/. Embryo/, exp. Morp/i. Vol. 31, 3, pp. 527-539, 1974
Printed in Great Britain
527
The morphogenetic properties of the node
in Clytia johnstoni
By L. J. HALE 1
From the Department of Zoology, University of Edinburgh
SUMMARY
1. The experiments show that:
(a) When a hydranth is picked off the stolon another normally regenerates from the
same spot.
(b) When the nodal coenosarc is exposed to sea water the probability of hydranth
regeneration is much reduced and stolons may grow.
(c) A similar exposure of the coenosarc in the middle of an internode results in branch
stolon growth at about the same frequency as at a node but hydranths do not grow.
(d) Cutting the coenosarc does not affect the structure which grows, but where the stolon
is cut at a node and a hydranth regenerates it does so at a distance of 0-3-0-6 mm from
the node.
(e) Grafting a hydranth into the side of the stolon at a node usually prevents hydranth
regeneration but the inhibition ceases when the graft is made at distances greater than
0-3 mm from the node.
(/) There is some evidence that hydranth regeneration is preceded by the development
of a contractile growth region in the stolon.
2. It is concluded that
(a) A node has a latent morphogenetic capacity to enable hydranths to grow there, a
capacity not demonstrated in the middle of an internode.
(b) Local exposure to sea water reduces the morphogenetic capacity of stolon coenosarc
for hydranth growth and increases it for stolon growth.
(c) Hydranth growth may require the earlier formation of a contractile growth zone.
(d) An existing hydranth usually inhibits the initiation of another hydranth at or close
to its junction with the stolon, perhaps by preventing the formation of such a growth zone
there.
INTRODUCTION
The nodes in Clytia johnstoni are created by the periodic production of
a hydranth branch by a growing stolon. It has been shown that these primary
hydranths are not produced at random (Hale, 1973*7). In addition, evidence
is produced in the same paper that secondary hydranths (which grow later
between the primary hydranths) tend to grow at a distance from one or the
other primary hydranth. No differences have been detected in the morphogenetic properties in isolated stolon from near to the nodes as compared with
isolated stolon from the middle of the internodes (Hale, 19736). It is thus
1
Author's address: The Centre for Human Ecology, University of Edinburgh, 57 George
Square, Edinburgh EH8 9JZ, U.K.
33
E M B
31
528
L. J. HALE
necessary to investigate the properties of the node itself and to attempt to
assess the part it plays in the pattern of secondary hydranths.
In this paper experiments are described which show that nodes do have
morphogenetic properties which might play an important part in controlling
secondary hydranth formation.
MATERIALS AND METHODS
The experiments were carried out on material grown in culture as described
in earlier papers. Further details are recorded under each experiment.
RESULTS
Experiment 1. Regeneration at a node after removal of the hydranth, with and
without severing the nodal stolon coenosarc
In this experiment the hydranths and their stalks were picked off leaving
a small round hole in the top of the stolonic perisarc. In some of the experiments
the coenosarc at the node was severed by pressure from a needle. The nodes
were observed for a period of three days and the results are recorded in Table 1.
Where the stolon coenosarc was severed it was repaired in about 3-6 h.
Table 1 shows that in almost 90 % of the experiments a hydranth grew from
the node, and just over 10 % failed to do so, there being no detectable difference
between the results of the two experiments. Where a hydranth grew it did so
through the hole left in the coenosarc.
Experiment 2. Regeneration at a node and in the middle of an internode with
severing of the stolonic coenosarc
The hydranths were removed in a slightly different way from that used in
the first experiment so that the treatment at a node could be equated to the
treatment at the middle of the internode. At the node two cuts were made
across the stolon, one on each side of the hydranth and as close as possible
to it. Tn this way the hydranth plus a very short length of stolon perisarc
was removed. A similar narrow slice of stolon perisarc was removed from the
centre of an adjacent internode. In each case the first cut severed the coenosarc
which retracted on both sides. The results of those experiments are recorded
in Table 2. The presence or absence of regeneration was again scored during
three days of observation. Repair of the cut stolon took place in all cases but
took longer than in the situation described in Expt. 1, about 12-20 h.
The experiment shows that hydranths grew from nodes but did not grow
in the internodes (a significant reduction; %2 = 50-8, D.F. = \, P < 0-0005).
Stolons sometimes grew from both these points and in about the same
proportion (11 %) in the two cases.
Compared with Expt. 1 the frequency of hydranth regeneration at nodes
was greatly reduced, from about 90 to 15 % (xz = 65-3, D.F. = 1, P < 0-0005).
