THE PECTORAL AND PELVIC GIRDLES AND THE SKELETON OF THE LIMBS.
_The Pectoral girdle._
Pisces. Amongst Fishes the pectoral girdle presents itself in its
simplest form in Elasmobranchii, where it consists of a bent band of
cartilage on each side of the body, of somewhat variable form, meeting
and generally uniting with its fellow ventrally. Its anterior border
is in close proximity with the last visceral arch, and a transverse
ridge on its outer and posterior border, forming the articular surface
for the skeleton of the limb, divides it into a dorsal part, which may
be called the scapula, and a ventral part which may be called the
coracoid.
In all the remaining groups of Fishes there is added to the
cartilaginous band, which may wholly or partially ossify, an osseous
support composed of a series of membrane bones.
In the types with such membrane bones the cartilaginous parts do not
continue to meet ventrally, except in the Dipnoi where there is a
ventral piece of cartilage, distinct from that bearing the
articulation of the limb. The cartilage is moreover produced into two
ventral processes, an anterior and a posterior, below the articulation
of the limb; which may be called, in accordance with Gegenbaur's
nomenclature, the præcoracoid and coracoid. Of these the præcoracoid
is far the most prominent, and in the majority of cases the coracoid
can hardly be recognised. The coracoid process is however well
developed in the Selachioid Ganoids, and the Siluroid Teleostei. In
Teleostei the scapular region often ossifies in two parts, the smaller
of which is named by Parker præcoracoid, though it is quite distinct
from Gegenbaur's præcoracoid. The membrane bones, as they present
themselves in their most primitive state in Acipenser and the
Siluroids, are dermal scutes embracing the anterior edge of the
cartilaginous girdle. In Acipenser there are three scutes on each
side. A dorsal scute known as the supra-clavicle, connected above with
the skull by the post-temporal; a middle piece or clavicle, and a
ventral or infra-clavicle (interclavicle), which meets its fellow
below.
In most Fishes the primitive dermal scutes have become subdermal
membrane bones, and the infra-clavicle is usually not distinct, but
the two clavicles form the most important part of the membranous
elements of the girdle. Additional membrane bones (post-clavicles) are
often present behind the main row.
The development of these parts in Fishes has been but little studied.
In Scyllium, amongst the Elasmobranchii, I find that each half of the
pectoral girdle develops as a vertical bar of cartilage at the front
border of the rudimentary fin, _and externally to the muscle plates_.
Before the tissue forming the pectoral girdle has acquired the
character of true cartilage, the bars of the two sides meet ventrally
by a differentiation _in situ_ of the mesoblastic cells, so that, when
the girdle is converted into cartilage, it forms an undivided arc,
girthing the ventral side of the body. There is developed in
continuity with the posterior border of this arc on the level of the
fin a horizontal bar of cartilage, which is continued backwards along
the insertion of the fin, and, as will be shewn in the sequel, becomes
the metapterygium of the adult (figs. 344, _bp_ and 348, _mp_). With
this bar the remaining skeletal elements of the fin are also
continuous.
The foramina of the pectoral girdle are not in the first instance
formed by absorption, but by the non-development of the cartilage in
the region of pre-existing nerves and vessels.
The development of these parts in Teleostei has been recently
investigated by 'Swirski (No. 472) who finds in the Pike (Esox) that
the cartilaginous pectoral girdle is at first continuous with the
skeleton of the fin. It forms a rod with a dorsal scapular and ventral
coracoid process. An independent mass of cartilage gives rise to a
præcoracoid, which unites with the main mass, forming a triradiate bar
like that of Acipenser or the Siluroids. The coracoid process becomes
in the course of development gradually reduced.
'Swirski concludes that the so-called præcoracoid bar is to some
extent a secondary element, and that the coracoid bar corresponds to
the whole of the ventral part of the girdle of Elasmobranchii, but his
investigations do not appear to me to be as complete as is desirable.
Amphibia and Amniota. The pectoral girdle contains a more or less
constant series of elements throughout the Amphibia and Amniota; and
the differences in structure between the shoulder girdle of these
groups and that of Fishes are so great that it is only possible to
make certain general statements respecting the homologies of the parts
in the two sets of types.
The generally accepted view, founded on the researches of Parker,
Huxley, and Gegenbaur, is to the effect that there is a primitively
cartilaginous coraco-scapular plate, homologous with that in Fishes,
and that the membrane bones in Fishes are represented by the clavicle
and interclavicle in the Sauropsida and Mammalia, which are however
usually admitted to be absent in Amphibia. These views have recently
been challenged by Götte (No. 466) and Hoffmann (No. 467), on the
ground of a series of careful embryological observations; and until
the whole subject has been worked over by other observers it does not
seem possible to decide satisfactorily between the conflicting views.
It is on all hands admitted that the scapulo-coracoid elements of the
shoulder girdle are formed as a pair of cartilaginous plates, one on
each side of the body. The dorsal half of each plate becomes the
scapula, which may subsequently become divided into a supra-scapula
and scapula proper; while the ventral half forms the coracoid, which
is not always separated from the scapula, and is usually divided into
a coracoid proper, a præcoracoid, and an epicoracoid. By the
conversion of parts of the primitive cartilaginous plates into
membranous tissue various fenestræ may be formed in the cartilage, and
the bars bounding these fenestræ both in the scapula and coracoid
regions have received special names; the anterior bar of the coracoid
region, forming the præcoracoid, being especially important. At the
boundary between the scapula and the coracoid, on the hinder border of
the plate, is placed the glenoid articular cavity to carry the head of
the humerus.
The grounds of difference between Götte and Hoffmann and other
anatomists concern especially the clavicle and interclavicle. The
clavicle is usually regarded as a membrane bone which may become to
some extent cartilaginous. By the above anatomists, and by Rathke
also, it is held to be at first united with the coraco-scapular plate,
of which it forms the anterior limb, free ventrally, but united
dorsally with the main part of the plate; and Götte and Hoffmann hold
that it is essentially _a cartilage bone_, which however in the
majority of the Reptilia ossifies directly without passing through the
condition of cartilage.
The interclavicle (episternum) is held by Götte to be developed from a
paired formation at the free ventral ends of the clavicles, but he
holds views which are in many respects original as to its homologies
in Mammalia and Amphibia. Even if Götte's facts are admitted, it does
not appear to me necessarily to follow that his deductions are
correct. The most important of these is to the effect that the dermal
clavicle of Pisces has no homologue in the higher types. Granting that
the clavicle in these groups is in its first stage continuous with the
coraco-scapular plate, and that it may become in some forms
cartilaginous before ossifying, yet it seems to me all the same quite
possible that it is genetically derived from the clavicle of Pisces,
but that it has to a great extent lost even in development its
primitive characters, though these characters are still partially
indicated in the fact that it _usually ossifies very early and
partially at least as a membrane bone_[210].
