Difference between revisions of "The Works of Francis Balfour 3-20"

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==CHAPTER XX. THE PECTORAL AND PELVIC GIRDLES AND THE SKELETON OF THE LIMBS==
 +
 +
 +
 +
TJie 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 praecoracoid
 +
and coracoid. Of these the praecoracoid is far the most
 +
 +
 +
 +
600 THE PECTORAL GIRDLE.
 +
 +
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 praecoracoid, though it is
 +
quite distinct from Gegenbaur's praecoracoid. 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 posttemporal ; a middle piece or clavicle, and a ventral or infraclavicle (inter-clavicle), 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 PECTORAL GIRDLE. 6oi
 +
 +
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 prascoracoid, 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 praecoracoid 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 inter-clavicle in the Sauropsida
 +
and Mammalia, which are however usually admitted to be
 +
absent in Amphibia. These views have recently been challenged
 +
by Gotte (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 praecoracoid, and an epicoracoid. By the conversion of parts
 +
of the primitive cartilaginous plates into membranous tissue
 +
various fenestrae may be formed in the cartilage, and the bars
 +
 +
 +
 +
602 THE NATURE OF THE CLAVICLE.
 +
 +
bounding these fenestrae both in the scapula and coracoid
 +
regions have received special names ; the anterior bar of the
 +
coracoid region, forming the praecoracoid, 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 Gotte and Hoffmann and
 +
other anatomists concern especially the clavicle and inter-clavicle.
 +
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 Gotte 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 Gotte 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 Gotte'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 coracoscapular 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 1 .
 +
 +
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.
 +
 +
1 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.
 +
 +
 +
 +
II IK PECTORAL GIRDLE.
 +
 +
 +
 +
603
 +
 +
 +
 +
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, Gotte), 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 scapulocoracoid 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 prassternum,' 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 praecoracoid, Gotte 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.
 +
 +
Gotte 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. Gotte 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
 +
 +
 +
 +
604 THK PECTORAL GIRDLE.
 +
 +
connecting the furculum with the sternum. The observations of Gotte,
 +
which tend to shew the keel of the sternum is really an interclavicle, appear
 +
to me of great importance.
 +
 +
A prascoracoid, 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 Dromaeus and Casuarius clavicles are present (fused with the scapula in the adult Dromaeus), though
 +
absent in other Ratitae (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 1 .
 +
 +
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 coracoscapular 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 Kolliker, and extended to other Mammalia by
 +
Gotte. 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 praecoracoid. Parker's
 +
so-called mesoscapula may ossify separately. The homologies of the episternum are much disputed. Gotte, 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 Gotte' 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 Gotte 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 Gotte to be a
 +
true interclavicle.
 +
 +
1 This process, known as the coracoid process, is held by Sabatier to be the
 +
pnecoracoid ; 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 praecoracoid in the Ornithodelphia is to my mind a serious difficulty in the way of
 +
Sabatier's view.
 +
 +
 +
 +
THE PECTORAL GIRDLE. 605
 +
 +
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 praecoracoid, but Gotte 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 praecoracoid 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. Gotte'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 prsecoracoid, 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 (praecoracoids) which Gotte 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., \\. 1853.
 +
 +
(464) A. Duges. " Recherches sur 1'osteologie et la myologie des Batraciens a
 +
leurs differens ages." Memoires des savants etrang. Academic royale des sciences de
 +
Finstitut de France^ Vol. vi. 1835.
 +
 +
(465) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomie der Wirbelthiere, 2 Heft. Schultergiirtel der Wirbelthiere. Bmstflosse der Fische. Leipzig,
 +
1865.
 +
 +
(466) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere : Brustbien u. Schultergiirtel." Archivf. mikr, Anat. Vol. xiv. 1877.
 +
 +
(467) C. K. Hoffmann. "Beitrage z. vergleichenden Anatomic d. Wirbelthiere." Niederlandisches Archivf. 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.
 +
 +
 +
 +
606 PELVIC GIRDLE.
 +
 +
 +
 +
(469) H. Rathke. Ueber die Entwicklung der Schildkrbten. Braunschweig,
 +
1848.
 +
 +
(470) H. Rathke. Ueber den Bau und die Entwicklung des Brustbeins der
 +
Saurier, 1853.
 +
 +
(471) A. Sabatier. Comparaison des ceinfures et des membres antMeurs et posttrtturs d. la Serie d. Vertttrh. Montpellier, 1880.
 +
 +
(472) Georg 'Swirski. Untersuch. iib. d. Entwick. d. Schultergiirtels n. 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 Chimaera 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
 +
 +
 +
 +
PELVIC GIRDLE. 607
 +
 +
 +
 +
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 vertebrae 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 praepubic 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
 +
 +
 +
 +
6o8 PECTORAL AND PELVIC GIRDLES.
 +
 +
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
 +
Uuck, 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 praepubis 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. Beckengiirtels d. Amphibien,
 +
Reptilien u. Vogel, Inaug. Diss. Dorpat, 1880.
 +
 +
(474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne
 +
d. Hiiftgelenkes." Morph. Jahrbuch, Vol. II. 1876.
 +
 +
(475) Th. H. Huxley. "The characters of the Pelvis in Mammalia, etc."
 +
Proc. of Roy. Soc., Vol. xxvm. 1879.
 +
 +
(476) A. Sabatier. Comparaison des ceintures et des membres anterieurs et
 +
posterieurs dans la Serie d. Vertebrcs. 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
 +
 +
 +
 +
LIMBS. 609
 +
 +
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 praecoracoid 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 praepubic 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 Gotte 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
 +
B. in. 39
 +
 +
 +
 +
6io
 +
 +
 +
 +
PAIRED FINS OF ELASMOBRANCHII.
