THE ORIGIN OF NEW CELLS AND TISSUES
There are many difficulties in the way of determining the origin of the
cells that make up the new part. The only means at present at our
command for studying their source is by serial sections of a number of
different stages taken at intervals from different animals. Since there
may be differences between the processes in different individuals, and
since we can only piece together the information gained from successive
stages, much uncertainty exists in regard to the changes that take place
during regeneration, even in some of those forms that have been examined
over and over again. Were it possible actually to follow out the
movements of the living cells in one and the same animal, the problem
would offer fewer difficulties, but this cannot be done. It will be more
profitable to consider first the better-known and simpler processes, and
afterward those that are less well-known.
The regeneration of the head and tail of lumbriculus and of certain
naids is a comparatively simple process, and has been studied by several
investigators, whose results agree, at least in regard to the most
essential features. Semper (’76) described the origin of the new organs
in the formation of new individuals by budding in nais. He found that
the new brain and nerve-cord develop from the ectoderm, the new mesoderm
also from ectoderm, and the new digestive tract from the old one, except
the pharynx, which arises by the fusion of two mesodermal “gill-slits.”
Bülow (’83) studied the regeneration of the tail of lumbriculus. He
found the ventral cord in the new part arising from a paired ectodermal
thickening, the mesoderm arising from a proliferation of cells. These
cells are invaginated in the region between ectoderm and endoderm--the
in-turning of the proctodæum being looked upon as an endodermal
invagination.[91] The more recent work of Randolph, Rievel, Michel,
Hasse, Hepke, and von Wagner on the same or related forms has served to
point out certain errors in the earlier work of Semper and Bülow, and
has added some new and important facts, especially in connection with
the origin of the mesoderm in the new part. Without attempting to give a
detailed account of these results,
[Illustration: FIG. 57.--After Hasse. Regeneration of _head_ of _Tubifex
rivulorum_. _A._ Sagittal section of anterior end. Six days after
cutting in two. _B._ Eleven days after cutting in two. _C._
Cross-section through new part. Five days after operation. _D._ Fourteen
days after operation. _E._ Sixteen days after operation.]
I shall describe the principal changes that have been found to take
place. When the anterior end of lumbriculus or of tubifex is cut off,
the cut-surface very quickly closes, as a result of the contraction of
the body wall. According to some investigators, the circular muscles are
chiefly concerned in the closing, but according to others the
longitudinal muscles bring about the result. The cut-end of the
digestive tract is pulled a little inward, and its end also closes (Fig.
57, _A_). For a day or two no important changes can be observed to take
place, but new ectoderm soon appears over the cut-surface. This ectoderm
arises in all cases from the old ectoderm, and as it increases in amount
the old ectoderm is pushed back from over the cut-end, leaving a layer
composed of a single row of cells over the end. Since nuclei in process
of division are rarely present before these initial processes begin, it
is probable that the changes are due, in large part, to an out-wandering
of ectodermal cells, or, what amounts to the same thing, to the leaving
behind of cells as the old ectoderm withdraws from the cut-end. In the
new ectoderm over the end, an active process of proliferation takes
place (Fig. 57, _B_), that leads to the production of a large number of
cells lying within the new part. The ectoderm has at this time begun to
bulge outward, so that the proliferated cells come to lie within the
dome-shaped beginning of the new head. There appears to be some
difference in the number and in the location of the proliferations in
different species. In general, the new cells arise from the ventral and
ventro-anterior region of the dome-shaped ectodermal covering of the new
part. Most of this new material gives rise to the brain, commissures,
and ventral nerve-cord (Fig. 57, _C_). The cells giving rise to these
structures in tubifex come from two ventral regions of proliferation
that extend along the sides and dorsally to the anterior end in front of
the digestive tract. Where the two masses meet above and in front, the
brain is formed.[92] The cells that do not take part in the formation of
the nervous system give rise to the muscles and connective tissue of the
new head. These cells lie especially at the outer sides of the
proliferated mass. The origin of the new muscles from ectoderm stands in
sharp contrast to the current ideas in regard to the origin of new
tissues, and yet it is a point on which the more recent investigators
are entirely in accord. Michel, Hepke, and von Wagner have arrived at
the same conclusion after a careful examination, and there seems to be
no reason for refusing to accept their results. The theoretical
importance of this discovery will be discussed later.
Soon after the proliferation from the ectoderm has begun, the blind end
of the digestive tract starts to push forward (Fig. 57, _D_). The cells
in the most anterior part of its wall begin to divide, and the end grows
in an anterior direction as a more or less solid rod. This rod extends,
in some species, as far forward as the ectoderm, meeting the latter on
the inner side of its antero-ventral surface. At this point an
in-turning of ectodermal cells, in the form of a blind pit, develops,
and later this pit, deepening to become a tube, forms the mouth cavity.
Its inner end is from the beginning in contact with the anterior end of
the digestive tract, or else it connects with the latter soon after its
formation. The two flatten against each other, the cells draw away in
the middle of the region of contact, and the cavity of the new mouth
becomes continuous with the cavity of the old digestive tract. The mouth
lies at first nearly terminal in position (Fig. 57, _E_), but by the
forward growth of the body wall over and in front of the mouth to form
the prostomium, the mouth comes later to lie more on the ventral
surface. The short tube produced by the in-turned ectoderm forms only a
short part of the digestive tract. It leads from the mouth opening to
the new pharynx, and forms, therefore, only the buccal cavity. A similar
ectodermal tube, the stomodæum, which develops in the egg-embryo,
becomes not only the buccal chamber, but also the lining of the pharynx.
The latter is, therefore, considered an ectodermal structure in the
embryo. On the other hand, in the regenerated head the lining of the new
pharynx arises from the anterior part of the endodermal digestive tract.
We find, therefore, that the same organ, the pharynx, may arise in the
same animal from distinct “germ-layers.” This result also has an
important bearing on our ideas concerning the value and meaning of the
so-called “germ-layers,” and has helped to bring about a revolution of
current opinion as to the importance of these layers.
The preceding account of the development of the head has shown that
while certain of the new organs and layers arise from the same organs of
the old part, yet this is not true for all of them. Thus while the
ectoderm gives rise to ectoderm, the new muscles do not appear to come
from the old ones, or even from other mesodermal tissues, but from the
ectoderm. The old digestive tract gives rise to the greater part of the
new one, but the new pharynx comes from the old endoderm, and not from
the in-turned ectoderm. The nervous system does not arise from the old
ventral cord, but from a proliferation of ectoderm. It has, thus, the
same origin as the nervous system of the embryo. The origin of the new
blood vessels has not been satisfactorily made out. The seta sacs arise
from ectodermal pits as in the embryo.
