, and it now becomes
necessary to consider what basis there is for this con-
clusion. As already pointed out (pp. 26, 27), some
sort of organization must be present in order that trans-
portative or chemical correlation may occur in a definite
and constant manner. If different regions of the body
produce specifically different substances they must be
specifically different, and if these substances act on
certain other parts in a definite specific way those parts
must possess a certain constitution. The data of
experimental reproduction discussed in earlier chapters
show that new individuals arise from parts of old indi-
viduals which either cannot possibly possess the ^'organi-
zation" of a complete individual or must possess an
indefinite number of such organizations. The latter
alternative leads to a conception of the Weismannian
sort, and I have tried to indicate how unsatisfactory
such conceptions are (pp. 22, 23).

If, on the other hand, the individual is primarily
a metaboHc gradient in a specific protoplasm, the only
primary difference between the dominant and other
levels of the gradient is a difference of metabolic rate.
At this time the products of metabolism at different

170

CONCLUSIONS AND SUGGESTIONS 171

levels of the gradient are not specifically different, but
differ in quantity. If the transportation of chemical
substances is the only means of correlation between
the different levels of the gradient, it is impossible to
understand either how the gradient can persist or how
a relation of dominance and subordination can arise
between levels of higher and those of lower metabolic
rate. Specific chemical correlation between parts is
possible only when specifically different parts are present,
and the definite space relations which we find associated
with physiological dominance do not usually appear in
such correlation. In short, I believe it is impossible
to conceive of the process of organic individuation with
the definite, constant, and orderly character which it
actually possesses as having its origin in transportative
or chemical correlation alone.

If, however, the metabolic gradient arises and is
maintained by the transmission of excitation from the
region of highest metabolic rate, this region becomes
dominant simply because its metabolic rate is so high
that it determines and maintains the gradient in rate,
and the differences in rate at different levels bring about
sooner or later differences in constitution and character
of the protoplasmic substratum. In regions of high
rate only certain relatively stable substances remain as
constituents of the substratum, and others are broken
down and eliminated. In regions of lower rate, on the
other hand, other substances accumulate as parts of
the substratum because under these conditions they are
less readily or less rapidly broken down than where the
rate is higher, and it is also probable that the character
of synthesis differs with the rate of metabolism. In

172 INDIVIDUALITY IN ORGANISMS

this way each level of the gradient develops a character-
istic protoplasm and the character of the protoplasm
in turn modifies and alters the character of the reactions,
and so specific, or what we call qualitative, differences
arise, and different specific substances may be produced
at different levels of the gradient. At the moment when
these specific differences first appear chemical correlation
in the commonly accepted sense becomes possible, and
from this time on it may play a part in determining the
character of further changes at the various levels. After
chemical correlation appears it is unquestionably a
factor of great importance in determining the character
of the various parts and so of the individual as a whole.
The point which I wish to emphasize is that chemical
or transportative correlation does not and cannot
account for the origin of the individual, because the
individual must exist as some sort of orderly and definite
relation or organization before orderly and definite
chemical correlation between its parts is possible. The
dynamic conception of the individual is primarily con-
cerned, not with the orderly specificities of chemical
correlation, but with the conditions in protoplasm which
make those orderly specificities possible.

The occurrence of transmission in living protoplasm
is a familiar fact. The existence of a transmission-
decrement and therefore of a limited range of effect-
iveness has been demonstrated for the transmission
of stimuli in plant tissues and in various animal nerves.
In many of the lower animals the range of effectiveness
in transmission can readily be observed by means of
the range of reaction to stimuH of different intensity.
In transportative correlation a definite range of effective-

CONCLUSIONS AND SUGGESTIONS 173

ness cannot exist unless transportation is uniform and
constant in rate in all parts at each level and the sub-
stance is gradually destroyed or transformed during
the transportation. The dynamic theory affords an
adequate basis for the very definite range of dominance
which we find in organisms, and a chemical theory
does not.

Tashiro's recent investigations on carbon-dioxide
production and my observations on susceptibility
gradients in the nerve indicate that physiological domi-
nance in the neuron, i.e., the direction of transmission, is
associated with the existence of a metabolic gradient.
Individuation in what is probably the most highly
specialized cell individual in the organism apparently
starts from the same condition, the metabolic gradient,
as in the simplest axiate animal or plant. It is certain
that dominance in the neuron depends primarily on
transmission and not on transportation. This argu-
ment from the highly specialized to the simple is
perhaps not of great value; still I cannot but believe
that the existence of an axial gradient in metabolic
rate in the neuron and in the simple axiate individuals
among the lower organisms is a fact of real significance.

It has been very generally believed by physiologists
that the nerve, at least the medullated nerve of verte-
brates, transmits excitations under normal conditions
without a decrement in energy or intensity. It is,
however, a well-known fact that even in these nerves a
decrement appears when transmission takes place at
low temperature or in partially narcotized or com-
pressed nerves; in fact, under various conditions which
decrease metabolic rate or irritability in the nerve.

174 INDIVIDUALITY IN ORGANISMS

Those who hold that the nerve in normal condition
transmits without a decrement have usually maintained
that under depressing conditions the nerve behaves
in a different way from the normal nerve and that the
decrement exists only under these conditions. In view
of the fact that in the nerves of the lower animals a
transmission-decrement undoubtedly occurs normally,
and that in protoplasmic transmission in the absence of
nerves the decrement is even more marked, the grounds
for the belief that transmission without a decrement
occurs in the vertebrate nerve do not appear to be ade-
quate. It seems scarcely probable that the higher
degree of specialization of the vertebrate nerve has
brought about a fundamental change in the character
of transmission of such a nature that the decrement is
reduced to zero and transmission to an indefinite or
infinite distance is possible. The experiments along
this line prove only that with the very limited lengths of
nerve available the decrement under normal conditions
is very slight or inappreciable. Evidently the nerve
of the vertebrate, and particularly of the higher verte-
brate, is a much better conductor than undifferentiated
protoplasm or even than the nerves of lower animals, and
within the limits of the individual vertebrate body the
decrement is undoubtedly slight or practically absent
when the nerve is in good metabolic condition, but the
conclusion that there is no decrement in such cases
seems unwarranted. It is also highly improbable that
the nature of transmission in the cooled, partially
narcotized, or compressed nerve is essentially different
from that in the same nerve under normal conditions,
and since a decrement appears under depressing condi-

CONCLUSIONS AND SUGGESTIONS 175

tions, the only conclusion justified by the facts seems
to be that a decrement must exist in normal transmission,
but is much less marked, and the range of transmission
is therefore much greater, than under depressing con-
ditions. Undoubtedly in the higher animals the range
of transmission is very much greater than the limits of
the individual body, for the size of the individual in
these forms is limited by other factors than the range
of dominance (see pp. 46, 47, 151), but that transmission
without decrement occurs is far from being demon-
strated and, as I have endeavored to show, there is
much evidence against such a view.

It is also a highly significant fact that the nervous
system, which is the chief conducting organ of the body
in those forms which possess it, develops in a definite
relation to the axial gradients. The dominant region
of the nervous system appears in the apical region of
the major axial gradient, and at other levels of the body
which contain the central nervous system it represents
the region of highest metabolic rate in the minor gradi-
ents. If the unity of the organism depends primarily
upon transportation, there is no apparent reason why
it should change to a unity depending on transmission
or why the dominant region of the central nervous
system should arise in the dominant region of the
primitive individual. If, however, organic unity is funda-
mentally and from the beginning dependent upon trans-
mission, the general plan and arrangement of the nervous
system are very evidently the expression in specialized
structure and function of the primary unity and relation
which was the starting-point of individuation, and domi-
nance or control by nervous transmission is merely

176 INDIVIDUALITY IN ORGANISMS

a specialized and more effective modification of the
dominance which is the foundation of organic unity and
order.

Moreover, the nervous system dominates or controls
the chemical activities of the organism to a very con-
siderable degree. If the primary dominance is purely
a matter of chemical correlation, it is difficult to con-
ceive how the functional dominance of the nervous
system has come about, but if the primary dominance
depends upon transmission of the same general char-
acter as nervous transmission, the functional dominance
of the nervous system is the natural and necessary
consequence of the primary relations.