 +
 +
 +
 +
by a line of columnar epiblast cells 1 . This connecting line of
 +
columnar epiblast is a very transitory structure, and after its
 +
disappearance the rudimentary fins become more prominent,
 +
consisting (fig. 343, &) 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.
 +
 +
For the remaining history it
 +
is necessary to confine ourselves
 +
to Scylliurn 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.
 +
 +
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.
 +
 +
 +
 +
 +
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 ; /. liver ; sd. opening of segmented duct into the body cavity ;
 +
mp. muscle plate ; ;. umbilical
 +
canal.
 +
 +
 +
 +
1 I''. M. I'alfour. Monograph on Elasmobranfh l-'hhes, pp. 1012.
 +
 +
 +
 +
 +
LIMBS. 6ll
 +
 +
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 1 .
 +
 +
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 2 .
 +
 +
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
 +
 +
1 Both Maclise arid Humphry {Journal of Anat. and Pkys., Vol. v.) had
 +
previously suggested that the paired fins were related to the unpaired fins.
 +
 +
2 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.
 +
 +
392
 +
 +
 +
 +
612
 +
 +
 +
 +
PAIRED FINS OF ELASMOBRANCHII.
 +
 +
 +
 +
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,
 +
 +
 +
 +
 +
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; /. foramen in pectoral girdle; pc. wall of peritoneal cavity.
 +
 +
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.
 +
 +
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 (inpi) 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
 +
 +
 +
 +
LIMBS.
 +
 +
 +
 +
613
 +
 +
 +
 +
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
 +
 +
 +
 +
 +
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.
 +
 +
 +
 +
in the embryo of the bar in question being formed by the
 +
coalesceace of rays, though the fact of its being perfectly
 +
continuous with the bases of the rays is somewhat in favour
 +
of this view 1 .
 +
 +
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
 +
 +
1 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.
 +
 +
 +
 +
614
 +
 +
 +
 +
THE PELVIC FIN.
 +
 +
 +
 +
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.
 +
 +
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
 +
 +
 +
 +
 +
FIG. 346. TRANSVERSE SECTION THROUGH THE PECTORAL FIN OF A YOUNG
 +
 +
EMBRYO OK SCYLLIUM STELLARE.
 +
mpt. basipterygial bar (metapterygium) ; fr. fin ray; m. muscles; hf. horny fibres.
 +
 +
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.
 +
 +
 +
 +
LIMBS.
 +
 +
 +
 +
6l 5
 +
 +
 +
 +
The pectoral fin. The earliest stage of the pectoral fin
 +
 +
 +
 +
 +
There
 +
 +
 +
 +
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 ; pit. pubis.
 +
 +
differs from that of the pelvic fin only in minor points,
 +
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,
 +
mpf). In front the bar is
 +
continuous with the pectoral girdle (figs. 344 and
 +
 +
348).
 +
 +
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. "' 3+8. F^OJJL ,,, v.
 +
 +
By the process spoken m p t me tapterygium (basipterygium of earlier
 +
 +
stage); me.p. rudiment of future pro- and mesopterygium ; sc. cut surface of scapular process ;
 +
cr. coracoid process;/;', foramen;/, horny fibres.
 +
 +
 +
 +
 +
of above, by which the
 +
attachment of the pec
 +
 +
 +
6l6 THE PECTORAL FIN.
 +
 +
toral 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
 +
 +
 +
 +
LIMBS.
 +
 +
 +
 +
6I 7
 +
 +
 +
 +
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 1 . 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 2 .
 +
 +
Gegenbaur's older view
 +
 +
 +
 +
 +
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 ;
 +
//. propterygium ; cr. coracoid process.
 +
 +
 +
 +
1 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.
 +
 +
2 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.
 +
 +
 +
 +
6l8 THE CHEIKOPTERYGIUM.
 +
 +
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.
 +
 +
Tlie ckeiropterygium.
 +
 +
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.
 +
 +
 +
 +
I 111: CHEIROPTERYGIUM.
 +
 +
 +
 +
619
 +
 +
 +
 +
In the Urodela (Gotte) the ulnar and fibular sides are
 +
primitively dorsal, and the radial and tibial ventral : in Mammalia however Kolliker 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, Gotte (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 Gotte 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 1 . In Triton the proximal row of
 +
carpal bones consists (using Gegenbaur's nomenclature) of (i) 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
 +
1 This seems to me clearly to follow from Gotte and Strasser's observations.
 +
 +
 +
 +
620 THE GHE1ROPTERYGIUM.
 +
 +
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.
 +
 +
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.
 +
 +
 +
 +
THE CIIF.IROPTERYGIUM. 621
 +
 +
Gotte, 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 Gotte 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 i, the centrale
 +
and carpal 4, and the distal of carpal 2 (consisting according to Gotte of two
 +
coalesced elements) and carpal 3.
 +
 +
The derivation of the cheiropterygium from the ichthyoptcrygium. 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.
 +
 +
 +
 +
622 THE CHEIROPTERYGIUM.
 +
 +
This view of the cheiropterygium corresponds in some respects with that
 +
put forward by Gotte 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. "Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen
 +
d. Fische I." Morphol. Jahrbuch, Vol. v. 1879.
 +
 +
(478) C. Gegenbaur. Untersuckungen 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 Zeitsckrift, Vol. V. 1870.
 +
 +
(480) C. Gegenbaur. " Ueb. d. Archipterygium." Jenaische Zeitschrift, Vol.
 +
vii. 1873.
 +
 +
(481) C. Gegenbaur. "Zur Morphologic d. Gliedmaassen d. Wirbelthiere."
 +
Morphologisches Jahrbuch, Vol. II. 1876.
 +
 +
(482) A. Gotte. Ueb. Entivick. 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. Extremitaten-Skelets bei einigen
 +
d. Reduction ihrer Gliedmaassen charakterisirten Wirbelthieren." Zeil.f. iviss. Zool.,
 +
Vol. xxin. 1873.