In regard to the origin of the new mesoderm, the evidence is still
insufficient, I think, to show that cells derived from the old muscles
or peritoneum take no part in the formation of the new muscles and
peritoneum; but that the greater part of the new muscles, etc., comes
from the proliferated cells can scarcely be doubted. This latter
discovery loses none of its significance, however, even if it should
prove true that the old muscles, etc., contribute something to the new
part. It is also not entirely disproven that the ventral nerve-cord does
not take a small share in the development of the new cord.
The regeneration of a new tail-end in these same forms appears to take
place in much the same way as the head. The cut-end quickly closes;
later a layer of ectoderm appears over the posterior surface, and the
new part bulges out and becomes dome-shaped. A paired, or in some
species a single, region of proliferation develops from the ectoderm,
that gives rise to the new ventral nerve-cord. Lateral proliferations
of ectoderm produce, according to some writers, the material out of
which the mesoderm of the new tail is formed. Randolph, on the other
hand, has described the new mesoderm as arising from the old, especially
from certain large peritoneal cells that are found throughout the body.
The cut-end of the digestive tract closes, and later new cells develop
at its posterior end. An in-turning of ectoderm, in the form of a pit,
fuses with the posterior end of the digestive tract and establishes
communication with the outside.
[Illustration: FIG 58.--After Hescheler. Regeneration of anterior end of
earthworm. _A._ After four days. _B._ After eleven days. _C._ After
twenty-five days. _D._ After twenty-one days (younger individual).]
The regeneration of the anterior end of the earthworm has been carefully
worked out by Hescheler, and although on account of the greater
complexity of the process the results are not so decisive as those just
described, yet in many respects they are in agreement. In Hescheler’s
experiments only four or five anterior segments were cut off. The
closing of the cut-end is somewhat different from that in lumbriculus. A
plug of cells soon forms over the end (Fig. 58, _A_). The new cells
appear to be lymph cells. Although this mass of cells may be quite
large, the cells do not seem to form later any of the organs in the new
head. The presence of these cells makes it very difficult to work out
the origin of the other cells that appear later. Owing to the absence of
this lymph plug in lumbriculus and nais it is easier to follow in them
the regenerative processes. In the midst of these lymph cells
spindle-like cells soon appear whose origin is obscure, but Hescheler
thinks it improbable that they are transformed lymph cells, although
they are completely intermixed with the latter. The spindle-cells
arrange themselves later in regular bands, that appear to be extensions
of the longitudinal muscles. A few days after the operation, the lymph
plug is covered over, beginning at the edge, by the ectoderm. The new
ectodermal cells arise from the old ectoderm, and seem to extend over
the lymph plug by a sort of migration process. Division of the cells
does not occur at this time. These covering cells are at first all
alike, the characteristic gland cells of the ectoderm being absent. The
digestive tract withdraws somewhat from the outer cut-surface, and its
end closes. The closed end abuts against the inner surface of the lymph
plug. The next changes are initiated by the appearance of karyokinetic
divisions in all the tissues of the new part, which lead to a rapid
growth and elongation. Dividing cells are found in the new, as well as
at the border of the old, ectoderm, where the new and the old parts are
continuous. At this stage there appears in the lymph plug another kind
of cell, that seems to arise, in part at least, from the ectoderm by an
in-wandering of new cells. Other new cells may come from the edge of the
old muscles, but it is not clear whether they come from a transformation
of muscle cells, or from undifferentiated cells lying in the old
muscles. In addition to these sources of new cells, it appears not
improbable that cells may separate from the end of the digestive tract.
Nerve fibres push out from the end of the ventral nerve-cord into the
new part, and groups of cells, often in process of division, appear in
the old ganglia, even in those that lie a long distance from the
anterior end. It is not improbable, Hescheler thinks, that new cells, as
well as fibres, grow forward from the most anterior end of the
nerve-cord into the new part. A mass of nerve cells and fibres appears
in front of the old nerve-cord, and extends upwards and around the
digestive tract, to meet over the anterior end of the latter in another
mass of cells that have arisen from an early in-wandering of ectodermal
cells. It is not improbable that the masses around the digestive tract
(the commissures) and also the new ventral cord may also include cells
that have had the same origin.
A tubular invagination of ectoderm is formed at this time at the
anterior end. It meets the anterior end of the digestive tract; the two
fuse, and the communication of the digestive tract with the outside is
established. The pharynx develops from the anterior part of the
digestive tract, which after Hescheler’s operation may contain some of
the original ectodermal stomodæum, since only five of the anterior
segments were cut off, and the embryonic stomodæum extends somewhat
behind this region. In another experiment, carried out by Kroeber,
somewhat more of the anterior end was removed, but the result was the
same (Fig. 59), so that it is clear that the new pharynx may be formed
from the old endoderm.
[Illustration: FIG. 59.--After Kroeber. Regeneration of anterior end of
_Allolobophora fœtida_, after removal of six segments. The first
stomodæal invagination had been destroyed. The new pharynx is developing
from the endoderm.]
Hescheler leaves several points still unsettled, more especially the
origin of the cells that give rise to the new musculature, but it is
almost impossible to make out their origin in this animal, owing to the
presence of the lymph cells. Hescheler’s discovery that the cells of the
lymph plug do not themselves, in all probability, contribute to the new
part, is an important result, and shows that these seemingly
undifferentiated cells do not possess the power of giving rise to the
different kinds of new tissues. The in-wandering of cells into this
solid plug from the ectoderm, and perhaps also from other sources, and
their subsequent union to produce the definitive organs, is also a point
of capital importance, especially as it puts us on our guard against a
too ready acceptation of the view that all cells in a mass that have the
same general and undifferentiated appearance have had a similar origin,
and in showing that apparently indifferent cells may really carry with
them into the new part those characters that determine their fate. Other
cells, apparently equally undifferentiated, and lying in the same
position, may have quite different possibilities.