Expt. 3 investigates these factors further.
The node of ClytiSL
529
Table 1. Regeneration at nodes after removal of the hydranths, with
and without severing the nodal stolonic coenosarc
No. of instances (%) in which
A
Treatment
Hydranths
regenerated
No hydranths
regenerated
Total
°
«> m
7(11)
66 (100)
22(88)
3(12)
25 (100)
^>
°
^
Table 2. Regeneration at nodes and in the middle of internodes
No. of instances (%) in which
Treatment
Hydranths
grew
Stolons
grew
Neither
hydranths nor
stolons grew
Totals
8(15)
6(11)
41(75)
55(101)
0(0)
3(11)
24(89)
27(100)
65
82
Experiment 3. Effect of severed coenosarc and hole size on regeneration at nodes
In this experiment the first four nodes along a stolon (from a growing end)
which has open hydranths were made the subject of the following operations:
A. The hydranth and its stalk were picked off leaving a small round hole
in the top of the stolon. The stolon coenosarc was left entire.
B. The hydranth and its stalk were picked off and the perisarc hole enlarged
to a size estimated to be comparable to that cut in C. The stolon coenosarc
was left entire.
C. The hydranth and stalk were removed with a small stolon annulus by
two cuts through the stolon close to the hydranth. The stolon coenosarc was
thus severed.
D. The hydranth and stalk were picked off leaving a small hole in the top of
the stolon. The coenosarc of the stolon was severed by pressure with a needle.
33-2
530
L. J. HALE
Table 3. Frequency of growth of hydranths and stolons from nodes after hydranth
removal, with and without enlarging the perisarc hole, and with and without
severing the coenosarc (matched samples)
(a) Basic Data
H = hydranth regeneration; S = stolon growth; O = no growth.
---
—^E_.
° <r'
•
•
—
—
Experiment
H
H
H
H
H
H
H
H
H
H
H
H
o
S
H S
H
S
S
O
H
H
o
o
0
H S
H
H
H
S
O
o
H
H
H
H
H
o
C/3
no.
1
2
3
4
5
6
7
8
9
10
11
12
H S
S
0
s
s
0
o
o0
H
s
(b) Data arranged according to the size of the hole made in the perisarc
and to whether the coenosarc was left entire
Hole size
A
Small
^
Large
\
Hydranth
hydranth
Hydranth
No
hydranth
Hydranth
0
12
0
0
3
0
6
3
3
12
6
3
0
7
0
5
1
4
2
5
1
11
2
10
0
19
0
5
4
4
8
8
4
23
8
13
No
State of
Entire
coenosarc
Stolon
No stolon
Severed
Stolon
No stolon
Totals
Stolon
No stolon
Totals
A.
No
hydranth
Twelve stolons were used for the experiment. In any one stolon the four
nodes were subjected one each to the four treatments A, B, C and D, i.e. every
stolon received all four operations. In order to estimate any possible effect
due to the position of a node relative to the growing point, each position was
subjected in equal frequency to each of the four treatments. The results of
The node of Oytia,
531
Table 4. Incidence of hydranth and stolon regeneration at nodes
in order from the stolon growing tip
Node
(•«- towards growing stolon tip)
1
2
3
4
No. of
Hydranths regenerated
Stolons regenerated
9
3
7
3
6
4
5
3
Totals
27
13
the experiment are recorded in Tables 3 a and 3 b and scored according to
regeneration or not of hydranths and regeneration or not of stolons from the
treated nodes; a node might grow a hydranth, or a stolon, or both, or neither.
Analysis (McNemar's test for matched samples) of the numerical data gives
the following results.
1. The effect of hole size.
(a) The incidence of hydranth regeneration was reduced from 19 (small hole)
to 8 (large hole) out of 24 in each case and this change was statistically significant (x2 = 6-7, D.F. = 1, 0-01 > P > 0-005). This was almost entirely due
to the situation where the coenosarc was left entire (x2 = 7-1, D.F. = 1,
0-01 > P > 0-005) there being no significant difference where the coenosarc
was severed (x2 = 0-17, D.F. = 1, 0-7 > P > 0-6).
(b) The incidence of stolon production was increased from 0 (small hole)
to 12 (large hole) out of 24 in each case and this change was also statistically
significant (x2 = 10-1, D.F. = 1, 0-005 > P > 0001). This was mostly due
to the situation where the coenosarc was left entire (^2 = 7-1, D.F. = 1,
0-01 > P > 0-005) there being no significant difference where the coenosarc
was severed C\'2 = 1-3, D.F. = 1, and 0-3 > P > 0-2).