 +
 +
(486) E. Rosenberg. "Ueb. d. Entwick. d. Wirbelsaule u. d. centrale carpi
 +
d. Menschen. " Morphologisches Jahrbuch, Vol. I. 1875.
 +
 +
(487) H. Strasser. "Z. Entwick. d. Extremitatenknorpel bei Salamandern u.
 +
Tritonen." Morphologisches Jahrbuch, Vol. V. 1879.
 +
 +
(488) G. 'S wirski. Untersitch. iib. d. Entwick. d. Schultergitrtels u. d. Skelcls 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. ill. 1877.
 +
 +
(490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut
 +
Acad., Vol. iv. 1877.
 +
 +
 +
 +
CHAPTER XXI.
 +
 +
 +
 +
THE BODY CAVITY, THE VASCULAR SYSTEM, AND THE
 +
VASCULAR GLANDS.
 +
 +
 +
 +
The Body cavity.
 +
 +
IN the Ccelenterata no body cavity as distinct from the
 +
alimentary cavity is present ; but in the remaining Invertebrata
 +
the body cavity may (i) take the form of a wide space separating
 +
the wall of the gut from the body wall, or (2) may be present in
 +
a more or less reduced form as a number of serous spaces, or
 +
(3) only be represented by irregular channels between the
 +
muscular and connective-tissue cells filling up the interior of the
 +
body. The body cavity, in whatever form it presents itself, is
 +
probably filled with fluid, and the fluid in it may contain special
 +
cellular elements. A well developed body cavity may coexist
 +
with an independent system of serous spaces, as in the Vertebrata and the Echinodermata ; the perihaemal section of the
 +
body cavity of the latter probably representing the system of
 +
serous spaces.
 +
 +
In several of the types with a well developed body cavity it
 +
has been established that this cavity originates in the embryo
 +
from a pair of alimentary diverticula, and the cavities resulting
 +
from the formation of these diverticula may remain distinct, the
 +
adjacent walls of the two cavities fusing to form a dorsal and a
 +
ventral mesentery.
 +
 +
It is fairly certain that some groups, e.g. the Tracheata, with
 +
imperfectly developed body cavities are descended from ancestors
 +
which were provided with well developed body cavities, but how
 +
far this is universally the case cannot as yet be definitely
 +
decided, and for additional information on this subject the
 +
 +
 +
 +
624 CIIORDATA.
 +
 +
 +
 +
reader is referred to pp. 355 360 and to the literature there
 +
referred to.
 +
 +
In the Chaetopoda and the Tracheata the body cavity arises
 +
as a series of paired compartments in the somites of mesoblast
 +
(fig. 350) which have at first a very restricted extension on the
 +
ventral side of the body, but eventually extend dorsalwards and
 +
vcntralwards till each cavity is a half circle investing the
 +
alimentary tract ; on the dorsal side the walls separating the two
 +
 +
 +
 +
 +
FIG. 350. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELINA
 +
LABYRINTHICA.
 +
 +
The section is taken slightly to one side of the middle line so as to shew the relation of the mesoblastic somites to the limbs. In the interior are seen the yolk
 +
segments and their nuclei.
 +
 +
i 16. the segments ; pr.l. procephalic lobe ; do. dorsal integument.
 +
 +
half cavities usually remain as the dorsal mesentery, while
 +
ventrally they are in most instances absorbed. The transverse
 +
walls, separating the successive compartments of the body
 +
cavity, generally become more or less perforated.
 +
 +
Chordata. In the Chordata the primitive body cavity is
 +
cither directly formed from a pair of alimentary diverticula
 +
(Cephalochorda) (fig. 3) or as a pair of spaces in the mesoblastic
 +
plates of the two sides of the body (fig. 20).
 +
 +
As already explained (pp. 294 300) the walls of the dorsal
 +
sections of the primitive body cavity soon become separated
 +
from those of the ventral, and becoming segmented constitute
 +
the muscle plates, while the cavity within them becomes
 +
 +
 +
 +
I
 +
 +
 +
 +
THE BODY CAVITY.
 +
 +
 +
 +
625
 +
 +
 +
 +
the
 +
 +
 +
 +
obliterated : they are dealt with in a separate chapter. The
 +
ventral part of the primitive cavity alone constitutes the
 +
permanent body cavity.
 +
 +
The primitive body cavity in the lower Vertebrata is at first
 +
continued forwards into the region of the head, but on the
 +
formation of the visceral clefts the cephalic section of the body
 +
cavity becomes divided into a series of separate compartments.
 +
Subsequently these sections of the body cavity become obliterated ; and, since their walls give rise to muscles, they may
 +
probably be looked upon as equivalent to the dorsal sections of
 +
the body cavity in the trunk, and will be treated of in connection
 +
with the muscular system.
 +
 +
As a result of its mode of origin the body cavity in
 +
trunk is at first divided into two
 +
lateral halves ; and part of the mesoblast lining it soon becomes distinguished as a special layer of epithelium, known as the peritoneal epithelium, of which the part bounding the
 +
outer wall forms the somatic layer,
 +
and that bounding the inner wall the
 +
splanchnic layer. Between the two
 +
splanchnic layers is placed the gut.
 +
On the ventral side, in the region of
 +
the permanent gut, the two halves
 +
of the body cavity soon coalesce,
 +
the septum between them becoming
 +
absorbed, and the splanchnic layers
 +
of epithelium of the two sides uniting
 +
at the ventral side of the gut, and
 +
the somatic layers at the median
 +
ventral line of the body wall (fig.
 +
 +
 +
 +
 +
In the lower Vertebrata the body
 +
cavity is originally present even in
 +
the post-anal region of the trunk, but
 +
usually atrophies early, frequently
 +
before the two halves coalesce.