In the vertebrates, the regeneration of the tail and limbs of amphibia
and of the tail of lizards has been studied by a number of
investigators. The regeneration of the tail of several urodeles and of
the larva of the frog was investigated more fully by Fraisse (’95) and
by Barfurth (’91). If we examine first the results of Fraisse’s study of
the tail of urodeles, which have bony vertebræ, we find the following
changes take place. The cut-surface is covered by the skin bending over
the exposed part, accompanied by a migration of cells from the edge of
the ectoderm. Only the unspecialized cells leave the old ectoderm to
wander out over the cut-surface; gland cells and sense cells are
entirely absent from the new ectoderm. These kinds of cells develop
later out of the undifferentiated cells over the new part. The
development of new vertebræ does not follow the embryonic method of
development. In the embryo the endodermal notochord is first laid down,
and around this and the nerve-cord mesodermal cells accumulate to form
the skeletal tissue. Later the notochord is largely obliterated, as the
vertebræ develop, pieces of it being left along the vertebral column. In
the regeneration of the tail of the adult animal, the remnants of the
old notochord (even if exposed by the cut) do not take any part in the
formation of new tissue. In fact, there is no notochord formed at all.
From the injured vertebræ, or at least from their covering of skeletal
tissue, cells are proliferated, out of which a cartilaginous tube
develops, enclosing the new nerve-cord, which is growing out from the
cut-end of the old cord. In this tube centres of deposition of
calcareous material are formed, and the new vertebræ are produced in
this way. The new nerve-cord develops from the cut-end of the old cord,
and more especially out of the cells of the lining epithelium of the
_canalis centralis_. The new muscles develop from cells that arise from
the old muscles.
In the tadpole of the frog the regeneration of the tail takes place
essentially in the way just described for the adult urodele, except
that, there being only a notochord in the tail, only a notochord is
regenerated. According to Fraisse, the new notochord develops from cells
that arise from the sheath of the old notochord, and not from the
vacuolated cells of the notochord itself. The notochord cells are, he
states, derived from the endoderm of the embryo,[93] while the sheath
arises from the mesoderm; hence the newly regenerated notochord that
arises from the sheath of the old one comes from a different germ-layer.
Exception may be taken to this statement, because in the frog’s embryo
the notochord develops from tissue that is at first perfectly continuous
with the mesoderm, and, in fact, may be called mesoderm; also because it
is probable, in the light of more recent research, that both the
notochord and its sheath have exactly the same origin.
It is known that the tail of lizards breaks off generally at a definite
region near the base, and that the break does not occur between the
vertebræ, but in the middle of a vertebra--in some species the seventh
caudal. The vertebræ are thicker at their ends than in the middle, and
are firmly held together by intervertebral cartilages. The centres of
the caudal vertebræ are the weakest links in the chain, or at least the
place at which the vertebral column is most easily broken in response to
the contraction of the tail-muscles.[94] Fraisse and others speak of
this arrangement as an adaptation for breaking off the tail.
The new tail that regenerates does not contain a new series of vertebræ,
as does the new tail of the salamander, but, instead, a cartilaginous
tube that is attached to the half of the broken seventh caudal vertebra.
The regeneration of the new tissues of the tail of the lizard takes
place as follows: A scab forms over the cut-surface, composed in part of
clotted blood, in part of broken-down tissues from the injured cells. In
the course of a week the necrotic tissue falls off, and a smooth surface
of ectoderm is found covering the end of the tail. The new ectoderm
appears to come from the old, but its method of development has not been
studied. The deeper layer of the skin of the lizard is composed of
mesodermal connective tissue, and in the new part this layer arises from
the connective tissue of the old part. The tissue that forms the
cartilaginous tube of the new tail develops from the skeletal tissue of
the broken vertebra. The remnants of the old notochord that are present
in the vertebra, have nothing to do with the new structure, nor does the
new tube represent in any way a notochord, but it appears to be a
structure _sui generis_. In later stages, osseous plates may be formed
in the cartilage, but these are too irregular to be compared to
vertebræ. A tube grows out from the cut-end of the nerve-cord, which in
some forms, as Fraisse shows, is only an extension of the lining
epithelium of the nerve-cord. In other forms it is possible that other
cells of the old cord may also grow backward, divide, and produce new
cells. The fine thread that is formed in this way does not send out any
nerve fibres into the surrounding parts. In _Anguis fragilis_, however,
a few ganglion cells are present in the new cord. It is probable,
Fraisse states, that while the new tube is morphologically a nerve-cord,
yet physiologically it is not functional in any of the reptiles.
The new muscles come from the old ones. Fraisse thinks that the new
muscle fibres come from the so-called “spindle fibres” that split off
from the primitive muscle bundles. These fibres, Fraisse believes,
originate normally during the process of physiological regeneration of
the muscles, and also after injury to the muscles. From these spindle
cells the new muscle fibres develop in the same way as the muscle cells
of the embryo.
Fraisse sums up the results of his studies of regeneration as follows:
(1) Both in amphibians and reptiles, injured tissues can only produce
new tissues like themselves. The leucocytes assume only the function of
nutrition and of devouring the broken-down parts of tissues. They never
become fixed tissues--neither connective tissue nor any other sort. (2)
All tissues are capable of regenerating themselves, either directly out
of their differentiated elements, or out of a matrix. As a matrix for
the epidermis, there is the Malpighian layer of the skin; for the
central nervous system, the epithelium of the central canal of the
nerve-cord; and for the musculature, the spindle fibres.
Fraisse also formulates the following general statements: (_a_)
Regeneration is neither a pure recapitulation of the ontogeny nor of the
phylogeny. The process is rather a hereditary one, with which
complicated adaptations of the tissues are often involved that follow
the laws of correlated development. (_b_) We cannot explain the
phenomenon of regeneration, as the result of wounding the tissues, or as
the outcome of an increase in the food supply, or as due to the removal
of a resistance to growth. Far more important are the principles covered
by the former paragraph, (_a_).
Barfurth has studied in detail the regeneration of the tail in some
amphibia; and his results, while not covering as much ground as do those
of Fraisse, yet give a more detailed account of the origin of the new
tissues. Barfurth’s results on triton and siredon are not essentially
different from those of Fraisse. In the tadpole of the frog, Barfurth
finds that the notochord regenerates from the sheath of the old
notochord. In the larval urodele, he finds that the new notochord arises
as in the tadpole, and not from the skeletal sheath, as Fraisse
maintains. In very young larvæ of siredon the chordal cells themselves
seem to give rise to the cells of the new notochord. In older larvæ, in
which the skeletal tissue is developed around the notochord,
regeneration takes place both from this tissue and also from the sheath
of the notochord. He concludes that in the regeneration of the new
notochord, and also of the skeleton, the origin of the cells depends
upon the developmental stage of the supporting tissues.
