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REGENERATION
Columbia University Biological Series.
EDITED BY
HENRY FAIRFIELD OSBORN
AND
EDMUND B. WILSON.
1. FROM THE GREEKS TO DARWIN.
By Henry Fairfield Osborn, Sc.D. Princeton.
2. AMPHIOXUS AND THE ANCESTRY OF THE VERTEBRATES.
By Arthur Willey, B.Sc. London Univ.
3. FISHES, LIVING AND FOSSIL. An Introductory Study.
By Bashford Dean, Ph.D. Columbia.
4. THE CELL IN DEVELOPMENT AND INHERITANCE.
By Edmund B. Wilson, Ph.D. J.H.U.
5. THE FOUNDATIONS OF ZOOLOGY.
By William Keith Brooks, Ph.D. Harv., LL.D. Williams.
6. THE PROTOZOA.
By Gary N. Calkins, Ph.D. Columbia.
7. REGENERATION.
By Thomas Hunt Morgan, Ph.D.
_COLUMBIA UNIVERSITY BIOLOGICAL SERIES. VII._
REGENERATION
BY
THOMAS HUNT MORGAN, PH.D.
PROFESSOR OF BIOLOGY, BRYN MAWR COLLEGE
[Illustration]
New York
THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., LTD.
1901
_All rights reserved_
COPYRIGHT, 1901,
BY THE MACMILLAN COMPANY.
_Norwood Press
J. S. Cushing & Co.--Berwick & Smith
Norwood, Mass., U.S.A._
To My Mother
PREFACE
This volume is the outcome of a course of five lectures on “Regeneration
and Experimental Embryology,” given in Columbia University in January,
1900. The subjects dealt with in the lectures are here more fully
treated and are supplemented by the discussion of a number of related
topics. During the last few years the problems connected with the
regeneration of organisms have interested a large number of biologists,
and much new work has been done in this field; especially in connection
with the regenerative phenomena of the egg and early embryo. The
development of isolated cells or blastomeres has, for instance, aroused
widespread interest. It has become clearer, as new discoveries have been
made, that the latter phenomena are only special cases of the general
phenomena of regeneration in organisms, so that the results have been
treated from this point of view in the present volume.
If it should appear that at times I have gone out of my way to attack
the hypothesis of preformed nuclear germs, and also the theory of
natural selection as applied to regeneration, I trust that the
importance of the questions involved may be an excuse for the criticism.
If I may be pardoned a further word of personal import, I should like to
add that it has seemed to me that far more essential than each special
question with which the biologist has to deal is his attitude toward the
general subject of biology as a science. Never before in the history of
biology has this been more important than at the present time, when we
so often fail to realize which problems are really scientific and which
methods are legitimate for the solution of these problems. The custom of
indulging in exaggerated and unverifiable speculation bids fair to dull
our appreciation for hypotheses whose chief value lies in the
possibility of their verification; but those who have spent their time
and their imagination in such speculations cannot hope for long to hold
their own against the slow but certain advance of a scientific spirit of
investigation of organic phenomena. The historical questions with which
so many problems seem to be connected, and for which there is no
rigorous experimental test, are perhaps responsible for the loose way in
which many problems in biology are treated, where fancy too often
supplies the place of demonstration. If, then, I have tried to use my
material in such a way as to turn the evidence against some of the
uncritical hypotheses of biology, I trust that the book may have a wider
bearing than simply as a treatment of the problems of regeneration.
I wish to acknowledge my many obligations to Professor H. F. Osborn and
to Professor E. B. Wilson for friendly criticism and advice; and in
connection with the revision of the text I am greatly indebted to
Professor J. W. Warren, to Professor W. M. Wheeler, to Professor G. H.
Parker, and to Professor Leo Loeb.
BRYN MAWR COLLEGE, PENNSYLVANIA,
June 11, 1901.
CONTENTS
CHAPTER I
GENERAL INTRODUCTION
PAGE
Historical Account of the Work on Regeneration of Trembley,
Bonnet, and Spallanzani 1
Some Further Examples of Regeneration 6
Definition of Terms 19
CHAPTER II
THE EXTERNAL FACTORS OF REGENERATION IN ANIMALS
The Effect of Temperature 26
The Effect of Food 27
The Effect of Light 29
The Effect of Gravity 30
The Effect of Contact 33
The Effect of Chemical Changes in the Environment 35
General Conclusions 36
CHAPTER III
THE INTERNAL FACTORS OF REGENERATION IN ANIMALS
Polarity and Heteromorphosis 38
Lateral Regeneration 43
Regeneration from an Oblique Surface 44
The Influence of Internal Organs at the Cut-surface 52
The Influence of the Amount of New Material 54
The Influence of the Old Parts on the New 62
The Influence of the Nucleus on Regeneration 65
The Closing in of Cut-edges 69
CHAPTER IV
REGENERATION IN PLANTS
Regeneration in Flowering Plants 71
Regeneration in Liverworts, Mosses, and Moulds 84
Hypothesis of Formative Stuffs 88
CHAPTER V
REGENERATION AND LIABILITY TO INJURY
Examples of Supposed Connection between Regeneration and
Liability to Injury 92
Regeneration in Different Parts of the Body 97
Regeneration throughout the Animal Kingdom 103
Regeneration and the Theory of Natural Selection 108
CHAPTER VI
REGENERATION OF INTERNAL ORGANS. HYPERTROPHY. ATROPHY
Regeneration of Liver, Eye, Kidney, Salivary Glands, Bones,
Muscles, Nerves,
Brain, and Cord of Vertebrates 111
Examples of Hypertrophy 115
Theories of Hypertrophy 118
Atrophy 123
Incomplete Regeneration 125
CHAPTER VII
PHYSIOLOGICAL REGENERATION
Supposed Relation between Physiological Regeneration and Restorative
Regeneration 128
Regeneration and Growth 131
Double Structures 135
CHAPTER VIII
SELF-DIVISION AND REGENERATION. BUDDING AND REGENERATION. AUTOTOMY.
