PHYSIOLOGICAL REGENERATION. REGENERATION AND GROWTH. DOUBLE STRUCTURES.
During the normal life of an individual many of the tissues of the body
are being continuously renewed, or replaced at definite periods. The
replacement of a part may go on by a process of continuous growth, such
as takes place in the skin and nails of man, or the replacement may be
abrupt, as when the feathers of a bird are moulted. It is the latter
kind of process that is generally spoken of as physiological
regeneration. In the same animal, however, certain organs may be
continually worn away, and as slowly replaced, and other organs replaced
only at regular intervals.
Bizozzero has made the following classification of the tissues of man,
on the basis of their power of physiological regeneration. (1) Tissues
made up of cells that multiply throughout life, as the parenchyma cells
of those glands that form secretions of a definite morphological nature;
the tissues of the testes, marrow; lymph glands, ovaries; the epithelium
of certain tubular glands of the digestive tract and of the uterus; and
the wax glands. (2) Tissues that increase in the number of their cells
till birth, and only for a short time afterward, as the parenchyma of
glands with fluid secretions, the tissues of the liver, kidney,
pancreas, thyroid, connective tissue, and cartilage. (3) Tissues in
which multiplication of cells takes place only at an early embryonic
stage, as striated muscles and nerve tissues. In these there is no
physiological regeneration.
There are many familiar cases of periodic loss of parts of the body. The
hair of some mammals is shed in winter and in summer. Birds renew their
feathers, as a rule, once a year. Snakes shed their skin from time to
time. The antlers of deer are thrown off each year, and new ones formed
accompanied by an increase in size and branching of the antlers. In
other cases similar changes may be associated with certain stages in the
life of the animal. The milk-teeth of the mammals are lost at definite
periods, and new teeth acquired.[61] The larval exoskeleton of insects
is thrown off at intervals, and after each moult the body increases in
size; but after the pupa stage is passed and the imago formed, there is
no further moulting. In the crustacea, on the other hand, the adult
animals moult from time to time, and the upper limit of size is less
well defined than in the insects. The larvæ also pass through a series
of moults.
An interesting case of physiological regeneration has been described by
Balbiani in a unicellular form, stentor. From time to time a new
peristome appears along the side, moves forward and replaces the old
peristome, that is absorbed as the new one comes into position. In other
infusoria the peristome may be absorbed before encystment, and a new one
appears when the animal emerges from the cyst. Schuberg states that when
division takes place in bursaria the new peristome develops on the
aboral piece in the same way as after encystment; and Gruber observed
that, when an aboral piece of an infusorian is cut off, a new peristome
develops in the same way as after normal division of the animal. These
observations indicate that the process of physiological regeneration may
follow the same course and probably involves the same factors as the
process of restorative regeneration.
Tubularia absorbs its old hydranth-heads if placed in an aquarium, and
regenerates new ones. It may even absorb the hydranth while growing in
an aquarium, as Dalyell has shown, and presumably, therefore, also under
natural conditions. After each regeneration the new stalk behind the
head increases in length.
In plants, in which there is a continuous apical growth, new parts are
being always added at the end of the stem, and old parts are continually
dying, as seen in palms. Most trees and shrubs in temperate climates
lose their leaves once a year and produce new ones in the spring. Since
the new leaves develop from the new shoots at the end of the stem and
branches, the old ones can, only in a general way, be said to be
renewed.
