Epiphysial Growth in the Branchial Skeleton
of Fishes.
By
R. Wheeler Haines,
Assistant Anatomist to the University of Cape Town.
With 7 Text-figures.
INTRODUCTION.
THE structure of bone and cartilage, and the relation between
them in the skeleton, have long claimed the attention of anatomists. Eggeling (1911) attempted to correlate the known facts on
a comparative basis, proposing three questions, of which the
third, " Wie stellt sich der stamrnesgeschichtliche Entwicklungsgang der enchondralen Ossifikation dar ?" is concerned with the
relation of endochondral bone to the cartilage which precedes
it. He worked on urodele amphibians, where he could find in
different species a series leading from the condition in P r o t e u s ,
where an unbroken cartilaginous rod runs from end to end of
each bone, to that in some Caducibranchiata where the cartilage
is reduced to two narrow zones at the articular ends. He was
followed by Frobose (1927), and Heidsieck (1928), who extended
his work to include the anuran amphibians and reptiles. Yet
as Eggeling himself realized, the U r o d e l a do not form an
entirely satisfactory starting-point for such a survey, for the
bones which at first sight appear to be in the most primitive
state are clearly derived from more complex structures by a
process of neoteny, and cannot therefore be used safely as
illustrations of a truly primitive condition. In fact Eggeling
quotes Gegenbaur to show that endochondral ossifications occur
in fish, and that here again can be found all the variations in
the degree of resorption of cartilage which he had studied in the
urodeles. So it has become desirable to re-examine the bones of
fish in the light of the newer work on other animals.
The most important work on the subject is that of Stephan
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B. WHEELER HAINES
(1900), who gives a full account of the earlier literature. He
found great variability in the degree and method of ossification
in various fish, and his results will be mentioned from time to
time in the body of this paper. Since, however, he was more
concerned with the structure of bone than with its relation to
cartilage, or with the arrangement of the cartilage itself, it has
been thought best to redescribe the cartilages fully here, even
where the description overlaps with that given by Stephan.
The branchial bones have been chosen for study as they are
comparable in shape to the long bones of tetrapods, can be
orientated accurately for longitudinal section, and are exceptionally easy to interpret as their growth is regular and predominantly longitudinal. Further, the bone and cartilage are
sharply differentiated from one another as in tetrapods, since
there is no trace of the' ossification mixte' described by Stephan.
The branchial bones used are the epibranchial and the ceratobranchial of the first branchial arch.
THE BBANCHIAL BONES OF TELEOSTEI.
A longitudinal section of a branchial bone taken from a
typical teleost, T r i g l a c a p en s i s , the Cape gurnard, stained
in bulk with haematin and eosin, and cut at 20/x, is shown in
Text-fig. 1. In this as in all subsequent diagrams the bone is
shown black, hyaline cartilage white, and connective tissue as
a network of lines representing fibres, and dots representing
nuclei. Patty spaces are left as white areas in the connective
tissue. The specimen was taken from a half-grown fish, so that
the bone was still in a state of active growth, and shows the
parts in their highest state of differentiation. It is in every
way a satisfactory example of the type of structure found in
teleosts, and both its anatomy and its mode of growth will be
described in detail.
At the end of the bone is a large mass of hyaline cartilage,
partly enclosed by the bone of the shaft, and partly projecting
freely beyond it. In this paper the whole mass of cartilage will
be called an epiphysis, following the usage of those comparative
anatomists who have described the somewhat similar structures
of Amphibia, Eggeling (1911) and Frobose (1927), rather than
dep
TEXT-FIG. 1.
Longitudinal section of ventral end of first epibranchial of T r i g la
c a p e n s i s . b.c, break in continuity of osseous cylinder; dep.,
depression in osseous cylinder; e.l., endosteal layer of marrow;
m.b., margin of osseous cylinder; o.cyl., osseous cylinder; pc,
perichondrium; p.L, parachondral layer of marrow; po., periosteum; I,, trabecula of bone; t.p.b., trabecula of periosteal bone;
z.f.c, zone of flattened cells; z.h.c-, zone of hypertrophied cells;
z.p., peripheral zone; z.u.c, zone of undifferentiated cells.