THEORIES OF AUTOTOMY
Review of Groups in which Self-division occurs 142
Division in Plane of Least Resistance 144
Review of Groups in which Budding occurs. Relation of Budding+
to Regeneration 149
Autotomy 150
Theories of Autotomy 155
CHAPTER IX
GRAFTING AND REGENERATION
Examples of Grafting in Hydra, Tubularia, Planarians, Earthworms,
Tadpoles 159
Grafting Pieces of Organs in Other Parts of the Body in
Higher Animals 178
Grafting of Parts of Embryos of the Frog 182
Union of Two Eggs to form One Embryo 188
CHAPTER X
THE ORIGIN OF NEW CELLS AND TISSUES
Origin of New Cells in Annelids 190
Origin of the New Lens in the Eye of Salamanders 203
The Part played by the “Germ-layers” in Regeneration 207
The Supposed Repetition of Phylogenetic and Ontogenetic
Processes in Regeneration 212
CHAPTER XI
REGENERATION IN EGG AND EMBRYO
Introduction 216
Regeneration in Egg of Frog 217
Regeneration in Egg of Sea-urchin 228
Regeneration in Other Forms: Amphioxus, Ascidian, Ctenophore,
Snail, Jelly-fish, Fish 236
CHAPTER XII
THEORIES OF DEVELOPMENT
Theories of Isotropy and of Totipotence of Cells 242
Theory of Qualitative Division of Nucleus 243
Theory of Equivalency of Cells 244
Theory of the Organized Structure of the Protoplasm 246
Theory of Cells as Units 250
Further Analysis of Theories of Qualitative Nuclear Divisions and
of the Equivalency of Blastomeres 252
Driesch’s Analytical Theory, Criticism, and Later Theories of
Driesch 253
Conclusions 256
CHAPTER XIII
THEORIES OF REGENERATION
Pre-formation Theory 260
Comparison with Growth of Crystal 263
Completing Theory 264
Theory of Formative Stuffs 265
Conclusions 269
Theory of Tensions controlling Growth 271
CHAPTER XIV
GENERAL CONSIDERATIONS AND CONCLUSIONS
Organization 277
Machine Theory of Development and of Regeneration 283
Teleology 283
“Action at a Distance” 284
Definition of Terms: Cause, Stimulus, Factor, Force, Formative
Force, Organization 287
Regeneration as a Phenomenon of Adaptation 288
LITERATURE 293
INDEX 311
REGENERATION
CHAPTER I
GENERAL INTRODUCTION
Although a few cases of regeneration were spoken of by Aristotle and by
Pliny, the subject first attracted general attention through the
remarkable observations and experiments of the Abbé Trembley. His
interest was drawn to certain fresh-water polyps, hydras, that were new
to him, and in order to find out if the organisms were plants or animals
he tried the effect of cutting them into pieces; for it was generally
known that pieces of a plant made a new plant, but if an animal were cut
into pieces, the pieces died. Trembley found that the polyp, if cut in
two, produced two polyps. Logically, he should have concluded that the
new form was a plant; but from other observations, as to its method of
feeding and of movement, Trembley concluded that the polyp was an
animal, and that the property of developing a new organism from a part
must belong to animals as well as to plants. “I felt,” he says,
“strongly that nature is too vast, and too little known, for us to
decide without temerity that this or that property is not found in one
or another class of organized bodies.”
Trembley’s first experiments were made in 1740, and the remarkable
results were communicated by letter to several other naturalists. It
came about in this way that before Trembley’s memoir had appeared, in
1744, his results were generally known, and several other observers had
repeated his experiments, and extended them to other forms, and had even
published an account of their own experiments, recognizing Trembley,
however, as the first discoverer. Thus Réaumur described, in 1742, a
number of other forms in which regeneration takes place; and Bonnet, in
1745, also described some experiments that he had made during the four
preceding years. Widespread interest was aroused by these results, and
many different kinds of animals were experimented with to test their
power of regeneration. Most important of these new discoveries were
those of Spallanzani, who published a short preliminary statement of his
results, in 1768, in his _Prodromo_.
Trembley found that when a hydra is cut in two, the time required for
the development of the new individuals is less during warm than during
cold weather. He also found that if a hydra is cut into three or four
parts, each part produces a new individual. If these new hydras are fed
until they grow to full size, and are then again cut into pieces, each
piece will produce a new polyp. The new animals were kept in some cases
for two years, and behaved in all respects as do ordinary polyps.
Trembley also found that if the anterior, or head-end, with its
tentacles, is cut off, it also will make a new animal. If a hydra is cut
lengthwise into two parts, the edges roll in and meet, and in an hour,
or less, the characteristic form may be again assumed. New arms may
appear later on the new individual. If a hydra is split lengthwise into
four pieces, each piece will also produce a new polyp.
If the head-end only of a hydra is split in two, each half becomes a new
head, and a two-headed hydra results. If each of the new heads is split
again, a four-headed hydra is produced; and if each of the four heads is
once more split in two, an eight-headed hydra is formed. A hydra of this
kind, in which seven heads had been produced in this way, is shown in
Fig. 1, _A_. Each head behaves as a separate individual, and all remain
united on the same stalk. If the foot-end of a hydra is split, a form
with two feet is produced.
One of the most ingenious and most famous experiments that Trembley made
consisted in turning a hydra inside out (Fig. 1, _B_, 1 and 2). The
animal tends to turn itself back again, but by sticking a fine bristle
through the body, Trembley thought that the turning back could be
prevented, and that the inner surface of the hollow body remained on the
outside, and the outer surface of the body came to line the new central
cavity. Each layer then changed, he thought, its original
characteristics, and became like that of the other layer. The details of
these experiments will be described in a future chapter, as well as more
recent experiments that have put the results in quite a different light.
Réaumur repeated Trembley’s experiment of cutting a hydra into pieces,
and obtained the same results. He found also that certain fresh-water
worms, as well as the terrestrial earthworm, regenerated when cut into
pieces. At his instigation two other naturalists[1] examined the
starfish and some marine polyps, and they concluded that it was highly
probable that these forms also could regenerate. Réaumur pointed out
that regeneration is more likely to occur in fragile forms which are
more exposed to injury.