[210] The fact of the clavicle going out of its way, so to speak,
to become cartilaginous before being ossified, may perhaps be
explained by supposing that its close connection with the other
parts of the shoulder girdle has caused, by a kind of infection,
a change in its histological characters.
In treating the development of the pectoral girdle systematically it
will be convenient to begin with the Amniota, which may be considered
to fix the nomenclature of the elements of the shoulder girdle.
Lacertilia. The shoulder girdle is formed as two membranous plates,
from the dorsal part of the anterior border of each of which a bar
projects (Rathke, Götte), which is free at its ventral end. This bar,
which is usually (Gegenbaur, Parker) held to be independent of the
remaining part of the shoulder girdle, gives rise to the clavicle and
interclavicle. The scapulo-coracoid plate soon becomes cartilaginous,
while at the same time the clavicular bar ossifies directly from the
membranous state. The ventral ends of the two clavicular bars enlarge
to form two longitudinally placed plates, which unite together and
ossify as the interclavicle.
Parker gives a very different account of the interclavicle in Anguis.
He states that it is formed of two pairs of bones 'strapped on to the
antero-inferior part of the præsternum,' which subsequently unite into
one.
Chelonia. The shoulder girdle of the Chelonia is formed (Rathke) of a
triradiate cartilage on each side, with one dorsal and two ventral
limbs. It is admitted on all hands that the dorsal limb is the
scapular element, and the posterior ventral limb the coracoid; but,
while the anterior ventral limb is usually held to be the præcoracoid,
Götte and Hoffmann maintain that, in spite of its being formed of
cartilage, it is homologous with the anterior bar of the primitive
shoulder-plates of Lacertilia, and therefore the homologue of the
clavicle.
Parker and Huxley (doubtfully) hold that the three anterior elements
of the ventral plastron (entoplastron and epiplastra) are homologous
with the interclavicle and clavicles, but considering that these
plates appear to belong to a secondary system of dermal ossifications
peculiar to the Chelonia, this homology does not appear to me probable.
Aves. There are very great differences of view as to the development
of the pectoral arch of Aves.
About the presence in typical forms of the coraco-scapular plate and
two independent clavicular bars all authors are agreed. With reference
to the clavicle and interclavicle Parker (No. 468) finds that the
scapular end of the clavicle attaches itself to and ossifies a mass of
cartilage, which he regards as the mesoscapula, while the
interclavicle is formed of a mass of tissue between the ends of the
clavicles where they meet ventrally, which becomes the dilated plate
at their junction.
Gegenbaur holds that the two primitive clavicular bars are simply
clavicles, without any element of the scapula; and states that the
clavicles are not entirely ossified from membrane, but that a delicate
band of cartilage precedes the osseous bars. He finds no
interclavicle.
Götte and Rathke both state that the clavicle is at first continuous
with the coraco-scapular plate, but becomes early separated, and
ossifies entirely as a membrane bone. Götte further states that the
interclavicles are formed as outgrowths of the median ends of the
clavicles, which extend themselves at an early period of development
along the inner edges of the two halves of the sternum. They soon
separate from the clavicles, which subsequently meet to form the
furculum; while the interclavicular rudiments give rise, on the
junction of the two halves of the sternum, _to its keel_, and to the
ligament connecting the furculum with the sternum. The observations of
Götte, which tend to shew the keel of the sternum is really an
interclavicle, appear to me of great importance.
A præcoracoid, partially separated from the coracoid by a space, is
present in Struthio. It is formed by a fenestration of a primitively
continuous cartilaginous coracoid plate (Hoffmann). In Dromæus and
Casuarius clavicles are present (fused with the scapula in the adult
Dromæus), though absent in other Ratitæ (Parker, etc.).
Mammalia. The coracoid element of the coraco-scapular plate is much
reduced in Mammalia, forming at most a simple process (except in the
Ornithodelphia) which ossifies however separately[211].
[211] This process, known as the coracoid process, is held by
Sabatier to be the præcoracoid; while this author also holds that
the upper third of the glenoid cavity, which ossifies by a
special nucleus, is the true coracoid. The absence of a
præcoracoid in the Ornithodelphia is to my mind a serious
difficulty in the way of Sabatier's view.
With reference to the clavicles the same divergencies of opinion met
with in other types are found here also.
The clavicle is stated by Rathke to be at first continuous with the
coraco-scapular plate. It is however soon separated, and ossifies very
early, in the human embryo before any other bone. Gegenbaur however
shewed that the human clavicle is provided with a central axis of
cartilage, and this observation has been confirmed by Kölliker, and
extended to other Mammalia by Götte. The mode of ossification is
nevertheless in many respects intermediate between that of a true
cartilage bone and a membrane bone. The ends of the clavicles remain
for some time, or even permanently, cartilaginous, and have been
interpreted by Parker, it appears to me on hardly sufficient grounds,
as parts of the mesoscapula and præcoracoid. Parker's so-called
mesoscapula may ossify separately. The homologies of the episternum
are much disputed. Götte, who has worked out the development of the
parts more fully than any other anatomist, finds that paired
interclavicular elements grow out backwards from the ventral ends of
the clavicles, and uniting together form a somewhat T-shaped
interclavicle overlying the front end of the sternum. This condition
is permanent in the Ornithodelphia, except that the anterior part of
the sternum undergoes atrophy. But in the higher forms the
interclavicle becomes almost at once divided into three parts, of
which the two lateral remain distinct, while the median element fuses
with the subjacent part of the sternum and constitutes with it the
presternum (manubrium sterni). If Götte's facts are to be trusted, and
they have been to a large extent confirmed by Hoffmann, his homologies
appear to be satisfactorily established. As mentioned on p. 563 Ruge
(No. 438) holds that Götte is mistaken as to the origin of the
presternum.
Gegenbaur admits the lateral elements as parts of the interclavicle,
while Parker holds that they are not parts of an interclavicle but are
homologous with the omosternum of the Frog, which is however held by
Götte to be a true interclavicle.