 +
 +
On the dorsal side of the gut the
 +
B. III.
 +
 +
 +
 +
FIG. 351. SECTION THROUGH
 +
THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN
 +
 +
28 F.
 +
 +
sp.c. spinal canal ; W. white
 +
matter of spinal cord ; pr. posterior nerve-roots ; cA. notochord ;
 +
x. sub-notochordal rod ; ao. aorta ;
 +
nip. muscle-plate ; nip 1 , inner layer
 +
of muscle-plate already converted
 +
into muscles; Vr. rudiment of
 +
vertebral body ; si. segmental
 +
tube ; sd. segmental duct ; sp.v.
 +
spiral valve ; v. subintestinal vein ;
 +
p.o. primitive generative cells.
 +
 +
40
 +
 +
 +
 +
626 ABDOMINAL PORES.
 +
 +
 +
 +
two halves of the body cavity never coalesce, but eventually the
 +
splanchnic layers of epithelium of the two sides, together with a
 +
thin layer of interposed mesoblast, form a delicate membrane,
 +
known as the mesentery, which suspends the gut from the dorsal
 +
wall of the body (figs. 119 and 351). On the dorsal side the
 +
epithelium lining of the body cavity is usually more columnar
 +
than elsewhere (fig. 351), and its cells partly form a covering for
 +
the generative organs, and partly give rise to the primitive
 +
germinal cells. This part of the epithelium is often known as
 +
the germinal epithelium.
 +
 +
Over the greater part of the body cavity the lining epithelium becomes in the adult intimately united with a layer of the
 +
subjacent connective tissue, and constitutes with it a special
 +
lining membrane for the body cavity, known as the peritoneal
 +
membrane.
 +
 +
Abdominal pores. In the Cyclostomata, the majority of the Elasmobranchii, the Ganoidei, a few Teleostei, the Dipnoi, and some Sauropsida
 +
(Chelonia and Crocodilia) the body cavity is in communication with the
 +
exterior by a pair of pores, known as abdominal pores, the external
 +
openings of which are usually situated in the cloaca 1 .
 +
 +
The ontogeny of these pores has as yet been but very slightly investigated.
 +
In the Lamprey they are formed as apertures leading from the body cavity
 +
into the excretory section of the primitive cloaca. This section would
 +
appear from Scott's (No. 87) observations to be derived from part of the
 +
hypoblastic cloacal section of the alimentary tract.
 +
 +
In all other cases they are formed in a region which appears to belong
 +
to the epiblastic region of the cloaca ; and from my observations on Elasmobranchs it may be certainly concluded that they are formed there
 +
in this group. They may appear as perforations (i) at the apices of
 +
papilliform prolongations of the body cavity, or (2) at the ends of cloacal
 +
pits directed from the exterior towards the body cavity, or (3) as simple
 +
slit-like openings.
 +
 +
Considering the difference in development between the abdominal pores
 +
of most types, and those of the Cyclostomata, it is open to doubt whether
 +
these two types of pores are strictly homologous.
 +
 +
In the Cyclostomata they serve for the passage outwards of the generative products, and they also have this function in some of the few Teleostei
 +
in which they are found ; and Gegenbaur and Bridge hold that the primitive
 +
mode of exit of the generative products, prior to the development of the
 +
Miillerian ducts, was probably by means of these pores. I have elsewhere
 +
 +
1 For a full account of these structures the reader is referred to T. W. Bridge,
 +
"Pori Abdominales of Vertebrata. " Journal of Anat. and Physiol. , Vol. XIV., 1879.
 +
 +
 +
 +
THE BODY CAVITY.
 +
 +
 +
 +
627
 +
 +
 +
 +
 +
suggested that the abdominal pores are perhaps remnants of the openings
 +
of segmental tubes ; there does not however appear to be any definite
 +
evidence in favour of this view, and it is more probable that they may have
 +
arisen as simple perforations of the body wall.
 +
 +
Pericardial cavity, pleural cavities, and diaphragm.
 +
 +
In all Vertebrata the heart is at first
 +
placed in the body cavity (fig. 353 A),
 +
but the part of the body cavity containing it afterwards becomes separated as
 +
a distinct cavity known as the pericardial cavity. In Elasmobranchii, Acipenser, etc. a passage is however left
 +
between the pericardial cavity and the
 +
body cavity ; and in the Lamprey a
 +
separation between the two cavities does
 +
not occur during the Ammoccete stage.
 +
In Elasmobranchii the pericardial
 +
cavity becomes established as a distinct
 +
space in front of the body cavity in the
 +
following way. When the two ductus
 +
Cuvieri, leading transversely from the
 +
sinus venosus to the cardinal veins, become developed, a horizontal septum,
 +
shewn on the right side in fig. 352, is
 +
formed to support them, stretching
 +
across from the splanchnic to the somatic side of the body cavity, and
 +
dividing the body cavity (fig. 352) in
 +
this part into (i) a dorsal section formed
 +
of a right and left division constituting
 +
the true body cavity (pp), and (2) a
 +
ventral part the pericardial cavity (pc).
 +
The septum is at first of a very small
 +
longitudinal extent, so that both in
 +
front and behind it (fig. 352 on the left
 +
side) the dorsal and ventral sections of the body cavity are in
 +
free communication. The septum soon however becomes prolonged, and ceasing to be quite horizontal, is directed obliquely
 +
upwards and forwards till it meets the dorsal wall of the body
 +
 +
40 2
 +
 +
 +
 +
-ht
 +
 +
 +
 +
FIG. 352. SECTION
 +
THROUGH THE TRUNK OF A
 +
SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN 28 F.
 +
 +
The figure shews the separation of the body cavity from
 +
the pericardial cavity by a
 +
horizontal septum in which
 +
runs the ductus Cuvieri ; on
 +
the left side is seen the narrow
 +
passage which remains connecting the two cavities.