Bonnet’s experiments were made on several kinds of fresh-water
[Illustration: FIG. 1.--_A_-_B_. After Trembley, _C_-_G’_. After
Bonnet. _A._ Seven-headed hydra made by splitting head-ends lengthwise.
_B._ Illustrating the method of turning hydra inside out by means of a
bristle: 1, foot being pushed through mouth; 2, completion of process.
_C._ Middle piece of an earthworm (cut into three pieces) with new
head and tail. _D._ Anterior part of an earthworm regenerating a new
“delicate” tail. _E._ Posterior third of a worm (lumbriculus) that
regenerated two heads. _F._ Middle piece of a worm (another species)
cut into three pieces. It made a tail at each end. _F’._ Anterior,
enlarged end (tail) of last. _G._ Small piece of a worm. _G’._
Regeneration of head and tail of same.]
worms, one of which, at least, seems to have been the annelid
lumbriculus. His first experiments (1741) showed that when the worm is
cut in two pieces, a new tail develops at the posterior end of the
anterior piece, and a new head at the anterior end of the posterior
piece. He found that if a worm is cut into three, four, eight, ten, or
even fourteen pieces, each piece produces a new worm; a new head
appearing on the anterior end of each piece, and a new tail on the
posterior end (Fig. 1, _G_, _G’_). The growth of the new head is limited
in all cases to the formation of a few segments, but the new tail
continues to grow longer, new segments being intercalated just in front
of the end-piece that contains the anal opening. In summer the
regeneration of a new part takes place in two to three days; in winter
in ten to twelve days, this difference not being due to the time of
year, but to the temperature. Bonnet found that if a newly regenerated
head is cut off, a new one regenerates, and if the second one is
removed, a third, new one develops, and in one case this occurred eight
times: the ninth time only a bud-like outgrowth was formed. In other
cases a new head was produced a few more times, but never more than
twelve. He thought that the capacity of a part to regenerate is in
proportion to the number of times that the animal is liable to be
injured under natural conditions.
Bonnet found that short pieces from the anterior or posterior end of the
body failed to regenerate, and usually died in a few days. Occasionally
two new heads appeared at the anterior end of a piece (Fig. 1, _E_), and
sometimes two tails at the posterior end.
Another kind of fresh-water worm[2] was found that gave a very
remarkable result. If it was cut in two pieces, the posterior piece
produced at its anterior end, not a new head, but a new tail. Thus there
is formed a worm with two tails turned in opposite directions, as shown
in Fig. 1, _F_, _F’_.
Spallanzani made many experiments on a number of different animals, but
unfortunately the complete account of his work was never published, and
we have only the abstract given in his _Prodromo_ (1768). He made a
large number of experiments with earthworms of several kinds, and found
that a worm cut in two pieces may produce two new worms; or, at least,
that the anterior piece produces a new tail, which increases in length
and may ultimately represent the posterior part of the body; the
posterior piece, however, produces only a short head at its anterior
end, but never makes good the rest of the part that was lost. A short
piece of the anterior end fails to regenerate; but in one species of
earthworm, that differs from all the others in this respect, a short
anterior piece or head can make a new tail at its posterior end.[3]
Spallanzani also found that if much of the anterior end is cut off, the
development of a new head by the posterior piece is delayed, and, in
some species, does not take place at all.
If a new head is cut off, another is regenerated, and this occurred, in
one case, five times. If, after a new head has developed, a portion only
is cut off, the part removed is replaced, and if a portion of this new
part is cut off it is also regenerated. If a worm is split
longitudinally into two pieces, the pieces die. If only a part of the
worm is split longitudinally and one part removed, the latter will be
regenerated from the remaining part.[4] Several contemporaries of
Spallanzani also made experiments on the earthworm.[5]
Spallanzani found that a tadpole can regenerate its tail; and if a part
of the new tail is cut off, the remaining part will regenerate as much
as is lost. Older tadpoles regenerate more slowly than younger ones. If
a tadpole is not fed, it ceases to grow larger, but it will still
regenerate its tail if the tail is cut off.[6] Salamanders also
regenerate a new tail, producing even new vertebræ. If a leg is cut off,
it is regenerated; if all four legs are cut off, either at the same time
or in succession, they are renewed. If the leg is cut off near the body,
an imperfectly regenerated part is formed. Regeneration of the legs was
found to take place in all species of salamanders that were known to
Spallanzani, but best in young stages. In full-grown salamanders,
regeneration takes place more promptly in smaller species than in larger
ones. Curiously enough, it was found that if the fingers or toes are cut
off, they regenerate very slowly. If the fingers of one side and the
whole leg of the opposite side are cut off at the same time, the leg may
be regenerated as soon as are the fingers of the other side. A year is,
however, often insufficient in some forms for a leg to become fully
formed. If an animal is kept without food for two months after a leg has
been cut off, the new leg will regenerate as rapidly as in another
salamander that has been fed during this time. If the animal is kept
longer without food, it will decrease in size, but nevertheless the new
leg continues to grow larger. Occasionally more toes or fewer toes than
the normal number are regenerated; but as a rule the fore leg renews its
four toes, and the hind leg its five toes.
In one experiment, all four legs and the tail were cut off six times
during the three summer months, and were regenerated. Spallanzani
calculated that 647 new bones must have been made in the new parts. The
regeneration of the new limbs was as quickly carried out the last time
as the first. Spallanzani also found that the upper and lower jaws of
salamanders can regenerate.
If the tentacles of a snail or of a slug are cut off, they are renewed;
and Spallanzani found that even if the entire head is cut off a new one
is regenerated. Also other parts of the snail, as the foot, or the
collar, may be regenerated. The head of the slug, it was found,
regenerates with more difficulty than does that of the snail.
These justly celebrated experiments of Trembley, Réaumur, Bonnet, and
Spallanzani furnished the basis of all later work. Many new facts, it is
true, have been discovered, and in many cases we have penetrated further
into the conditions that influence the regeneration, but many of the
important facts in regard to regeneration were made known by the work of
these four naturalists.
_SOME FURTHER EXAMPLES OF REGENERATION_
So many different phenomena are included at the present time under the
term “regeneration,” that it is necessary, in order to get a general
idea of the subject, to pass in review some typical examples of the
process.