2. The effect of breaking the coenosarc.
(a) Breaking the coenosarc did not alter significantly the incidence of
hydranth regeneration; the reduction from 15 to 12 out of 24 in each case is
not statistically significant (x2 = 0*44, D.F. = 1, 0-6 > P > 0-5).
(b) Breaking the coenosarc did not alter significantly the incidence of stolon
regeneration; the reduction from 9 to 3 out of 24 in each case is not statistically
significant (x2 = 2-5, D.F. = 1, 0-2 > P > 0-1).
To summarize, hydranth regeneration was significantly reduced by enlarging
the hole at the node but leaving the coenosarc entire. Enlarging the hole also
significantly increased the incidence of stolons growing from the node, providing the coenosarc was unbroken.
In four cases both a hydranth and a stolon grew from an operated node.
A test of association between growth of these two kinds of branches can be
made using the data in the bottom right-hand box in Table 3(b) giving, for
the null hypothesis, x2 = 2-29, D.F. = 1 and 0-2 > P > 0-1. This result is
532
L. J. HALE
supported if the figures for the Marge hole' situation are considered; both
kinds of branch only grew together in these circumstances. Therefore under
the conditions of this experiment there is no evidence that growth of hydranth
and stolon are associated.
The number of hydranths regenerated in these experiments at each node, in
order from the growing stolon tip, is given in Table 4. In the case of hydranth
regeneration, the apparent trend is not significant (x2 = 2-86, D.F. = 1,
0-1 > P > 0-05); although the probability is small, other tests do not support
significance. There is clearly no trend in the case of the stolons.
When a hole is made in the perisarc it is repaired by the cells of the coenosarc
inside. The first reaction to a perisarc hole is for the adjacent ectoderm cells
to swell a little and plug the hole. Next the ectoderm cells secrete a thin layer
of chitin to seal the hole. The coenosarc then slowly resumes its normal
appearance.
If the coenosarc is severed when the hole is made the repair is of course
delayed until after the coenosarc has regrown to the hole. The amount of
retraction on cutting is variable but most commonly between 0-15 and 0-30 mm.
Regrowth starts almost immediately, slowly gathering speed up to about
0-15-0-25 mm per hour.
Growth up the tube is accompanied by pulsations in the ends of the growing
stolons. The ectoderm at the tips also showed some tendency towards the
thickened layer characteristic of growing stolon ends.
If the nodal cells are marked by staining with Nile blue sulphate and the
coenosarc then severed in the middle of the marked node, the coloured cells
remain at the tip during retraction and the subsequent growth up the tubes,
and then return to their original place at fusion. If a hydranth or stolon branch
grows from the repaired node the marked cells move into and form a significant
part of the growing tip, just as they do if the coenosarc is not severed.
There is a distinct point of difference in the repair of the coenosarc if the
perisarc hole is relatively large, i.e. when a thin annulus of perisarc is removed:
repair of the coenosarc takes 12-20 h as compared with 3-6 h if a small hole
is made. The difference occurs when the coenosarc has grown back to the
hole. When there is a large hole the rate of growth suddenly slows and the
last short distance across the break (ca. 0-1 mm) is only slowly repaired; with
the small hole there is no delay in the rate of repair.
Another point of difference in regeneration after making a relatively large
perisarc hole is that, after the repair of the coenosarc, regeneration of a hydranth
(when it took place) occurred from a position slightly displaced from the
node. The distance was 0-4-0-6 mm either to one or other side of the node
(3 towards and 2 away from the stolon growing tip). The cells at the node did
not take part in the growth of these displaced regenerations but remained
at the repaired node. If a branch stolon grew it did so at the node and nodal
cells took part.
The node of Clytia
533
Table 5. Frequency of regeneration at nodes after hydranth removal, with or
without enlargement of hole in perisarc and severance of coenosarc. {Confirmatory
experiment, matched samples)
Treatment:
hole enlarged,
coenosarc severed
No
hydranths Hydranths
Treatment:
Hole small,
coenosarc entire
Totals
Hydranths
8
6
14
No hydranths
1
0
1
9
6
15
Totals
A second confirmatory experiment was carried out. Hydranths were removed
by cuts through the stolon, close to them, on either side as before and, as
controls, the adjacent hydranths just picked off leaving a small hole and the
coenosarc entire. The two nodes nearest a growing stolon tip with open
hydranths were used reciprocally. Table 5 is the record of the results.