As regards the role of the nervous system in develop-
ment and reconstitution, there has been much differ-
ence of opinion. Many biologists have maintained
that the nervous system exerts a specific formative
influence on various parts and so determines their
course of development and differentiation, while others
deny the existence of any such influence. In the case
of certain organs and parts, e.g., striated muscle, it has
been definitely demonstrated that embryonic develop-
ment may occur without nervous connection, but in
the mature condition frequent nervous stimulation is
necessary for maintenance of structure and function.
And as regards reconstitution, some investigators have
found that certain parts, such as the amphibian leg,
regenerate incompletely or not at all in the absence of
nerves, while others have maintained that connection
with nerves is unnecessary for complete regeneration
of these parts. These apparently contradictory and
confusing results can, I believe, be very simply inter-

CONCLUSIONS AND SUGGESTIONS 177

preted and harmonized. If the metabolic rate in the
organ or part in question is sufficiently high, it is ca-
pable of undergoing its characteristic development and
differentiation without nervous stimulation, assuming
of course that its other relations as a part of the indi-
vidual are not fundamentally altered; but when its
intrinsic metabolic rate falls below a certain level its
development does not occur, or is incomplete, or it
undergoes atrophy unless its rate is further increased
by nervous stimulation. In the case of striated muscle
during the earlier stages of development the intrinsic
metabohc rate is high enough to permit without nervous
stimulation the accumulation of structural material
and the characteristic course of differentiation deter-
mined by other correlative conditions, but as differ-
entiation and senescence progress the metabolic rate
falls, and finally the muscle is not even able to maintain
itself in the absence of the accelerating influence of
nervous stimulation upon its metabohc rate, because
when its rate falls below a certain level it does not replace
its losses by new muscle substance. In the regenera-
tion of the amphibian leg and other cases where the
influence of the nervous system is in dispute, the relations
are without doubt essentially the same.

There is no reason to believe that the nerve impulse
is anything more than an acceleration of metabolism.
The appearance of the nervous system does not consti-
tute the addition of something new to the organism; it
is merely the visible expression of relations already
existing and, as the facts indicate, of the relations
which constitute the foundation and starting-point of
individuation.

178 INDIVIDUALITY IN ORGANISMS

The question whether metabohc gradients involving
different metabolic processes may exist at the same time
in the same protoplasm must at least be raised. So far
as gradients depending on transmission are concerned,
this question is really the question whether different
sorts of changes or excitations may be transmitted
through the same protoplasm and whether different
metabolic effects result. Any answer to this question
at present is little more than a guess. It is perhaps
conceivable that at least in undifferentiated or slightly
differentiated protoplasm some degree of difference in
the character of the transmitted change may exist under
different conditions of excitation, etc. If such differ-
ences do exist, they must of course be important factors
in development and differentiation, but they merely
complicate and do not alter fundamentally the character
of unity and order in the individual. At present there
seems to be no real evidence that they exist.

THE NATURE OF INHIBITION

In chaps, iv and v, I have pointed out that the
inhibition or retardation of new individuation by the
dominant region of an individual occurs when the origi-
nal gradient is sufficiently fixed in the protoplasm, or the
metabolic rate at the levels concerned is sufficiently high
to prevent the establishment of a gradient in another
direction or to obliterate more or less completely or
prevent the further development of a gradient in another
direction. In Tubularia the inhibiting influence of the
apical region on the development of a hydranth at the
basal end of a piece is apparently simply the obHterating
effect of the original gradient on the gradient in the

CONCLUSIONS AND SUGGESTIONS 179

opposite direction. If the latter attains a sufficiently
high rate it interferes with or obHterates the other and
the hydranth develops, though partial inhibition may
be evident in its shortness and slow development.

In the case of a lateral bud of a plant, the develop-
ment of which is inhibited by the main growing tip,
the relation is probably the same. As long as the bud
is within the range of dominance of the growing tip its
own gradient from apex to base is more or less com-
pletely obliterated by a gradient from base to apex
determined by the main growing tip. This may in
time alter the protoplasmic gradient in the bud deter-
mined in the earlier stages of its individuation so that
it becomes incapable of development or develops only
into a short branch, a spine, or some other rudimentary
structure. It is interesting to note that Mogk in his
studies of plant correlation finds that when the axillary
shoots of a seedHng are allowed to grow until they
attain dominance over the main shoot (see pp. 152, 153),
the latter often dies and the death gradient is in the
reverse direction from that of death from lack of water
or other conditions in* an uninhibited shoot. Leaves
and roots probably represent partially inhibited gradi-
ents under certain conditions, and some of the specialized
outgrowths on the animal body, such as appendages, may
perhaps in some cases represent somewhat similar rela-
tions, though I know of no definite evidence bearing on
this point.

So far as the evidence goes, it indicates that all
inhibition of this sort is a matter of interference between
gradients in opposite or nearly opposite directions, the
one gradient reducing, obliterating, or even reversing

i8o INDIVIDUALITY IN ORGANISMS

the other. This interference is in certain respects
analogous to physical interference in the transmission
of water waves, sound waves, light waves, etc., but the
protoplasmic substratum in the organism represents a
factor not concerned in physical interference in non-
solid media. Undoubtedly a gradient which is originally
dynamic becomes more or less stably fixed or estab-
lished in the protoplasm as a gradient in irritabihty,
structure, or differentiation, because the effects of the
transmitted excitations modify the protoplasmic condi-
tion and this modification may become more or less
persistent. Temporary inhibition may result from
temporary interference between metabolic gradients,
but for permanent or long-enduring inhibition the
protoplasmic condition determined by one gradient
must be reduced or obliterated or its direction reversed
by the action on the protoplasm of another gradient.
In the cases of obliteration or reversal of the axial
gradients by other gradients this factor undoubtedly
plays a more or less important part, and the increasing
stability of the protoplasmic substratum with the prog-
ress of individual development and evolution^ deter-
mines that such obliteration and reversal occur much
more readily in the lower than in the higher organisms.
Since conduction in the nerve is apparently asso-
ciated with an axial gradient, it is at least an interesting
question whether nervous inhibition may not be funda-
mentally a similar relation of gradients, either in differ-
ent neurons or in the innervated organ. The mechanism
of nervous inhibition is still obscure, but if the nervous

^ Child, Senescence and Rejuvenescence, 1915, pp. 50, 53, 194, 267,
463-65.

CONCLUSIONS AND SUGGESTIONS i8i

system is really the final expression of the primitive
dominance in the individual, it is conceivable that the
highly specialized nervous inhibition may have some-
thing in common with the primitive form of inhibition
in the lower animals and plants.

ORIGIN OF METABOLIC GRADIENTS AND OF DOMINANCE

The data of reconstitution discussed in chaps, iv
and V show very clearly that new metabolic gradients
arise in relation to various external factors : in Tubularia
the cut end (pp. 132-37); in Corymorpha the difference
between a free surface and one in contact (pp. 142-46) ;
in Harenactis difference in the character of a wound
determining more or less growth of new tissue and so
the localization of a new apical region. As regards the
plants, the evidence from adventitious buds (pp. 83-86)
also indicates that the axes of such buds arise anew,
slight differences in metabolic rate between different
cells apparently often determining whether a new indi-
vidual shall arise in one place or another. As regards
various plants, we know that certain of the minor axes,
and in some cases the major axis, are determined by
the differential action of light. I beHeve we are justi-
fied in saying that whenever a new metabolic gradient
of sufficiently high rate is established by an external
factor a new individuation occurs.

It is of course easy to assume, as is often done, that
polarity and symmetry are self-determined in the in-
dividual, and that these self-determined relations are
simply altered and modified by external factors. But
the evidence for self-determination is lacking, and the
evidence for external determination is abundant and

1 82 INDIVIDUALITY IN ORGANISMS

highly conclusive. The assumption of self-determined
polarity and symmetry in protoplasm is simply super-
fluous, and the burden of proof is upon its supporters.

Of course the metabolic gradients present in one
individual may persist in the parts when that individual
divides, so that in such cases the axial relations of the
new individual are predetermined. This is the case in
fission in Planaria (pp. 92-96) and in many other forms.
Apparently also the gradient in a reproductive body,
e.g., many eggs, is often determined by its relations of
attachment, nutrition, etc., to the parent body.