 +
 +
sp.c. spinal canal ; w. white
 +
matter of spinal cord ; pr.
 +
commissure connecting the
 +
posterior nerve-roots ; ch. notochord ; x. sub-notochordal
 +
rod ; ao. aorta ; sv. sinus venosus ; cav. cardinal vein ; ht.
 +
heart ; pp. body cavity ; pc.
 +
pericardial cavity ; as. solid
 +
oesophagus ; /. liver ; nip. muscle-plate.
 +
 +
 +
 +
628 THE PERICARDIAL CAVITY.
 +
 +
Anteriorly all communication is thus early shut off between the
 +
body cavity and the pericardial cavity, but the two cavities still
 +
open freely into each other behind.
 +
 +
The front part of the body cavity, lying dorsal to the pericardial cavity, becomes gradually narrowed, and is wholly
 +
obliterated long before the close of embryonic life, so that in
 +
adult Elasmobranch Fishes there is no section of the body cavity
 +
dorsal to the pericardial cavity. The septum dividing the body
 +
cavity from the pericardial cavity is prolonged backwards, till it
 +
meets the ventral wall of the body at the point where the liver
 +
is attached by its ventral mesentery (falciform ligament). In
 +
this way the pericardial cavity becomes completely shut off from
 +
the body cavity, except, it would seem, for the narrow communications found in the adult. The origin of these communications
 +
has not however been satisfactorily worked out.
 +
 +
The septum between the pericardial cavity and the body
 +
cavity is attached on its dorsal aspect to the liver. It is at first
 +
nearly horizontal, but gradually assumes a more vertical position,
 +
and then, owing to the obliteration of the primitive anterior
 +
part of the body cavity, appears to mark the front boundary of
 +
the body cavity. The above description of the mode of formation of the pericardial cavity, and the explanation of its relations
 +
to the body cavity, probably holds true for Fishes generally.
 +
 +
In the higher types the earlier changes are precisely the
 +
same as those in Elasmobranch Fishes. The heart is at first
 +
placed within the body cavity attached to the ventral wall of
 +
the gut by a mesocardium (fig. 353 A). A horizontal septum is
 +
then formed, in which the ductus Cuvieri are placed, dividing
 +
the body cavity for a short distance into a dorsal (/./) and
 +
ventral (p.c) section (fig. 353 B). In Birds and Mammals, and
 +
probably also in Reptilia, the ventral and dorsal parts of the
 +
body cavity are at first in free communication both in front of
 +
and behind this septum. This is shewn for the Chick in
 +
fig- 353 A an d B, which are sections of the same chick, A being
 +
a little in front of B. The septum is soon continued forwards
 +
so as completely to separate the ventral pericardial and the
 +
dorsal body cavity in front, the pericardial cavity extending at
 +
this period considerably further forwards than the body cavity.
 +
 +
Since the horizontal septum, by its mode of origin, is
 +
 +
 +
 +
THE BODY CAVITY.
 +
 +
 +
 +
629
 +
 +
 +
 +
necessarily attached to the ventral side of the gut, the dorsal
 +
part of the primitive body space is divided into two halves by a
 +
median vertical septum formed of the gut and its mesentery
 +
(fig- 353 B). Posteriorly the horizontal septum grows in a
 +
slightly ventral direction along the under surface of the liver
 +
(fig- 354)j till it meets the abdominal wall of the body at the
 +
insertion of the falciform ligament, and thus completely shuts
 +
off the pericardial cavity from the body cavity. The horizontal
 +
septum forms, as is obvious from the above description, the
 +
dorsal wall of the pericardial cavity 1 .
 +
 +
A. B.
 +
 +
 +
 +
 +
 +
FIG. 353. TRANSVERSE SECTIONS THROUGH A CHICK EMBRYO WITH TWENTYONE MESOBLASTIC SOMITES TO SHEW THE FORMATION OF THE PERICARDIAI,
 +
CAVITY, A. BEING THE ANTERIOR SECTION.
 +
 +
p.p. body cavity; p.c. pericardial cavity; al. alimentary cavity ; au. auricle; v. ventricle; s.v. sinus venosus; d.c. ductus Cuvieri ; ao. aorta; nip. muscle-plate; me.
 +
medullary cord.
 +
 +
With the complete separation of the pericardial cavity from
 +
the body cavity, the first period in the development of these
 +
parts is completed, and the relations of the body cavity to the
 +
 +
1 Kolliker's account of this septum, which he calls the mesocardium laterale (No.
 +
298, p. 295), would seem to imply that in Mammals it is completed posteriorly even
 +
before the formation of the liver. I doubt whether this takes place quite so early as
 +
he implies, but have not yet determined its exact period by my own observations.
 +
 +
 +
 +
630
 +
 +
 +
 +
THE PERICARDIAL CAVITY.
 +
 +
 +
 +
pericardial cavity become precisely those found in the embryos
 +
of Elasmobranchii. The later changes are however very different. Whereas in Fishes the right and left sections of the body
 +
cavity dorsal to the pericardial cavity soon atrophy, in the
 +
higher types, in correlation with the relatively backward situation of the heart, they rapidly become larger, and receive the
 +
lungs which soon sprout out from the throat.
 +
 +
The diverticula which form the lungs grow out into the
 +
splanchnic mesoblast, in front of
 +
the body cavity ; but as they
 +
grow, they extend into the two
 +
anterior compartments of the body
 +
cavity, each attached by its mesentery to the mesentery of the
 +
gut (fig. 354, lg). They soon moreover extend beyond the region of
 +
the pericardium into the undivided
 +
body cavity behind. This holds
 +
not only for the embryos of the
 +
Amphibia and Sauropsida, but
 +
also for those of Mammalia.