These results are similar to those in the experiment above. First, McNemar's
test for matched samples supports the significance of the reduction in the
frequency of hydranth regeneration in this experimental situation as compared
with the controls C\'2 = 6-1, D.F. = 1, 0-025 > P > 0-01). Secondly, the six
regenerated hydranths were displaced as before, between 0-3 and 0-5 mm from
the node, whereas in the controls hydranths regenerated at the nodes.
The second experiment was carried out to give an opportunity to observe
whether or not hydranth regeneration was accompanied by pulsations in the
adjacent stolon coenosarc. In some cases the pulsations were clear and occurred
at about 4£ min intervals, i.e. at a similar periodicity to a stolon growing tip.
In other cases pulsations were less certainly identified although the increased
flagellar activity seen in the clearer cases (and which also occurs in the growing
region of a stolon) made one suspect that it might be taking place. Pulsations
were also seen to occur at nodes where hydranth regeneration failed to take
place.
The foregoing three experiments were based on the fact that removal of an
open hydranth nearly always resulted in regeneration of a new hydranth. In
the next two experiments (4 and 5) regeneration at a node has been tested in
the presence of another hydranth grafted into the stolon at or near the node.
Experiment 4. Four adjacent nodes (those occupied by the first four open
hydranths) were again used. Four different experimental situations were set up:
534
L. J. HALE
Table 6. Frequency of hydranth regeneration at nodes with
and without grafting (matched samples)
Treatment: hydranth
grafted at node
Hydranth
not
Hydranth
regenerated regenerated
Treatment:
no hydranth
grafted at node
Hydranth
regenerated
Hydranth not
regenerated
Totals
Totals
19
0
19
6
0
6
25
0
25
(i) The hydranth and stalk were removed and a small (additional) hole made
in the side of the stolon at the node.
(ii) The same as (i) except that an open hydranth was grafted into the
original hole of the hydranth; the hole in the side being untouched.
(iii) The same as (i) except that an open hydranth was grafted into the hole
in the side; the hole in the top being untouched.
(iv) Hydranths were grafted into both holes.
Seven groups of four, arranged at random, were successfully treated.
In all situations, grafted hydranths fused with the host stolon, remained
open and feeding and behaved in every way like normal hydranths. In
situation (iv) each node possessed two open hydranths.
In situations (ii) and (iii) no regeneration of hydranths occurred from the
unoccupied holes, although a stolon grew from the hole in the side in one case
in group (ii). In four cases in group (i) hydranths regenerated and in the other
three no regeneration took place.
Informative results might come from (i) and (iii) and a further two series
were carried out using only these two experimental situations on each stolon.
The combined results are recorded in Table 6 which shows that where
a hydranth was grafted to a node no regeneration took place although on
six occasions no regeneration took place in the control either (McNemar's test
confirms that the effect of the grafted hydranth was to significantly reduce
hydranth regeneration; x2 = 17-1, D.F. = 1,P < 0-001).
Experiment 5. The hydranth was removed from each of two adjacent internodes (the first two on a stolon with open hydranths); one of the removed
hydranths was grafted into the side of the parent stolon at various distances
from one node and, as the control, the other hydranth was grafted into the
535
The node of Clytia
Table 7. Frequency of hydranth regeneration at nodes after hydranth
grafting at, or displaced from the node (matched samples)
(a) Data arranged in contingency table
Treatment: hydranth
grafted at node
Hydranth
not
regenerated
Treatment:
Hydranth graft
displaced from
node
Hydranth
regenerated
Hydranth not
regenerated
Totals
Hydranth
regenerated
Totals
9
4
13
7
0
7
16
4
20
(b) Data arranged according to distance of grafted
hydranth from node (matched pairs)
Distance of
grafted
hydranth
from node
(mm)
0-1-0-2
0-3-0-4
0-5-0-6
0-7-0-8
0-9-1-0
Totals
Experiment
Control
A
A
Distance of
grafted
No
hydranth
No
hydranth
Hydranth
hydranth from node Hydranth
(mm)
regenerated regenerated
regenerated regenerated
0
3
4
5
1
5
0
13
7
1
1
0
0
0
0
0
0
Totals
0
0
2
2
0
5
3
3
4
1
4
16
side of the stolon at the other node itself. The results are summarized in
Table 7(a). There are three points of importance; first, out of 20 experiments
there were nine cases where hydranths grew from a node when the graft was
displaced with no regeneration in the controls, in contrast to zero cases where
hydranths grew from the controls with no regeneration in the experiments.