That a very close relation exists between the process of physiological
regeneration and restorative regeneration will be sufficiently evident
from the preceding illustrations. We do not gain any insight into either
of the processes, so far as I can see, by deriving the one from the
other, for the process of restorative regeneration may be, in point of
time, as old as that of physiological regeneration. This does not mean,
of course, that the same factors may not be present in both cases. So
similar are the two processes that several naturalists have attempted to
show how the process of restorative regeneration has been derived from
physiological regeneration. Barfurth, recognizing the resemblance
between the two processes, speaks of restorative regeneration as a
modification of physiological regeneration, and Weismann also supports
this point of view. He says: “Physiological and pathological
regeneration obviously depend on the same causes, and often pass one
into the other, so that no real line of demarcation can be drawn between
them. We nevertheless find that in those animals in which the power of
regeneration is extremely great physiologically, it is very slight
pathologically. This proves that a slight power of pathological
regeneration cannot possibly depend on a general regenerative force
present within the organism, but rather that this power can be provided
in those parts of the body which require a continual, periodic
regeneration; in other words, the regenerative power of a part depends
on adaptation.” It is, I think, erroneous to state “that in those
animals in which the power of regeneration is extremely great
physiologically, it is very slight pathologically.” All that we are
justified in concluding from the evidence is that in some cases in which
physiological regeneration takes place, as in the vertebrates,
pathological (restorative) regeneration may not be well developed; but
even in these forms restorative regeneration is certainly present, and
present especially in internal organs, as in the salivary gland, in the
liver, and in the eye, which are little exposed to injury. How far
physiological regeneration takes place in the tissues of the lower
animals we do not know at present, except in a few cases, but far from
supposing it to be absent, it may be as well developed as in higher
forms. Weismann’s further conclusion, that because in some animals
physiological regeneration is very great and restorative regeneration
very slight, therefore the latter cannot “depend on a general
regenerative force within the organism,” is, I think, quite beside the
mark. In this connection we should not fail to notice a difference
between these two regenerative processes that several writers have also
called attention to, viz. that the power of cell-multiplication and the
formation of new cells in each kind of tissue does not carry with it the
power of restorative or even of physiological regeneration, in cases
where several kinds of tissue make up an organ. For instance, if the leg
of the mammal is cut off, the old cells may give rise to new ones, but
the processes that would bring about the formation of the new leg are
not present, or, rather, if present, cannot act. Thus, although the
production of new cells from each of the different parts of the leg of a
mammal may take place, yet the conditions are unfavorable to the
subsequent formation of a new leg out of the proliferated cells. We
should not infer that this power does not exist, but that under the
conditions it cannot be carried out. The assumption that physiological
regeneration is the forerunner of restorative regeneration, in the sense
that historically the former preceded the latter and furnished the basis
for the development of the latter, cannot be shown, I think to be even
probable. This way of looking at the two processes puts them, I believe,
in a wrong relation to each other. We find both processes taking place
in the simplest forms as in the unicellular protozoa, and present
throughout the entire animal kingdom without any connection, excepting
so far as they both depend on the general processes of growth
characteristic of each organ and of each animal. This leads us to
consider the general question of regeneration in its relation to the
phenomena of growth.
_REGENERATION AND GROWTH_
It has been pointed out in several cases in which external factors
influence the growth of a plant, or of an animal, that the same factors
play a similar part in the regeneration. The action of gravity on the
growth of plants has been long known, and that it is a factor in the
regeneration of a piece of a plant has also been shown. The only animal
in which gravity has been definitely shown to be an important factor
during growth is antennularia, and it has been found that gravity is
also a factor in the regeneration of the same form. Not only is this
influence shown in the growth of the new part that has developed, but
the same influence seems to be one of the factors that determines where
the new growth takes place. This latter relation is known in only a few
cases, for instance in plants, according to Vöchting, and in
antennularia, according to Loeb, so that, until further evidence is
forthcoming, it is best not to extend this generalization too far; but
it seems not impossible that it may be generally true. How an external
factor may determine the location of new growth, as well as the
subsequent development of the new part, we do not know at present.
In regard to the internal factors that influence the growth and the
regeneration of new parts, we are almost completely in the dark. In
cases of hypertrophy of the kidney, etc., the evidence seems to show
that a specific substance, urea, that is normally taken from the blood
by this organ may, if present in more than average amounts, excite the
cells to greater activity and to growth, but whether the urea itself
does this directly, or only indirectly through the greater functional
activity of the cells, has not, as we have seen, been ascertained. That
growth is influenced by internal factors can be shown, at least in
certain cases, even although we cannot refer to the definite chemical or
physical factors in the process. Some experiments that I have made on
the tails of fish show very clearly the action of an internal factor. If
the tail of fundulus is cut off obliquely, as indicated by the line 2-2
in Fig. 40, _A_, new material appears in a few days along the outer
cut-edge. It appears to be at first equal in amount along the entire
edge. As the material increases in width, it grows faster over
[Illustration: FIG. 40.--_A._ Tail of _Fundulus heteroclitus_. Lines
indicate levels at which _B_ and _C_ were cut off. _B._ Regenerating
from cross-cut. _C._ Regenerating from oblique cut. _D, E._ Regenerating
from two oblique surfaces. _G._ Tail of stenopus. _H, I._ Tail of last
cut off squarely and obliquely.]
that part of the edge that is nearer the base of the tail (Fig. 40,
_C_). This growth continues to go on faster on the lower side, until the
rounded form of the tail is produced. If we make the oblique cut so that
the part nearer the base of the tail is on the upper side, the result is
the same in principle; the upper part of the new material grows faster
than any other part. If we make two oblique cuts on the same tail, as
shown in Fig. 40, _D_, or as in _E_, the new part grows faster in each
case on that part of the cut-edge that lies nearer the base of the tail.
