J. Embryol. exp. Morph., Vol. 17, 1, pp. 11-25, February 1967
Printed in Great Britain
\ \
The analysis of species specific variations in form
and growth pattern in Hydra
By JULIAN F. HAYNES 1
From the Department of Biology, University of Notre Dame
INTRODUCTION
The importance of internal factors in determining the polarity of hydroids was
first emphasized by Child (1907 a). He stated that organic form was the result of
a 'reaction complex' which was present throughout the organism but was
expressed only in the region where it was the most concentrated. According to
Child, when a segment is cut from a hydroid, the character of the regenerate is
dependent on the amount and nature of this physico-chemical complex. Later
Child (19076) stated more specifically that the 'reaction complex' was an axial
gradient in the physiological activity of the hydroid. Initially his basis for the
existence of physiological gradients was purely theoretical and depended on the
behaviour of regenerating Tubularia. However, Child & Hyman (1919) and
Child (1947) showed an axial gradient in the ability of Hydra to reduce methylene blue and an axial gradient in the susceptibility of Hydra to KCN poisoning.
They attributed this axial gradient to a metabolic or physiological gradient in
Hydra, a gradient in the respiratory activity of the animal.
Weimer (1928, 1932, 1934) interpreted regeneration and reconstitution in
terms of the physiological gradient described by Child & Hyman. He stated that
each hypostome formed by a reaggregate was produced by a small piece of the
original hypostome which had retained its high physiological activity. He
attributed differences in rate of regeneration following cuts at different levels in
terms of the metabolic activity of the animal at the level of the cut. Thus Weimer
interpreted the physiological gradient as the causative agent of polarity.
The demonstration by Browne (1909), Rand (1911), and Rand, Bovard &
Minich (1926) that the regions of high metabolic activity, the hypostome and the
bud, were capable of inducing hypostome and tentacle formation, suggested an
alternate interpretation of the metabolic gradient. This interpretation states
that the metabolic gradient is another polarized character, and itself is not
causing polarity. This was confirmed by Yao (1945 a, b, c), who showed that the
hypostome could induce in the absence of oxygen and with its oxidative enzymes
poisoned.
1
Author's address: Department of Biology, University of Notre Dame, Notre Dame,
Indiana 46556, U.S.A.
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J. F. HAYNES
Burnett (1961, 1962) has further shown that the regions of high metabolic
activity result from a double gradient of form regulating principles and that
these principles are also determining polarity. One of these factors is a growthinducing principle arising from the hypostome, and the second is a growthinhibiting principle produced by the regions of active growth. According to
Burnett (1961) the hypostome induces a region of active growth beneath the
tentacles. This active growing region produces a growth inhibitor which prevents any additional growth in its immediate vicinity. As the two factors pass
down the body column, the growth-inhibiting factor is lost or destroyed more
rapidly. At the budding region there is a deficiency in the growth-inhibiting
substance and still a large amount of the growth-stimulating factor. This excess
of stimulator results in the production of a second region of high growth rate—
the bud. The second growth region then inhibits growth in the peduncle. The
metabolic activity of Hydra rather than causing polarity is being controlled by
the factors which also control polarity.
The growth region was initially described by Tripp (1928) and rediscovered
by Brien & Reniers-Decoen (1949) and Brien (1953). The presence of a subhypostomal growth region has been confirmed by observing the apical-basal
movement of segments of tissue marked with dyes (Burnett, 1959), marked
with intracellular algae (Burnett & Garofalo, 1960), and by the use of a genetic
marker (Brien & Reniers-Decoen, 1956; Haynes & Burnett, 1964). It has also
been confirmed by actual mitotic counts (Shostak, Patel & Burnett, 1965;
Burnett, 1966). However, Campbell (1965) was unable to demonstrate a definite
growth region using H 3 radioautographs. This apparent conflict stems from two
factors. First, Campbell reports total mitotic counts while Brien & ReniersDecoen and Burnett consider only the divisions of the epithelial-muscular and
digestive cells. They exclude the interstitial cells and the secretory cells which are
not attached to the mesogloea and do not contribute to the apical-basal growth
of the animal. Secondly, Campbell's counts may have been elevated by the
incorporation of H 3 thymidine into the numerous intracellular bacteria occupying vacuoles in the digestive cells. These vacuoles with heavily labelled bacteria
could be interpreted in radioautographs as labelled nuclei. It should also be
noted that neither Brien & Reniers-Decoen nor Burnett has ever denied the
occurrence of mitosis in the gastric region. They have simply stated that mitosis
is less frequent in the gastric region than in the subhypostomal growth zone and
that this results in the apical-distal growth pattern.