The regeneration of different parts of the salamander shows some
characteristic methods of renewal of lost parts. If the foot is cut off
a new foot is regenerated; if more than the foot is cut off, as much is
renewed as was lost. For instance, if the cut is made through the fore
leg, as much of the fore leg as was removed, and also the foot, are
regenerated; if the cut is made through the upper part of the leg, the
rest of that part of the leg and the fore leg and the foot are
regenerated. The new part is at first smaller than the part removed,
although it may contain all the elements characteristic of the leg. It
gradually increases in size until it has grown to the same size as the
leg on the other side of the body, and then its growth comes to an end.
Other parts of the body of the salamander also have the power of
regeneration. If a part of the tail is cut off, as much is renewed as
has been removed; if a part of the lower or upper jaw is cut off, the
missing part is regenerated; if a part of the eye is removed, a new eye
is formed from the part that remains; but if the whole eye is
extirpated, or the whole limb, together with the shoulder girdle, is
removed, neither structure is regenerated.
In other vertebrates the power of regeneration is more limited. A lizard
can regenerate its tail, but not its limbs. A dog can regenerate neither
its limbs nor its tail.
It has been stated that the new limb of the salamander is at first
smaller than the one removed, but it may contain all the elements of the
original limb. We find this same phenomenon in other forms, and since it
is a point of some theoretical interest, a few other examples may be
given. If the tail of a fish that has a bilobed form is cut off near the
base, as indicated in Fig. 40, _G_, there appears over the exposed edge
a narrow band of new material. The new part
[Illustration: FIG. 2.--_A. Allolobophora fœtida._ Normal worm. _B-F._
Anterior ends of worms, which, after the removal of one, two, three,
four, and five segments, have regenerated the same number. _G._
Anterior third cut off. Only five head-segments regenerated. _H._ Worm
cut in two in middle. A head-end of five segments regenerated. _I._
Worm cut in two posterior to middle. A heteromorphic tail regenerated
at anterior end.]
now begins to grow faster at two places than at intermediate points, as
shown in Fig. 40, _H_. The new tail, although very short, assumes, as a
result, the characteristic bilobed form. The point of special interest
is that the new material that appears over the exposed edge does not
first grow out at an equal rate at all points until it reaches the level
of the original fork, and then continue to grow faster in two regions
to form the lobes of the tail, but the two regions of most rapid growth
are very soon established in the new tail. Subsequent growth in all
parts of the new tail enlarges it to the full size.
[Illustration: FIG. 3.--_A_, _B_. Short head-ends of _A. fœtida_ that
did not regenerate at posterior surface. _C_, _D_, _E_. Longer anterior
pieces, that made new segments at their posterior ends. _F._ After
Hazen. A piece consisting of five (3 to 7) anterior segments grafted,
in a reversed position, upon the anterior end of another worm. A
heteromorphic head of about two segments regenerated at the free end,
which is the posterior end of the piece.]
In some cases of regeneration, in which the new part is at first smaller
than the part removed, the new part represents at first only the distal
portion of the body, and although the new part may grow to the full
size, the whole of the part removed may never come back. This is
illustrated in the regeneration of the anterior end of the earthworm;
for example, in the red-banded earthworm, or brandling (_Allolobophora
fœtida_).[7] If one segment of the anterior end is cut off, one segment
is very quickly regenerated (Fig. 2, _B_); if two segments are cut off,
two come back (Fig. 2, _C_); if three segments are cut off, as many are
regenerated (Fig. 2, _D_); if four are cut off, generally four come back
(Fig. 2, _E_); when five are cut off, four or five come back (Fig. 2,
_F_); but if six or more are cut off, only four or five are regenerated
(Fig. 2, _G_). It is found in this case that a limit is soon reached
beyond which fewer segments are produced than have been removed. The new
segments form the anterior end or head that enlarges to the
characteristic size; but the missing segments behind the new head are
never regenerated, and the worm remains shortened throughout the rest of
its life. If the reproductive region has been removed with the anterior
part, new reproductive organs are never formed and the worm remains
incapable of reproducing itself.
This same relation between the number of segments cut off from the
anterior end and the number that is regenerated seems to hold good
throughout the whole group of annelids, although the maximum number that
comes back may be different in different species. Thus in lumbriculus
six or seven or even eight new segments come back if more than that
number have been removed.
If we examine the method of regeneration from the posterior end of a
piece of an earthworm, we find that when several or many posterior
segments have been removed a new part comes back, composed at first of a
very few segments. The terminal segment contains the new posterior
opening of the digestive tract. New segments are now formed just in
front of the terminal segment, the youngest being the one next to the
end-segment. The process continues until the full complement of segments
is made up (Fig. 3, _C_, _D_, _E_). Comparing these results with those
described above for the anterior end, we find, in both cases, that only
a few segments are at first formed, but in the posterior regeneration
new segments are intercalated near the posterior end. This process of
intercalation is the characteristic way in which many annelids add new
segments to the posterior end, as they grow larger and longer.
Amongst the flatworms the fresh-water planarians show remarkable powers
of regeneration. If the anterior end is cut off at any level, a new head
is produced (Fig. 4, _C_). The new worm is at first too short, _i.e._
the new head is too near the pharynx, but changes take place in the
region behind the new head that lead to the development of new material
in this part. The new head is, in consequence, carried farther and
farther forward until the typical relations of the parts have been
formed, when the growth in the region behind the head comes to an end
(Fig. 4, _C¹_). Similar changes take place when the posterior end is
cut off, as shown in Fig. 4, _B, B¹_. The new part contains the new
pharynx that is proportionately too near the head, but the pharynx is
carried farther backwards by the formation of new material in front of
it, until it has reached its typical distance from the head. In these
planarians the results are somewhat complicated, owing to the old part
changing its form, especially if the piece is not fed; but the main
facts are given above, and a more complete account of the changes that
occur will be given in another place.
[Illustration: FIG. 4.--_A-E. Planaria maculata._ _A._ Normal worm. _B,
B¹._ Regeneration of anterior half. _C, C¹._ Regeneration of posterior
half. _D._ Cross-piece of worm. _D¹, D², D³, D⁴._ Regeneration of
same. _E._ Old head. _E¹, E², E³._ Regeneration of same. _F._ _P.
lugubris._ Old head cut off just behind eyes. _F¹._ Regeneration of new
head on posterior end of same.]