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R. WHEELER HAINES
that of anatomists who have applied the word epiphysis to
only a part of the whole cartilage, such as Eoux (1895) and
Hintzsche (1927) in their work on mammals, and Fell (1925) in
her work on birds. Even in fish the cartilage can, as will be
shown later, be broken up into several zones by a study of the
shape, size, and distribution of the cartilage cells; but these
zones are not sufficiently differentiated from one another to
warrant a major distinction between any part as a true epiphysis and another part as a constituent of the shaft. Further,
the nomenclature here adopted will be found very convenient
in the description of embryos of mammals and birds in early
stages before the cartilages have become fully differentiated
into their various parts.
The whole epiphysis is made of hyaline cartilage which nowhere shows any sign of calcification of the matrix, but differences in the structure of the cells allow various zones to be
distinguished within the mass. Most of the cartilage which
projects beyond the bone of the shaft is characterized by
rounded cells of medium size which lie scattered in the matrix.
This tissue shows the typical structure of hyaline cartilage and
may be called the zone of undifferentiated cells (z.u.c). Immediately under the perichondrium the cells are smaller and
more closely spaced, so that a peripheral zone (z.p.) can be
distinguished. This fades into the surrounding perichondrium
on the one hand and into the zone of undifferentiated cells on
the other without sharp demarcation. There is no typical
articular cartilage with flattened cells at the surface such as is
found in the joints of land animals, for in the joint between
one branchial and the next there is no joint cavity. Movement
is allowed by a layer of loose connective tissue between the two
bones. At the level of the margin of the bone of the shaft
(m.b.) there lies a zone characterized by flattened cells which
all lie with their flattened surfaces in the transverse plane of
the bone, the zone of flattened cells (z.f.c). This forms a lenticular mass lying partly within the end of the shaft and partly
outside it. Towards the shaft the flattened cells round off,
enlarge, and become widely scattered, forming the zone of
hypertrophied cells (z.h.c.) which lies next to the marrow cavity.
EPIPHYSES IN FISHES
81
The names given to these zones of the cartilaginous epiphysis
follow as far as possible those used by Pell (1925) in her description of the histogenesis of cartilage and bone in the chick. They
are strictly descriptive and do not involve any theory of growth
or function. Since they describe the kind of cells rather than
their arrangement or the nature of the matrix in which they are
embedded, they can be applied more widely than those used in
mammalian anatomy, for the structure of the cells is more
constant in each zone than is the structure of the matrix. For
instance all bony animals have a zone of flattened cells in their
epiphyses, though these may be irregularly scattered, or arranged in clumps or columns, and the name 'zone of flattened
cells' is therefore more generally applicable than the name ' zone
of cartilage columns'. Again in the zone of cartilage lying next
to the medullary cavity of the shaft the matrix may be hyaline
or it may be calcined, so that the name ' zone of. hypertrophied
cells' is preferable to the name ' zone of calcified cartilage'.
The shaft, where it surrounds the epiphysis, consists of a
simple tube of bone, the osseous cylinder of Fell (1925) (oxyl.),
which has a sharply denned margin (m.b.) where the bone gives
place to cartilage. The bone is compact and lamellated, but, as
is typical of the bones of higher Teleostei, contains no bone
corpuscles. Below the epiphysis the bone surrounds a medullary
cavity which is crossed by irregular trabeculae of spongy bone
(t.), springing from the osseous cylinder. In places the cylinder
shows irregular depressions (dep.) filled with bone marrow, and
towards the centre of the shaft its continuity is broken (b.c).
Outside the cylinder trabeculae of periosteal bone (t.p.b.) lie
in the neighbouring fatty connective tissue.
The medullary cavity is filled with a loose mass of fatty
marrow. This is slightly condensed on the surface of the bone
to form an endosteal layer (e.l.), and a rather better marked
condensation forms a parachondral layer (p.l.) which lies in
contact with the base of the epiphysis. Surrounding the bone
is a periosteum (po.) Avhich passes directly into the perichondrium {fc.) at the border where bone and cartilage meet.
This bone shows one solution of the biological problem
of providing a mechanism which will allow two bones to
NO.
303
G
82
B. WHEELER HAINES
function as rigid articulating structures and at the same time
to grow.