These results may be supposed to be due to the better nourishment of the
new tissues nearer the base of the tail; but it is not difficult to show
that the difference in the rate of growth over different parts of the
cut-edge is not due to this factor. If, for example, we cut off the
tail of one fish squarely near the outer end, as shown in Fig. 40, _F_,
1-1, and the tail of a second near the base of the tail, as shown in
Fig. 40, _F_, 2-2, and of a third by an oblique cut that corresponds to
a cut extending from the upper side of the cut-edge of the tail of the
first fish to the lower cut-edge of the tail of the second fish, as
shown in Fig. 40, _F_, we find that the rate of growth over the first
and second tails is about the same as that of the lower side of the
third tail. In other words, the maximum rate of growth that is possible
for the entire oblique edge is carried out only near the lower edge, and
the growth of the rest of the new material is held in check. By means of
another experiment a similar phenomenon can be shown. If the bifurcated
tail of a young scup (_Stenopus chrysops_) is cut off by a cross-cut
(Fig. 40, _G_, 1-1), it will be found that at first the new material is
produced at an equal rate along the entire cut-edge; but it soon begins
to grow faster at two points, one above and the other below, so that the
characteristic swallow-tail is formed at a very early stage (Fig. 40,
_H_) and before the new material has grown out to the level of the notch
of the old tail. If the tail of another individual is cut off by an
oblique cut (Fig. 40, _G_, 2-2), we find, as shown in Fig. 40, _I_, that
at two points the new tail grows faster, but the lower lobe faster than
the upper one.
These results show very clearly that in some way the development of the
typical form of the tail influences the rate of growth at different
points. The more rapid growth takes place in those regions at which the
lobes of the tail are developing. In other words, although the
physiological conditions would seem to admit of the maximum rate of
growth over the entire cut-edge, this only takes place in those parts
that give the new tail its characteristic form. The growth in other
regions is held in check. The same explanation applies to the more rapid
growth at that part of an oblique cut that is nearest the base of the
tail, for by this means the tail more nearly assumes its typical form.
These results demonstrate some sort of a formative influence in the new
part. We can refer this factor at present only to some structural
feature that regulates the rate of growth. We find here one of the
fundamental phenomena behind which we cannot hope to go at present,
although it may not be beyond our reach to determine in what way this
influence is carried out in the different parts. This topic will be more
fully considered in a later chapter.
Another illustration may be given from certain experiments in the
regeneration of _Planaria lugubris_. If the posterior end is cut off
just in front of the genital pore, as indicated in Fig. 41, new material
develops at the anterior cut-edge, and in a few days a new head is
formed out of this new material. A new pharynx appears in the new
tissue immediately in front of the old part. It lies, therefore, just
behind the new head. The proportions of the new worm are at this time
very different from those of a typical worm, since the head is much too
near to the new pharynx and to the old genital pore. New material is now
produced in the region behind the head and in front of the pharynx, so
that the head is carried further forward until the new worm has fully
assumed the characteristic proportions. As the new head is formed the
old part loses its material, so that it becomes flatter and narrower,
and if the worm is not fed the old part may lose also something of its
former length. If the worm is fed, however, as soon as the pharynx
develops the old part loses less and the new part grows forward more
rapidly. The most striking phenomenon in the growth of the new worm is
the formation of new material in the region behind the head. The result
of this growth is to carry the head forward and produce the
characteristic form of the animal. This change is all the more
interesting since the growth does not take place at a free end, but in
the middle of the new material. It is only by the formation of new
material in this region that the head is carried to its proportionate
distance from the pharynx. It appears that in some way the growth is
regulated by influences that determine the form of the new organism.