Amphibia. In Amphibia the two halves of the shoulder girdle are each
formed as a continuous plate, the ventral or coracoid part of which is
forked, and is composed of a larger posterior and a smaller anterior
bar-like process, united dorsally. In the Urodela the two remain
permanently free at their ventral ends, but in the Anura they become
united, and the space between them then forms a fenestra. The anterior
process is usually (Gegenbaur, Parker) regarded as the præcoracoid,
but Götte has pointed out that in its mode of development it strongly
resembles the clavicle of the higher forms, and behaves quite
differently to the so-called præcoracoid of Lizards. It is however to
be noticed that it differs from the clavicle in the fact that it is
never segmented off from the coraco-scapular plate, a condition which
has its only parallel in the equally doubtful case of the Chelonia.
Parker holds that there is no clavicle present in the Amphibia, while
Gegenbaur maintains that an ossification which appears in many of the
Anura (though not in the Urodela) in the perichondrium on the anterior
border of the cartilaginous bar above mentioned is the representative
of the clavicle. Götte's observations on the ossification of this bone
throw doubt upon this view of Gegenbaur; while the fact that the
cartilaginous bar may be completely enclosed by the bone in question
renders Gegenbaur's view, that there is present both a clavicle and
præcoracoid, highly improbable.
No interclavicle is present in Urodela, but in this group and in a
number of the Anura, a process grows out from the end of each of the
bars (præcoracoids) which Götte holds to be the clavicles. The two
processes unite in the median line, and give rise in front to the
anterior unpaired element of the shoulder girdle (omosternum of
Parker). They sometimes overlap the epicoracoids behind, and fusing
with them bind them together in the median line. Parker who has
described the paired origin of the so-called omosternum, holds that it
is not homologous with the interclavicle, but compares it with his
omosternum in Mammals.
BIBLIOGRAPHY.
(463) Bruch. "Ueber die Entwicklung der Clavicula und die Farbe des
Blutes." _Zeit. f. wiss. Zool._, iv. 1853.
(464) A. Dugès. "Recherches sur l'ostéologie et la myologie des
Batraciens à leurs différens âges." _Mémoires des savants étrang.
Académie royale des sciences de l'institut de France_, Vol. VI. 1835.
(465) C. Gegenbaur. _Untersuchungen zur vergleichenden Anatomie der
Wirbelthiere_, 2 Heft. _Schultergürtel der Wirbelthiere. Brustflosse
der Fische._ Leipzig, 1865.
(466) A. Götte. "Beiträge z. vergleich. Morphol. d. Skeletsystems d.
Wirbelthiere, Brustbein u. Schultergürtel." _Archiv f. mikr. Anat._
Vol. XIV. 1877.
(467) C. K. Hoffmann. "Beiträge z. vergleichenden Anatomie d.
Wirbelthiere." _Niederländisches Archiv f. Zool._, Vol. V. 1879.
(468) W. K. Parker. "A Monograph on the Structure and Development of
the Shoulder-girdle and Sternum in the Vertebrata." _Ray Society_,
1868.
(469) H. Rathke. _Ueber die Entwicklung der Schildkröten._
Braunschweig, 1848.
(470) H. Rathke. _Ueber den Bau und die Entwicklung des Brustbeins der
Saurier_, 1853.
(471) A. Sabatier. _Comparaison des ceintures et des membres
antérieurs et postérieurs d. la Série d. Vertébrés._ Montpellier,
1880.
(472) Georg 'Swirski. _Untersuch. üb. d. Entwick. d. Schultergürtels
u. d. Skelets d. Brustflosse d. Hechts._ Inaug. Diss. Dorpat, 1880.
_Pelvic girdle._
Pisces. The pelvic girdle of Fishes is formed of a cartilaginous band,
to the outer and posterior side of which the basal element of the
pelvic fin is usually articulated. This articulation divides it into a
dorsal iliac, and ventral pubic section. The iliac section never
articulates with the vertebral column.
In Elasmobranchii the two girdles unite ventrally, but the iliac
section is only slightly developed. In Chimæra there is a well
developed iliac process, but the pubic parts of the girdle are only
united by connective tissue.
In the cartilaginous Ganoids the pelvic girdle is hardly to be
separated from the skeleton of the fin. It is not united with its
fellow, and is represented by a plate with slightly developed pubic
and iliac processes.
In the Dipnoi there is a simple median cartilage, articulated with the
limb, but not provided with an iliac process. In bony Ganoids and
Teleostei there is on each side a bone meeting its fellow in the
ventral line, which is usually held to be the rudiment of the pelvic
girdle; while Davidoff attempts to shew that it is the basal element
of the fin, and that, except in Polypterus, a true pelvic girdle is
absent in these types.
From my own observations I find that the mode of development of the
pelvic girdle in Scyllium is very similar to that of the pectoral
girdle. There is a bar on each side, continuous on its posterior
border with the basal element of the fin (figs. 345 and 347). This bar
meets and unites with its fellow ventrally before becoming converted
into true cartilage, and though the iliac process (_il_) is never very
considerable, yet it is better developed in the embryo than in the
adult, and is at first directed nearly horizontally forwards.
Amphibia and Amniota. The primitive cartilaginous pelvic girdle of the
higher types exhibits the same division as that of Pisces into a
dorsal and a ventral section, which meet to form the articular cavity
for the femur, known as the acetabulum. The dorsal section is always
single, and is attached by means of rudimentary ribs to the sacral
region of the vertebral column, and sometimes to vertebræ of the
adjoining lumbar or caudal regions. It always ossifies as the ilium.
The ventral section is usually formed of two more or less separated
parts, an anterior which ossifies as the pubis, and a posterior which
ossifies as the ischium. The space between them is known as the
obturator foramen. In the Amphibia the two parts are not separated,
and resemble in this respect the pelvic girdle of Fishes. They
generally meet the corresponding elements of the opposite side
ventrally, and form a symphysis with them. The symphysis pubis, and
symphysis ischii may be continuous (Mammalia, Amphibia).
The observations on the development of the pelvic girdle in the
Amphibia and Amniota are nearly as scanty as on those of Fishes.
Amphibia. In the Amphibia (Bunge, No. 473) the two halves of the
pelvic girdle are formed as independent masses of cartilage, which
subsequently unite in the ventral line.
In the Urodelous Amphibia (Triton) each mass is a simple plate of
cartilage divided into a dorsal and ventral section by the acetabulum.
The ventral parts, which are not divided into two regions, unite in a
symphysis comparatively late.
The dorsal section ossifies as the ilium. The ventral usually contains
a single ossification in its posterior part which forms the ischium;
while the anterior part, which may be considered as representing the
pubis, usually remains cartilaginous; though Huxley (No. 475) states
that it has a separate centre of ossification in Salamander, which
however does not appear to be always present (Bunge). There is a small
obturator foramen between the ischium and pubis, which gives passage
to the obturator nerve. It is formed by the part of the tissue where
the nerve is placed not becoming converted into cartilage.