In a recent publication Burnett (1966) presents a generalized model explaining the growth pattern and the gradient of metabolic activity found in the
hydra in terms of this double gradient of stimulator and inhibitor. However,
when the various species of hydra are studied it is apparent that, although they
all conform to the same general form, no two species are identical in their
pattern of growth. If the stimulator-inhibitor model is valid, it must be possible to
explain the various growth patterns as modifications of this general model.
Specific variations in Hydra
13
Since Haynes & Burnett (1964) have shown that in Hydra viridis a relatively
slight modification of the stimulator-inhibitor balance can significantly change
the form of the animal, a likely modification of the generalized model would
be one which altered the relative amounts of stimulator and inhibitor in a fixed
way for each species. In order to support this hypothesis that each species of
hydra has different form and growth pattern because of a specific modification
in its stimulator-inhibitor ratio, there must be some way of quantifying the
stimulator and inhibitor ratio in each species. The direct measurement of these
substances is impossible at the present time and this makes it necessary to
evaluate their relative amounts by observing their biological effects. Since the
metabolic gradient is a function of the relative amount of stimulator and
inhibitor at any level (Burnett, 1966), it is possible that measuring the gradient
of metabolic activity in each species will provide a measure of the stimulatorinhibitor gradient for each species. Using this information it should be possible
to derive from the generalized model a series of specific models. These models
will differ from each other only in the relative amounts of stimulator and
inhibitor present. If these models are correct they should then explain the form
and growth pattern for each of the species of hydra, and they should also aid
in the interpretation of the histological differentiation which occurs in the body
column of a hydra.
This paper proposes to study the metabolic gradient in four species of hydra
by determining the succinic dehydrogenase activity in various regions and to
attempt to use the enzymic activity as a measure of the stimulator-inhibitor
ratio in each species. It will then interpret the form and growth pattern of each
species as a result of its specific stimulator-inhibitor balance.
MATERIALS AND METHODS
Cultures of Hydra fusca, H. pseudoligactis, H. viridis and H. pirardi were
maintained in a medium prepared by adding CaCl2 to distilled water containing
Versene (Loomis & Lenhoff, 1956). The animals were fed daily on newly hatched
larvae of Anemia salina. Since Hydra viridis possesses an algae in the gastrodermal cells, animals devoid of algae were used to make a positive reaction
show up more clearly. These were obtained by rearing green animals in 0-5 %
glycerine solution in normal medium for 2-4 weeks (Browne, 1909). The
animals void their algae in response to this treatment, and once this process is
completed the animals may be maintained indefinitely without recurrence of the
algal infection.
Succinic dehydrogenase activity was demonstrated by placing live animals,
starved for 24 h, in a substrate containing 1 part 0-2 M sodium succinate, 1 part
0-2 M phosphate buffer of pH 7-6 and two parts of distilled water, plus 5 mg
nitro BT for each 20 cc of medium. This is the medium of Nachlas et al. (1957)
with a reduced amount of tetrazolium. Prior to incubation the animals were
14
J. F. HAYNES
relaxed in 1-0% urethane, and rinsed in a phosphate buffer (0-1 M, pH 7-6)
before being transferred to the substrate.
The animals were incubated in the substrate at temperatures varying from 10°
to 37° C, and the deposition of the blue diformazan was observed with a dissecting microscope at a magnification of x 15-20. The time required for each
region to show activity was recorded. The animals were removed periodically
from the substrate, rinsed in distilled water and fixed in absolute alcohol or
Bouin's fluid. The animals were mounted as whole mounts unstained or lightly
stained with either phloxine or methylene blue. Some animals were embedded in
paraffin and sectioned at 10 fi. The slides were deparaffinized, rinsed in absolute
alcohol and then cleared and mounted with permount.