In pieces of Tubularia, Corymorpha, Planaria, and
many other forms, the original polarity gradually dis-
appears as the length of the isolated piece decreases
until it becomes practically apolar, and new polarities
arise in relation to conditions at the cut ends (pp. 97-101) .
This fact indicates that polarity is rather a matter of
relation of parts than a fundamental property of pro-
toplasm, for in fractions of the axis below a certain
length it disappears.

In nature a particular kind of individual shows certain
characteristic axial relations; it is radially or bilaterally
symmetrical, or a combination in a characteristic way
of radial and bilateral arrangements. But the char-
acteristic axial relations are not invariable; they appear
regularly merely because events follow the same course in
successive generations. In plants the axial relations
can be altered in many ways and by many external
factors. Bilateral symmetry may be transformed into
radial or radial into bilateral, the position of branches
may be altered from alternate to opposite or to whorled,
and so on. The bilateral tentacle groups on the rings

CONCLUSIONS AND SUGGESTIONS 183

in Harenactis (Fig. 82, p, 147) show that the radial
arrangement characteristic of the animals in nature is
not invariably determined in the protoplasm, but is
only one of various possibiHties, which may or may not
be realized according to conditions.

If my conception of the relation between the meta-
bolic gradient and dominance is correct, then of course
the origin of a new gradient is the origin of a new domi-
nance, and if such a gradient is uninhibited by gradients
in other directions, and if its metabolic rate is high
enough, it becomes the major axis of an individual and
its region of highest rate the dominant apical region.

MORPHOLOGICAL DIFFERENTIATION IN RELATION TO
METABOLIC RATE

The belief that qualitative differences of some sort
in the fundamental constitution of the organism must
underlie the morphological and physiological differences
which arise during development in different parts of
the individual has been so widespread among biologists
that any attempt at even a statement of the problem
of differentiation in anything like quantitative terms
is sure to meet with serious objection and criticism in
some quarters. Nevertheless, the simplest and most
satisfactory, and, I believe, the only adequate, interpre-
tation of the data of reconstitution which have been
discussed in preceding chapters is that the starting-point
of differentiation is in differences in metabolic rate.
The attempt to interpret these facts on any other basis
very soon becomes involved, either in the barren assump-
tions of the hypotheses which simply postulate an
invisible organization to account for a visible, or else

i84 INDIVIDUALITY IN ORGANISMS

in the equally barren neo-vitalistic assumptions of some
non-mechanistic controlling or determining principle,
entelechy, or whatever we please to call it.

The head of Planaria will serve to illustrate the
point. I have shown that a series of different forms
of head occur in reconstitution, ranging from the normal
to the headless condition (pp. 106-8). These differ-
ent forms represent various degrees of inhibition and
they result, not only from the inhibitory influence of
other parts (pp. 108-14), but can be produced experi-
mentally by a great variety of conditions. In a lot of
similar pieces from animals in similar physiological
condition a decrease in head-frequency or a shift toward
the headless condition can be induced by low tempera-
ture, narcotics, carbon dioxide, etc., although in certain
cases, as we have seen (pp. 1 12-13), the results are com-
plicated by the metabolic relations between the head-
forming region and other parts of the piece. On the
other hand, conditions which accelerate metabolism,
such as high temperature or increased motor activity,
increase the head-frequency or shift it toward the normal
end of the series. We cannot believe that differences
in temperature or motor activity alter the fundamental
^'organization" in the head-forming region, but it is
a fact that such conditions according to their degree
may determine any or all of the various kinds of head
between the normal and headless extremes.

Again, how does either an *' organization" or an
entelechy aid us in interpreting the structures formed
on rings in Harenactis (pp. 146-49) ? Here results
range from various bilateral arrangements of parts to
the characteristic radial symmetry, and from single

CONCLUSIONS AND SUGGESTIONS 185

tentacles to normal animals. Either the plan of organi-
zation or the purpose of entelechy must be very different
in different tentacle groups on such rings. We know,
however, that the pieces will not form rings except
under certain experimental conditions, and that when
they do not they undergo reconstitution in the usual
way to animals of the usual form. Evidently the
development of these structures on the rings results from
certain experimental conditions, but if simple experi-
mental conditions can alter the fundamental axial
relations in the individual, what is the necessity of the
postulated organization, or entelechy, or other similar
principle ? And does not the obliteration in Corymorpha
of the original axial relations and the establishment of
new relations in their place, by means of experimental
conditions whose action upon metabolism is primarily
quantitative (pp. 142-46), indicate that the axes them-
selves are primarily quantitative relations? Similarly
the fact that the localization of experimental reproduc-
tion may be determined as a resultant of different axes
or by a minor axis in the absence of the major axis
(pp. 163-68) forces us to the conclusion that the
different axes are fundamentally identical and therefore
represent quantitative relations.

Moreover, the conception of the organic axis as a
metabolic gradient enables us not only to interpret, but
to control and to predict. In recent work on the oligo-
chete annelids, by Dr. Hyman, it has been possible on
the basis of the metabolic axial gradient to predict and
control experimental results, and this is possible among
the flatworms to an even greater degree. As regards
the manner in which physiological and morphological

i86 INDIVIDUALITY IN ORGANISMS

specialization results from difference in metabolic rate
there are various possibilities. In a physico-chemical
complex like living protoplasm a change in tempera-
ture of a certain amount alters the rate of chemical
reaction to a certain degree, but it also alters many
other conditions in protoplasm, e.g., osmotic conditions,
surface-tension, aggregate condition of colloids, etc.,
and it alters some in a greater, others in a less, degree.
In such a case the change in each particular process or
condition in the living protoplasm may be quantitative,
but since different factors are altered in different degree
the total change may determine qualitative differences
in the reactions or their products. Changes of this
sort may result, not merely from differences in tempera-
ture, but from other primarily quantitative changes.
In fact, it is very doubtful whether we can alter metabolic
rate to any great extent without bringing such changes
in quality somewhere in the complex.

Elsewhere I have called attention to various facts
which have as yet received but little attention, but
which indicate that a relation exists between morpho-
logical structure and metabolic rate.^ Structural fea-
tures which are stable with a certain metabolic rate are
eliminated when the rate increases, while decrease in
rate may determine the addition of new structural sub-
stances, and so on. Metabolic rate is apparently a
factor, though of course by no means the only one, in
determining what substance or substances accumulate
in the living cell as structural substratum, and the
structural substratum is an important factor in determin-
ing the character of the reactions which occur in it.

* Child, Senescence and Rejuvenescence, 1915, pp. 47-54, 226-27.

CONCLUSIONS AND SUGGESTIONS 187

The lack of specificity in the action of a great variety
of experimental conditions upon development and
morphology has often been noted. For example, the
aberrations or abnormalities in development, or more
properly the partial inhibitions of development pro-
duced by low temperature, various narcotics and poisons,
and many other conditions are essentially the same.
The reason for the lack of specificity undoubtedly lies
in the fact that the action of these various substances
and conditions is primarily quantitative, yet a greater
or less degree of differentiation, various differences in
form and arrangement, and even the presence or absence
of specific organs may be determined by their action.

The results of the quantitative changes in living
protoplasm in a particular case must of course depend
upon its specific constitution. The kind of specializa-
tion or differentiation which arises at a particular level
of a metabolic gradient must depend upon this constitu-
tion, and the developmental and morphological resem-
blances between different forms must of course depend
in general upon similarities of constitution. The
development of the region of highest metabolic rate
in the major gradient as a growing tip in plants and as
a central nervous system or brain in animals must result
from differences in constitution and dynamic processes
in the plant and animal protoplasm, but growing tips
in general and central nervous systems in general have
certain common characteristics.

We must, I believe, conclude that the conception of
the metabolic gradient, a gradient primarily quantitative,
originating in and primarily determined by the dominant
region, as the basis of physiological and morphological

i88 INDIVIDUALITY IN ORGANISMS

order, of ''organization/' specialization, and differentia-
tion in the organic individual, not only presents no
fundamental difficulties, but is supported by a great
body of experimental and observational evidence from
various biological fields.