There is a peculiar cartilage in the ventral median line in front of
the pubis, which is developed independently of and much later than the
true parts of the pelvic girdle. It may be called the præpubic
cartilage.
Reptilia. In Lacertilia the pelvic girdle is formed as a somewhat
triradiate mass of cartilage on each side, with a dorsal (iliac)
process, and two ventral (pubic and ischiad) processes. The acetabulum
is placed on the outer side at the junction of the three processes,
each of which may be considered to have a share in forming it. The
distal ends of the pubis and ischium are close together when first
formed, but subsequently separate. Each of them unites at a late stage
with the corresponding process of the opposite side in a ventral
symphysis. A centre of ossification appears in each of the three
processes of the primitive cartilage.
Aves. In Birds the parts of the pelvic girdle no longer develop as a
continuous cartilage (Bunge). Either the pubis may be distinct, or, as
in the Duck, all the elements. The ilium early exhibits a short
anterior process, but the pubis and ischium are at first placed with
their long axes at right angles to that of the ilium, but gradually
become rotated so as to lie parallel with it, their distal ends
pointing backwards, and not uniting ventrally excepting in one or two
Struthious forms.
Mammalia. In Mammalia the pelvic girdle is formed in cartilage as in
the lower forms, but in Man at any rate the pubic part of the
cartilage is formed independently of the remainder (Rosenberg). There
are the usual three centres of ossification, which unite eventually
into a single bone--the innominate bone. The pubis and ischium of each
side unite with each other ventrally, so as completely to enclose the
obturator foramen.
Huxley holds that the so-called marsupial bones of Monotremes and
Marsupials, which as shewn by Gegenbaur (No. 474) are performed in
cartilage, are homologous with the præpubis of the Urodela; but
considering the great gap between the Urodela and Mammalia this
homology can only be regarded as tentative. He further holds that the
anterior prolongations of the cartilaginous ventral ends of the pubis
of Crocodilia are also structures of the same nature.
BIBLIOGRAPHY.
(473) A. Bunge. _Untersuch. z. Entwick. d. Beckengürtels d. Amphibien,
Reptilien u. Vögel. Inaug. Diss._ Dorpat, 1880.
(474) C. Gegenbaur. "Ueber d. Ausschluss des Schambeins von d. Pfanne
d. Hüftgelenkes." _Morph. Jahrbuch_, Vol. II. 1876.
(475) Th. H. Huxley. "The characters of the Pelvis in Mammalia, etc."
_Proc. of Roy. Soc._, Vol. XXVIII. 1879.
(476) A. Sabatier. _Comparaison des ceintures et des membres
antérieurs et postérieurs dans la Série d. Vertébrés._ Montpellier,
1880.
_Comparison of Pectoral and Pelvic girdles._
Throughout the Vertebrata a more or less complete serial homology may
be observed between the pectoral and pelvic girdles.
In the cartilaginous Fishes each girdle consists of a continuous band,
a dorsal and ventral part being indicated by the articulation of the
fin; the former being relatively undeveloped in the pelvic girdle,
while in the pectoral it may articulate with the vertebral column. In
the case of the pectoral girdle secondary membrane bones become added
to the primitive cartilage in most Fishes, which are not developed in
the case of the pelvic girdle.
In the Amphibia and Amniota the ventral section of each girdle becomes
divided into an anterior and a posterior part, the former constituting
the præcoracoid and pubis, and the latter the coracoid and ischium;
these parts are however very imperfectly differentiated in the pelvic
girdle of the Urodela. The ventral portions of the pelvic girdle
usually unite below in a symphysis. They also meet each other
ventrally in the case of the pectoral girdle in Amphibia, but in most
other types are separated by the sternum, which has no homologue in
the pelvic region, unless the præpubic cartilage is to be regarded as
such. The dorsal or scapular section of the pectoral girdle remains
free; but that of the pelvic girdle acquires a firm articulation with
the vertebral column.
If the clavicle of the higher types is derived from the membrane bones
of the pectoral girdle of Fishes, it has no homologue in the pelvic
girdle; but if, as Götte and Hoffmann suppose, it is a part of the
primitive cartilaginous girdle, the ordinary view as to the serial
homologies of the ventral sections of the two girdles in the higher
types will need to be reconsidered.
_Limbs._
It will be convenient to describe in this place not only the
development of the skeleton of the limbs but also that of the limbs
themselves. The limbs of Fishes are moreover so different from those
of the Amphibia and Amniota that the development of the two types of
limb may advantageously be treated separately.
In Fishes the first rudiments of the limbs appear as slight
longitudinal ridge-like thickenings of the epiblast, which closely
resemble the first rudiments of the unpaired fins.
These ridges are two in number on each side, an anterior immediately
behind the last visceral fold, and a posterior on the level of the
cloaca. In most Fishes they are in no way connected, but in some
Elasmobranch embryos, more especially in Torpedo, they are connected
together at their first development by a line of columnar epiblast
cells[212]. This connecting line of columnar epiblast is a very
transitory structure, and after its disappearance the rudimentary fins
become more prominent, consisting (fig. 343, _b_) of a projecting
ridge both of epiblast and mesoblast, at the outer edge of which is a
fold of epiblast only, which soon reaches considerable dimensions. At
a later stage the mesoblast penetrates into this fold and the fin
becomes a simple ridge of mesoblast, covered by epiblast. The pectoral
fins are usually considerably ahead of the pelvic fins in development.
[212] F. M. Balfour. _Monograph on Elasmobranch Fishes_, pp.
101-2.
For the remaining history it is necessary to confine ourselves to
Scyllium as the only type which has been adequately studied.
The direction of the original ridge which connects the two fins of
each side is nearly though not quite longitudinal, sloping somewhat
obliquely downwards. It thus comes about that the attachment of each
pair of limbs is somewhat on a slant, and that the pelvic pair nearly
meet each other in the median ventral line a little way behind the
anus.
[FIG. 343. SECTION THROUGH THE VENTRAL PART OF THE TRUNK OF A YOUNG
EMBRYO OF SCYLLIUM AT THE LEVEL OF THE UMBILICAL CORD.
_b._ pectoral fin; _ao._ dorsal aorta; _cav._ cardinal vein; _ua._
vitelline artery; _u.v._ vitelline vein; _al._ duodenum; _l._ liver;
_sd._ opening of segmented duct into the body cavity; _mp._ muscle
plate; _um._ umbilical canal.]