_LATERAL REGENERATION_
Not only does regeneration take place in an antero-posterior direction,
but in many animals also at the side. The regeneration of the limb of
the salamander is, of course, a case of lateral regeneration in
relation to the animal as a whole, but in a longitudinal direction in
regard to the limb itself. Lateral regeneration of the limb would take
place if the limb was split lengthwise into two parts and one of the
parts removed. If the entire salamander were cut in two lengthwise, each
half would most certainly die without regeneration, if for no other
reason than that the integrity of the median organs is necessary for the
life of the different parts. If, however, a planarian is cut lengthwise
into a right and left half, each piece will complete itself laterally
and make a new worm (Fig. 13½, _A-D_). Even a narrow piece cut from
the side will produce a new worm by regenerating laterally, as shown in
Fig. 19, _a_, _b_, _c_. In hydra, also, a half-longitudinal piece
produces a new animal, but in this case not by the addition of new
material at the side, but by the cut-edges meeting to make a tube of
smaller diameter. Subsequently the piece changes its form into that
characteristic of hydra.
_REGENERATION OF TERMINAL PORTIONS OF THE BODY_
In most of the preceding examples the behavior of the larger piece of
the two that result from the operation has been described; but there are
some important facts in connection with the regeneration of the smaller
end-pieces. The leg, or the tail, that has been cut from the salamander
soon dies without regenerating. The life of the leg can be maintained
only when the part is supplied with certain substances from the body of
the animal. It does not follow, of course, that, could the leg or the
tail be kept alive, they would regenerate a salamander. In fact, there
is evidence to show, in the tail at least, that, although it may
regenerate a structure at its anterior end, the structure is not a
salamander, but something else. This has been definitely shown in
certain experiments with the tail of the tadpole. It is possible to
graft the tail of one tadpole in a reversed position, _i.e._ with its
anterior end free, on the tail of another tadpole (Fig. 54, _A-D_), or
even on other parts of the body. Regeneration takes place from the free
end, _i.e._ from the proximal end of the grafted tail. The new structure
resembles a tail, and not a tadpole. If it be objected that the
experiment is not conclusive because of the presence of the old tail, or
of the use of the newly developing part, the objection can be met by
another experiment. If, as shown in Fig. 56, _A_, a triangular piece is
cut out of the base of the tail of a young tadpole, the cut being made
so deep that the nerve-cord and notochord are cut in two, there develops
from the proximal end of the tail a new tail-like structure that is
turned forward, or sometimes laterally. In this case the objections to
the former experiment do not apply, and the same sort of a structure,
namely, a tail, is produced.
[Illustration: FIG. 5.--_Hydra viridis_. _A._ Normal hydra. Lines
indicate where piece was cut out. _B_, 1-4. Changes in a piece of _A_,
as seen from the side. _C_, 1-4. Same as seen from the end. _D, E, F._
Later stages of same piece, drawn to same scale.]
In the earthworm also we find some interesting facts connected with the
regeneration of the terminal pieces. If one, two, three, four, or five
segments are cut from the anterior end, they will die without
regenerating. Pieces that contain more segments, six to ten, for
example, may remain alive for a month or longer, but do not regenerate
(Fig. 3, _A_, _B_). That this lack of power to regenerate at the
posterior end is not due to the smallness of the piece can be shown by
removing from a piece of five segments one or two of its anterior
segments. These will be promptly regenerated. Another experiment has
shown, however, that if these small pieces can be kept alive for a long
time, and also supplied with nourishment, regeneration will take place
at the posterior end. If, for instance, a small piece of eight or ten
segments has its anterior three or four segments cut off, and is grafted
by its anterior end to the anterior end of another worm, as shown in
Fig. 3, _F_, the piece will begin, after several months, to regenerate
at its exposed posterior end, but in the one instance in which this
experiment has been successfully carried out, a new head, and not a
tail, appeared on the exposed free end. The result is not due to the
grafting, or to the anterior position of the posterior end, but to some
peculiarity in the piece itself. We find the converse of this result in
an experiment with the tail region of the earthworm, where the outcome
is more clearly seen to be connected with the nature of the piece
itself. If a piece less than half the length of the worm is cut off from
the posterior end, there is generally formed from its anterior
cut-surface, not a head, but another tail (Fig. 2, _I_). The result is
similar to that described by Bonnet for one of the fresh-water annelids.
A parallel case to that of the head of the earthworm is found in one of
the planarians. If the head of _Planaria lugubris_ is cut off just
behind the eyes (Fig. 4, _F_), there is produced, at the posterior
cut-edge of the head, a new head turned in the opposite direction, as
shown in Fig. 4, _F¹_.
_REGENERATION BY TRANSFORMATION OF THE ENTIRE PIECE_
[Illustration: FIG. 6.--_A._ Piece of _Bipalium kewense_. Middle pigment
stripe injured at two points (see circles in _A_). _B._ Regeneration of
same piece.]
In the regeneration of some of the lower animals, the transformation of
a piece into a new animal of smaller size is brought about by a change
in form of the piece itself, rather than through the production of new
material at the cut-ends. If a ring is cut from the body of hydra, as
shown in Fig. 5, _A_, the open ends of the ring are soon closed by the
contraction of the sides of the piece, and in the course of a few hours
the ring has become a hollow sphere; or, if the piece is longer, a
closed cylinder. After a day or two, the piece begins to elongate, and
four tentacles appear near one end (Fig. 5, _B_, _C_, _D_). The piece
continues to elongate until it forms a small polyp, having the typical
proportions of length to breadth (Fig. 5, _E_, _F_). It has changed into
a new cylinder that is longer than the piece cut off, but
correspondingly narrower. In this case there cannot be said to be a
replacement of the missing parts, but rather, through the transformation
of the old piece, the formation of a new whole. In planarians also the
formation of a new worm from a piece involves a change in the form of
the old part, as well as the addition of new material at the cut-end. If
a cross-piece is cut out, as shown in Fig. 4, _D_, new material appears
at the ends, but the old piece also becomes narrower and longer (Fig. 4,
_D¹-D⁴_). If the old head is cut off, it produces new material at its
posterior end (Fig. 4, _E_, _E¹_), and also becomes smaller as the new
part grows larger (Fig. 4, _E²_, _E³_). In a land planarian, _Bipalium
kewense_, a piece is transformed into a new worm, as shown in Fig. 6,
_A_, _B_. In this case the old pigment stripes of the piece are carried
directly over into the new worm, the piece elongating during the
transformation.