In addition, the epidermis and gastrodermis of each species were isolated
according to the method of Haynes & Burnett (1963) and the two layers were
incubated in the substrate separately. These were studied in whole mounts and
in sections. Some animals were transected longitudinally and incubated so that
the epidermis and gastrodermis were exposed simultaneously to the medium.
Control animals were incubated in the substrate plus sodium malonate.
RESULTS
Effect of temperature
The optimum temperature for carrying out the incubation was found to be
between 20° and 25° C. Although the reaction would proceed more rapidly at
higher temperatures, the animals showed increased autolysis and cellular
damage. Between 20° and 25° C, although the speed was slightly slower, the
localization of the reduced tetrazolium and the condition of the tissue following
incubation were considerably improved. Therefore temperatures in the 20-25°
range were employed routinely.
Table 1. Time in minutes required for various regions of Hydra
to reduce tetrazolium salt
Hypostome
Growth region
Distal gastric region
Proximal gastric region
Budding region
Peduncle
Basal disc
I. fusca
H. viridis
H. pirardi
2-5
9
12
14
8-10
20-45
6-8
12-13
15
16
25
25
30
6-8
5
5
5
5
5
5
5
H. pseudo
ligactis
5-8
15
18
25
14
40 +
25
Distribution ofsuccinic dehydrogenase in the epidermis
(a) Hydra fusca (Fig. 1). Of the four species studied, H. fusca showed the
greatest dehydrogenase activity. After 10 min incubation in the substrate the
Specific variations in Hydra
15
reduced tetrazolium began to appear in three regions: the tentacle bases and the
hypostome (2-5 min), the basal disc (6-8 min) and the budding region (8-10 min).
The reduction of the tetrazolium then progressed from the hypostome proximally through the gastric region (that is, towards the budding region). The distal
portion of the gastric region became coloured first (12-13 min) and the proximal
portion adjacent to the budding region showed the positive reaction last
(13-14 min).
Fig. 1. Hydra fusca. A, The distribution of succinic dehydrogenase activity. There
are three regions of high activity: the hypostome, the budding region and the basal
disc. B, A hypothetical model showing the relative amounts of stimulator (S) and
inhibitor (/). The growth stimulator is shown on the left and the growth inhibitor on
the right. The growth inhibitor is produced at the two regions of growth: the subhypostomal growth region and the budding zone. The gradient in growth-zone
inhibitor and the gradient budding-zone-inhibitor are inverted in respect to each
other and produce a constant level of inhibitor in the gastric region. This is represented as a rectangle and the components of the rectangle are separated by a dashed
line. C, A graphic representation of the observed SDH activity at the various levels
of the body column. The dotted line represents the points at which the high metabolic activity results from the presence of stimulator unopposed by inhibitor.
The reactions then spread from the budding region and the basal disc into the
peduncle. Since H. fusca has a small peduncle this region is not always evident
in the preparations. In animals with extended peduncles, the peduncle was the
least active region in the animal and required 20-45 min to react.
(b) Hydra viridis (Fig. 2). The reduction of tetrazolium by H. viridis was con-
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J. F. HAYNES
siderably slower than in H. fusca. The reduced tetrazolium appeared first in the
basal disc (10-12 min) and then in the hypostome (12-13 min). After the hypostome reacted, the proximal portions of the tentacles began to reduce the
tetrazolium (15 min). During the period from 12-25 min the reaction spread
down the body column, appearing first in the growth region and then in the
gastric region. The portion of the gastric region adjacent to the budding region
was the last part of the gastric region to react. The peduncle showed a positive
reaction after about 30 min.
Fig. 2. Hydra viridis. A, The distribution of succinic dehydrogenase activity. There
is a simple gradient of enzyme activity from the hypostome to the peduncle. There is
a secondary elevation in the basal disc. The budding region is not distinguishable
enzymically. B, C, as in Fig. 1.