The elongated ridge, forming the rudiment of each fin, gradually
projects more and more, and so becomes broader in proportion to its
length, but at the same time its actual attachment to the side of the
body becomes shortened from behind forwards, so that what was
originally the _attached border_ becomes in part converted into the
_posterior border_. This process is much more completely carried out
in the case of the pectoral fins than in that of the pelvic, and the
changes of form undergone by the pectoral fin in its development may
be gathered from figs. 344 and 348.
Before proceeding to the development of the skeleton of the fin it may
be pointed out that the connection of the two rudimentary fins by a
continuous epithelial line suggests the hypothesis that they are the
remnants of two continuous lateral fins[213].
[213] Both Maclise and Humphry (_Journal of Anat. and Phys._,
Vol. V.) had previously suggested that the paired fins were
related to the unpaired fins.
Shortly after the view that the paired fins were remnants of
continuous lateral fins had been put forward in my memoir on
Elasmobranch Fishes, two very interesting papers were published by
Thacker (No. 489) and Mivart (No. 484) advocating this view on the
entirely independent grounds of the adult structure of the skeleton of
the paired fins in comparison with that of the unpaired fins[214].
[214] Davidoff in a Memoir (No. 477) which forms an important
contribution to our knowledge of the structure of the pelvic fins
has attempted from his observations to deduce certain arguments
against the lateral fin theory of the limbs. His main argument is
based on the fact that a variable but often considerable number
of the spinal nerves in front of the pelvic fin are united, by a
longitudinal commissure, with the true plexus of the nerves
supplying the fin. From this he concludes that the pelvic fin has
shifted its position, and that it may once therefore have been
situated close behind the visceral arches. If this is the
strongest argument which can be brought against the theory
advocated in the text, there is I trust a considerable chance of
its being generally accepted. For even granting that Davidoff's
deduction from the character of the pelvic plexus is correct,
there is, so far as I see, no reason in the nature of the lateral
fin theory why the pelvic fins should not have shifted, and on
the other hand the longitudinal cord connecting some of the
spinal nerves in front of the pelvic fin may have another
explanation. It might for instance be a remnant of the time when
the pelvic fin had a more elongated form than at present, and
accordingly extended further forwards.
In any case our knowledge of the nature and origin of nervous
plexuses is far too imperfect to found upon their character such
conclusions as those of Davidoff.
The development of the skeleton has unfortunately not been as yet very
fully studied. I have however made some investigations on this subject
on Scyllium, and 'Swirski has also made some on the Pike.
In Scyllium the development of both the pectoral and pelvic fins is
very similar.
In both fins the skeleton in its earliest stage consists of a bar
springing from the posterior side of the pectoral or pelvic girdle,
and running backwards parallel to the long axis of the body. The outer
side of this bar is continued into a plate which extends into the fin,
and which becomes very early segmented into a series of parallel rays
at right angles to the longitudinal bar.
In other words, the primitive skeleton of both the fins consists of a
longitudinal bar running along the base of the fin, and giving off at
right angles series of rays which pass into the fin. The longitudinal
bar, which may be called the basipterygium, is moreover continuous in
front with the pectoral or pelvic girdle as the case may be.
[FIG. 344. PECTORAL FIN OF A YOUNG EMBRYO OF SCYLLIUM IN
LONGITUDINAL AND HORIZONTAL SECTION.
The skeleton of the fin was still in the condition of embryonic
cartilage.
_b.p._ basipterygium (eventual metapterygium); _fr._ fin rays;
_p.g._ pectoral girdle in transverse section; _f._ foramen in
pectoral girdle; _pc._ wall of peritoneal cavity.]
The primitive skeleton of the pectoral fin is shewn in longitudinal
section in fig. 344, and that of the pelvic fin at a slightly later
stage in fig. 345.
A transverse section shewing the basipterygium (_mpt_) of the pectoral
fin, and the plate passing from it into the fin, is shewn in fig. 346.
Before proceeding to describe the later history of the two fins it may
be well to point out that their embryonic structure completely
supports the view which has been arrived at from the consideration of
the soft parts of the fin.
My observations shew that the embryonic skeleton of the paired fin
consists of a series of parallel rays similar to those of the unpaired
fins. These rays support the soft part of the fin which has the form
of a longitudinal ridge, and are continuous at their base with a
longitudinal bar, which may very probably be due to secondary
development. As pointed out by Mivart, a longitudinal bar is also
occasionally formed to support the cartilaginous rays of unpaired
fins. The longitudinal bar of the paired fins is believed by both
Thacker and Mivart to be due to the coalescence of the bases of
primitively independent rays, of which they believe the fin to have
been originally composed. This view is probable enough in itself, but
there is no trace in the embryo of the bar in question being formed by
the coalescence of rays, though the fact of its being perfectly
continuous with the bases of the rays is somewhat in favour of this
view[215].
[215] Thacker more especially founds his view on the adult form
of the pelvic fins in the cartilaginous Ganoids; Polyodon, in
which the part which constitutes the basal plate in other forms
is divided into separate segments, being mainly relied on. It is
possible that the segmentation of this plate, as maintained by
Gegenbaur and Davidoff, is secondary, but Thacker's view that the
segmentation is a primitive character seems to me, in the absence
of definite evidence to the reverse, the more natural one.
[FIG. 345. PELVIC FIN OF A VERY YOUNG FEMALE EMBRYO OF SCYLLIUM
STELLARE.
_bb._ basipterygium; _pu._ pubic process of pelvic girdle; _il._
iliac process of pelvic girdle.]
A point may be noticed here which may perhaps appear to be a
difficulty, viz. that to a considerable extent in the pectoral, and to
some extent in the pelvic fin the embryonic cartilage from which the
fin-rays are developed is at first a continuous lamina, which
subsequently segments into rays. I am however inclined to regard this
merely as a result of the mode of conversion of the indifferent
mesoblast into cartilage; and in any case no conclusion adverse to the
above view can be drawn from it, since I find that the rays of the
unpaired fin are similarly segmented from a continuous lamina. In all
cases the segmentation of the rays is to a large extent completed
before the tissue in question is sufficiently differentiated to be
called cartilage by an histologist.
Thacker and Mivart both hold that the pectoral and pelvic girdles have
been evolved by ventral and dorsal growths of the anterior end of the
longitudinal bar supporting the fin-rays.
There is, so far as I see, no theoretical objection to be taken to
this view, and the fact of the pectoral and pelvic girdles originating
continuously, and long remaining united with the longitudinal bars of
their respective fins is in favour of rather than against this view.