Secondly, grafting a hydranth at a node did not always prevent regeneration
- i t occurred four times in the controls; hydranths also grew from the corresponding nodes with displaced grafts in these cases.
Thirdly, in seven cases no regeneration took place even though the graft
was displaced; there was no regeneration in the controls in these cases. The
536
L. J. HALE
results are also instructive if arranged according to the distance of the grafted
hydranths from the node, as in Table 7(6).
Thus inhibition of regeneration occurred where the grafted hydranth was
within 0-2 mm of the node. There is no evidence of any inhibition when grafts
were made at distances of 0-3 mm or more; experiments and controls then
gave the same results.
DISCUSSION
Hydranth initiation
The experiments have shown that a node is predisposed to regenerate a
hydranth, since removal of an existing hydranth usually results in the growth
of a new one at the same point on the parent stolon within a few hours. The
situation at a node is in contrast with that between nodes, where hydranth
growth does not take place in otherwise similar experimental conditions.
An existing hydranth inhibits the growth of a second hydranth from the
same point on the stolon, but it does not affect a mature hydranth grafted at
the node. When regeneration takes place complete hydranths are formed; they
do not partially grow in these conditions. The inhibition is thus an effect on
initiation.
Whatever the nature of this inhibition of hydranth initiation, it is not the
only factor involved. Firstly, removing a hydranth does not always result in
its replacement; sometimes there is no regeneration. This could be explained
by an insufficient supply of cells for growth, a phenomenon discussed elsewhere
(Hale, 1973 a).
Secondly, grafting a hydranth to the side of a node does not always prevent
regeneration of a hydranth on the top of the node. Thus in some circumstances
the inhibition can be over-ridden and a second hydranth can be initiated and
grow in the presence of an existing one. As this happened in both experimentals
and controls the feature must be a function of the particular experimental
stolon rather than of the nodes alone. The observations gave no indication
as to the nature of this over-riding principle.
The fact that the removal of a hydranth nearly always results in the regeneration of a new hydranth might indicate that there is an embryonic region
at the node. This is supported by the fact that if the coenosarc is severed at
the node, a hydranth still grows after the two cut ends have rejoined. In
addition, staining experiments show that the nodal cells return to the node
during the repair. Whether or not the coenosarc is severed the nodal cells
form a significant part of the tip of the regenerating hydranth.
However, it is doubtful if a permanent embryonic region is present since
the observations on stained nodes also show that where no hydranth regeneration took place the nodal cells dispersed. Also, it might be expected
that regeneration, when it occurred, would commence quickly if a permanent
embryonic region were present; a delay in some hours in fact occurs before
The node of Clytia
537
any new structure becomes visible. The possibility that this period of time is
required to organize a growth zone is discussed below.
Relation between hydranths and stolons
In the conditions of these experiments not only did hydranths grow at nodes
but so also did stolons; the incidence of the two kinds of structure were not
shown to be associated. This was a little surprising since it has been shown
(Hale, 1973 a) that stolon branches tend to originate near to hydranth branches.
The situation is not, however, quite the same; in this work the initiation of
both kinds of branch together has been scored, whereas in the earlier work
the counts made were mostly of the initiation of stolons relative to existing
mature hydranths.
Effect of exposed nodal coenosarc
The particular experimental situation which most affected the type of branch
and the frequency of regeneration from nodes was the exposure of the stolonic
coenosarc by a relatively large hole in the perisarc. The possibility that the
sea water caused this change requires consideration. Other workers have
investigated this, especially on Tubularia. Barth (1944 and earlier papers) found
that the rate of regeneration increased linearly with the logarithm of the oxygen
concentration in the sea water. Goldin (19426) agreed with this finding although
he thought the effectiveness of oxygen in regeneration was limited (Goldin,
1942 a). The effect that these two workers were concerned with was growth
and not the initiation of regeneration. Zwilling (1939) and Rose & Rose (1941)
were concerned with the effect of oxygen concentration on the initiation of
regeneration. They removed perisarc in Tubularia and found regeneration was
promoted in such areas due, they suggested, to a higher oxygen concentration
at the exposed surface. The Roses also suggested that inhibitor escaped into
the water in these areas and thus inhibition was reduced.