[Illustration: FIG. 41.--Posterior end of _Planaria lugubris_, cut off
between pharyngeal and genital pores. Figure to left shows the piece
after removal. The four figures to the right show the regeneration of
the same piece, drawn to scale. As soon as the new pharynx had
developed, the worm was fed. The experiment extended from November 17 to
January 8.]
Another experiment on the same animal gives also a somewhat similar
result. If a worm is cut in two obliquely (Fig. 21, _B_) and the
regeneration of the posterior piece is followed, it is found that the
new material appears at first evenly along the entire cut-surface. It
then begins to grow faster on one side (Fig. 21, _b_), and a head
appears in this region with its axis at right angles to the cut-edge. As
the head grows larger the growth is more rapid _on one side_, and as a
result the head is slowly turned forward (Fig. 21, _b_). This more rapid
growth on one side brings the new head finally into its typical position
with respect to the rest of the piece. The end result of these changes
is to produce a new worm having a typical form. If the oblique cut is
made behind the old pharynx, as in Fig. 22, _A_, the new pharynx that
appears in the new material along the cut-edge lies obliquely at first,
indicating that the new median line is very early laid down in the new
part, and connects the middle line of the old part with the middle of
the new head. As the region behind the new head grows larger and broader
the pharynx comes to lie more and more in an antero-posterior direction,
and finally, when the new part is as broad as the old,[62] the pharynx
lies in the middle line of a symmetrical worm.
These results show that the new growth may even take place more rapidly
on one side of the structural median line than on the other, and on that
side that must become longer in order to produce the symmetrical form of
the worm. Here also we find that a formative influence of some sort is
at work that regulates the different regions of growth in such a way
that a typical structure is produced. The more rapid growth on one side
is, however, in this case clearly connected with the relatively smaller
development of the organs on that side, and perhaps this same principle
may explain all other cases. If so the phenomenon appears much less
mysterious than it does when the growth is referred to an unknown
regulative factor.
_DOUBLE STRUCTURES_
A structure that is single in the normal animal may become double after
regeneration, and in some cases the special conditions that lead to the
doubling have been determined. Trembley showed that if the head of hydra
is split lengthwise into two parts, each part may complete itself and a
two-headed form is produced. If the posterior end of a hydra is split,
an animal with two feet is made. It is true that the two-headed forms
may subsequently separate after several weeks into two individuals, and
even the form with two feet may lose one of them by constriction, as
Marshall and King have shown. Driesch has produced a tubularian hydroid
with two heads by splitting the stem partially into two pieces. Each
head is perfect in all respects, and although each has fewer tentacles
than
[Illustration: FIG. 42.--_Planaria lugubris._ _A._ Two heads produced
after operation similar to that in Fig. 24. Each head about half size.
_B._ Worm split in half through level of pharynx. New half-worms larger
than half of normal worm.]
the head that regenerates from an undivided stem, yet the number of
tentacles on each head is more than half the average number. This is
connected apparently with the fact that the circumference of each half
is greater than half the circumference of the original stem. Planarians
with double tails, produced by partial splitting, have been described by
Dugès and by Faraday, and it has also been shown that by partial
splitting of the anterior end of the worm two heads can be produced. Van
Duyne, Randolph, and Bardeen and I have obtained the same result. Each
half completes itself on the cut-side and produces a symmetrical
anterior end. If one of the heads is cut off, it will be again
regenerated. If the heads are united very near to the trunk, as in Fig.
42, _A_, they may never grow to the full size of the original head, as I
have found; but if the pieces have been split posteriorly, so that each
head has a long anterior end, then each one may become nearly as large
as the original head (Fig. 42, _B_). We see in these cases the influence
of the region of union on the growth of the new part. If the new part is
near the region of attachment, the smaller size of the latter restrains
the growth of the new head; but if the region of union is farther
distant, the head may grow more nearly to its full size despite the
influence of the region of union. King has found in the starfish that if
the arm is split lengthwise, each half may complete itself laterally and
a forked arm result. An additional entire arm may be formed by splitting
the disk partially in two between two arms. If the cut-edges do not
reunite a new arm will grow out from each cut-surface (Fig. 38, _E_). In
this case the development of the new arm cannot be accounted for on the
assumption that the typical form completes itself, since a sixth arm
cannot be supposed to be a typical structure in the starfish. The result
must depend on other factors, such as the presence of an open surface in
a region where the cells have the power of making new arms.