THE FUNDAMENTAL REACTION SYSTEM

If the dynamic conception of the organic individual
is correct, the starting-point lies, not in a certain organi-
zation, but in a certain reaction system. This is a
protoplasm of specific constitution with a corresponding
metabolic specificity, or one may say that this specificity
is the expression of a specific constellation of conditions
and that this in turn has been determined by the specific
constellation of factors external to itself to which each
organism, individual, or part has been subjected in the
past. It is this reaction system, not an organization,
which constitutes the basis of inheritance, and it is
in this system that differences in metabolic rate initiate
the process of organization. We may for convenience
regard the embryonic or undifferentiated cell of the
species as representing this fundamental reaction system,
although even there the system is doubtless not reduced
to its lowest terms. The developmental changes in this
system fall into two groups, the self-determined' changes

^It is perhaps desirable to indicate just what is meant by self-
determination in this connection. All that the word is intended to
imply here is that the region of highest metabolic rate may undergo
certain progressive changes, which are determined by its own
constitution and by continued metabolism in it. These changes may
in time make this region dififerent structurally and physiologically
from what it was originally, even though it is independent of other
parts.

CONCLUSIONS AND SUGGESTIONS 189

characteristic of the dominant region and the correlatively
determined changes characteristic of subordinate regions.
It is a very significant fact that the self-determined
changes in animals always result, where they proceed far
enough, in the development of a nervous system. Of
course as a matter of fact the changes which occur in the
development of a central nervous system are not all ab-
solutely self-determined, for if they were all cells of
the nervous system would be alike. We may say, how-
ever, that in the animal the nervous system or its apical
"portion represents more nearly than any other part of
the body the result of self-determined progressive changes
in the fundamental reaction system of the species, while
other parts represent the result of changes determined by
correlation and dependence. From this point of view
the animal organism is fundamentally nervous system;
all other parts represent lower levels of metabolism and
independence. The central nervous system represents
more nearly than any other part of the individual the
product of the fundamental reaction system at its highest
level. The cephalic nervous system is, so to speak, the
organism at its best.

In the plant, however, the self-determining dominant
region remains, at least during growth, in an undiffer-
entiated or relatively undifferentiated condition as the
growing tip, and growth and cell division are its chief
activities. In consequence of this condition its domi-
nance over other regions is slight, the degree of indi-
viduation in the plant remains low, and the life of the
plant remains simple and narrowly limited in character.

This difference between animals and plants, in the
one the development of the dominant region into the

I go INDIVIDUALITY IN ORGANISMS

central nervous system, the most stable structure physi-
ologically of the body, and in the other its persistence
indefinitely as an embryonic cell or a group of cells, must
be an expression of the fundamental difference between
the two groups of organisms. Evidently this difference
is primarily a difference in relation between the proto-
plasmic substratum and the metabolic reactions. Stable
morphological structure and differentiation in the plant
consist largely in the deposition of carbohydrates and
other non-proteid substances within or about the cells,
while in the animal morphological differentiation very
generally has its origin and foundation in the accumula-
tion and specialization of protoplasm itself. Apparently
the protoplasmic substratum of the plant is much less
stable physiologically than that of the animal. The
plant seems to be incapable or almost incapable of syn-
thesizing proteid molecules which are physiologically
stable where the metabolic rate is high. The protoplasm
of the plant cell is certainly much more directly and
intimately involved in the chemical reactions of metab-
oHsm than that of most animal cells; consequently
in regions of high metabolic rate no persistent proto-
plasmic structure like that of the animal cell can arise,
because there is no accumulation of relatively stable
substances in the cell. In regions where the metabolic
rate is lower, substances may accumulate in the cell
as structure which with a higher metabolic rate would
be decomposed. In the plant, therefore, morphological
differentiation increases with increasing distance from
the growing tip and decreasing metabolic rate, while
in the animal differentiation begins and is most stable
in the apical region — the region of highest reaction rate —

CONCLUSIONS AND SUGGESTIONS 191

and progresses from this to other parts. Animal metab-
olism evidently synthesizes highly stable molecules, even
where metabolic activity is most intense.

In the plant the whole substratum may apparently
be mobilized to some extent when the metabolic rate
is high, and only as the rate becomes lower do substances
accumulate as structure. In nearly all if not all animals,
on the other hand, certain protoplasmic substances are
relatively more stable under the existing metabolic
conditions than in the plant and therefore accumulate,
and a progressive structural development and differ-
entiation occur even when the metabolic rate is highest.
In the animals the morphological structure which de-
velops in the region of highest metabolic rate is physio-
logically the most stable structure of the body, because
the less stable substances are decomposed in the intense
metabolic activity and so do not form permanent con-
stituents of the substratum. In regions of lower meta-
bolic rate substances accumulate which are readily
removed by an increase in metabolic rate. These parts
may therefore undergo dedifferentiation and rediffer-
entiation. The head-region, however, or, more specifi-
cally, the central nervous system, is almost or quite
incapable of dedifferentiation under ordinary conditions,
because its structure has developed under conditions
of more intense metabolic activity than any other
part of the body and is therefore more stable. If the
metabolic rate could be increased sufficiently above the
rate in the developing nervous system without bringing
about death, doubtless dedifferentiation of the nervous
system would occur to some extent. To refer briefly
to the analogy between the organism and the flowing

192 INDIVIDUALITY IN ORGANISMS

stream which I have used elsewhere/ the plant is some-
what like a stream flowing in an alluvial channel, capable
of shifting and removing previous structural deposits,
and, when its rate is highest, of holding all its sediment
in suspension. The animal, on the other hand, repre-
sents a condition like that in the stream when deposition
of sediment is going on and giving rise to stable structure,
even where the rate of flow is highest. In such a stream
the most stable structure develops where the rate of
flow is highest, while the structure developed with a
low rate of flow is readily altered or eliminated by an
increase in rate.

The fundamental differences in behavior between
plant and animal are of course associated with this
difference. Since the plant is to a large extent incapable
of developing morphological colloid structures, such as
nerve and muscle, its reactions to external factors are
limited very largely to growth reactions, instead of being
motor reactions like those in most animals. The low
degree of individuation and physiological efficiency in
the plant as compared with the animal must also depend
on this low degree of physiological stability in the pro-
toplasmic substratum.

AGAMIC REPRODUCTION IN RELATION TO PHYSIOLOGICAL
ISOLATION

The occurrence of reproduction in consequence of
physiological isolation of parts under experimental con-
ditions makes it highly probable that at least many of
the processes of agamic reproduction in nature are like-

^ Child, "The Regulatory Processes in Organisms," Jour, of
MorphoL, XXII, 191 1.

CONCLUSIONS AND SUGGESTIONS 193

wise the result of physiological isolation. Elsewhere I
have endeavored to show that physiological isolation is
a fundamental factor in asexual reproduction in both
plants and animals, and that reproduction results from
physiological isolation because the isolated part loses
to a greater or less extent its differentiation as a part,
becomes physiologically younger, and undergoes a new
individuation.^ In chap, iv above it was also pointed
out that agamic reproduction in Tubularia and Planaria
is readily interpreted as the result of physiological
isolation. Moreover, in the discussion of the data of
experimental reproduction we have seen that physio-
logical isolation and reproduction may result, not only
from increase in size beyond the range of dominance,
but also from decrease in the range of dominance in
consequence of decrease in metabolic rate in the domi-
nant region, from decrease in conductivity in the path of
transmission, and finally from a decrease in receptivity
of a subordinate part, brought about by the action of
local factors, which determine the establishment of new
gradients in it or make it otherwise more independent.
Undoubtedly all these different forms of physiological
isolation occur in nature, and in many reproductive
processes more than one of them are probably concerned.
Reproduction in consequence of increase in size is
one of the commonest forms of reproduction in organic
individuals from the single cell to complex organisms
among both animals and plants. Reproduction also
occurs very commonly under conditions unfavorable to

^ Child, " Die physiologische Isolation von Teilen des Organismus,"
Vortrdge und Aufsatze uber Entwickehmgsmechanik , H, XI, 191 1; Senes-
cence and Rejuvenescence, 1915, pp. 228.

194 INDIVIDUALITY IN ORGANISMS

growth or active life; that is, under conditions which
undoubtedly decrease metabolic rate and so decrease
the range of dominance. Under such conditions unicel-
lular forms often fragment into a number of small
individuals, and some of the simple plants break up
into their constituent cells, which then grow and divide
to form small individuals, even under the same conditions
which made impossible the persistence of the original
larger individual. Other plants give rise to adventi-
tious buds, sometimes in great numbers, under such
conditions, while still others break up into quiescent
forms, and so on. In my study of senescence and
rejuvenescence I have pointed out that the decrease in
metabolic rate with advancing senescence in the lower
animals and plants often leads automatically by decreas-
ing dominance to physiological isolation of parts and
so to rejuvenescence and reproduction of new individuals.