In contrast to H. fusca, the budding region of H. viridis did not possess a high
succinic dehydrogenase activity. It was impossible to distinguish between the
gastric region and the budding region on the basis of a differential ability to
reduce Nitro-BT.
(c) Hydrapseudoligactis (Fig. 3). The reduced tetrazolium initially appeared in
the hypostome and tentacles (5-8 min). The next region to react was the budding
region (14 min) and was followed by the growth and gastric regions (15-25 min).
At 25 min the basal disc showed a slight activity which was not significantly
increased after 40 min incubation. The peduncle was inactive and failed to
reduce the dye after 40 min incubation.
Specific variations in Hydra
17
In 75 % of the H. pseudoligactis observed, the growth and gastric regions
reacted simultaneously, but in the other 25 % there was a distinct gradient with
the growth region, distal gastric region and the proximal gastric region reacting
in that order.
(d) Hydra pirardi (Fig. 4). The least evidence for a gradient in succinic dehydrogenase activity was found in this species. In most animals observed the entire
body reacted within 5 min of the initial reduction of the tetrazolium. The initial
reduction was always in the tentacles and the hypostome. The reaction in the
Fig. 3. Hydra pseudoligactis. A, The distribution of succinic dehydrogenase
activity. There are two regions of high activity: the hypostome and the budding
region. The gastric region shows a low activity and the peduncle is inactive. There is
. a slight activity in the basal disc. B, C, as in Fig. 1.
other regions followed so rapidly that it was impossible to state that one of these
regions showed a greater activity than the others.
Distribution of succinic dehydrogenase in the bud
The activity in the bud was similar in all species observed and will be described
only in Hydra fusca.
In the initial stage of budding (Fig. 5 A), the entire bud shows a high succinic
dehydrogenase activity (approximately equal to that of the parent's hypostome).
As soon as the bud has pushed away from the parent's body column enough to
2
JEEM 17
18
J. F. HAYNES
form a distinct tubular outgrowth, a differential activity becomes apparent. At
this time the heaviest activity is in the base of the bud where the bud attaches
to the parent animal (Fig. 5B). As the growth of the bud progresses to the point
of tentacle production, the region of highest succinic dehydrogenase activity
Fig. 4. Hydra pirardi. A, The distribution of succinic dehydrogenase activity. In
most animals there is no gradient. This indicates one of the occasional animals in
which the hypostome and basal disc showed an elevation of activity. B, C, as in
Fig. 1.
Fig. 5. Budding. A-D, The distribution of succinic dehydrogenase activity in four
successive stages of a developing bud.
begins to switch from the base of the bud to the hypostome and developing
tentacles. Frequently buds are found which are devoid of any demonstrable
succinic dehydrogenase activity. These can be tentatively interpreted as buds
Specific variations in Hydra
19
which have lost the basal activity but have not yet established a region of apical
dominance.
Once the region of activity is established in the distal end of the bud (Fig. 5 C,
D), the base transforms into a typical peduncle. At this stage the bud consists of
a hypostome and tentacles which react at the same rate as the parent's hypostome (5 min), and a body column reacting at the same rate as the distal portion
of the parent's gastric region (11 min). The base of the bud is a reduced peduncle
and reacts at the same rate as the parent's peduncle.
Succinic dehydrogenase activity in sexual animals
A number of Hydra viridis bearing testes and eggs were incubated in the substrate. The activity of the testes was equal to that of the hypostome and they
reduced the tetrazolium within 12-15 min. The presence of the testes did not
alter the behaviour of the animals. In contrast to the testes the eggs showed a
very slight activity and reacted more slowly than the epidermis of adjacent
gastric region.
Cellular localization of the epidermal enzymic activity
Sections from the four species which had reacted throughout their epidermis
all showed the same cellular localization. The activity was confined to the
epitheliomuscular cells and to cnidoblasts. The enzymic activity is frequently
concentrated in the apical cytoplasm of the epitheliomuscular cells, and this
results in the appearance of the discrete blue spots seen in the whole mounts.