[Illustration: FIG. 7.--_Stentor cœruleus._ _A._ Normal, fully expanded
individual. _A¹._ Same contracted. Line _a-a_ indicates where it was
cut in two. _B, C._ Pieces after division. _B¹, B², B³._ Regeneration
of three distal pieces (_B_) containing old peristome. _C¹, C²._
Regeneration of two proximal or foot pieces (_C_).]
A similar change takes place in pieces of unicellular animals, as best
shown by cutting off pieces of stentor. If _Stentor cœruleus_ is cut in
two pieces, as indicated in Fig. 7, each piece makes a new individual of
half size, but of proportionate form. The old peristome remains on the
anterior piece, but becomes reduced in size as the piece changes its
shape, and although it may be at first too large for the length of the
new piece, it ultimately reaches a size about proportionate to the rest
of the animal. The posterior piece is at first too long for the size of
the new peristome that is formed, but the latter becomes larger, until
the characteristic form has been reached. The change in form of the
stentor may take place in a few hours, and the result is brought about,
not by the development of new protoplasm over the cut-end, but by a
change of the old protoplasm into the new form. A similar experiment is
shown in Fig. 8, in which a stentor was cut into three pieces, each
piece containing a part of the old nucleus.
[Illustration: FIG. 8.--After Gruber. _Stentor cœruleus._ _A._ Cut into
three pieces. _B._ This row shows regeneration of anterior piece. _C._
This row shows regeneration of middle piece. _D._ This row shows
regeneration of posterior piece.]
_REGENERATION IN PLANTS_
In the higher plants the production of a new plant from a piece takes
place in a different way from that by which in animals a new individual
is formed. The piece does not complete itself at the cut-ends, nor does
it change its form into that of a new plant, but the leaf-buds that are
present on the piece begin to develop, especially those near the distal
end of the piece, as shown in Fig. 32, _A_, and roots appear near the
basal end of the piece. The changes that take place in the piece are
different from those taking place in animals, but as the principal
difference is the development of the new part near the end, rather than
over the end, and as in some cases the new part may even appear in new
tissue that covers the end, and, further, since the process seems to
include many factors that appear also in animals, we are justified, I
think, in including this process in plants under the general term
regeneration.
[Illustration: FIG. 9.--After Vöchting. _A_, _A¹_, _A²_. Pieces of
thallus of _Lunularia communis_ regenerating at the apical end. _B._
Piece of thallus cut in two in the middle line. _B¹._ Same split at
side of middle. _C._ An oblique piece extending to middle line. _C¹_,
_C²_. Oblique pieces not extending to middle line. _D._ Fruiting stalk
stuck into sand, producing new thallus above sand. _D¹._ Same laid
horizontally regenerating near base. _E._ Same with fruiting head cut
off. Regenerating at base. _E¹._ Twisted piece regenerating at two
points. _F._ Piece of ray of head regenerating near base. _F¹._ Same
with distal end of ray cut off. Also regenerating at base.]
In the lower plants, such as the mosses, the liverworts, the moulds, and
the unicellular forms, regeneration also takes place. Vöchting has shown
that pieces from any part of the thallus of a liverwort[8] produce new
plants. If a cross-piece is cut off, there appears a small outgrowth
from the middle of the anterior cut-edge, as shown in Fig. 9, _A_,
_A²_, that gradually enlarges to form a new thallus. It will be seen
from the figures that the whole anterior edge does not grow forward, but
a new thallus arises from a group of cells at, or near, the anterior
edge. These cells are the least-differentiated cells in the piece, and
have softer cell walls than have the other cells.
[Illustration: FIG. 10.--After Pringsheim. _A._ A piece of seta of
sporophore of _Hypnum cupressiforme_, sending out protonema-threads.
_B._ Longitudinal section of a piece of the seta of sporophore of _Bryum
cæspitosum_. _C._ Piece of same of _Hypnum cupressiforme_. Moss-plant
arising from new protonema. _D._ Piece of same of _Hypnum serpens_ with
protonema and moss-plant arising from it.]
Pringsheim has shown that if a piece of the stalk of the sporangium of
certain mosses is cut off, it produces at its ends thread-like
outgrowths which are like the protonema-stage of the moss, and from this
protonema new moss-plants may arise (Fig. 10, _A_, _B_, _C_, _D_).
Braefeld has obtained a somewhat similar result in one of the moulds, in
which a piece of the sporangium stalk gives rise to a mycelium from
which new sporangia may be produced.
_REGENERATION IN EMBRYOS AND EGGS_
Regeneration takes place not only in adult organisms, but also in
embryos, and larvæ of many animals. It is often stated that the power of
regeneration is more highly developed in embryos than in adults, but the
facts that can be advanced in support of this view are not numerous. One
of the few cases of this sort known to us is that of the leg of the
frog, that does not regenerate, while the leg of the tadpole is capable
of regenerating.
[Illustration: FIG. 11.--_A._ Blastula of Sea-urchin. Dotted lines
indicate where pieces of wall were cut off. To the right are shown
stages in the development of these pieces. _B._ Two-cell stage of egg of
sea-urchin. One blastomere isolated. Its development shown in figures to
right of _B_. _C._ Fertilized but unsegmented egg. Dotted line indicates
where it was cut in two. Upper row of figures to right shows development
of nucleated piece; lower row shows the fertilization and development of
non-nucleated piece.]
The early stages in the development of the sea-urchin, or of the
starfish, may be taken to illustrate the power of regeneration in
embryos. If the hollow blastula of the sea-urchin is cut into pieces
(Fig. 11, _A_), each piece, if not too small, may produce a new
blastula. The edges of the piece come together, and fuse in the same
way in which a piece of hydra closes. A new hollow sphere of small size
is formed, which then passes through the later stages of development as
does the whole normal blastula.