Functionally the mass of cartilage which projects beyond the
shaft forms a resilient bearing surface capable of slight deformation so as to offer a large area of resistance to a force impinging
upon it, in a manner similar to that in which a rubber tyre
transmits forces from the road surface to the wheels of a car.
Such a massive cartilaginous epiphysis minimizes any sudden
shock to the bony system, and at the same time allows the
transmission of muscular forces from one bone to another. The
cartilage which lies within the end of the osseous cylinder acts
as the means of fixation of the cartilage to the bone, working
like the part of a cork which is inserted into the neck of a bottle.
Other solutions of the problem of fixation, more economical in
material, have been evolved by other animals, but this primitive
mechanism, which is found in the adult only in fish, is seen in
the early stages of development of all bony animals.
A special mechanism to allow bone to grow is necessary because it is one of the structures which have no power of interstitial growth, and so must be surrounded by tissues which lay
down new bone on its surface. In the case of a long bone the
surface of the shaft is clothed by periosteum, a membrane containing osteoblasts which form the new bone, but at the ends of
these bones a more complex mechanism is necessary owing to
the mechanical conditions there prevailing, for a thin periosteum
overlying hard bone would make a bad wearing surface.
Three successive stages in the growth of a branchial bone from
a fish are shown in Text-fig. 2. The nature of each of the kinds
of cell found in cartilage has been worked out fully in the
epiphyses of more specialized animals, and has been confirmed
recently by the detailed histological work of Carey (1922) and
Dodds (1930) on mammals and Fell (1925) on birds. The processes going on in the epiphyses of these animals are easier to
interpret than those of less developed epiphyses, for the parts
not actively growing are heavily calcified or are replaced by
bone, leaving the growing parts clearly differentiated as hyaline
cartilage. It is possible, however, to trace such an epiphysis
back to an embryonic stage in which it is comparable to that
EPIPHYSES IN FISHES
83
of a fish, and so to determine the processes occurring in a cartilage even when it is all hyaline. For though the epiphyses of the
fish are more simple in structure than those of other groups,
the types of individual cartilage cells which compose them are
identical.
The essential region of proliferation providing for the growth
in length of the bone lies in the zone of flattened cells, which
are in active division. The stimulus which makes these cells
divide rather than those of the rest of the epiphysis, the reason
TEXT-FIG. 2.
Diagram showing the growth of a branchial bone from a typical
bony fish, b.c, break in continuity of osseous cylinder; dep.,
depression in osseous cylinder; m.b., margin of osseous cylinder;
o.cyl., osseous cylinder; t., trabecula of bone; t.p.b., trabecula of
periosteal bone; z./.c, zone of flattened cells; z.h.c, zone of
hypertrophied cells; z.u.c, zone of undifferentiated cells.
why they are flattened, and the forces which cause their regular
orientation with their flat surfaces in the transverse plane of the
bone are all unknown. In all bony animals, however, such a
layer of flattened cells is found, and always in the same morphological position. Often the cells are arranged in nests or in
columns, but young embryos usually show an irregular arrangement similar to that of the fish. By the proliferation and subsequent growth of the cartilage in the zone of flattened cells, the
epiphysis grows in length. Since its basal part, or zone of hypertrophied cells, is firmly embedded in the bone of the shaft,
and since this again is a rigid structure, the projecting part of
the epiphysis, including both the zone of undifferentiated cells
and the peripheral zone, is displaced away from the shaft and so
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R. WHEELER HAINBS
the bone as a whole grows in length. At the same time the
flattened cells lying nearest the shaft change character. They
stop dividing and become rounded and large, passing gradually
into the typical hypertrophied cells. In doing so they greatly
increase the amount of matrix around them, and so help to
add to the longitudinal growth of the bone. If these processes
were continued indefinitely without other change, the zone of
hypertrophied cells would become greatly elongated, but in fact
at the same time as the epiphysis is gaining in length by the
division of the flattened cells and by the enlargement of these
cells and their matrix to form the zone of hypertrophied cells,
it is also losing in length by the destructive action of the parachondral layer of the bone marrow. This dissolves away the
matrix, opening up one by one the capsules in which the cartilage
cells lie. This erosion of the basal surface of the zone of hypertrophied cells keeps pace with the new cartilage formation at
its other surface, so that the zone remains constant in length
from age to age, and only projects as far into the shaft as serves
for the fixation of the epiphysis.