The same may be said of the fact that the first part of each girdle to
be formed is that in the neighbourhood of the longitudinal bar
(basipterygium) of the fin, the dorsal and ventral prolongations being
subsequent growths.
The later development of the skeleton of the two fins is more
conveniently treated separately.
[FIG. 346. TRANSVERSE SECTION THROUGH THE PECTORAL FIN OF A YOUNG
EMBRYO OF SCYLLIUM STELLARE.
_mpt._ basipterygial bar (metapterygium); _fr._ fin ray; _m._
muscles; _hf._ horny fibres.]
The pelvic fin. The changes in the pelvic fin are comparatively
slight. The fin remains through life as a nearly horizontal lateral
projection of the body, and the longitudinal bar--the basipterygium--at
its base always remains as such. It is for a considerable period
attached to the pelvic girdle, but eventually becomes segmented from
it. Of the fin rays the anterior remains directly articulated with the
pelvic girdle on the separation of the basipterygium (fig. 347), and
the remaining rays finally become segmented from the basipterygium,
though they remain articulated with it. They also become to some
extent transversely segmented. The posterior end of the basipterygial
bar also becomes segmented off as the terminal ray.
The pelvic fin thus retains in all essential points its primitive
arrangement.
The pectoral fin. The earliest stage of the pectoral fin differs from
that of the pelvic fin only in minor points. There is the same
longitudinal or basipterygial bar to which the fin-rays are attached,
whose position at the base of the fin is clearly seen in the
transverse section (fig. 346, _mpt_). In front the bar is continuous
with the pectoral girdle (figs. 344 and 348).
[FIG. 347. PELVIC FIN OF A YOUNG MALE EMBRYO OF SCYLLIUM STELLARE.
_bp._ basipterygium; _m.o._ process of basipterygium continued into
clasper; _il._ iliac process of pectoral girdle; _pu._ pubis.]
[FIG. 348. PECTORAL FIN OF AN EMBRYO OF SCYLLIUM STELLARE.
_mp._ metapterygium (basipterygium of earlier stage); _me.p._
rudiment of future pro- and mesopterygium; _sc._ cut surface of
scapular process; _cr._ coracoid process; _fr._ foramen; _f._ horny
fibres.]
The changes which take place in the course of the further development
are however very much more considerable in the case of the pectoral
than in that of the pelvic fin.
By the process spoken of above, by which the attachment of the
pectoral fin to the body wall becomes shortened from behind forwards,
the basipterygial bar is gradually rotated outwards, its anterior end
remaining attached to the pectoral girdle. In this way this bar comes
to form the posterior border of the skeleton of the fin (figs. 348 and
349, _mp_), constituting what Gegenbaur called the metapterygium, and
eventually becomes segmented off from the pectoral girdle, simply
articulating with its hinder edge.
The plate of cartilage, which is continued outwards from the
basipterygium, or as we may now call it, the metapterygium, into the
fin, is not nearly so completely divided up into fin-rays as in the
case of the pelvic fin, and this is especially the case with the basal
part of the plate. This basal part becomes in fact at first only
divided into two parts (fig. 348) a small anterior part at the front
end (_me.p_), and a larger posterior along the base of the remainder
of the fin. The anterior part directly joins the pectoral girdle at
its base, resembling in this respect the anterior fin-ray of the
pelvic girdle. It constitutes the rudiment of the mesopterygium and
propterygium of Gegenbaur. It bears four fin-rays at its extremity,
the anterior not being well marked. The remaining fin-rays are borne
by the edge of the plate continuous with the metapterygium.
The further changes in the cartilages of the limb are not important,
and are easily understood by reference to fig. 349 representing the
limb of a nearly full-grown embryo. The front end of the anterior
basal cartilage becomes segmented off as a propterygium, bearing a
single fin-ray, leaving the remainder of the cartilage as a
mesopterygium. The remainder of the now considerably segmented
fin-rays are borne by the metapterygium.
The mode of development of the pectoral fin demonstrates that, as
supposed by Mivart, the metapterygium is the homologue of the basal
cartilage of the pelvic fin.
From the mode of development of the fins of Scyllium conclusions may
be drawn adverse to the views recently put forward on the structure of
the fin by Gegenbaur and Huxley, both of whom consider the primitive
type of fin to be most nearly retained in Ceratodus, and to consist of
a central multisegmented axis with numerous rays. Gegenbaur derives
the Elasmobranch pectoral fin from a form which he calls the
archipterygium, nearly like that of Ceratodus, with a median axis and
two rows of rays; but holds that in addition to the rays attached to
the median axis, which are alone found in Ceratodus, there were other
rays directly articulated to the shoulder-girdle. He considers that in
the Elasmobranch fin the majority of the lateral rays on the posterior
(median or inner according to his view of the position of the limb)
side have become aborted, and that the central axis is represented by
the metapterygium; while the pro- and mesopterygium and their rays
are, he believes, derived from those rays of the archipterygium which
originally articulated directly with the shoulder-girdle.
Gegenbaur's view appears to me to be absolutely negatived by the facts
of development of the pectoral fin in Scyllium; not so much because
the pectoral fin in this form is necessarily to be regarded as
primitive, but because what Gegenbaur holds to be the primitive axis
of the biserial fin is demonstrated to be really the base, and it is
only in the adult that it is conceivable that a second set of lateral
rays could have existed on the posterior side of the metapterygium. If
Gegenbaur's view were correct we should expect to find in the embryo,
if anywhere, traces of the second set of lateral rays; but the fact is
that, as may easily be seen by an inspection of figs. 344 and 346,
such a second set of lateral rays could not possibly have existed in a
type of fin like that found in the embryo[216]. With this view of
Gegenbaur's it appears to me that the theory held by this anatomist to
the effect that the limbs are modified gill arches also falls; in that
his method of deriving the limbs from gill arches ceases to be
admissible, while it is not easy to see how a limb, formed on the type
of the embryonic limb of Elasmobranchs, could be derived from a
visceral arch with its branchial rays[217].
[216] If, which I very much doubt, Gegenbaur is right in
regarding certain rays found in some Elasmobranch pectoral fins
as rudiments of a second set of rays on the posterior side of the
metapterygium, these rays will have to be regarded as structures
in the act of being evolved, and not as persisting traces of a
biserial fin.
[217] Some arguments in favour of Gegenbaur's theory adduced by
Wiedersheim as a result of his researches on Protopterus are
interesting. The attachment which he describes between the
external gills and the pectoral girdle is no doubt remarkable,
but I would suggest that the observations we have on the vascular
supply of these gills demonstrate that this attachment is
secondary.