The activity of the cnidoblasts was restricted to the region of the cytoplasm
adjacent to the base of the nematocyst capsule.
Distribution of succinic dehydrogenase in the gastrodermis
In intact animals of all species incubated until the epidermis showed a complete reaction, no succinic dehydrogenase activity was demonstrable in the
gastrodermis. However, when the isolated gastrodermis was incubated, or in
the gastrodermis of animals excised to expose the gastrodermal cells to the substrate, the gastrodermis of all species showed a succinic dehydrogenase activity
slightly less than that of the gastric region epidermis. The failure of the gastrodermis of an intact animal to react is the result of the failure of the substrate to
reach the cells of the gastrodermis. The epidermis plus the oral sphincter provides a barrier to the entrance of the surrounding medium into the gut cavity.
This undoubtedly allows the animal to provide a controlled environment for its
gastrodermal cells.
It was impossible to determine with absolute certainty which gastrodermal
cells were giving a positive reaction. From the distribution of the reduced
tetrazolium in the sections and in whole mounts it appears that most of the
activity was confined to the digestive cells. This interpretation is based primarily
20
J. F. HAYNES
on observations made on the gastrodermis of Hydra viridis, since the gastrodermis of this species retained cellular structure following incubation to a
greater extent than did any of the other species.
DISCUSSION
In all the species examined there was a demonstrable axial gradient of succinic
dehydrogenase activity. The most active regions were always the tentacles and
the hypostome and, in three species, the basal disc. In Hydra fusca and H.
pseudoligactis the proximal distal gradient was interrupted by a second region of
high activity in the budding region. In H. viridis and H. pirardi these secondary
regions of high activity were not present. The gradient in succinic dehydrogenase activity is similar to the gradient described by Child & Hyman (1919)
employing methylene blue and KCN.
This distribution of succinic dehydrogenase does not agree with that found
by Lentz & Barnett (1961). They found no gradient in enzymic activity and no
distinction between the cells of the epidermis and the gastrodermis. We were able
to obtain results similar to theirs with animals which were incubated at 37 °C for
prolonged periods. Since all the cells of a hydra possess mitochondria (Haynes,
unpublished observation) they all undoubtedly possess some succinic dehydrogenase. The gradient is only apparent when the reaction is carried out at a
slow rate and observed carefully during the incubation. The point at which the
entire animal shows a positive response corresponds to the point where an
animal completely disintegrates in KCN.
It is impossible to interpret the gradient in succinic dehydrogenase activity
in terms of either a stimulating or inhibiting factor alone. If the gradient in
enzyme activity reflects a gradient in stimulator it is impossible to explain the
high rate of activity in either the bud or the basal disc. In both these places there
should be a lower activity. On the other hand if the enzymic gradient is explained in terms of the inhibitor alone, then it is impossible to explain why there
is a region of high activity at the distal end or in the budding region. As Burnett
(1966) has shown, the only way in which Child's physiological gradient may be
related to the developmental gradient is for the physiological gradient to express
an interaction of the two developmental factors, and he has proposed the model
discussed in the introduction. The question now arises as to whether the basic
model can be used to explain the differences of form and differences in the
succinic dehydrogenase gradient found in different species of hydra. At the
present an absolute answer to this question cannot be given, and its answer will
require precise identification and quantification of the growth-regulating factors.
However, it is possible to use the activity of succinic dehydrogenase as a
measure of the relative amounts of these factors and make the model species
specific.
This can be accomplished by assuming that the metabolic gradient is a
Specific variations in Hydra
21
function of the ratio of the stimulator to the inhibitor. A graphic representation
(Figs. 1C-4C) of the observed enzyme activity will be an approximation of a
gradient in the stimulator/inhibitor (St/I) ratio. From this gradient we can
propose hypothetical levels of the stimulator and the inhibitor (Figs. 1B-4B).
While these levels are not measures of the actual amounts, they should accurately
reflect the relative levels of stimulator and inhibitor for each species. Thus a
species with a negligible gradient, as in H. pirardi (Fig. 2 A), would represent an
animal in which the level of stimulator is very high compared to the inhibitor.