In regard to the regeneration of the muscles, Barfurth comes to the
following conclusions: In very young larvæ of siredon, the degenerative
changes in the muscle cells are often very slight. Regeneration takes
place by growth from and the displacement of the old muscles. During
this time bud-like terminal and lateral formations occur in the muscle
fibres. These outgrowths contain nuclei and form sarcoblasts; and these
pass into the new part, where they make the new muscle fibres in the
same way as do the cells of the embryo. In older larvæ of the frog, and
in mature animals in general, the changes are more complicated. Two
processes can be distinguished: (_a_) degenerative and (_b_)
regenerative. (_a_) Broken-down muscle fibres that have been cut, and
torn-off pieces of muscle fibres, are found present. There follows an
accumulation of leucocytes and of giant cells. The nuclei in the
degenerating muscle fibres atrophy, and the substance of the fibres
breaks down. (_b_) The muscle fibres split lengthwise to form spindle
fibres, and there is an increase in the number of nuclei at the same
time. Sarcoblast-like outgrowths of the old muscle fibres are formed,
which produce the sarcoblasts that become new muscle fibres.
Barfurth agrees with Fraisse in two main points, viz. that all the
tissues of the tail have the power of regeneration, and that each tissue
produces only tissue like itself. The law which Kölliker attempted to
establish, viz. that the elements of the formed tissues have lost the
power of producing other kinds of tissue,--the law of the specification
of the tissue,--is supported by these results of Fraisse and of
Barfurth, but is contradicted, as has been shown above, by the results
on the earthworm, and also as we shall see even in the amphibia, as for
instance in the regeneration of the lens of the eye.
Spallanzani[95] was the first to study the regeneration of the limb in
salamanders, and found that the skeleton in the new part is like that in
the normal limb. Bonnet, Philipeaux,[96] as well as other
naturalists,[97] also examined the regeneration of the limbs of
salamanders. Götte (’79) has studied the embryonic development and the
regeneration of the limb of triton, especially in regard to the origin
of the new bones. He found that the skeleton develops in much the same
way in the embryonic limb and in the regenerated limb, and the process
in the latter may be said to repeat that in the former. This is
especially true for the regeneration of the limb of a very young larva,
but the older the larva the more it departs from the embryonic type of
development. If the limb is cut off through the upper arm, or through
the thigh, new tissue develops over the cut-end. If the larva is quite
young, so that formation of the cartilages in the leg has not gone very
far, the new tissue differs very little from the old; but if the leg of
an older larva is amputated, the difference between the old and the new
parts is more striking. If the bones of the leg have become ossified,
the transition from the old to the new part is at first very sharp. The
new tissue, that will make the new cartilages of the new limb, develops
as a cap over the cut-end of the old bone. Götte does not give an
explicit statement in regard to the origin of the new cartilage, but his
account leads one to suppose that it develops from the old cartilage or
from some part of the bone. This is, in fact, the case, as I have
observed in preparations of the regenerating leg of _Plethodon
cinereus_, in which the new cartilaginous tissue comes from the
periosteum of the old bone. Götte shows that two long rods of tissue are
formed, that are separate for the greater part of their length. They
give rise to the two bones of the lower leg, or forearm, as the case may
be. The broken end of the femur or humerus also completes itself by a
short cartilaginous cap, which is at first continuous with the two rods
just described. The ends of these two rods break up into a series of
pieces that form the tarsalia, or the carpalia, and the digits. Two
digits are first formed, and the others are added as outgrowths from the
side of one of the two rods. It is important to note that the new
cartilages are formed, in large part, out of a continuous substratum (or
rather of two) which separates into proportionate parts to produce the
elements of the new limb.
The regeneration of the muscles of the limb of an adult animal,
plethodon, has been recently worked out by Towle. The leg was cut off in
the middle of the forearm. Extensive changes take place in all the
muscles that extend across the level of the cut. The old fibres in the
lower end of the muscle, _i.e._ those near the cut-end, disintegrate,
and the number of nuclei greatly increases. The division of the nuclei
seems to be direct, each retaining some of the old muscle substance
about itself. From some of these cells the new muscle tissue is formed
in the new part. Higher up in the forearm the muscle fibres break down
to a smaller extent, and still higher up some of the old fibres may
remain intact. New muscle fibres are also formed in the old muscle,
especially in the region near the cut-end.
The process of regeneration has not been so fully worked out in any
other vertebrates as in those described in the preceding pages, although
the regeneration of _single tissues_ or organs in the vertebrates has
been extensively investigated. In all such cases it is found that like
tissues give rise to like.
In the planarians it has been found that during regeneration the
ectoderm covers the exposed surface, and from it arises the new
ectoderm; the digestive tract appears to come in part from the old tract
and in part from the middle-layer cells; the nervous system appears also
to develop out of the middle-layer cells that are found scattered
through the body. These cells seem to form a sort of reserve supply
that gives rise to the digestive tract, nervous system, and middle-layer
cells in the new parts. From them also arise the new pharynx, and the
lining of the pharynx chambers, as well as some other structures. It is
impossible to say at present whether one and the same kind of cell may
give rise to all these structures, or whether different kinds of cells
are present in the middle layer, that cannot be distinguished from each
other by the methods at present at our command.
The changes taking place in the tissues of those animals that regenerate
by morphallaxis have been only quite recently carefully investigated.
Bickford stated that in tubularia the old differentiated tissue changes
over directly into the tissue of the new part, and Driesch confirmed
this statement. Stevens has studied by means of serial sections the
different changes that take place. Division of both ectodermal and
endodermal cells is found to occur, but especially the ectodermal.
Whether all the ectodermal cells divide, or only some of them, is
difficult or impossible to state, but whether this happens or not, all
the old region goes over into the new hydranth.
The changes that take place in hydra have been recently worked out in my
laboratory by Rowley, who finds that a certain amount of division takes
place in the old cells, especially in the ectoderm. The division of the
cells is not a very active process, and it seems not improbable that
many of the old cells go over without dividing into the new part.
One of Trembley’s most celebrated experiments was that in which hydras
were turned inside out (Fig. 1, _A_, _B_), so that the ectoderm came to
line the inner cavity and the endoderm to cover the outer wall. The
tentacles were not everted but remained sticking out of the mouth of the
everted animal. Their openings, or arm-holes, therefore, appear on the
outer surface of the body. In order to prevent the everted hydra from
turning itself back again, as it tends to do, Trembley pushed a small
bristle crosswise through the wall of the body. Finding the hydras still
sticking on the bristles the next day, he concluded that they had not
returned to their former condition, but that the outer layer (the
endoderm) had changed its character so that it became ectoderm, and the
inner layer (the ectoderm) became endoderm.[98] The experiment seemed to
show that the two layers could change their specific character and be
transformed into each other according to their position in the animal.