[FIG. 349. SKELETON OF THE PECTORAL FIN AND PART OF PECTORAL GIRDLE
OF A NEARLY RIPE EMBRYO OF SCYLLIUM STELLARE.
_m.p._ metapterygium; _me.p._ mesopterygium; _pp._ propterygium;
_cr._ coracoid process.]
Gegenbaur's older view that the Elasmobranch fin retains a primitive
uniserial type appears to me to be nearer the truth than his more
recent view on this subject; though I hold that the fundamental point
established by the development of these parts in Scyllium is that the
posterior border of the adult Elasmobranch fin is the primitive base
line, _i.e._ the line of attachment of the fin to the side of the
body.
Huxley holds that the mesopterygium is the proximal piece of the axial
skeleton of the limb of Ceratodus, and derives the Elasmobranch fin
from that of Ceratodus by the shortening of its axis and the
coalescence of some of its elements. The secondary character of the
mesopterygium, and its total absence in the embryo Scyllium, appears
to me as conclusive against Huxley's view, as the character of the
embryonic fin is against that of Gegenbaur; and I should be much more
inclined to hold that the fin of Ceratodus has been derived from a fin
like that of the Elasmobranchii by a series of steps similar to those
which Huxley supposes to have led to the establishment of the
Elasmobranch fin, but in exactly the reverse order.
With reference to the development of the pectoral fin in the Teleostei
there are some observations of 'Swirski (No. 488) which unfortunately
do not throw very much light upon the nature of the limb.
'Swirski finds that in the Pike the skeleton of the limb is formed of
a plate of cartilage, continuous with the pectoral girdle; which soon
becomes divided into a proximal and a distal portion. The former is
subsequently segmented into five basal rays, and the latter into
twelve parts, the number of which subsequently becomes reduced.
These investigations might be regarded as tending to shew that the
basipterygium of Elasmobranchii is not represented in Teleostei, owing
to the fin rays not having united into a continuous basal bar, but the
observations are not sufficiently complete to admit of this conclusion
being founded upon them with any certainty.
_The cheiropterygium._
Observations on the early development of the pentadactyloid limbs of
the higher Vertebrata are comparatively scanty.
The limbs arise as simple outgrowths of the sides of the body, formed
both of epiblast and mesoblast. In the Amniota, at all events, they
are processes of a special longitudinal ridge known as the Wolffian
ridge. In the Amniota they also bear at their extremity a thickened
cap of epiblast, which may be compared with the epiblastic fold at the
apex of the Elasmobranch fin.
Both limbs have at first a precisely similar position, both being
directed backwards and being parallel to the surface of the body.
In the Urodela (Götte) the ulnar and fibular sides are primitively
dorsal, and the radial and tibial ventral: in Mammalia however
Kölliker states that the radial and tibial edges are from the first
anterior.
The exact changes of position undergone by the limbs in the course of
development are not fully understood. To suit a terrestrial mode of
life the flexures of the two limbs become gradually more and more
opposite, till in Mammalia the corresponding joints of the two limbs
are turned in completely opposite directions.
Within the mesoblast of the limbs a continuous blastema becomes
formed, which constitutes the first trace of the skeleton of the limb.
The corresponding elements of the two limbs, viz. the humerus and
femur, radius and tibia, ulna and fibula, carpal and tarsal bones,
metacarpals and metatarsals, and digits, become differentiated within
this, by the conversion of definite regions into cartilage, which may
either be completely distinct or be at first united. These
cartilaginous elements subsequently ossify.
The later development of the parts, more especially of the carpus and
tarsus, has been made the subject of considerable study; and important
results have been thereby obtained as to the homology of the various
carpal and tarsal bones throughout the Vertebrata; but this subject is
too special to be treated of here. The early development, including
the succession of the growth of the different parts, and the extent of
continuity primitively obtaining between them, has on the other hand
been but little investigated; recently however the development of the
limbs in the Urodela has been worked out in this way by two
anatomists, Götte (No. 482) and Strasser (No. 487), and their results,
though not on all points in complete harmony, are of considerable
interest, more especially in their bearing on the derivation of the
pentadactyloid limb from the piscine fin. Till however further
investigations of the same nature have been made upon other types, the
conclusions to be drawn from Götte and Strasser's observations must be
regarded as somewhat provisional, the actual interpretation of various
ontological processes being very uncertain.
The forms investigated are Triton and Salamandra. We may remind the
reader that the hand of the Urodela has four digits, and the foot
five, the fifth digit being absent in the hand[218]. In Triton the
proximal row of carpal bones consists (using Gegenbaur's nomenclature)
of (1) a radiale, and (2 and 3) an intermedium and ulnare, partially
united. The distal row is formed of four carpals, of which the first
often does not support the first metacarpal; while the second
articulates with both the first and second metacarpals. In the foot
the proximal row of tarsals consists of a tibiale, an intermedium and
a fibulare. The distal row is formed of four tarsals, the first, like
that in the hand, often not articulating with the first metatarsal,
the second supporting the first and second metatarsals; and the fourth
the fourth and fifth metatarsals.
[218] This seems to me clearly to follow from Götte and
Strasser's observations.
The mode of development of the hand and foot is almost the same. The
most remarkable feature of development is the order of succession of
the digits. The two anterior (radial or tibial) are formed in the
first instance, and then the third, fourth and fifth in succession.
As to the actual development of the skeleton Strasser, whose
observations were made by means of sections, has arrived at the
following results.
The humerus with the radius and ulna, and the corresponding parts in
the hind limb, are the first parts to be differentiated in the
continuous plate of tissue from which the skeleton of the limb is
formed. Somewhat later a cartilaginous centre appears at the base of
the first and second fingers (which have already appeared as
prominences at the end of the limb) in the situation of the permanent
second carpal of the distal row of carpals; and the process of
chondrification spreads from this centre into the fingers and into the
remainder of the carpus. In this way a continuous carpal plate of
cartilage is established, which is on the one hand continuous with the
cartilage of the two metacarpals, and on the other with the radius and
ulna.
In the cartilage of the carpus two special columns may be noticed, the
one on the radial side, most advanced in development, being continuous
with the radius; the other less developed column on the side of the
ulna being continuous both with the ulna and with the radius. The ulna
and radius are not united with the humerus.
In the further growth the third and fourth digits, and in the foot the
fifth digit also, gradually sprout out in succession from the ulnar
side of the continuous carpal plate. The carpal plate itself becomes
segmented from the radius and ulna, and divided up into the carpal
bones.