It will be seen that if the state of an epiphysis at one time is
compared to its state after an interval in which growth has
occurred, the cells which constitute the zone of undifferentiated
cells are the same in both cases, some of the flattened cells have
become hypertrophied and have been replaced by the division of
others, while the old hypertrophied cells have all been destroyed
by the erosive action of the marrow, and new ones substituted
for them by change of the flattened cells from the area of proliferation.
Growth in width of the epiphysis and growth of the zone of
undifferentiated cells are very much slower processes than the
longitudinal growth already described, and can only occur by
division of the undifferentiated cells themselves or by addition
of new cells from the perichondrium and peripheral zone. The
connective tissue cells of the periehondrium pass gradually into
those of the peripheral zone, which resemble them both in their
smallness and in their close distribution, and have probably been
derived from them by the development of the cartilage matrix
which masks the fibres of the connective tissue. By the further
EPIPHYSES IN FISHES
85
development of matrix and the enlargement of the cells to form
typical cartilage, the zone of undifferentiated cells increases at
the expense of the peripheral zone, which is in turn replenished
from the perichondrium. But the cells of the undifferentiated
zone itself are not too specialized to divide, and studies of the
distribution of mytotic figures in the cartilage of mammals by
Harris and Eussel (1933) show that much of its growth is interstitial.
The new formation of bone follows that of the cartilage. As
the zone of hypertrophied cells grows by the addition of new
cells from the area of proliferation, new bone is laid down at
the margin of the old, always keeping pace with the change in
the cartilage. Thus the osseous cylinder is moulded round the
epiphysis, which determines its shape, and here it is smooth
both on its inner surface, where it is applied to the cartilage,
and on its outer surface, where it is covered by the periosteum
which forms it. Towards the centre of the shaft trabeculae are
laid down both inside (t) and outside (t.p.b.) the cylinder, the
one set by the activity of the marrow and the other by that of
the periosteal tissues. The formation of this new bone is accompanied by the destruction of part of the original osseous cylinder
by the erosive action of the bone marrow, Avhich can in different
places destroy cartilage or bone or build up new bone to take
the place of the old. Thus are formed the depressions (dep.)
seen on the inner surface of the osseous cylinder and the breaks
in continuity (b.c), always accompanied by the formation of
new trabeculae which take over the mechanical functions of the
old cylinder.
To illustrate the mechanism of endochondral bone formation
a section from another teleost, D e n t e x a r g y r o z o n a , the
silver-fish, is shown in Text-fig. 3. The same conventions are
used as before, with the addition of areas of even stippling to
show calcified cartilage (i.e.). A reconstruction of a portion of
this bone, built up in blotting-paper impregnated with wax, is
shown in Text-fig. 4.
Over half of its extent the parachondral layer of the marrow
lies as before on the surface of the zone of hypertrophied cells,
but here the erosion is more irregular. At A, B, and c in Text-
K. WHEELER HAINES
TEXT-FIG. 3.
Longitudinal section of ventral end of first epibranohial of
D e n t e x a r g y r o z o n a . c.p., closing plate of bone; i.e.,
isolated mass of cartilage; m.s., marrow space; t., trabecula
of bone.
fig. 3 isolated masses of marrow tissue are seen in the cartilage.
In the reconstruction B and c are seen to be continuous with the
main marrow cavity behind the tongue of cartilage D and the
BPIPHYSES IN FISHES
87
TEXT-FIG. 4.
Beconstruction of a thick section of the branchial bone from
D e n t e x a r g y r o z o n a shown in Text-fig. 3.
mass of bone E, while A appears as a shallow depression which
would have become continuous with the main cavity had the
reconstruction been carried a few sections farther. Of these
processes of the marrow c is completely surrounded by bone,
00
E. WHEELER HAINES
while B is only partly surrounded by bone and A is surrounded
entirely by cartilage. Over part of its extent the parachondral
layer of the marrow is separated from the cartilage by a layer
of bone, which takes the form of a plate partly closing off the
marrow cavity from the epiphysis (c.p.).