These remarkable results were not challenged until 1887, when Nussbaum
repeated the experiment and showed that Trembley had overlooked an
important fact. It was found that even the bristle pushed through the
body does not prevent the hydra from regaining its original condition,
although it may delay the turning back. If the turning back can be
prevented, the animal dies. Nussbaum showed how the turning-back takes
place in an animal while it remains on the bristle. The everted foot-end
begins first to turn back, pushing into the central cavity. When it
comes to the bristle it passes to one side of it, and continuing to turn
back the foot passes out of the mouth, drawing the rest of the body
after it.[99] The last act of the turning can take place only by tearing
away through one or both sides, and this is often done. The bristle may
still remain sticking to the body through one side, or even remain
through both sides if the body has, after tearing through, healed up
around the bristle. The process of turning back may take place quite
quickly, and had been overlooked by Trembley, who trusted too
confidently to the presence of the bristle sticking through the animal.
The method by which the turning back of the layers takes place was not,
it appears, clearly described by Nussbaum in his first paper, for his
account seems to imply, in certain passages, that the ectoderm may slide
over the endoderm during the process, rather than that both layers
always turn together. Ischikawa, who studied the problem later, gave a
clearer account of the method of turning back. Nussbaum has stated in a
later paper that he had described essentially the same process.
In conclusion, it can be definitely stated that a transformation of
ectoderm into endoderm cannot take place in hydra. Ischikawa also tried
removing the endoderm from a piece by spreading it out and then killing
the inner layer by weak acid applied with a brush, but pieces of this
sort failed to regenerate a new endoderm.
Tower has recently stated that if a living hydra is put into a strong
light from an arc lamp of 52 volt 12 ampere capacity, that is focussed
on the animal (after passing through an alum cell), the ectoderm cells
fly off, but if the animal is kept, it subsequently produces a new
ectoderm. Whether all the ectoderm is lost, or only the larger
neuro-muscular cells, was not made out.
One of the most unexpected discoveries of recent times in connection
with the problem of regeneration is the renewal of the extirpated eye of
triton and salamandra. Colucci first discovered in 1891 that if the eye
is partially removed a new eye develops from the piece that remains and
that _the new lens develops from the margin of the bulb_. Wolff, a few
years later, not knowing of Colucci’s results, also found that after
extirpation of the lens of triton, by making an incision in the cornea,
a new lens develops from the edge of the old iris. Wolff pointed out the
great theoretical importance of this result. The experiment has been
repeated and confirmed by a number of more recent workers, so that
there remains no question as to its accuracy.
[Illustration: FIG. 60.--After Wolff. Regeneration of lens of eye of
Triton. _A._ Edge of iris with beginning lens. _B, C, D._ Later stages
of same. _E._ After Fischel. Whole eye with regenerating lens.]
After the removal of the old lens the wound in the cornea quickly heals,
and in the course of two or three weeks a thickening appears at one
point at the edge of the iris (Fig. 60, _A_). The cells that produce
this thickening are the ordinary deeply pigmented cells of the iris,
where the outer layer of cells of the iris becomes continuous with the
inner layer. The cells increase in number and produce a spheroidal ball
that hangs down into the space formerly occupied by the lens (Fig. 60,
_E_). The cells become clearer by absorbing their pigment and arrange
themselves concentrically as in the normal lens. When fully formed the
new lens separates from the iris and occupies the normal position.
The most surprising fact in connection with the development of the new
lens is that it arises from a part of the body from which the lens of
the eye never develops in the embryo of this form or of any other
vertebrate. In the embryo the lens develops from the ectoderm at the
side of the head and only secondarily unites with the optic cup, that
has come from an evagination of the anterior wall of the fore brain. In
the regeneration of the adult lens, however, the ectoderm covering the
eye takes no part in the formation of the new lens,--in fact, it is
separated from the eye by the thick inner, mesodermal layer of the
cornea. The lens develops, as has been stated, from the already
differentiated layers of the iris. It is a point of further interest to
notice that the cells that form the transparent lens come from the iris
cells that are in part at least filled with black pigment. If this
pigment remained in the cells the new lens, while it might be
structurally perfect, would be physiologically useless. The pigment
disappears, however, as the lens develops. In this case we find a highly
specialized organ, the lens, developing out of tissue also specialized
in another direction. It does not simplify the problem to point out that
the lens and the iris are both parts of the eye, since they have arisen
from different parts of the body and have only secondarily come into
apposition with each other. Colucci was contented to point out that both
the embryonic lens and the regenerated one come from ectoderm and that
the result can be brought into harmony with the “germ layer” hypothesis.
Wolff has called attention to the fact that the new lens arises from the
upper edge of the iris, and that this is obviously the most advantageous
position in which it could develop from the iris, since by its own
weight it falls into place as it develops. If the lens had developed
from any other point of the margin, its position would be less
advantageous, as it might not be brought into its proper position.
Fischel, who has more recently studied the regeneration of the lens in
the larvæ of _Salamandra maculata_, finds that after the removal of the
lens the iris is thrown into wrinkles or folds and may stick at first to
the cut-edge of the cornea. After the cornea has healed, the iris
returns to its normal position. He finds that the first changes are more
or less alike around the entire rim of the iris and involve a partial
absorption of the pigment, a separation of the inner and outer layers at
the edge, and a swelling of the margin. These changes go only a little
way in those parts that do not produce a lens, but at the upper edge of
the iris they go farther and lead to the formation of a lens in that
region. He finds also that a new lens develops in animals kept in the
dark as well as in those kept in the light, and in the same way.
Fischel also tried the effect of removing a part of the upper edge of
the iris at the time when the lens was extirpated, in order to see if,
in the absence of this part, the lens would develop from other parts of
the uninjured margin of the iris. He found that the new lens still comes
from the upper edge of the iris from the part left after the operation
and not from the intact edge in other parts. This seemed to show that an
injury to the iris is in itself a stimulus that starts the formation of
a lens. This conclusion is made probable by the results of other
experiments in which the iris was stuck at several points, when new
lenses began to develop at several of these regions of injury. In some
cases Fischel found that two or more lenses began to develop when the
iris had not been intentionally injured; but it is not improbable that
some sort of injury may have been effected when the lens was removed.