The original radial column is divided into three elements, a proximal
the radiale, a middle element the first carpal, and a distal the
second carpal already spoken of. The first carpal is thus situated
between the basal cartilage of the second digit and the radiale, and
would therefore appear to be the representative of a primitive middle
row of carpal bones, of which the centrale is also another
representative.
The centrale and intermedium are the middle and proximal products of
the segmentation of the ulnar column of the primitive carpus, the
distal second carpal being common both to this column and to the
radial column.
The ulnar or fibular side of the carpus or tarsus becomes divided into
a proximal element--the ulnare or fibulare--the ulnare remaining
partially united with the intermedium. There are also formed from this
plate two carpals to articulate with digits 3 and 4; while in the foot
the corresponding elements articulate respectively with the third
digit, and with the fourth and fifth digits.
Götte, whose observations were made in a somewhat different method to
those of Strasser, is at variance with him on several points. He finds
that the primitive skeleton of the limb consists of a basal portion,
the humerus, continued into a radial and an ulnar ray, which are
respectively prolonged into the two first digits. The two rays next
coalesce at the base of the fingers to form the carpus, and thus the
division of the limb into the brachium, antebrachium and manus is
effected.
The ulna, which is primitively prolonged into the second digit, is
subsequently separated from it and is prolonged into the third; from
the side of the part of the carpus connecting the ulna with the third
digit the fourth digit is eventually budded out, and in the foot the
fourth and fifth digits arise from the corresponding region. Each of
the three columns connected respectively with the first, second, and
third digits becomes divided into three successive carpal bones, so
that Götte holds the skeleton of the hand or foot to be formed of a
proximal, a middle, and a distal row of carpal bones each containing
potentially three elements. The proximal row is formed of the radiale,
intermedium and ulnare; the middle row of carpal 1, the centrale and
carpal 4, and the distal of carpal 2 (consisting according to Götte of
two coalesced elements) and carpal 3.
_The derivation of the cheiropterygium from the ichthyopterygium._ All
anatomists are agreed that the limbs of the higher Vertebrata are
derived from those of Fishes, but the gulf between the two types of
limbs is so great that there is room for a very great diversity of
opinion as to the mode of evolution of the cheiropterygium. The most
important speculations on the subject are those of Gegenbaur and
Huxley.
Gegenbaur holds that the cheiropterygium is derived from a uniserial
piscine limb, and that it consists of a primitive stem, to which a
series of lateral rays are attached on one (the radial) side; while
Huxley holds that the cheiropterygium is derived from a biserial
piscine limb by the "lengthening of the axial skeleton, accompanied by
the removal of its distal elements further away from the
shoulder-girdle and by a diminution in the number of the rays."
Neither of these theories is founded upon ontology, and the only
ontological evidence we have which bears on this question is that
above recorded with reference to the development of the Urodele limb.
Without holding that this evidence can be considered as in any way
conclusive, its tendency would appear to me to be in favour of
regarding the cheiropterygium as derived from a uniserial type of fin.
The humerus or femur would appear to be the basipterygial bars
(metapterygium), which have become directed outwards instead of
retaining their original position parallel to the length of the body
at the base of the fin. The anterior (proximal) fin-rays and the pro-
and mesopterygium must be supposed to have become aborted, while the
radius or ulna, and tibia or fibula are two posterior fin-rays
(probably each representing several coalesced rays like the pro- and
mesopterygium) which support at their distal extremities more numerous
fin-rays consisting of the rows of carpal and tarsal bones.
This view of the cheiropterygium corresponds in some respects with
that put forward by Götte as a result of his investigations on the
development of the Urodele limbs, though in other respects it is very
different. A difficulty of this view is the fact that it involves our
supposing that the radial edge of the limb corresponds with the
metapterygial edge of the piscine fin. The difficulties of this
position have been clearly pointed out by Huxley, but the fact that in
the primitive position of the Urodele limbs the radius is ventral and
the ulna dorsal shews that this difficulty is not insuperable, in that
it is easy to conceive the radial border of the fin to have become
rotated from its primitive Elasmobranch position into the vertical
position it occupies in the embryos of the Urodela, and then to have
been further rotated from this position into that which it occupies in
the adult Urodela and in all higher forms.
BIBLIOGRAPHY _of the Limbs_.
(477) M. v. Davidoff. "Beiträge z. vergleich. Anat. d. hinteren
Gliedmaassen d. Fische I." _Morphol. Jahrbuch_, Vol. V. 1879.
(478) C. Gegenbaur. _Untersuchungen z. vergleich. Anat. d.
Wirbelthiere._ Leipzig, 1864-5. Erstes Heft. Carpus u. Tarsus. Zweites
Heft. Brustflosse d. Fische.
(479) C. Gegenbaur. "Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im
Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere."
_Jenaische Zeitschrift_, Vol. V. 1870.
(480) C. Gegenbaur. "Ueb. d. Archipterygium." _Jenaische Zeitschrift_,
Vol. VII. 1873.
(481) C. Gegenbaur. "Zur Morphologie d. Gliedmaassen d. Wirbelthiere."
_Morphologisches Jahrbuch_, Vol. II. 1876.
(482) A. Götte. _Ueb. Entwick. u. Regeneration d. Gliedmaassenskelets
d. Molche._ Leipzig, 1879.
(483) T. H. Huxley. "On Ceratodus Forsteri, with some observations on
the classification of Fishes." _Proc. Zool. Soc._ 1876.
(484) St George Mivart. "On the Fins of Elasmobranchii." _Zoological
Trans._, Vol. X.
(485) A. Rosenberg. "Ueb. d. Entwick. d. Extremitäten-Skelets bei
einigen d. Reduction ihrer Gliedmaassen charakterisirten
Wirbelthieren." _Zeit. f. wiss. Zool._, Vol. XXIII. 1873.
(486) E. Rosenberg. "Ueb. d. Entwick. d. Wirbelsäule u. d. centrale
carpi d. Menschen." _Morphologisches Jahrbuch_, Vol. I. 1875.
(487) H. Strasser. "Z. Entwick. d. Extremitätenknorpel bei Salamandern
u. Tritonen." _Morphologisches Jahrbuch_, Vol. V. 1879.
(488) G. 'Swirski. _Untersuch. üb. d. Entwick. d. Schultergürtels u.
d. Skelets d. Brustflosse d. Hechts._ Inaug. Diss. Dorpat, 1880.
(489) J. K. Thacker. "Median and paired fins. A contribution to the
history of the Vertebrate limbs." _Trans. of the Connecticut Acad._,
Vol. III. 1877.
(490) J. K. Thacker. "Ventral fins of Ganoids." _Trans. of the
Connecticut Acad._, Vol. IV. 1877.