Barfurth has been able to produce a double tail in the tadpole by the
following method: A hot needle is thrust into one side of the tail, so
that the notochord and the nervous system are injured. The tail is then
cut off just posterior to the region injured by the needle. A new tail
grows out from the cut-end, and also in some cases another tail grows
out at the side where the notochord was injured by the needle. The
injury to the notochord and the removal of tissue immediately about it
leads to a proliferation of cells, around which other tissues are added
and the new tail produced.
Lizards with double tails have often been described,[63] and it now
appears that all these cases are due to injuries to the normal tail.
Tornier has succeeded, experimentally, in producing double and even
triple tails. If the end of the tail is broken off, and the tail is then
injured near the end, two tails may regenerate, one from the broken end
and one from the region of injury (Fig. 43). Under natural conditions
this might occur if the tail were partially bitten off and the end of
the tail lost at the same time. A regenerated tail may produce another
tail if it is wounded. A three-tailed lizard may be made by cutting off
the tail and then making two injuries proximal to the broken end. Two of
the new tails may be included in the same outer covering if they arise
near together, as shown in Fig. 43, _B_. Lizards with two or three tails
may be produced in another way. If the tail is cut off very obliquely,
so that two or three vertebræ are injured, there arises from each
wounded vertebra a cartilaginous tube that forms the axis of a new tail.
Tornier thinks that the regeneration is the result of overnourishment of
the region where the injury has been made, but this does not seem in
itself a sufficient explanation. Tornier has also been able to produce,
experimentally, double limbs in _Triton cristatus_ in the following way:
The limb is cut off near the body, and, after the cut-end has formed new
tissue, a thread is tied over the end in such a way that it is divided
into two parts. As the new material begins to bulge outward it is
separated into halves by the constricting thread, and each part produces
a separate leg (Fig. 43, _D_). The soles of the two feet in the
individual represented in Fig. 43, _D_, are turned toward each other.
The femur is bifid at its outer end, and to each end the lower part of
one leg is attached. The bones in this part are fused together at the
knee, so that only the foot portions can be separately moved.
[Illustration: FIG. 43.--After Tornier. _A._ _Lacerta agilis._ Produced
by partly breaking off old tail. New tail arises at place of breaking.
Old tail also remains. _B._ Three-tailed form--two tails being united in
a common covering. Old tail had been cut off (it regenerated the lower
branch from cut-end) and two proximal vertebræ that had been injured.
_C._ Additional limb of _Triton cristatus_ produced by wounding femur.
_D._ Double foot of _triton cristatus_ produced by tying thread over
regenerating stump. _E._ Foot of _Triton cristatus_. Dotted lines
indicating how foot was cut off. _F._ Regeneration of same. _G._ Another
way of cutting off foot. _H._ Result of last operation.]
The same method used to produce double tails in the lizard can also be
used to produce double legs. The femur is broken in the vicinity of the
hip-joint, and the soft parts are cut into over the break. Then, or
better somewhat later, the leg is amputated below the broken part. A new
limb regenerates from the cut-end, and at the same time another limb
grows out from the broken femur (Fig. 43, _C_). The same result is
reached if the femur has a slit cut into it in the region of the
hip-joint, so that it is much injured. Later the leg is cut off below
the place of injury. A double leg is the result.
Feet with supernumerary digits can also be produced by artificial
wounds. If the first and second and then the fourth and fifth toes are
cut off, as indicated by the lines in Fig. 43, _E_, so that a part of
the tarsus and a part of the tibia and fibula are cut away (the third
finger being left attached to the remaining middle portion), more toes
grow out from the wounded surface than were removed, as shown in Fig.
43, _F_. A similar result may be obtained in another way. If the first
and second toes are cut off by an oblique cut (Fig. 43, _G_), and then
after the wound has healed the third, fourth, and fifth toes are also
cut off by another oblique cut (a part of the tarsus being removed each
time), more toes are regenerated than were cut off[64] (Fig. 43, _H_).
Tornier suggests that the double feet that are sometimes formed in
embryos--even in the mammalia--have resulted from a fold of the amnion
constricting the middle of the beginning of the young leg, in the same
way as is brought about artificially by tying a string over the growing
end of the regenerating leg of triton.