Fischel, as has been said, removed extensive portions of the upper part
of the iris and found that a new lens could be formed at the cut-edge,
even in the region of the _pars ciliaris_; and, even after the removal
of the entire upper part of the iris, lens-like structures may appear in
the inner or retinal layer of the remaining region.
If instead of removing the lens it is displaced by pressing on the
cornea until the lens leaves its normal position and comes to lie in the
vitreous humor, a new lens develops from the edge of the iris, as though
the old lens had been entirely removed from the eye, but in the
experiments in which this was done the new lens was not well developed.
The result shows that it is not necessary that the old lens be removed
from the eye in order to induce the regeneration of a new one, but only
that the lens lose its normal position in the eye.
In regard to the stimulus that determines the development of the lens,
Fischel agrees with Wolff that gravity has a share in producing the
result. The absence of the old lens from its normal position, as well as
the wrinkling of the cornea, may also enter in as factors. Fischel takes
issue with Wolff as to the interpretation of the result as an
adaptation, and states that “the organism always responds to a change of
relation in only one way, whose direction is already determined by
internal structural relations, without regard to whether the result is
adaptive or not. The response follows each stimulus in a way determined
by the limited possibilities of the cells. With such a uniformity in the
reaction, the idea of a fundamental adaptability cannot be connected,
since the reaction that appears to us to be adaptive in a series of
complicated changes may be non-adaptive in another series.”
Whether Fischel has here really met Wolff’s argument is, I think, open
to question. It does not alter the result to show that factors already
existing enter into the process, so long as the organism is so
constructed that just those factors are present that bring about a
useful response. That the response may be sometimes imperfect does not
affect seriously the argument--in fact, it makes the case all the more
remarkable if these imperfect attempts are in the direction of useful
responses. Fischel sums up his conclusions as follows: “It is not
necessary, and it is irreconcilable with the facts, to describe the
formation of the lens in a teleological sense, and to bring this case
forward as a proof of the universal application of a teleological
principle. As has been already stated, the facts in regard to this case
show much more clearly that the organism reacts to each change always in
a manner that corresponds to its limited possibilities without regard to
a teleological principle. A planarian, for instance, responds to a
stimulus and makes a new head, even when it possesses one or more
already; a tubularian produces a hydranth at its basal end, if this end
is freely surrounded by water; an actinian forms a new mouth on the side
of its body, etc.; so also do the cells of the _pars ciliaris_, and the
_pars iridica retinæ_ differentiate into lens fibres. Working blindly,
without respect to the consequences as far as they concern the whole,
the one thing only is produced for which the conditions are present that
bring about its formation in the cells.”
_THE PART PLAYED BY THE “GERM-LAYERS” IN REGENERATION_
Our examination of the origin of the tissues and organs in the new parts
has shown that in most cases the old tissues give rise to the same kind
of tissue in the new part; or in some other cases, as in the nervous
system, the regenerating organs arise from the same “layer” as that from
which they develop in the embryo. These facts have led many writers to
state that the tissues and organs in the regenerated part arise from the
same germ-layers as do the same parts in the embryo. It is supposed that
ectoderm gives rise to ectoderm, and to those structures that arise from
the ectoderm in the embryo, as, for instance, the nervous system,
stomodæum, etc. The endoderm is supposed to give rise to endoderm, and
to endodermal structures, and the mesoderm to mesoderm and its
derivates. So fixed has this opinion become that it is not uncommon to
find investigators proclaiming the triumphant success of their results,
because they have been able to trace the organs in the regenerated part
to the same germ-layers that give rise to these organs in the embryo.
Before deciding as to the value of this point of view, let us examine
briefly the foundations of the so-called germ-layer hypothesis.
The origin of this hypothesis goes back at least to 1759, when C. F.
Wolff maintained his thesis that the digestive tract of the chick exists
as a flat, leaf-like structure that subsequently rolls up into a tube.
He thought it probable that other embryonic organs might arise in the
same way. His views made at the time no impression on his
contemporaries, and lay buried until 1812, when Meckel republished
Wolff’s work in a German translation. Pander, in 1817, distinguished two
layers in the early embryo, a serous and a mucous, and stated that later
a third, vascular layer appears between the other two. Von Baer
published in 1829 his celebrated memoir on the development of the chick,
in which he made out two primary layers in the germ, the animal and the
vegetative layer, and held that each of these separates into two to
produce the four embryonic layers. Remak, in 1851-1855, gave a more
precise description of the germ-layers, and stated that from the
innermost layer, the epithelium and glandular cells of the digestive
tract arise (including the lining of the glands that open into the
digestive tract). From the outermost layer he showed that the integument
and sense organs and the nervous system develop, and from the two middle
layers develop the muscles, blood, excretory, and reproductive organs.
By the term “germ-layers” was meant at this time only that the embryo is
formed out of sheets.
Huxley in 1849 pointed out that a medusa is made up of two layers, an
outer and an inner, and called attention to their possible equivalency
to von Baer’s serous and mucous layers. This idea of a resemblance
between the layers of an embryo and of an adult of a lower form
furnished the starting-point for the more modern formulation of the
germ-layer hypothesis. Kowalevsky’s work on the development of a number
of the lower animals showed that there is present in many forms a
two-layered stage, or gastrula, formed by an in-turning of the wall of
the hollow blastula. In this way two germ-layers are established, an
outer and an inner, that correspond to the ectoderm and to the lining of
the digestive tract, or endoderm. While Kowalevsky’s work did much
toward laying the foundation of the modern study of embryology, he
himself indulged in very little of the sort of speculation that came
into vogue a few years later. Kowalevsky’s discovery of the gastrula
stage in the embryos of many different groups has been fully confirmed
and extended, but the elaborate speculations that have been built up on
this as a basis have gone far beyond the evidence, and, for a time, drew
the attention of embryologists away from more important problems.
Haeckel took a more extreme position than most of his contemporaries,
and assumed that the gastrula stage that occurs in so many of the groups
of metazoa corresponds to an ancestral, two-layered adult animal, the
gastræa, from which all the higher forms have descended. The presence of
the gastrula in the development was interpreted as a “repetition” of
this ancestral adult stage. Thus the two primary layers are supposed to
have an historical meaning.[100] Embryologists soon began a search for a
similar mode of interpreting the middle germ-layer, or layers, which
led, amongst other views, to the formulation of the “gut-pouch
hypothesis.” From this point of view the body cavities, or cœlomes, are
supposed to have been originally sac-like outgrowths from the digestive
tract of an ancestral adult animal. Later, these cœlome sacs are
supposed to have been shut off from connection with the digestive
tract--their cavities becoming the body cavities, and their walls giving
rise to the mesodermal organs. The formation of pouches from the walls
of the archenteron of the embryo in several groups of animals has been
interpreted as a repetition of the ancestral adult animal.