Within the shaft the bone mostly takes the form of transverse
trabeculae (£.) crossing the marrow cavity from side to side,
but other trabeculae form an irregular network enclosing the
marrow spaces (m.s.). Besides the bone there are found in the
shaft isolated masses of calcified cartilage (i.e.), each completely
or nearly completely surrounded by bone. The calcification is
dense, and neither in this nor in the other sections of the series
are there any recognizable remains of cartilage cells.
The mode of growth of enchondral bone may be deduced
from a study of this specimen (Text-figs. 3 and 4), and is
shown in the diagrams of Text-fig. 2.
The closing plate (cp.) shown in Text-fig. 3 lies in contact
with the cartilage of the epiphysis, and has been laid down by
the parachondral layer of the marrow which from time to time
can give up its erosive activity and form bone instead. At A the
cartilage above the plate is being eroded by the marrow, and if
the fish had lived the cartilage would have been completely
removed from the upper surface of the bony plate. The bone
would then have been covered on both its surfaces by marrow,
and would have formed one of the series of transverse trabeculae
seen in this specimen.
The formation of transverse trabeculae from closing plates
occur for the most part in the inner part of the shaft near the
axis of the bone. Nearer the periphery a modified process is
substituted. At B a process of the marrow has made its way into
the cartilage inside the periosteal bone of the osseous cylinder.
At c is a similar process, but here the marrow has laid down a
sheet of bone which separates it from the cartilage. If the fish
had lived this cartilage would have been eroded by the marrow
at A, leaving the bone as part of the network which encloses
the marrow spaces lining the shaft.
The presence of isolated masses (i.e.) of calcified cartilage
within the bone is due to an irregularity in the processes of
EPIPHYSES IN FISHES
09
erosion, whereby a block of cartilage has been cut out from the
main mass of the epiphysis by destruction of the cartilage
around it. It has then been preserved from further erosion by
the formation on its surface of a sheet of bone separating it
from the marrow. It has calcined and lost its cells, so that it
eventually appears as a deeply stained isolated mass sharply
differentiated from the bone which surrounds it. Since the eroding tissue of the marrow which carved out the block grew for
the most part longitudinally into the epiphysis, the block itself
has a longitudinal direction, and since the block in its turn has
determined the arrangement of the bone which is laid down on
its surface, the whole structure has a longitudinal arrangement,
and joins together two transverse trabeculae.
In the branchial bone shown in Text-fig. 3 there is only one
longitudinal structure containing a core of calcified cartilage,
but in the head of an articular bone from T e t r a d o n r e t i c u l a t u s described and figured by Stephan (1900) these structures are numerous and form the main mass of the endochondral
bone. Functionally they are well suited to resist longitudinal
forces and the calcined masses form a series of pegs which attach
the cartilaginous epiphysis firmly to the bone of the shaft, and
transmit forces directly from one to the other. Thus their
formation allows the efficient performance of the two functions
of epiphysial structures, the transmission of forces and the
elongation of the bone. Similar formations have been described
by Heidsieck (1928) in lizards, and according to my own
observations they occur regularly in tortoises and crocodiles,
though in these tetrapods the cartilage cells can still be recognized
in the calcified matrix. Finally, in the mammals the mechanism
has been, refined by the reduction of the cartilage to thin plates
without cells which form the basis of the 'primary trabeculae'
of young bone.
Growth in width of the bone is brought about by two separate
mechanisms. At the ends the increase in width of the cartilaginous epiphysis itself as it grows older leads to an increase in
the diameter of the bony cylinder laid down around it, so that
the whole cylinder becomes wider at its ends than at the middle
of the bone. This would produce a dumb-bell-shaped structure
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R. WHEELER HAINES
if it were not for the second mechanism which comes into play,
the formation of new periosteal bone around the old bony
cylinder.
The ends of two articulating branchial bones from another
teleost, S p a r u s d u r b a n e n s i s , the white stumpnose, are
shown in Text-fig. 3. The same conventions are used in this
diagram as in the figures given before, with the addition of
irregular black areas representing muscle. Bone marrow is not
represented and the space it occupied is left blank. The loose
connective tissue lying between the two epiphyses is typical in
these joints.