The St/I ratio will remain sufficiently high to maintain a maximum level of
enzymic activity through the entire animal. This results in a low St/I ratio in the
portion of the gastric region adjacent to the budding zone and therefore a
depressed metabolic activity in this region. Considering individual species in
this fashion, a St/I ratio can be hypothesized for each.
In three species there is a definite distal-to-proximal gradient from the
hypostome at least to the budding region. This is a reflexion of the decrease in
the growth-stimulating factor in the presence of a relatively constant degree of
inhibition. There is an abrupt drop in the amount of inhibitor in the budding
region and this results in H.fusca (Fig. 1B) and in H. pseudoligactis (Fig. 3B) in
the region of high activity seen in the budding zone. The low level seen in the
peduncle is a reflexion of the inhibitor from the budding region. The buddingregion inhibitor is the dominant factor in the peduncle and prevails until the
most proximal portion, the basal disc, is reached. Here a small amount of
growth-stimulating factor unopposed by inhibitor induces a region of high
enzymic activity.
The level of stimulator at the peduncle, while sufficient to elevate the metabolic
gradient, is not concentrated enough to induce growth. The cells of this region
are capable of mitosis and growth when supplied with additional stimulator
(Burnett, 1961), or when subjected to severe experimental trauma (Tokin &
Gorbunova, 1934).
There are four exceptions to this scheme. The failure of H. viridis (Fig. 2)
to show a high activity in the budding region; the failure of the basal disc of if.
pseudoligactis (Fig. 3) to react except after prolonged incubation; the reduction
of the size of the peduncle in H. fusca; and the absence of a gradient in H.
pirardi. Each of these exceptions may be explained by altering the levels of the
growth stimulating and growth inhibiting substances.
The failure of the budding region of H. viridis to react at a high level is an
excellent example of the metabolic and developmental gradients as independent
manifestations of the same regulatory system. The developmental system of H.
viridis can function perfectly well without an elevation of the level of metabolism
in the budding region. A possible explanation of the absence of an elevated
metabolism in the budding zone can be found if one looks at the overall form of
H. viridis (Fig. 2A). When compared to a species such as H. pseudoligactis
(Fig. 3 A) it has an indistinct peduncle. The gastric region and the peduncle blend
22
J. F. HAYNES
gradually together. Since the peduncle is produced through the action of the
inhibitor on the cells proximal to the budding region, it is possible that the
reduced peduncle is the result of the reduction in the amount of inhibitor present
in H. viridis. There is enough inhibitor present to retard budding in the gastric
region and in the peduncle but not enough to affect the enzyme levels significantly. Therefore H. viridis is demonstrating enzymically only the gradient in
growth stimulator. From the point of view of form, the body plan of H. viridis
is the result of a high St/I ratio (Fig. 2C).
The situation found in Hydra pirardi is very similar to that described in H.
viridis. However, in H. pirardi the excess of stimulator is even greater than in
H. viridis, and the result is a negligible gradient in the St/I ratio (Fig. 2C). This
prediction of a high level of growth stimulator in these species is supported by
the findings of Lesh & Burnett (1964) that H. viridis and H. pirardi are the best
sources of the growth stimulator.
In contrast, H. pseudoligactis possesses a pronounced peduncle (Fig. 3 A). This
indicates a high level of inhibitor. In fact, it is so high that it persists further
prdximally than the stimulator, and in this species the basal disc is inactive
(Fig. 3B). The body plan is the result of a high inhibitor-stimulator ratio
(Fig.3C).