Still earlier stages of the sea-urchin, or of the starfish, have the
power of producing embryos if they are cut into pieces. If the
segmenting egg is separated into a few parts, each part will continue to
develop. Even the first two blastomeres or cells will, if separated,
produce each a whole embryo (Fig. 11, _B_). The power of development of
a part does not even end here, for, if the undivided, fertilized egg is
cut into pieces, the part that contains the nucleus will segment and
produce a whole embryo (Fig. 11, _C_, upper row). If the egg is cut in
two or more pieces before fertilization, and then each part is
fertilized, it has been found that not only the nucleated, but even the
non-nucleated fragments (if they are entered by a single spermatozoon)
may produce embryos (Fig. 11, _C_, lower row).
It may be questioned whether the development of parts of the embryo, or
of the egg, into a whole organism can be included in the category of
regenerative processes. There are, it is true, certain differences
between these cases and those of adult forms, but as there are many
similarities in the two cases, and as the same factors appear in both,
we cannot refuse, I think, to consider all the results from a common
point of view.
_PHYSIOLOGICAL REGENERATION_
Finally, there are certain normal changes that occur in animals and
plants that are not the result of injury to the organism, and these have
many points in common with the processes of regeneration. They are
generally spoken of as processes of physiological regeneration. The
annual moulting of the feathers of birds, the periodic loss and growth
of the horns of stags, the breaking down of cells in different parts of
the body after they have been active for a time, and their replacement
by new cells, the loss of the peristome in the protozoon, stentor, and
its renewal by a new peristome, are examples of physiological
regeneration. This group of phenomena must also be included under the
term “regeneration,” since it is not sharply separated from that
including those cases of regeneration after injury, or loss of a part,
and both processes appear to involve the same factors.
_DEFINITION OF TERMS_
The older writers used such terms as “replacement of lost parts,”
“renewal of organs,” and “regeneration” to designate processes similar
to those described in the preceding pages. The term regeneration has
been for a long time in general use to include all such phenomena as
those referred to, but amongst recent writers there is some diversity of
opinion as to how much is to be included in the term, and the question
has arisen as to the advantage of applying new names to the different
kinds of regeneration. There can be little doubt of the advantage, for
the sake of greater clearness, of the use of different terms to
designate different phenomena, but I think that there is at the same
time the need of some general term to cover the whole field, and the
word regeneration, that is already in general use, seems to fulfil this
purpose better than any other.
Roux[9] points out that Trembley, and later Nussbaum, showed that a
piece of hydra regenerates without the formation of new material. Roux
adds that since during development the piece takes no nourishment, the
_regeneration_ must be brought about by the rearrangement of the cells
present in the piece.[10] The change may, or may not, involve an
increase in the number of the cells through a process of division. In
consequence of this method of development a re-differentiation of the
cells that have been already differentiated takes place. This process of
regeneration, Roux points out, is very similar to the “post-generation”
of the piece of the blastula of the sea-urchin embryo, and he concludes
that “regeneration may be brought about entirely, or very largely,
through the rearrangement and re-differentiation of cells without any,
or with very little, proliferation taking place.” In the adults of
higher animals regeneration by proliferation preponderates, but
rearrangement and re-differentiation of cells occur in all processes of
regeneration, even in higher vertebrates. The two kinds of regeneration
that Roux distinguishes are, he says, essentially quantitative.[11]
Barfurth[12] has defined regeneration as “the replacement of an
organized whole from a part of the same.” If the part is given by
nature, there is a process of physiological regeneration; if the part is
the result of an artificial injury, the process is one of pathological
regeneration. Barfurth includes in the latter category the production of
a new, entire individual from a piece, as in hydra; regeneration by
proliferation, as in the earthworm; and also the development of pieces
of an egg or of an embryo.
Barfurth’s definition of regeneration is unsatisfactory, since an egg is
itself a portion of an organism that makes a new whole, and this sort of
development is not, of course, as he himself points out, to be included
in the term regeneration. Nor does the use of the word “replacement”
save the definition, since in many cases the kind of part that is lost
is not replaced. The use of the word “pathological” to distinguish
ordinary regeneration from physiological regeneration is, I think, also
unfortunate, since it implies too much. There is nothing necessarily
pathological in the process, especially in such cases as hydra, or as in
the development of a piece of an egg where the piece is transformed
directly into a new organism. Furthermore, in those cases in which (as
in some annelids and planarians) a new head is formed after or during
the process of natural division, there is little that suggests a
pathological process; and in this instance the regeneration takes place
in the same way as after artificial section.
Driesch, in his _Analytische Theorie_, states that Fraisse and Barfurth
have established that during regeneration each organ produces only its
like. Driesch defines regeneration, therefore, as the re-awakening of
those factors that once more bring into play, by means of division and
growth, the elementary processes that had ceased to act when the
embryonic development was finished. This is regeneration in the
restricted sense, but Driesch also points out that this definition must
be enlarged, since, when a triton, for example, regenerates its leg, not
only does each tissue produce its like, but later a reconstruction and
differentiation takes place, so that a leg and foot are formed, and not
simply a stump containing all of the typical tissues. Driesch holds that
regeneration should include only those cases in which a _proliferation_
of new tissue precedes the development of the new part, and suggests
that other terms be used for such cases as those of pieces of hydra,
pieces of the egg, etc., in which the change takes place in the old part
without proliferation of new tissue. It seems to me unwise to narrow the
scope of the word regeneration as Driesch proposes, for it has neither
historical usage in its favor, nor can we make any fundamental
distinction between cases in which proliferation takes place and those
in which it does not. As will be shown later, the factors that are
present in the two cases appear to be in large part the same, and while
it may be convenient to put into one class those cases in which
proliferation precedes the formation of the new organs, and into another
class those cases in which the change takes place without proliferation,
yet, since the distinction is one of subordinate value, it is necessary
to have one word to include both groups of cases; and no better word
than regeneration has, I think, been as yet suggested.