Thisfishwas more fully grown than the gurnard and silver-fish
described earlier, and since. the rate of growth was already
slowed down in comparison with that of the younger specimen,
the various parts of the epiphysis are not so well differentiated
from one another. The zone of undifferentiated cells is characterized as before, and the peripheral zone is also recognizable.
But the distinction between the perichondrium and the peripheral zone is sharper, showing that the transformation of
perichondral tissue into cartilage had slowed down or stopped
altogether. In the larger of the two epiphyses shown in the
diagram the position of the zone of flattened cells can be recognized only by the smallness and close spacing of the cell bodies,
for they are not clearly differentiated by their shape. In the
zone of hypertrophied cells the cell bodies are larger than those
in the other zones, but the boundaries are less clear than in the
young gurnard described above. The bone as well as the cartilage shows changes characteristic of maturity and arrested
growth. On the surface of the larger epiphysis is laid down a
sheet of bone, closing off the medullary cavity and separating
the marrow from the cartilage of the epiphysis. The plate is
not quite complete, and at its edges a small part of the cartilage
still lies in contact with the marrow, which has by its destructive
action eroded bays in the cartilage (b.e.). In the smaller epiphysis
there are a few flattened cells in their characteristic position,
and most of the basal surface of the zone of hypertrophied cells
is still exposed to the action of bone marrow. But here again
zones are not well marked off from each other, and growth is slow.
EPIPHYSES IN FISHES
91
The formation of a closing plate of bone shutting off the
cartilage of the zone of hypertrophied cells from the erosive
TEXT-FIG. 5.
Longitudinal section of articulating ends of first epibranchial and
ceratobranchial of S p a m s d u r b a n e n s i s . b.e., bay eroded in
cartilage; 2./.C., zone of flattened cells; z.h.c, zone of hj^pertrophied
cells; z.u.c, zone of undifferentiated cells.
action of the marrow is the most primitive method of stoppage
of growth found in any bone. This stoppage is not necessarily
final, for the bony plate itself can be eroded, the cartilage
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K. WHEELER HAINES
resume its activity and growth go on. In general it may be
said that fish never complete their growth, for throughout life
growth goes on though it becomes slower when the fish is old.
Often it is seasonal, involving alternate periods of activity and
rest, but the structure of the bones in these periods has not been
examined. In many animals there is a more complex mechanism
for stopping growth when adult life is reached, a mechanism
which involves the complete destruction of the proliferating
zone of flattened cells.
A pair of epiphyses from a fourth teleost, S c i a e n a h o l o 1 e p i d o t a, the kabeljau, are shown in Text-fig. 6. The larger
of the two epiphyses is of the type already described, and though
a well-formed closing plate almost completely separates the zone
of hypertrophied cells from the marrow, the epiphysis still shows
clear differentiation of its various zones. The smaller epiphysis
however shows a definite advance in structure. The several zones
can be distinguished as before, but within the zone of undifferentiated cells much of the cartilage has become heavily calcified,
and two irregular areas (c.e.) are seen in the section from which
the figure was drawn. In this calcified cartilage there are very
few cells in comparison with the number in the cartilage which
surrounds it, so that it appears that in the process of calcification
many of the cells have become atrophied and lost in the matrix.
Centres of calcification similar to that found in this, the most
highly developed epiphysis described in a fish, are commonly
found in other groups of animals. Though these centres have
been developed independently in the fish and in the tetrapods, by
their position they confirm the identification of the zone of
undifferentiated cells with the corresponding zone of other
animals, an identification which would otherwise depend only
on the evidence of the shapes of the cells themselves. Further,
the occurrence of this centre shows that even if Parsons' (1905)
definition of an articular epiphysis as a centre of calcification
or ossification occurring in the cartilage at the end of a long bone
were retained, some fish would have to be counted among the
animals which possessed them. On the other hand, if the epiphysis is defined as the whole mass of the cartilage at the end of
a long bone together with any secondary centres of calcification
93
EPIPHYSES IN FISHES
zFc
TEXT-FIG. 6.
Longitudinal section of articulating ends of first epibranchial and
ceratobranchial of Sciaena h o l o l e p i d o t a . c.e., area of calcification of epiphysis; z.f.c, zone of flattened cells; z.h.c, zone of
hypertrophied cells; z.u.c, zone of undifferentiated cells.
or ossification developed in it, all fish may be said to have
epiphyses, though only some have a centre of calcification.