Goldin (1942a, b) found also that pH changes (between 6 and 8) affected
the rate of regeneration. Those experiments in which fragments of a hydroid
were placed in glass tubes (Goetsch, 1929; Goldin, 1942a; Miller, 1942) showed
that regeneration only took place at the orifice but this might be due to a
general lowering of oxygen concentration or of pH within the tube.
The interpretation of the exposure experiments of Zwilling and the Roses
apparently depends on the suggestion that the chitinous perisarc of Tubularia
is relatively impermeable to dissolved oxygen or to inhibitor. In Clytia the
perisarc is very thin; in addition, the animal has a high ratio of surface area
to volume as well as an effective hydroplasmic circulation. It seems unlikely
therefore that there could be a significant oxygen or pH gradient across the
perisarc.
The effect of exposing nodal coenosarc to sea water is different from that
described for Tubularia. In Clytia it causes a reduction and not an increase
538
L. J. HALE
in the likelihood of hydranth regeneration at nodes. A further result of such
exposure in Clytia (not reported in Tubular id) is the growth of stolons (Expt. 3)
but since stolons grow from internode regions in about the same numbers as
from nodes (Expt. 2) the effect must be one on the stolonic coenosarc generally
and not a property of the nodes alone.
In Clytia the exposure of the stolon coenosarc results in a small amount of
swelling of the exposed ectoderm, giving an impression of some ionic or osmotic
effect, although this is just supposition. The coenosarc returns slowly to normal;
if no branch grows the hole in the perisarc is sealed; if a hydranth or stolon
grows (or both grow) these secrete their normal chitinous envelope and re-cover
the coenosarc.
The exposure of the coenosarc perhaps causes that part of the stolon to be
more like the growing areas of a stolon. In the latter the chitin is newly secreted
and soft and might have different properties with respect to sea water than the
hardened chitin characteristic of non-growing stolon. There are difficulties in
this suggestion when normal stolon branching is considered; this occurs
through hardened perisarc.
Growth zones
An interesting observation was the detection (in some cases) of a pulsating
region in the stolon at a node after removal of a hydranth. These pulsations took
place at about 4^ min intervals, a periodicity characteristic of the growing stolons
in Clytia (Hale, 1960). In the growing point of a stolon the pulsating region
is also the growth region, and it would be interesting if regeneration was found
to take place only after the formation of a pulsating growth region. The time
interval between the removal of a hydranth and the beginning of its regeneration
might, at least partially, be required to create such a growth area.
The possible importance of contractile growth areas in the growth and
differentiation of isolates of parts of Clytia are reported and discussed elsewhere
(Hale, 19736).
Contractile areas have also been detected in the stolon coenosarc after
cutting it; growth back up the perisarc tube is accompanied by contractions
in the two ends of the coenosarc. This is the normal means of stolon growth.
If a hydranth has been removed from the node and the stolon coenosarc
severed here, the stolon has already developed two contractile regions which
later fuse into one and could form the regenerating hydranth growth region.
Since the hole in the perisarc is small its separate repair poses no problems, as
the hydranth regenerates by growth through it.
In the case of the large hole situation the contractile area (again if confirmed)
is seen to be much more likely to result in stolon growth from the hole in the
perisarc, although hydranths do sometimes regenerate. If, as suggested above,
exposure to sea water is a stolonizing influence the contractile property would
be expected to be more directed to stolon growth and less to hydranth growth.
The node 0/Clytia
539
A possible explanation of the displaced hydranth regeneration after making
a relatively large perisarc hole and dividing the coenosarc follows from the
relatively long period of time required by the regrowing ends of the cut stolon
to bridge and seal the gap exposed to the sea water. With two such stolons
growing to meet each other, both having become 'stolonized' by the influence
of sea water, the two growing regions are slow in fusing. Each might therefore
tend to produce a hydranth branch in the normal manner of a growing stolon,
that is, at a few millimetres from the tip, the latter in this case being at the
node. Competition for materials might be the reason why only one grew
(Spiegelman, 1945).
Verification of the occurrence of contractile areas in regeneration situations
is required however, as adequate techniques for this were not used in this
work. Not the least of the reasons for more detailed inquiries is that pulsating
areas which did not result in any regeneration have been fairly certainly detected.
The possibility that regeneration might require the preformation of a growth
area, and that hydranths prevent the formation of such growth areas within
about 0-3 mm from them, offers at least a partial explanation of the spacing
of secondary hydranths from existing hydranths described elsewhere (Hale,
1973fl). These experiments do not provide any obvious pointers to explain
the longer spacing between primary hydranths.
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(Received 25 June 1973, revised 1 November 1973)