Reproduction under depressing conditions has often
been interpreted in a teleological way as an attempt of
the organism to avoid extinction by producing new
individuals, some of which might succeed in finding
favorable conditions for continued existence. As a
matter of fact, however, such reproduction is merely
the expression of physiological weakness ; the individual
can no longer maintain itself as a unity in its original
size, and as the original unity disappears, new unities
arise as local metabolic conditions determine.

Regarding the part played by changes in the con-
ductivity of the path of transmission in bringing about
physiological isolation and reproduction in nature, we
know little. It is undoubtedly a fact that the increase
in conductivity during development of the individual

CONCLUSIONS AND SUGGESTIONS 195

brings about an extension of dominance and so inhibits
or retards physiological isolation (see pp. 149-51), and
it is probable that sooner or later with advancing
senescence a decrease in conductivity occurs in at least
some cases. It is also probable that decrease in con-
ductivity occurs in the lower organisms under external
conditions which decrease metabolic rate in the organism
in general. Such changes, where they occur, may play
a part in determining physiological isolation and repro-
duction.

Local external conditions undoubtedly assist in the
physiological isolation of subordinate parts in many
cases. In various plants local conditions very favorable
to metabolic activity and growth may determine the
development of buds in spite of the inhibiting influence
of the dominant region. We have seen how in pieces
of Tubularia stem the presence of the wound at the
basal end assists in estabHshing the new gradient, even
in spite of the presence of the old (see pp. 132-37). This
is a good case of physiological isolation by the action of
local factors.

Further analytic investigation along these lines is
greatly needed to enable us to determine the part played
by the various factors in different cases of reproduction,
but the mere observation of various reproductive
processes — such, for example, as the production of a new
plant by a strawberry runner, after it has attained a
certain length — will enable us to learn much concern-
ing the range of dominance and its changes under
different conditions.

The redupHcation of parts in an organism, such as
leaves and roots in the plant and segments and various

196 INDIVIDUALITY IN ORGANISMS

other parts in the animal, belongs in the same category
with the reproductive processes which give rise to new
whole organisms. In such cases physiological isolation
may be partial or with reference to a specialized con-
stituent individual of the organism.

The localization of reproduction in the individual
may be determined by various other factors besides
distance from the dominant region. Some parts less
distant than others may be physiologically isolated
earher because of lower conductivity of paths, or because
of other correlative conditions within the organism, or
because of certain external conditions. In isolated
parts the least differentiated cells or regions, or those
with the highest metabolic rate, are likely to react earlier
than others and so determine the localization of the
reproductive process. Sometimes, particularly among
plants, in reproductions which occur with advancing
age or under depressing conditions, it is the original
dominant region which separates from other parts as
a smaller individual and so becomes the reproductive
body, spore, or whatever it may be called.

Special unrecognized factors may play a part in
certain cases, but it seems impossible to doubt that, in
general, agamic reproduction in organisms results from
physiological isolation of parts of the individual. Indi-
viduation is a physiological integration depending pri-
marily on the dominance and subordination of parts in
relation to an axial gradient or gradients, and agamic
reproduction is a physiological disintegration of this
unity which makes possible new integrations.

The fundamental similarity in individuation and
reproduction in the lower animals and plants is well

CONCLUSIONS AND SUGGESTIONS 197

illustrated by a comparison of certain corals with the
plants. Wood-Jones^ has recently found from a study
of living animals under natural conditions that in the
staghorn corals there is a radially symmetrical, apical
zooid at the tip of the stem which gives rise by budding
to the bilaterally symmetrical, lateral zooids, while
these do not reproduce as long as the apical zooid is
present and active. At a certain distance from the
apical zooid one of the bilaterally symmetrical zooids
may become radially symmetrical and begin to reproduce
new zooids and so become the apical zooid of a branch.
If the apical stem-region with the apical zooid is removed,
several branches may arise by the transformation of
bilateral into radial, reproducing zooids. Moreover,
the apical zooid of stem and branches remains young
indefinitely, while the lateral zooids which do not
reproduce undergo senescence and die. In other corals
various degrees of composite individuation are found
to exist. The relation of the dominant apical zooid
to other parts in the staghorn corals is very evidently
essentially the same as that between the growing tip
and other parts in plants, and it is impossible to doubt
that the same fundamental principle underlies and
determines the relation, not only in these two cases,
but in organisms in general.

GAMETIC REPRODUCTION

Sexual or gametic reproduction, with rare exceptions
the only reproductive process giving rise to whole new
organisms among the higher animals, is commonly

^ F. Wood- Jones, Coral and Atolls, London, 191 2, chaps, viii, ix.

198 INDIVIDUALITY IN ORGANISMS

regarded as very different from the agamic reproductive
processes. Actually, however, there are certain funda-
mental similarities between the two processes. I have
discussed this matter at some length elsewhere,^ and
need only review certain important points here. The
evidence indicates that the gametes, the two cells which
unite in sexual reproduction and which in their more
highly specialized forms we call egg and spermatozoon,
are physiologically subordinate parts of the body and
undergo differentiation with other parts ^ instead of being
composed of a mysterious, independent substance, the
germ plasm, as Weismann and many others have be-
lieved. Gametic maturity occurs at a relatively ad-
vanced physiological age in the organism, and the
gametes, like other parts of the body, are physiologi-
cally old cells with a low metabolic rate and are evi-
dently approaching death. Their isolation from other
parts of the body in those multicellular forms in which
complete isolation occurs has apparently no relation
to the range of dominance, but seems rather to be asso-
ciated with the completion of their period of growth
and differentiation. So far as the parent organism is
physiologically concerned, the isolation of the sex cells
may be compared with the casting off of other old
cells which have played their part and are approaching
death. In many cases, however, the egg remains in
the parent body until an earher or later stage of embry- ■
onic development is reached, but even in such cases
the egg, after completing its developmental period, seems
to have little physiological relation to other parts of the
parent body.

^ Child, Senescence and Rejuvenescence, 1915, Part IV.

CONCLUSIONS AND SUGGESTIONS 199

Except in the case of parthenogenic eggs, which
develop without fertilization, neither of the gametes
undergoes dedifferentiation and a new development by
itself, but in some way their union, or conditions asso-
ciated with it, or in various cases certain experimental
conditions ("artificial parthenogenesis"), initiates the
process of dedifferentiation and rejuvenescence which
makes possible the development of a new individual and
a new period of differentiation and senescence. The
increasing metabolic rate and the loss of differentiation
in the early stages of embryonic development indicate
clearly that rejuvenescence is occurring-, but sooner or
later the intake of nutrition results in renewed accumula-
tion of substratal substance and senescence begins again.
The period of dedifferentiation and rejuvenescence is
short, and during most of its development the sexually
produced organism is growing old.

As I have endeavored to show, the development of
the individual in gametic reproduction is fundamentally
the same process as in agamic and experimental repro-
duction. In most cases the polarity, i.e., the major
axial gradient, and in some cases the minor gradients,
are determined in the eggs before embryonic develop-
ment begins, usually, so far as observation permits
definite conclusions, by their relations to the parent
body, but in some of the lower plants the major axis is
apparently determined after the egg leaves the plant-
body by the direction or differential action of light or
other external factors. The point of entrance of the
sperm seems in many cases among animals to be a
factor in determining the symmetry gradients, where
they are not already determined. In at least many

200 INDIVIDUALITY IN ORGANISMS

plants, however, and doubtless in some animals, the
symmetry gradients are determined in later stages.

From this point of view the chief difference between
agamic and gametic reproduction is that in the latter
the mere isolation of the reproductive body from the
parent individual is not sufficient to start the process of
dedifferentiation and new development. The gametes
do not react except under special conditions, because
they have become so highly specialized and differentiated
as parts of the parent individual that they are incapable
of such reaction. But when the special conditions are
present, dedifferentiation begins and development pro-
ceeds. Certain eggs develop parthenogenically, and
these in many cases are very evidently less highly differ-
entiated than eggs which require fertilization. It is
probable that they or some of them represent a stage
in gametic development in which the egg is still capable
of reacting to isolation like the physiologically or physi-
cally isolated part of the body of Tubularia or Planaria
by undergoing dedifferentiation and a new course of
development. If this conclusion is correct, these par-
thenogenic eggs represent a condition intermediate be-
tween the parts of the body of lower forms which
undergo agamic reproduction when isolated and the
more highly specialized gametes for which fertiliza-
tion is a necessary condition of further activity. At
least many of the eggs in which development can be
initiated experimentally by other means than fertili-
zation are apparently almost capable of natural par-
thenogenesis, and so are probably less highly specialized
than eggs which are not susceptible to experimental
treatment.