 +
 +
To understand the further
 +
 +
rrianfrps in rhp nerirardial ravitv FlG> 354- SECTION THROUGH
 +
 +
pencaraiai cavity THECARDIACREGION OF AN EMBRYO
 +
 +
it is necessary to bear in mind its OF LACERTA MURALIS OF 9 MM. TO
 +
 +
, ,. ,, ,. . . SHEW THE MODE OF FORMATION OF
 +
 +
relations to the adjoining parts. THE PERICARDIAL CAVITY.
 +
 +
 +
 +
 +
'-/it
 +
 +
 +
 +
It lies at this period completely
 +
ventral to the two anterior pro
 +
 +
 +
ht. heart ; pc. pericardial cavity ;
 +
al. alimentary tract; lg. lung; /.
 +
liver ; pp. body cavity ; md. open
 +
longations of the body Cavity COn- end of Mullerian duct ; wd. Wolffian
 +
. . duct; vc. vena cava inferior; ao.
 +
 +
taming the lungs (fig. 354). Its aorta; ch. notochord; me. medullary
 +
 +
dorsal wall is attached to the gut, cord>
 +
 +
and is continuous with the mesentery of the gut passing to the
 +
dorsal abdominal wall, forming the posterior mediastinum of
 +
human anatomy.
 +
 +
The changes which next ensue consist essentially in the
 +
enlargement of the sections of the body cavity dorsal to the
 +
pericardial cavity. This enlargement takes place partly by the
 +
elongation of the posterior mediastinum, but still more by the
 +
two divisions of the body cavity which contain the lungs
 +
extending themselves ventrally round the outside of the peri
 +
 +
 +
THE BODY CAVITY.
 +
 +
 +
 +
631
 +
 +
 +
 +
cardial cavity. This process is illustrated by fig. 355, taken
 +
from an embryo Rabbit. The two dorsal sections of the body
 +
cavity (pl.p] finally extend so as completely to envelope the
 +
pericardial cavity (pc\ remaining however separated from each
 +
other below by a lamina extending from the ventral wall of the
 +
pericardial cavity to the body wall, which forms the anterior
 +
mediastinum of human anatomy.
 +
 +
By these changes the pericardial cavity is converted into a
 +
closed bag, completely surrounded at its sides by the two lateral
 +
halves of the body cavity, which were primitively placed
 +
 +
 +
 +
SJ3. C.
 +
 +
 +
 +
 +
FIG. 355. SECTION THROUGH AN ADVANCED EMBRYO OF A RABBIT TO SHEW
 +
HOW THE PERICARDIAL CAVITY BECOMES SURROUNDED BY THE PLEURAL
 +
CAVITIES.
 +
 +
ht. heart; pc. pericardial cavity; //./ pleural cavity; Ig. lung; al. alimentary
 +
tract; ao. dorsal aorta; ch. notochord; rp. rib; st. sternum; sp.c. spinal cord.
 +
 +
dorsally to it. These two sections of the body cavity, which in
 +
Amphibia and Sauropsida remain in free communication with
 +
the undivided peritoneal cavity behind, may, from the fact of
 +
their containing the lungs, be called the pleural cavities.
 +
 +
In Mammalia a further change takes place, in that, by the
 +
formation of a vertical partition across the body cavity, known
 +
as the diaphragm, the pleural cavities, containing the lungs,
 +
 +
 +
 +
632 THE VASCULAR SYSTEM.
 +
 +
become isolated from the remainder of the body or peritoneal
 +
cavity. As shewn by their development the so-called pleurae or
 +
pleural sacks are simply the peritoneal linings of the anterior
 +
divisions of the body cavity, shut off from the remainder of
 +
the body cavity by the diaphragm.
 +
 +
The exact mode of formation of the diaphragm is not fully
 +
made out ; the account of it recently given by Cadiat (No. 491)
 +
not being in my opinion completely satisfactory.
 +
 +
BIBLIOGRAPHY.
 +
 +
(491) M. Cadiat. "Du developpement de la partie cephalothoracique de 1'embryon, de la formation du diaphragme, des pleures, du pericarde, du pharynx et de
 +
1'cesophage." Journal de F Anatomic et de la Physiologic, Vol. xiv. 1878.
 +
 +
 +
 +
Vascular System.
 +
 +
The actual observations bearing on the origin of the vascular
 +
system, using the term to include the lymphatic system, are
 +
very scanty. It seems probable, mainly it must be admitted on
 +
d priori grounds, that vascular and lymphatic systems have
 +
originated from the conversion of indefinite spaces, primitively
 +
situated in the general connective tissue, into definite channels.
 +
It is quite certain that vascular systems have arisen independently in many types ; a very striking case of the kind being
 +
the development in certain parasitic Copepoda of a closed
 +
system of vessels with a red non-corpusculated blood (E. van
 +
Beneden, Heider), not found in any other Crustacea. Parts of
 +
vascular systems appear to have arisen in some cases by a
 +
canalization of cells.
 +
 +
The blood systems may either be closed or communicate
 +
with the body cavity. In cases where the primitive body cavity
 +
is atrophied or partially broken up into separate compartments
 +
(Insecta, Mollusca, Discophora, etc.) a free communication
 +
between the vascular system and the body cavity is usually
 +
present ; but in these cases the communication is no doubt
 +
secondary. On the whole it would seem probable that the
 +
vascular system has in most instances arisen independently of
 +
the body cavity, at least in types where the body cavity is
 +
 +
 +
 +
THE VASCULAR SYSTEM. 633
 +
 +
present in a well-developed condition. As pointed out by the
 +
Hertwigs, a vascular system is always absent where there is not
 +
a considerable development of connective tissue.
 +
 +
As to the ontogeny of the vascular channels there is still much to be
 +
made out both in Vertebrates and Invertebrates.