A comparison of the germ-layers in different forms very soon led to an
attempt to “homologize” the layers in different animals. If the layers
have had historically the same origin, or appear in the same way in the
embryos, or give rise to the same organs, they are said to be
homologous. In the absence of a knowledge of the first two of these
conditions it is generally considered sufficient, if it can be shown
that similar organs arise from a layer, to “homologize” that layer in
the two forms. The study of embryology soon became a search for
homologies. The results led to inextricable difficulties and innumerable
contradictions until, a reaction setting in, many embryologists became
sceptical in regard to the value of this entire method of study.
The results of a detailed study of the process of cleavage in a number
of groups have helped, perhaps, to clear the way for a sounder
conception. It has been found that the cleavage of the egg in members of
the groups of annelids, mollusks, and turbellarians is extremely
similar--so similar, in fact, that it seems hardly possible that they
could be due to chance, especially as the series of cleavages is quite
complicated. The discovery of these similarities led at once to
comparison, and comparison to the establishment once more of homologies,
and the homologies led again to contradictions, until at present
scarcely any two workers agree as to a criterion of homology.[101]
Leaving this question aside, however, and fixing our attention only on
the similarity of the process of cleavage, we are justified, I think, in
looking for an explanation of the similarity in some sort of an
historical connection. We can eliminate, I think, without discussion the
possibility of this type of cleavage representing an ancestral adult
animal. So far as the question of descent enters the problem, we can
infer with some degree of probability that the groups in question may
have come from a common group in which the egg divided in much the same
way as we find it dividing at the present time. As a formal hypothesis
this view meets with no serious difficulty, since a chain of forms, or a
continuous living substance, connects the present animals with those
living in the past; and we may assume that the same factors peculiar to
the egg of the ancestors are still present in the eggs of their
descendants. This sort of explanation gives us no causal knowledge of
the way in which the egg divides, nor does it preclude the possibility
of new changes coming in that may entirely alter the form of the
cleavage. Moreover, since we are dealing with a question of historical
probability only, we cannot be certain that the same type of cleavage
may not have arisen quite independently in each group.
The argument in favor of the gastrula stage also representing an
ancestral larval stage may be admitted as a remote possibility, but on
evidence even far less satisfactory than that for the similarities of
cleavage being accounted for by a common descent. That this gastrula was
ever an adult form we have no means of deciding, even as a matter of
probability, and even if this could be made plausible it by no means
follows that such an adult stage would become an embryonic stage of
later forms. Consequently that part of the germ-layer theory that rests
on such a supposed connection cannot be looked upon as much more than a
fiction.
But even granting that there is an historical, embryonic[102]
connection, its small importance for the scientific problems connected
with embryonic development, and budding and regeneration has been shown
by a number of recent discoveries, and nowhere more clearly than in the
cases of the formation of new individuals by budding. As an example may
be cited the method of development of the ascidian from the egg, and by
means of buds. The work of Kowalevsky, Della Valle, Seeliger, and Van
Beneden on the budding process of ascidians showed that there are some
discrepancies between the bud development and the embryonic development.
The more recent papers of Hjort, Oka, Pizon, Salensky, Lefevre, and
others have shown very clearly that the germ-layer theory is
inapplicable to the bud development in this group. The bud arises as a
double-walled tube, or rather a tube within a tube, with a space
between. The outer tube comes in all cases from the ectoderm of the
animal; the inner tube has a different origin in different species. In
perophora, didemnum, and clavellina, the inner tube comes from endoderm;
in botryllus it arises from the ectoderm of the larval peribranchial or
atrial cavity. In all these forms the inner tube gives rise to the new
pharyngeal cavity of the bud, while this same cavity comes from the
endoderm of the archenteron of the embryo. In the bud embryo the
peribranchial space is also derived from the inner tube; hence it is
endodermal in the first series, and ectodermal in botryllus. In the egg
embryo it is ectodermal. In regard to the development of the nervous
system there is some difference of opinion. A number of investigators
have found that the new brain arises from the outer part of the inner or
branchial tube, which has in most cases an endodermal origin. Seeliger
and Lefevre believe the nervous system to arise from mesodermal cells
that lie between the two tubes. It appears, nevertheless, that in
several forms the brain really comes from the inner tube, which also
gives rise to the branchial sac. Therefore, in those cases in which the
inner tube is endodermal the brain has the same origin, and in the case
in which the inner tube is ectodermal, the brain is ectodermal, but the
pharyngeal sac has also an ectodermal origin. There is obviously no
definite relation between the origin of these structures in the bud and
in the egg embryo.
A similar difficulty is met with in the Bryozoa in regard to the
development of the egg embryo and the bud embryo.
Braem, who has made a critical examination of the germ-layer
theory,[103] has found it impossible to give a morphological definition
of a germ-layer, and has adopted a physiological criterion. He thinks
that in whatever way a germ-layer arises, whether by folding, or by
delamination, etc., it exists independently of its method or place of
origin. A layer is not endodermal because it forms the inner wall of a
gastrula, but it is endodermal because it develops into the digestive
tract. The germ-layers of different forms are only similarly placed, but
whether they are homologous will depend on other things. On this view
the inner tube of the ascidian bud that gives rise to both digestive
tract and to the nervous system is simply an indifferent layer until it
gives rise to these structures. Its cells may be looked upon as
indifferent, as are those of the blastula. Thus the difficulty of the
morphologist is not solved, but the knot is cut. For Braem the
germ-layers are convenient terms, since he rejects any historical
significance that they may have, and it is just this side of the
question that the morphologist has attempted to work out. While the
evidence shows that the germ-layers cannot have any such final
attributes as embryologists have attempted to assign to them, and that
Braem has called attention to the real and important problems connected
with the study of development, yet it may still be admitted without
endangering the newer point of view, that there may be also an
historical question in connection with the germ-layers, if not in the
sense of a repetition of an ancestral adult gastræa, yet in the sense
that similarity in embryonic development may in some cases find its
historical explanation in a common descent.