CONCLUSIONS AND SUGGESTIONS 201

If we accept this view, we must regard gametic
reproduction merely as a more highly specialized form
of reproduction which occurs in more advanced life
or in more highly differentiated individuals than agamic
reproduction, but which involves essentially the same
cycle of differentiation and senescence, followed by
dedifferentiation and rejuvenescence, the production of a
new individual, and another period of differentiation
and senescence.

From this standpoint the egg and the embryo are
in general the most unfavorable material that could be
found for the investigation and analysis of the processes
of reproduction and individuation, for in most cases the
gametes are formed in the parent organism under con-
ditions which do not permit of extensive and exact ex-
perimental control. Moreover, they consist of single
cells, and so cannot be divided experimentally before de-
velopment begins, and the egg has usually attained a
certain, often a very high, degree of individuation before
it is isolated. The agamic and experimental reproduc-
tions afford a much wider range of control, and we can
analyze the beginnings of individuation there as we can-
not in the egg. The only logical procedure is, in my
opinion, to interpret gametic reproduction, as I have
attempted to do, on the basis of our knowledge of the
experimental and agamic processes, and not vice versa.
Our slow progress toward an adequate conception of
organic individuality has undoubtedly been due in con-
siderable part to the fact that we have confined our
attention so largely to gametic reproduction, and have
neglected the simpler processes in which, if anywhere,
the key to the problem is to be found.

202 INDIVIDUALITY IN ORGANISMS

HEREDITY, EVOLUTION, AND OTHER PROBLEMS FROM THE
DYNAMIC STANDPOINT

If the organism is fundamentally a specific reaction
system in which quantitative differences initiate physio-
logical individuation, development, and differentiation,
nothing can be more certain than that it acts essentially
as a unit in inheritance. It is the fundamental reaction
system which is inherited, not a multitude of distinct,
qualitatively different substances or other entities with
a definite spatial localization. Development is not a
distribution of the different qualities to different regions,
but simply the realization of possibilities, of capacities
of the reaction system. The process of realization differs
in different regions because the conditions are differ-
ent. Neither characters nor factors as distinct entities
are inherited, but rather possibilities, which are given
in the physico-chemical constitution of the fundamental
reaction system, but not necessarily localized in this
or that part of it.

The fact that in the past investigation of inheritance
has been almost entirely limited to the special aspects
of heredity and development connected with gametic
reproduction has contributed very largely to delay our
progress and limit and distort our conceptions of the
processes of inheritance. This, the most highly special-
ized form of reproduction, is the most unfavorable
point of attack upon the problems involved, for the
possibilities of control of the earlier stages of individu-
ation are narrowly limited, and many factors which
are not really essential to reproduction and develop-
ment are characteristically present in this reproductive
process.

CONCLUSIONS AND SUGGESTIONS 203

The process of inheritance is involved to exactly
the same extent in the reconstitutional development
of a new individual from a piece of Tuhularia stem or
of the planarian body, or in the formation of a new grow-
ing tip from the differentiated cells of a leaf (Figs. 38, 39),
from callus tissue (Fig. 40), or from any other part of
the plant, as it is in the reproduction of a new individual
from the egg, with or without fertilization, in any of
these forms. The simple agamic and experimental re-
productions, moreover, afford very much greater poss-
ibilities for the analysis and control of the processes
and mechanism of inheritance and development than
gametic reproduction. Any adequate conception of in-
heritance and development must be based upon ana-
lytic investigation of these simple reproductions and
synthesis of the results, and it must interpret inheritance
in gametic reproduction in terms of the simpler processes.
Continued sexual breeding and hybridization under
controlled conditions and with pedigreed individuals has
contributed much and undoubtedly will contribute
further toward the solution of certain special problems
of inheritance, and also affords results which possess a
statistical value, but this method of procedure alone
can never carry us very far toward the solution of the
fundamental problem of inheritance. The key to this
problem also will be found in the simpler reproductive
processes.

If the organism is a unit in inheritance and develop-
ment we must expect to find that so-called ''acquired
characters" may be impressed on the organism to such
a degree that sooner or later the reaction system may
give rise to these characters without the action of the

204 INDIVIDUALITY IN ORGANISMS

particular external factor which originally produced
them. The reaction of the organism to a sufficient
local excitation is not simply a local reaction, but a
reaction more or less of the whole organism, and we know
that in the case of many physiological reactions the
repetition of the reaction in response to repeated external
excitation alters the reaction system so that response
occurs more readily or more rapidly or with a lower
intensity of stimulus. We say that the irritability of the
protoplasm is increased, its *' threshold" for stimulation
is lowered, etc. If this change goes far enough the
reaction may occur in the absence of the external factor
which first produced it, simply because the condition
or constitution of the protoplasm has been so altered
by the repetition of the reaction that it occurs auto-
matically when any condition determines a sufficiently
high metabolic rate in the reaction system. The
^'inheritance of acquired characters" then belongs in
the same general category as the increase in irritability
resulting from repeated excitation, but it may in many
cases require thousands or hundreds of thousands of
generations before a condition approaching auto-
maticity in its production is attained. In the face
of the physiological facts it is difficult to understand
how biologists can continue to maintain the distinction
between soma and germ plasm, and to content them-
selves with the assertion that natural selection is ade-
quate to account for adaptation in the organic world.
If the organism is in any sense a dynamic entity, then
its evolution must be a reaction determined, on the
one hand, by its physico-chemical constitution, and
on the other, by its relation with the external world,

CONCLUSIONS AND SUGGESTIONS 205

and its adaptations are simply special features of this
relation.

Evolution is not directly concerned with morpho-
logical characters, but with the physico-chemical con-
stitution of the reaction system, and so with the rate
and character of its reactions and the conditions under
which they occur. I have called attention elsewhere'
to the resemblance between the progress of evolution
and the progress of senescence and development in the
individual, and have suggested that evolution, like
senescence and other processes in nature, may be essen-
tially a change from a less stable to a more stable condi-
tion in the dynamic reaction system which constitutes
the organism.

The significance of this dynamic conception of the
organism for various other biological problems will be
apparent without further discussion, and I believe it
may possess a certain significance for certain problems
of comparative psychology and sociology. It is at
least a matter of some interest to be able to trace the
fimdamental identity in individuation from the simple
unicellular organism to the highest plants in the one
direction and to conscious man in the other, and to show
that the growing tip of the plant and the brain of man
have something in common. Moreover, to find the
same principle of individuation in the egg and in the
adult organism and again in the single nerve cell and
its fiber is at least highly suggestive. The recognition
of the fact that individuation in the organism is a rela-
tion of dominance and subordination of parts removes
much of the difficulty in accounting for the high degree

^ Child, Senescence and Rejuvenescence, 1915, pp. 144, i93j A^2r^S'

2o6 INDIVIDUALITY IN ORGANISMS

of definiteness and the constancy of character of the
developmental processes and other activities of living
things. It also has a certain bearing upon the problem
of the origin of individuations whose component parts
are human beings or groups of human beings. Between
the organic individual and the state there is, from this
point of view, a real analogy, for control or government
is the essential feature in the individuation in both, and
the relations are in certain respects similar in both
cases. It is not a mere fanciful analogy to conceive
the organism as a state or the state as an organism,
since both are dynamic individuals and some degree
of dominance or government exists in both. These
suggestions are an indication of some of the broader
bearings of the dynamic conception of the organic
individual, but discussion along these lines must be
postponed.