 +
 +
The smaller channels often rise by a canalization of cells. This process
 +
has been satisfactorily studied by Lankester in the Leech 1 , and may easily
 +
be observed in the blastoderm of the Chick or in the epiploon of a newlyborn Rabbit (Schafer, Ranvier). In either case the vessels arise from a network of cells, the superficial protoplasm and part of the nuclei giving rise
 +
to the walls, and the blood-corpuscles being derived either from nucleated
 +
masses set free within the vessels (the Chick) or from blood-corpuscles
 +
directly differentiated in the axes of the cells (Mammals).
 +
 +
Larger vessels would seem to be formed from solid cords of cells, the
 +
central cells becoming converted into the corpuscles, and the peripheral cells
 +
constituting the walls. This mode of formation has been observed by
 +
myself in the case of the Spider's heart, and by other observers in other
 +
Invertebrata. In the Vertebrata a more or less similar mode of formation
 +
appears to hold good for the larger vessels, but further investigations are
 +
still required on this subject. Gotte finds that in the Frog the larger vessels
 +
are formed as longitudinal spaces, and that the walls are derived from the
 +
indifferent cells bounding these spaces, which become flattened and united
 +
into a continuous layer.
 +
 +
The early formation of vessels in the Vertebrata takes place in the
 +
splanchnic mesoblast ; but this appears due to the fact that the circulation
 +
is at first mainly confined to the vitelline region, which is covered by
 +
splanchnic mesoblast.
 +
 +
The Heart.
 +
 +
The heart is essentially formed as a tubular cavity in the
 +
splanchnic mesoblast, on the ventral side of the throat, immediately behind the region of the visceral clefts. The walls of this
 +
cavity are formed of two layers, an outer thicker layer, which has
 +
at first only the form of a half tube, being incomplete on its
 +
dorsal side; and an inner lamina formed of delicate flattened
 +
cells. The latter is the epithelioid lining of the heart, and the
 +
cavity it contains the true cavity of the heart. The outer layer
 +
gives rise to the muscular wall and peritoneal covering of the
 +
heart. Though at first it has only the form of a half tube (fig.
 +
 +
1 "Connective and vasifactive tissues of the Leech." Quart. J. of Micr. Science,
 +
Vol. XX. 1880.
 +
 +
 +
 +
634
 +
 +
 +
 +
THE HEART.
 +
 +
 +
 +
356), it soon becomes folded in on the dorsal side so as to form
 +
for the heart a complete muscular wall. Its two sides, after thus
 +
meeting to complete the tube of
 +
the heart, remain at first continuous
 +
with the splanchnic mesoblast surrounding the throat, and form a provisional mesentery the mesocardium which attaches the heart to
 +
the ventral wall of the throat. The
 +
superficial stratum of the wall of
 +
the heart differentiates itself as the
 +
peritoneal covering. The inner epithelioid tube takes its origin at the
 +
time when the general cavity of the
 +
heart is being formed by the separation of the splanchnicmesoblastfrom
 +
the hypoblast. During this process
 +
(fig. 357) a layer of mesoblast remains close to the hypoblast, but connected with the main mass
 +
 +
 +
 +
 +
FIG. 356. SECTION THROUGH
 +
THE DEVELOPING HEART OF AN
 +
EMBRYO OF AN ELASMOBRANCH
 +
(Pristiurus).
 +
 +
al. alimentary tract ; sp. splanchnic mesoblast ; so. somatic mesoblast ; ht. heart.
 +
 +
 +
 +
 +
FIG. 357. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE
 +
HEAD OF AN EMBRYO CHICK OF THIRTY HOURS.
 +
 +
hb. hind-brain; vg. vagus nerve; ep. epiblast; ch. notochorcl ; x. thickening of
 +
hypoblast (possibly a rudiment of the sub-notochordal rod) ; al. throat; ht. heart;
 +
//. body cavity; so. somatic mesoblast; sf. splanchnic mesoblast; Ay. hypoblast.
 +
 +
 +
 +
THE VASCULAR SYSTEM.
 +
 +
 +
 +
635
 +
 +
 +
 +
of the mesoblast by protoplasmic processes. A second layer
 +
next becomes split from the splanchnic mesoblast, connected
 +
with the first layer by the above-mentioned protoplasmic
 +
processes. These two layers form together the epithelioid lining
 +
of the heart ; between them is the cavity of the heart, which soon
 +
loses the protoplasmic trabeculae which at first traverse it. The
 +
cavity of the heart may thus be described as being formed by a
 +
hollowing out of the splanchnic mesoblast, and resembles in its
 +
mode of origin that of other large vascular trunks.
 +
 +
The above description applies only to the development of
 +
the heart in those types in which it is formed at a period after
 +
the throat has become a closed tube (Elasmobranchii, Amphibia,
 +
Cyclostomata, Ganoids (?)). In a number of other cases, in
 +
which the heart is formed before the conversion of the throat
 +
into a closed tube, of which the most notable is that of Mammals
 +
(Hensen, Gotte, Kolliker), the heart arises as two independent
 +
 +
A.
 +
 +
 +
 +
 +
B.
 +
 +
 +
 +
mes fir
 +
 +
 +
 +
 +
FIG. 358. TRANSVERSE SECTION THROUGH THE HEAD OF A RABBIT OF THE
 +
 +
SAME AGE AS FIG. 144 B. (From Kolliker.)
 +
B is a more highly magnified representation of part of A.
 +
 +
rf. medullary groove; mp. medullary plate; riv. medullary fold; h. epiblast ;
 +
dd. hypoblast; dd' . notochordal thickening of hypoblast; sp. undivided mesoblast;
 +
^.somatic mesoblast; dfp. splanchnic mesoblast; ph. pericardial section of body
 +
cavity; ahh. muscular wall of heart; ihh. epithelioid layer of heart; vies, lateral
 +
undivided mesoblast ; sw. part of the hypoblast which will form the ventral wall of
 +
the pharynx.