Hydra fusca represents a condition intermediate between H. viridis and H.
pseudoligactis. In this case neither the stimulator nor the inhibitor is in excess
(Fig. 1B). The stimulator is produced by the hypostome and the inhibitor by
the growth region in amounts such that their initial functional interaction persists for almost the entire length of the animal (Fig. 1C). A result is the extremely
lengthened gastric region found in H. fusca. The failure of a peduncle to develop
basal to the budding region may be explained by assuming that the animal is
unable to exceed a certain size. Although there is no evidence that a physiological
limitation on the size of H. fusca exists, there is definite evidence of a limit to the
length of H. viridis (Haynes & Burnett, 1964). A mutation in H. viridis which
results in the elongation of the animal causes the periodic detachment of portions of the animal proximal to the budding region. This detachment serves
periodically to return the animal to the length of non-mutant H. viridis. In
H. fusca the balance between the stimulator and growth-region inhibitor is such
that a long gastric region results. The limitation on the size of the animal prevents the development of an extensive peduncle. The form of H. fusca is the result
of physiologically balanced amounts of growth stimulator and inhibitor.
The question of why the inhibitor does not act on the budding region or on
the subhypostomal growth region which produce it cannot be adequately
answered. There is conclusive evidence (Rand, 1911; Rand et al. 1926; Burnett,
1961, 1966) that an inhibitor is produced by these regions, and that these are
simultaneously the regions of more active growth (Brien & Reniers-Decoen,
1949; Shostak et al. 1965; Burnett, 1966). This is analogous to the production
of auxin in the apical meristem of a growing plant. Here a growing region pro-
Specific variations in Hydra
23
duces an inhibitor which blocks the establishment of additional growth centres
(Audus, 1959). In both instances a mechanism exists which will retard the
establishment of secondary growth centres without affecting those regions
already in active growth.
Although the present study shows that the enzymic activity corresponds to
the St/I ratio at any level, it does not permit the determination of the nature
of the control mechanisms. It is impossible to ascertain if the control mechanism
is a direct or an indirect one. It is also possible that they act by controlling
the permeability of the membranes of the cells. However, the elucidation of the
mechanisms controlling the enzymic activity are of secondary importance to the
demonstration that there are factors acting on an organismal level responsible
for the polarity and ultimately through a series of undetermined steps for
form. Since one of these steps must involve the differentiation of cells and the
specific distribution of cell types through the body column of the hydra we are
currently analysing histological differences in the body column in terms of the
same St/I ratios which have explained the gross body form.
SUMMARY
1. The ability of four different species of Hydra to oxidize succinate has
been measured. Each species studied shows a gradient in the activity of succinic
dehydrogenase, which appears to be identical to the metabolic gradients
described by Child & Hyman.
2. Each species may be recognized by its unique metabolic gradient.
3. The relationship between the growth-regulating system proposed by
Burnett for Hydra and the metabolic gradient is discussed. The control of the
level of enzyme activity is shown to be by a pair of organismal factors which are
also responsible for the maintenance of polarity in Hydra.
4. By using the level of enzymic activity as a measure of the relative amounts
of these two factors at any level in the animal, it is possible to explain the
differences in form and metabolic gradient of the different species in terms of
modifications in the relative amounts of the growth stimulator and inhibitor.
RESUME
U analyse des variations specifiques de la forme et des
modalites de croissance chez Hydra
1. La capacite d'oxyder le succinate a ete mesuree chez quatre especes
differentes du genre Hydra. Chaque espece etudiee montre un gradient d'activite de la deshydrogenase succinique, qui apparait etre identique aux gradients
metaboliques decrits par Child & Hyman.
2. Chaque espece peut etre reconnue par son gradient metabolique particulier.
3. La relation entre le systeme regulateur de la croissance propose par Burnett
24
J. F. HAYNES
chez Hydra et le gradient metabolique est discute. II est montre que le controle
du niveau de l'activite enzymatique est sous la dependance d'un couple de
facteurs de l'organisme qui est egalement responsable du maintien de la polarite
chez Hydra.
4. En utilisant le niveau de l'activite enzymatique comme mesure des taux
relatifsdeces deux facteurs a chaque niveau de l'animal, il est possible d'expliquer
les differences de forme et de gradient metabolique des differentes especes en
termes de modifications dans les taux relatifs du facteur stimulateur de la
croissance et du facteur inhibiteur.
This work was supported by N.S.F. grant no. GB 3262.
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Hydra fusca. Bull. biol. 82, 293-386.
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{Manuscript received 29 March 1966, revised 1 August 1966)