In conclusion it is perhaps permissible to call atten-
tion to the simplification and unification of viewpoint
which this conception accomplishes. The separation of
morphological from physiological investigation and
thought, particularly in zoology, which followed the
acceptance of the theory of evolution, and the fact that
the morphologists, rather than the physiologists or
biochemists, have chiefly concerned themselves with the
great problems of heredity, development, and evolution,
have brought it about that biological theory in these
fields has been to some extent a world apart. While
proclaiming their acceptance of the mechanistic or
physico-chemical conception of life, the theorists of this
group and their followers have not only made but few
attempts to apply physico-chemical conceptions to

CONCLUSIONS AND SUGGESTIONS 207

the organism, but have often decried the value of such
attempts. It is still true, therefore, to a large extent
that to grasp these theories we must enter a new world
of symbols, which only too often appear to have no
resemblance or relation to any other symbols commonly
in use in scientific thought. When we have become
familiar with our new world, we can perform marvelous
feats with its symbols and fill our pages with formulae of
gametic constitution or what not, but so far as any real
connection between this world and the other world of
science is concerned, such theories and their symbols
leave us, at least in most cases, exactly where we were
at the beginning. We can discuss the topographic
location of hereditary factors in the chromosome, and
we can arrange them in any way necessary to account
for the observed facts. In fact, we can invent symbols to
describe development or any other process in the organ-
ism. But some of the discussions which have to do
with these static, morphological symbols remind us
irresistibly of that old problem of the angels and the
needle point.

Being entirely unable to find any degree of intellec-
tual satisfaction in those static conceptions of the
organism which seem to have no relation to anything
else in the world and which raise many questions but
answer none, and being forced by my own experimental
investigations to conclusions very different from these,
I have attempted to apply dynamic conceptions to cer-
tain biological problems, with the results which have
been considered in the preceding pages. Whatever
other value the dynamic viewpoint may possess, it
serves as a basis for the synthesis and ordering of many

2o8 INDIVIDUALITY IN ORGANISMS

facts in various fields which heretofore have seemed to
have Httle or nothing in common, and I think we may
say that it aids in bringing certain aspects of biology at
least within hailing-distance of physico-chemical con-
ceptions.

INDEX

Note. — References give the number of the page on which the matter referred to begins.

Anophthalmic form in Planaria,
io6, 141.

Axis, organic: occurrence of, 8;
apical and basal ends of, 10; ter-
minology of, 19; simplest form
of, 35; susceptibility gradients
in relation to, 53, 60; independ-
ence of apical region of, 96, 113;
dominance of apical region of,
102; control of space relations
in, 128; experimental oblitera-
tion and determination of, 142;
as resultant of two pre-existent
axes, 164; quantitative charac-
ter of, 167, 185; time of deter-
mination of in egg and embryo,
199. See also Dominance; Gra-
dients; Individual; Polarity;
Symmetry.

Begonia, adventitious buds in, 83.

Biaxial forms: in Tubularia, gy,
133; in Planaria, 99, 117; ex-
perimental transformation of, in
Corymorpha, 144; experimental
determination of, in Planaria,
149. See also Axis; Gradients;
Individual.

Conductivity: in relation to trans-
mission, 40; increase in, during
development, 150; in relation to
physiological isolation, 194. See
also Transmission.

Corals, individuation in, 197.

Correlation, physiological: differ-
ent kinds of, 4, 27; occurrence
of transportative, 26, 44, 170;
conditions determining trans-
portative, 26, 170, 172. See also
Axis; Dominance; Gradients;
Individual; Transmission;
Transportation.

Corymorpha: description of, 92;
metabolic gradients in, 132; ob-
literation and determination of
gradients in, 142.

Crystal, compared with organic in-
dividual, 24.

Cyclamen persicum, dominance
and subordination in leaf of, 156.

Dedifferentiation: in agamic re-
production, 7, 9, 91; in formation
of adventitious individuals in
plants, 83; in reconstitution of
Planaria, 109; capacity for, in
lower and higher animals, 120;
in embryonic development, 199.
See also Differentiation.

Differentiation: occurrence of, 6;
orderly character of, 7; different
degrees of, in eggs and embryos,
121; in relation to metabolic
gradient, 171; in relation to
metabolic rate, 183, 190. See
also Dedifferentiation.

Dominance, physiological: origin
of, 36, 181; in relation to meta-
bolic gradients, 37, 88, 171;
range of, 45, 127, 133, 138, 149,
162, 172; in experimental re-
production of Tubularia, 102,
133; in experimental reproduc-
tion of Planaria, 102, 114; of
apical region in plants, 104, 152;
experimental control of, in Tu-
bularia, 134; experimental ob-
literation and determination of,
142; extension of, during devel-
opment, 149; in relation to size
of individual, 151, 193; in rela-
tion to adventitious buds in
plants, 154; of growing tip in
conifers, 154; direction of, in
plants, 155; in leaf of Cyclamen,

209

2IO

INDIVIDUALITY IN ORGANISMS

156; in root system, 157; nature
of, 170; in the neuron, 173;
decrease of, in relation to physio-
logical isolation, 193; in corals,
197. See also Gradients; Indi-
vidual; Isolation.

Entelechy, 23, 137, 184,

Evolution: increasing stability of
order in, 6; in relation to envi-
ronment, 204; as an equilibra-
tion process, 205.

Fertilization, 199.

Fission; in Stenostomum, 79; in
Planaria, 92, 140, 141.

Frog, developmental gradient in
early development of, 66.

Ginkgo: developmental gradient in
embryo of, 73; formation of
growing tip of, 77.

Gradients, developmental: in re-
lation to metabolic gradients,
65; in early embryo of frog, 66;
in fiatworm, 67; in chick em-
bryo, 69; in relation to rate of
growth, 72; in embryo of moss,
73; in embryo of Ginkgo, 73;
in plant axes, 73; in bilater-
ally symmetrical plants, 77; in
agamic reproduction of Fen-
naria 79; in reconstitution of
Planaria, 81; in Metzgeria, 83;
in adventitious buds of Bego-
nia, 83; in buds on callus, 86.
See also Gradients, metabolic.

Gradients, metabolic: origin of,
29, 181; as simplest expression
of order, 35, 187; in relation to
physiological dominance and
subordination, 36, 170; inter-
ference between, 39, 178; efifect
of, on protoplasm, 40; inherit-
ance of, 41, 182; as basis of
qualitative differences, 42; dem-
onstration of, as susceptibility
gradients, 52; in animals, 53,
59; in Stentor, 55; in starfish

egg, 56; in parts and organs, 57;
demonstration of, by differen-
tial inhibition, 58; in relation to
axes, 60; in plants, 61; as
gradients in carbon-dioxide pro-
duction, 62; in neuron, 62, 151,
173; in relation to differences in
electrical potential, 63; demon-
stration of, by differential
staining, 64; in relation to
developmental gradients, 65,
79; in experimental repro-
duction in Marchantia, 86, 165;
in Tuhularia, 91; in agamic
reproduction of Planaria, 93;
independence of apical regions
of, 96; in reconstitution of
Tuhularia, 130; control of
length of, in Planaria, 140; ex-
perimental obliteration and de-
termination of, 142; localization
as resultant of different, 164;
problem of different kinds of,
178; relation of, to inhibition,
178. See also Axis; Domi-
nance; Individual.

Growing tip: as feature of plant
individual, 73; in relation to de-
velopmental gradients, 74; in
adventitious individuals, 83; in
relation to range of dominance,
150; dominance of, in plants,
152; localization of, as resultant
of different axes, 165; self-
determination in, 189; condi-
tions determining character of,
190.

Harenactis, control of reconstitu-
tion in, 146.

Head-determination, in Planaria,
III.

Headless form; in Planaria, 106;
conditions determining, 118,
141.

Head frequency: in pieces of Pla-
naria, 108; experimental altera-
tion of, 108; interpretation of,
119; relation of, to metabolic
rate, 184.

INDEX

211

Individual, organic: fundamental
characteristics of, 2; nature of
unity in, 3, 48, 175; various
theories of, 3, 22; character of
order in, 8, 17, 35; reproduction
in relation to, 12; terminology
of, 18; comparison of, with
social individual, 21, 26, 206;
formulation of the problem of
the, 29; dynamic conception of
the, 29, 88, 172; as one or more
metabolic gradients, 40, 170;
limitation of size of, 45, 47, 151;
as result of relation between pro-
toplasm and environment, 49;
origin of adventitious, in plants,
83, 154, 194; size of, in relation
to range of dominance, 151;
fundamental reaction system of,
188; difference between plant
and animal, 189; in relation to
inheritance, 202; significance of
dynamic conception of, 205. See
also Axis; Dominance; Indi-
viduality; Individuation.