If in the light of this discussion we turn to the phenomena of
regeneration, we again find evidence showing that the germ-layer theory
fails to apply in all cases. It has been pointed out that in
lumbriculus, and in the naids, the new mesoderm is derived from the
ectoderm, and does not come from the old mesodermal tissues. The
mesoderm of the embryo in annelids is derived from one, and later from
two, superficial cells of the blastula,[104] that push in about the time
of gastrulation. They cannot, at this time, be referred to one layer
rather than to the other. It cannot be affirmed, therefore, that in
regeneration, the mesoderm arises from a different layer from that in
the embryo, but neither can this be denied. The most important point in
this connection is that the new mesoderm comes from the ectoderm that is
already differentiated, and not from the mesodermal tissues. It is
clear, however, that while the lining of the pharynx in the embryo is
ectodermal, it is endodermal in the regenerated part.
It is true that these cases are very exceptional, and that generally the
new organs come from similar organs in the old part, but one established
exception is sufficient to show that the traditional conception of the
germ-layers may be of little value, and since the hypothesis itself, out
of which the idea in regard to regeneration from definite germ-layers
has been formed, has been proven to be insufficient in other directions,
the time is ripe to look for a more secure footing. It need hardly be
added that the idea of a supposed necessity for an organ to arise from a
definite germ-layer is so empty of all significance that we may well
rejoice to be able to set it aside as a naïve view that has had its day.
Furthermore, a new series of problems has arisen in connection with the
experimental work to be described in a later chapter. If, as seems
probable, the question of the germ-layers will be merged into the much
broader question of the origin of the specification of the tissues, we
can in the future more profitably direct our attention to the
experimental evidence that bears on the latter question.
_THE SUPPOSED REPETITION OF PHYLOGENETIC AND ONTOGENETIC PROCESSES IN
REGENERATION_
It has been claimed that at times ontogenetic, and even phylogenetic,
processes are repeated during regeneration. Fraisse, for instance, who
advocates this point of view, thinks that it has been too much
neglected, and calls attention to several instances of what he believes
to be cases in point. He thinks that Bülow is correct in his comparison
between the method of development of the new tissue at the end of the
tail in certain naids, and the method of gastrulation and formation of
the mesoderm in the embryo. Later results have shown, however, that in
several points Bülow’s observations are incorrect. The in-turning of
ectoderm that Bülow compares with the process of gastrulation is
connected with the formation of the ectodermal proctodæum, and is not
comparable with the development of the endoderm in the embryo.
Götte also, as we have seen, cites a case of resemblance between the
regeneration of the limbs of the salamander and their mode of embryonic
development. He finds the resemblances less marked as the animal becomes
older. The resemblance is, however, not very close and of a rather
general sort, and since the same structures develop in both cases out of
the same kind of substance, it is not surprising that there should be
some resemblances in the processes. This evidence is counterbalanced by
the mode of regeneration of the tail in the adult of certain forms, and
in the regeneration of the lens of the eye from the iris.
Carrière finds that the eye of snails regenerates from the ectoderm in
much the same way as the young eye develops. Granted that the eye is to
come from the ectoderm in both cases, and that the same structure
develops, it is not to be wondered at that the two processes have much
in common.
The mistake, I think, is not in stating that the two processes are
sometimes similar, or even identical, but in stating the matter as
though the regenerative process repeats the embryonic method of
development. If the same conditions prevail, then the same factors that
bring about the embryonic development may be active in bringing about
the regenerative processes. In fact, we should expect them to coincide
oftener than appears to be the case, but this may be due to the
conditions being different in the young and in the adult.
It has been claimed also that in some cases there is regenerated a
structure like that possessed by the ancestors of the animal. The stock
example of this process is Fritz Müller’s result on the regeneration of
the claw of a shrimp, _Atypoida protimirum_.[105] Fraisse and Weismann
and others have brought forward this case as demonstrative. The animal
is said to regenerate a claw different from any of those in the typical
form, and one that resembles the claw of another related genus,
_Carodina_. The value of evidence of this sort is not above question.
Przibram has shown in other crustacea that when a maxilliped is cut off
a structure different in kind often regenerates, but that after several
months the typical structure returns. Do we find here an ancestral organ
that first appears, and then gives way to its more modern
representative? If it _resembled_ the maxilliped of any other
crustacean, the evidence would, no doubt, be accepted by those who
accept the evidence furnished by Müller. What then shall we say to the
case, first discovered by Herbst, in which the eye of certain prawns
being cut off, an antenna-like organ regenerates? Since these antennæ
are similar to those possessed by the same animal, shall we assume that
it once had antennæ in place of eyes?
Another comparison, that Fraisse has made, is worth quoting as showing
how far credulity may be carried. In the regeneration of the tail of
certain lizards pigment first appears in the ectoderm of the new part
and then sinks deeper into the layers. Fraisse found a lizard on Capri
in which the tail is pigmented throughout life, and although he did not
know whether or not the pigment is in the skin he suggests that this
lizard represents an ancestral condition, that is repeated by the
regenerating tails of other forms.
Boulenger (’88) pointed out that the scales over the regenerated tail of
several lizards have a different arrangement from that of the normal
tail, and furthermore, the new arrangement is sometimes like that found
in other species. He claims that this shows that such forms are related,
even where no evidence of their relation is forthcoming. That the
conditions in the new tail may be different from those in the normal
tail is shown by the absence of a vertebral column, etc.; therefore that
the scales also should have a new arrangement is not surprising, but the
facts fail, I think, to show that there need be any genetic relation
between the forms in question. That the conditions in the new tail might
be like those in an ancestral form may be admitted, but this is very
different from assuming that the results show a genetic relation
actually to exist. The main point is that, even if the results should be
nearly identical, it may be entirely misleading to infer that ancestral
characters have reappeared.
In some cases an extra digit or toe may regenerate on the leg of a
salamander, and this too has been interpreted as a return to an
ancestral condition. But Tornier has shown, as has been stated, that
several additional digits, or even a whole extra hand, may be produced
by wounding the leg in certain ways, and these too would have to be
interpreted as ancestral, if the hypothesis is carried out logically. It
has been shown by King that one or more additional arms may be produced
in a starfish by splitting between the arms already present, and if we
accepted evidence of this sort as having any value in interpreting lines
of descent we should conclude[106] that the ancestors of the starfish
had six, seven, or more arms according to the number that can be
produced artificially, etc. Therefore, until further evidence of a more
convincing kind is forthcoming, we can safely, I think, decline to
accept the results, so far known, as having any value in interpreting
the relationships or the descent of the animals.