In many of these cases, in which the double structure is the result of
splitting the part in the middle line, the completion of the new part is
exactly the same as though the parts had been entirely separated. The
only special problem that we meet with in these instances is that this
doubling is possible while the piece remains a part of the rest of the
organism. This shows that there is a great deal of independence in the
different parts of the body in regard to their regenerative power, and
that local conditions may often determine the formation of double
structures.
It has been shown during the last decade that double embryos may be
produced artificially by incomplete separation of the first two
blastomeres. Driesch, Loeb, and others have demonstrated that if the
first two cells of the egg of the sea-urchin be incompletely separated,
each may produce a single embryo and the two remain sticking together.
Wilson has shown in amphioxus that the same result occurs if the first
two cells are partially separated by shaking. Schultze has shown in the
frog that if at the two-cell stage the egg is held in an inverted
position, _i.e._ with the white hemisphere turned upwards, each
blastomere gives rise to a whole embryo--the two embryos being united,
sometimes in one way, sometimes in another, as shown in Fig. 63. In this
case it appears that the results are due to a rotation of the contents
of each blastomere, so that like parts of the two blastomeres become
separated. In the egg of the sea-urchin, and of amphioxus, gravity does
not have a similar action on the egg, but the results seem to be due to
a mechanical separation of the blastomeres. These cases of double
structures, produced by the segmenting egg, appear to belong to the
same category as those described above for adult forms--especially in
those cases where pieces regenerate by morphallaxis.
[Illustration: FIG. 44.--_A._ _Planaria lugubris_, cut in two as far
forward as region between eyes, regenerating half-heads. _B._ Same cut
in two at one side of middle line. Smaller piece produced a new head.
_C._ _Planaria maculata_, split in two. It produced two heads in angle.
_D._ Another, that produced a single head in angle.]
In connection with the production of double structures there should be
mentioned a peculiar method of formation of new heads, first discovered
by Van Duyne in a planarian. He found that if the animal is cut in two
in the middle line, the halves being left united only at the head-end,
as shown in Fig. 44, _D_, _C_, there may appear one or two new heads in
the angle between the halves. I have repeated this experiment with the
same result, and have found that it may also occur when only a piece is
partially split from the side of the body, as shown in Fig. 44, _B_. In
Van Duyne’s experiment the two new heads do not appear unless the cut
extends far forward, but if the division extends into the region between
the two eyes there may be formed, as I have found, a single eye on each
side that makes a pair with the old eye of that side (Fig. 44, _A_). It
is evident in this case that each head has completed itself on the
cut-side, the completion including the eye and the side of the head also
with its “ear-lobe.” The result, in this case, is the same as though the
pieces had been completely cut in two. If the cut does not extend quite
so far forward there are usually formed one or two heads near the angle,
each with a pair of eyes and a pair of ear-lobes (Fig. 44, _C_).
Sometimes a single head develops in the angle itself (Fig. 44, _D_), and
it is difficult to tell whether it belongs to one or to the other side,
or whether it is common to both sides. Van Duyne spoke of the double and
single head of the latter kind which he obtained as heteromorphic
structures in Loeb’s use of the term. According to this definition,
heteromorphosis is the replacement of an organ by one that is
morphologically and physiologically unlike the original one, but this
statement has been made to cover a number of different phenomena. The
examples of heteromorphosis that Loeb gives by way of illustration of
the phenomenon are: the production of a hydranth on the aboral end of
tubularia, and the formation of roots in place of a stem in
antennularia, etc. The formation of the heads in the angle in planarians
does not appear to me to belong in this category. It seems rather that
the phenomenon is of the same sort as the formation of a new head at the
side of a longitudinal piece, and if so the new heads in the angle are,
therefore, in their proper structural position for new heads belonging
to the posterior halves. Even if it should prove true that a single head
may develop exactly in the angle itself, and belong to both sides, it
can be interpreted by an extension of the same principle.[65] The
position of this median head turned backward suggests an obvious
comparison with the production of the heteromorphic head in _Planaria
lugubris_, but a closer examination will show, I think, that the two
cases are different. The heteromorphic head is produced only when the
head is cut off close behind the eyes. If cut off slightly behind this
region, a posterior end is generally formed. But in the worms split
lengthwise the head in the angle may be formed at a level much farther
posteriorly than the eyes. If the split extends into the head, then the
eyes that develop are the supplements of those of the old part. Our
analysis leads, therefore, to the conclusion that the heads, or parts of
heads, in the split worms are not heteromorphic structures but
supplementary heads.