Driesch has made use of two other descriptive terms. The word
“reparation” is used to describe the development of the hydranth of
tubularia. The new hydranth is formed in this case out of the old tissue
at the end of the piece (Fig. 20, _A_). The change appears to be the
same as that which takes place in a piece of hydra, etc. The word
“reparation” does not seem to me to express very satisfactorily this
sort of change, or sharply separate it from those cases in which the
animal is _repaired_ by adding what has been taken away; but in this
latter sense Driesch does not use the term. I have not made use of the
word, in general, except as applied to Driesch’s work.
Another term, “regulation,” used by Roux,[13] and also by Driesch and
others, is used in a sort of physiological sense to express the
_readjustments_ that take place, by means of which the typical form is
realized or maintained. By inference we may extend the use of the word
to include the changes that take place in the new material, that is
proliferated in forms that regenerate by this method. Driesch uses this
term, regulation, to include a much more general class of phenomena than
those included in the term regeneration, as for instance, the regulation
of metabolism and of adaptation, etc. One of the subdivisions of the
term regulation is called “restitution.” This word also is used where I
should prefer to use the word regeneration as a general term, and the
word reorganization when reference is made to the internal changes that
lead to the production of a typical form.
Both Roux and Driesch also speak of “self-regulation,” by which is
meant, I suppose, that the changes taking place are due to readjustments
in the part itself, and are not induced by outside factors. The
expression “self-regulation” is not, I think, a very happy one, since
all change is ultimately dependent upon a relation between inside and
outside conditions.
Hertwig[14] defines regeneration as the power of replacement of a part
of the organism. He states that in all cases the beginning of the
process is the same, viz. the appearance of a small protuberance
composed of cells, that is the rudiment of the new part. It is evident
that Hertwig has taken into account only one side of the process. Those
cases in which a rearrangement or reorganization takes place in the old
part are not even considered.[15] Goebel[16] points out that in plants
the fully formed cells are, as a rule, incapable of further growth after
they have once served as a basis of an organ of the body, but often some
of the cells may remain in a latent condition, and grow again, when the
intercellular interactions are disturbed. This is the case, he thinks,
in regeneration. Goebel speaks of regeneration by means of adventitious
buds in those cases in which the buds had not previously existed before
the removal of the part. In those cases in which the buds are in
existence before the piece is removed, as in the leaves of Asplenium,
Begonia, etc., the development is not the result of regeneration, Goebel
thinks, but the buds represent a stage in the development of the
species. It may be pointed out, however, that it is certainly a
remarkable fact that often the conditions that lead to the unfolding of
an existing bud are the same as those that lead to the development of a
new bud.
The preceding account will suffice to illustrate some of the principal
ideas that are held in regard to the process of regeneration. Since many
new facts have come to light in the last few years, it may not be amiss
to point out what terms will be used in the following pages to include
each kind of process.
The word “regeneration” has come to mean, in general usage, not only the
replacement of a lost part, but also the development of a new, whole
organism, or even a part of an organism, from a piece of an adult, or of
an embryo, or of an egg. We must include also those cases in which the
part replaced is less than the part removed, or even different in kind.
At present there are known two general ways in which regeneration may
take place, although the two processes are not sharply separated, and
may even appear combined in the same form. In order to distinguish
broadly these two modes I propose to call those cases of regeneration in
which a proliferation of material precedes the development of the new
part, “epimorphosis.” The other mode, in which a part is transformed
directly into a new organism, or part of an organism without
proliferation at the cut-surfaces, “morphallaxis.”
In regard to the form of the new part, certain terms may be used that
will enable us to characterize briefly different classes. When the new
part is like that removed, or like a part of that removed, as when a leg
or a tail is regenerated in a newt, the process is one of
[Illustration: FIG. 12.--After Herbst. Diagram showing brain, eye, and
“heteromorphic” antenna (in place of eye of one side) of palæmon. The
animal had lived in a dark aquarium for five months.]
“homomorphosis.”[17] Under this heading we may distinguish two cases, in
one of which the entire lost part is at once, or later,
replaced--holomorphosis; in the other the new part is less than the part
removed--meromorphosis. When the new part is different from the part
removed the process has been called by Loeb “heteromorphosis,” but there
are at least two different kinds of processes that are covered by this
definition. In one case the new part is not only different from the part
removed, but is also an organ that belongs to a different part of the
body (or it may be unlike any organ of the body). This we may call
“neomorphosis.” As an illustration of this process may be cited the
development of an antenna, when the eye of a crab or of a prawn is cut
off near the base (Fig. 12); and as an example of an organ different in
kind from any organ of the same animal, may be cited the case of
_Atyoïda potimirum_, in which the new leg is unlike any other leg on the
body. The name “heteromorphosis” can be retained for those cases in
which the new part is the mirror figure of the part from which it
arises, or more generally stated, where the new part has its axes
reversed as compared with the old part. As an example of this may be
cited the development of an aboral head on the posterior end of a piece
of the stem of Tubularia (Fig. 15, _B_), or the development of a tail at
the anterior end of a posterior piece of an earthworm (Fig. 2).
The term “physiological regeneration” I shall use in the ordinary sense
to include such changes as the moulting and replacement of the feathers
of birds, the replacement of teeth, etc.,--changes that are a part of
the life-cycle of the individual. In some cases it can be shown that
these processes are closely related to ordinary regeneration, as when a
feather pulled out is formed anew without waiting for the next moulting
period, and formed presumably out of the same rudiment that would have
made the new feather in the ordinary moulting process.
It is sometimes convenient to contrast the process of physiological
regeneration with all other kinds. The use of the term “pathological
regeneration” for the latter seems to me, as has been said,
unsatisfactory. The two terms proposed by Delage,[18] viz. “regular
regeneration” and “accidental regeneration,” have certain advantages,
although there is nothing accidental, or at least occasional, in regard
to the process itself, as it is entirely regular, although it may only
occur after an accident to the animal. The term “regular regeneration”
is, I think, more satisfactory than “physiological regeneration,” but
the latter has the advantage that it has come into current use. For what
is known as pathological or accidental regeneration, I propose the term
“restorative regeneration,” and I shall continue to use the term
“physiological regeneration” as generally understood.