The four fish described above all belong to one order of
the bony fish, the Teleostei. They all show the same general
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R. WHEELER HAINES
structure, though there are slight differences due to their varying
ages and species. The same structure was found in the branchial
bones of other fish of this order, D e n t e x r u p e s t r i s , the
steenbras, and P a g r u s l a n i a r i u s , the panga.
An epiphysis from a museum specimen of L e p i d o s t e u s ,
a fish from an entirely different order, is shown in Text-fig. 7.
The general shape of the epiphysis is similar to those described
before, and the various zones of cartilage can be distinguished
though their boundaries are blurred. The closing plate {cp.) of
bone is quite complete, stretching from one side of the shaft
to the other. Next to the closing plate the cartilage of the zone
of hypertrophied cells is irregularly calcified, forming a plate of
varying thickness (c.z.h.c). Enclosed in it are several islands
of hyaline cartilage (i.h.), a fact confirmed by the study of serial
sections which shows that the islands were completely closed
off by the calcified area. Cartilage cell-bodies are also found in
the area (c.c.b.).
Calcification of the zone of hypertrophied cells is constant in
most animals, but is only occasionally found in fish. Stephan
(1900) describes a similar section from the lower jaw of T e t r a don r e t i c u l a t u s . Apart from this specialization the epiphysis of L e p i d o s t e u s is very similar to that of the other
bony fishes examined.
It has been seen that in all the fish described the structure
of the epiphysis is essentially the same, though these fish belong
to two separate orders not closely related. It is therefore probable that in the common stock which gave rise to these orders
the epiphyses were similar to those of modern bony fish, and
that they have been inherited from this stock with slight
modifications. The essentials would be: a wide zone of undifferentiated cells with a peripheral zone, a proliferative zone of
flattened cells, and a zone of hypertrophied cells derived from
the flattened cells and progressively destroyed during growth
by the young marrow of the shaft. This kind of epiphysis is
characteristic both of the young and of the adult fish, and also
of a few adult animals of other groups. But at some stage in
their development it is found in the long bones of all gnathostome vertebrates with very few exceptions (Elasmobranchs,
EPIPHYSBS IN PISHES
95
TEXT-FIG. 7.
Longitudinal section of a branchial bone from L e p i d o s t e u s . c.c.b.,
cartilage cell-body; c.p., closing plate; c.z.h.c, calcified part of
zone of hypertrophied cells; i.h., island of hyaline cartilage;
z./.c, zone of flattened cells; z.h.c, zone of hypertrophied cells;
z.u.c, zone of undifferentiated cells.
Dipnoi and some Urodeles). Thus considerations both of comparative anatomy and of embryology lead to the belief that
this is a primitive form of epiphysis.
As mentioned in the introduction to this paper, Stephan
(1900) has shown that the fish are more variable in their mode
of ossification than are the tetrapods. Here only those variations
which are found in the branchial bones are discussed, and the
96
R. WHEELER HAINBS
equally interesting forms of 'ossification mixte' described by
Stephan have not been studied. Yet these forms may be more
primitive than those found in the branchial arches, so that it
would be dangerous to make any suggestions as to the evolution
of epiphyses until they have been redescribed in the light of the
more recent advances in the study of bone and cartilage.
My thanks are due to Professor M. R. Drennan and to Professor T. A. Stephenson for the interest they have taken in this
paper and for their encouraging criticism of my work.
SUMMARY.
The branchial bones of several bony fishes were studied in
longitudinal section, and in one case a reconstruction model was
made. A massive cartilaginous epiphysis lies partly enclosed
within the shaft of the bone and partly beyond it. In the cartilage there can be distinguished a zone of undifferentiated cells
lying beyond the bony shaft, a proliferative zone of flattened
cells at the level of the end of the shaft, and a zone of hypertrophied cells no longer undergoing division enclosed within the
shaft. In some cases there is a secondary centre of calcification
in the zone of undifferentiated cells.
The mechanism of growth of the epiphysis and the formation
of endochondral bone are similar to those found in the long bones
of tetrapods, but in their details they are more simple in the fish.
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