Individuality: different kinds of,
48; superficial origin of organic,
49. See also Axis; Dominance;
Individual; Individuation.

Individuation: in Amoeba proto-
plasm, 6; in experimental re-
production, 14; nature of, 41,
48; in "rings" in Harenactis,
146; conditions determining
low degree of, in plants, 189; in
corals, 197; degree of, in egg,
201; fundamental identity of,
in organisms, 205. See also
Axis; Dominance; Individual;
Individuality.

Inheritance: of metabolic gradi-
ents, 41, 182; in relation to or-
ganic individual, 202; in relation
to different reproductive pro-
cesses, 202; of "acquired char-
acters," 204.

Inhibition: of head-formation in
Planaria, 112, 141; in reconsti-
tution of Tubularia, 135; of
growing shoots in plants, 153;
in apical direction in plants,

155; by leaves in plants, 156;
of root-formation by roots, 159;
non-specific character of, in
plants, 168; nature of, 178.

Irritability: increase of, by re-
peated excitation, 33, 204; gra-
dient of, 34, 180.

Isolation, physiological: condi-
tions determining, 45; effect of,
46; infrequency of, in higher
forms, 47; in agamic reproduc-
tion in Tubularia, 92; in agamic
reproduction in Planaria, 94;
experimental, in Tubularia, 135;
experimental, in Planaria, 141;
experimental, in plants, 152; as
the basis of agamic reproduc-
tion, 192; different conditions
determining, 193; in relation to
reduplication of parts, 195. See
also Dominance.

Lumbriculus, experimental control
of reconstitution in, 118.

Marchantia: experimental repro-
duction in, 86; localization in,
as resultant of different axes,
165.

Metabolism: characteristics of,
15; relation of, to protoplasm,
16; susceptibility in relation to,
5 1 ; increase in rate of, after sec-
tion in Planaria, no; rate of, in
relation to differentiation, 183,
190; rate of, in relation to sta-
bility of structure, 191. See
also Gradients; Individual; Ir-
ritability.

Metzgeria, agamic reproduction in,
83.

Moss, developmental gradient in
embryo of, 73.

Nervous system: in relation to
metabolic gradients, 40, 61, 175;
superficial origin of, 49; meta-
bolic gradient in cells of, 62, 151,
173; independent formation of,
in reconstitution, 114; supposed

212

INDIVIDUALITY IN ORGANISMS

formative influence of, 119, 176;
self-determination of, in devel-
opment, 120, 188; extension of
dominance in, 151; dominant
region of, 175; functional domi-
nance of, 176; possible nature of
inhibition in, 180; in relation to
fundamental reaction system,
188; animal organism in rela-
tion to, 189; possibility of de-
differentiation in, 191. See also
Conductivity; Transmission.

Organization: theories of, 22; as
a condition of chemical correla-
tion, 26; not the basis of orgaaic
individuality, 41; in relation to
minimal size in reconstitution,
124; in relation to experimental
conditions, 184.

Parthenogenesis, 199, 200.

Pennaria, developmental gradients
in agamic reproduction of, 79.

Planaria dorolocephala: suscepti-
bility gradients in, 52; develop-
mental gradients in experimental
reproduction of, 81; agamic re-
production of, 92; experimental
reproduction in short pieces of,
99; dominance and subordina-
tion in, 102; reconstitution in,
105; different forms of head in,
106; head-frequency in experi-
mental reproduction of, 108; ex-
perimental control of head-
frequency in, 108; control of
range of dominance in, 138; de-
termination of biaxial forms in,
149; extension of dominance in,
149; localization as resultant of
different axes in, 164.

Planaria macidata, head-frequency
in reconstitution of, 113.

Polarity: occurrence of, 8; theo-
ries of, 28; obliteration of, in
experimental reproduction, 100;
origin of, 181; nature of, 182.
See also Axis; Dominance;
Gradients; Individual.

Poplar, development of buds on
callus in, 86.

Protoplasm: in relation to metabo-
lism, 16; as a metabolic pro-
duct, 1 7 ; metabolic gradients in,
34; differentiation of, in rela-
tion to metabolic gradients, 171;
effect of quantitative external
factors on, 186.

Reconstitution: in relation to
metabolic gradients in Planaria,
81; independence of apical re-
gion in, 97; dominance and sub-
ordination in, 102; the process
of, in Planaria, 105; limiting
factors in, 1 1 7 ; progressive limi-
tation of, in animals, 120; in
embryonic stages, 121; propor-
tional relations of parts in, 122;
limit of size in, 1 24; of hydranth
in Tubularia, 128; in long pieces
of Tubularia, 132; of ''rings" in
Harenactis, 146. See also Indi-
vidual.

Rejuvenescence: nature of, 46, 90;
in reconstitution of Planaria, 89;
in posterior zooids of Planaria,
94; capacity for, in lower and
higher animals, 120; in relation
to physiological isolation, 193;
in early embryonic develop-
ment, 199. See also Senescence.

Reproduction, agamic: occurrence
of, 12, 89; of parts, 13, 195; in
relation to physiological isola-
tion, 45, 192; in Pennaria, 79;
in Stenostomum, 79; in Metz-
geria, 83; of adventitious buds
in Begonia, 83; in Tubularia,
92; in Planaria, 92; different
conditions determining, 193;
localization of, 196; difference
between and gametic, 198. See
also Isolation; Reproduction,
gametic.

Reproduction, experimeittal: sig-
nificance of, 14, 88; in Planaria,
81, 105; in poplar, 86; in Mar-
chantia, 86, 165; in short pieces

INDEX

213

of Tubularia, 97; in short pieces
of Planaria, 99; in long pieces
of Tubularia, 132; in plants,
152; in relation to shape of
piece in Planaria, 114. See also
Reconstitution; Reproduction,
agamic, gametic.

Reproduction, gametic: occur-
rence of, 13; nature of, 47, 198;
difference between, and agamic,
200. See also Reproduction,
agamic, experimental.

Roots of plants : as subordinate in-
dividuals, 104, 162; dominance
and physiological isolation in,
157; relation of, to other parts
of plant, 159; conditions deter-
mining formation of, 162.

Sea-urchin, control of proportions
in larva of, 58.

Senescence: nature of, 46, 90; in
relation to self-maintenance of
parts, 177; in relation to game-
tic reproduction, 198; in rela-
tion to evolution, 205. See also
Rejuvenescence.

Starfish: axial relations in, 8; sus-
ceptibility gradient in egg of, 56.

Stenostomum: agamic reproduc-
tion in, 79; extension of domi-
nance in, 150.

Stentor, susceptibility gradient in,
55-

Subordination, physiological: ori-
gin of, 36; of basal to apical
levels in Planaria, 102, 115; of
basal to apical levels in plants,
104. See also Dominance.

Susceptibility: in relation to me-
tabolism, 51; gradients of, 52;
gradient of, in Stentor, 55; gra-

dient of, in starfish egg, 56;
gradient of, in plants, 61.

Symmetry: occurrence of, 8; of
starfish, 8; susceptibility gradi-
ents in axes of, 57; bilateral, in
certain plants, 77, 86; in "rings"
in Harenactis, 148; in conifers,
155; origin of, 181; significance
of experimental alterations of,
182; conditions determining,
199. See also Axis; Gradients;
Polarity.

Teratomorphic form, in Planaria,
106, 141.

Teratophthalmic form, in Pla-
naria, 106, 141.

Transmission: decrement in, 29,
32, 45, 47, 150, 172, 173; nature
of, 31, 44, 177; in relation to
conductivity, 40; possibility of
different kinds of, 43, 178; range
of, 45; increase in range of, dur-
ing development, 149; in rela-
tion to dominance, 170. See
also Conductivity.

Transportation: occurrence of, in
organisms, 4,27; conditions de-
termining, 26, 170, 172; from
roots of plants, 161. See also
Correlation.

Tubularia: description of, 91;
agamic reproduction in, 92;
experimental reproduction in
short pieces of, 97, 133; domi-
nance in experimental reproduc-
tion of, 102; control of distance
relations in reconstitution of,
128; reconstitution of hydranth
of, 128; reconstitution in long
pieces of, 132; range of domi-
nance in, 133; inhibition in re-
constitution of, 135.

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