 +
 +
 +
 +
636
 +
 +
 +
 +
THE HEART.
 +
 +
 +
 +
tubes (fig. 358), which eventually coalesce into an unpaired
 +
structure.
 +
 +
In Mammals the two tubes out of which the heart is formed appear at
 +
the sides of the cephalic plates, opposite the region of the mid- and hindbrain (fig. 358). They arise at a time when the lateral folds which form
 +
the ventral wall of the throat are only just becoming visible. Each half of
 +
the heart originates in the same way as the whole heart in Elasmobranchii,
 +
etc. ; and the layer of the splanchnic mesoblast, which forms the muscular
 +
wall for each part (ahh), has at first the form of a half tube open below to
 +
the hypoblast.
 +
 +
On the formation of the lateral folds of the splanchnic walls, the two
 +
halves of the heart become carried inwards and downwards, and eventually
 +
 +
 +
 +
 +
FlG. 359. TWO DIAGRAMMATIC SECTIONS THROUGH THE REGION OF THE
 +
HIND-BRAIN OF AN EMBRYO CHICK OF ABOUT 36 HOURS ILLUSTRATING THE
 +
FORMATION OF THE HEART.
 +
 +
fib. hind-brain ; nc. notochord ; E. epiblast ; so. somatopleure ; sp. splanchnopleure ; d. alimentary tract ; hy. hypoblast ; hs. heart ; of. vitelline veins.
 +
 +
 +
 +
THE VASCULAR SYSTEM.
 +
 +
 +
 +
637
 +
 +
 +
 +
meet on the ventral side of the throat. For a short time they here remain
 +
distinct, but soon coalesce into a single tube.
 +
 +
In Birds, as in Mammals, the heart makes its appearance as two tubes,
 +
but arises at a period when the formation of the throat is very much more
 +
advanced than in the case of Mammals. The heart arises immediately
 +
behind the point up to which the ventral wall of the throat is established
 +
and thus has at first a A -shaped form. At the apex of the A , which forms
 +
the anterior end of the heart, the two halves are in contact (fig. 357),
 +
though they have not coalesced; while behind they diverge to be continued
 +
as the vitelline veins. As the folding in of the throat is continued backwards the two limbs of the heart are brought together and soon coalesce
 +
from before backwards into a single structure. Fig. 359 A and B shews the
 +
heart during this process. The two halves have coalesced anteriorly (A)
 +
but are still widely separated behind (B). In Teleostei the heart is formed
 +
as in Birds and Mammals by the coalescence of two tubes, and it arises
 +
before the formation of the throat.
 +
 +
The fact that the heart arises in so many instances as a
 +
double tube might lead to the supposition that the ancestral
 +
Vertebrate had two tubes in the place of the present unpaired
 +
heart.
 +
 +
The following considerations appear to me to prove that this
 +
conclusion cannot be accepted. If the folding in of the splanchnopleure to form the throat were deferred relatively to the
 +
formation of the heart, it is clear that a modification in the
 +
development of the heart would occur, in that the two halves of
 +
the heart would necessarily be formed widely apart, and only
 +
eventually united on the folding in of the wall of the throat. It
 +
is therefore possible to explain the double formation of the heart
 +
without having recourse to the above hypothesis of an ancestral
 +
Vertebrate with two hearts. If the explanation just suggested
 +
is the true one the heart should only be formed as two tubes
 +
when it arises prior to the formation of the throat, and as a single
 +
tube when formed after the formation of the throat. Since this
 +
is invariably found to be so, it may be safely concluded that the
 +
formation of the heart as two cavities is a secondary mode of
 +
development, which has been brought about by variations in the
 +
period of the closing in of the wall of the throat.
 +
 +
The heart arises continuously with the sinus venosus, which in
 +
the Amniotic Vertebrata is directly continued into the vitelline
 +
veins. Though at first it ends blindly in front, it is very soon
 +
connected with the foremost aortic arches.
 +
 +
 +
 +
638 THE HEART.
 +
 +
 +
 +
The simple tubular heart, connected as above described, grows
 +
more rapidly than the chamber in which it is contained, and is
 +
soon doubled upon itself, acquiring in this way an S-shaped
 +
curvature, the posterior portion being placed dorsally, and the
 +
anterior ventrally. A constriction soon appears between the
 +
dorsal and ventral portions.
 +
 +
The dorsal section becomes partially divided off behind from
 +
the sinus venosus, and constitutes the relatively thin-walled
 +
auricular section of the heart; while the ventral portion, after
 +
becoming distinct anteriorly from a portion continued forwards
 +
from it to the origin of the branchial arteries, which may be called
 +
the truncus arteriosus, acquires very thick spongy muscular
 +
walls, and becomes the ventricular division of the heart.
 +
 +
The further changes in the heart are but slight in the case of the Pisces.
 +
A pair of simple membranous valves becomes established at the auriculoventricular orifice, and further changes take place in the truncus arteriosus.
 +
This part becomes divided in Elasmobranchii, Ganoidei, and Dipnoi into a
 +
posterior section, called the conus arteriosus, provided with a series of
 +
transverse rows of valves, and an anterior section, called the bulb us
 +
arteriosus, not provided with valves, and leading into the branchial
 +
arteries. In most Teleostei (except Butirinus and a few other forms) the
 +
conus arteriosus is all but obliterated, and the anterior row of its valves
 +
alone preserved ; and the bulbus is very much enlarged 1 .
 +
 +
In the Dipnoi important changes in the heart are effected, as compared
 +
with other Fishes, by the development of true lungs. Both the auricular
 +
and ventricular chamber may be imperfectly divided into two, and in the
 +
conus a partial longitudinal septum is developed in connection with a
 +
longitudinal row of valves 2 .
 +
 +
In Amphibia the heart is in many respects similar to that of the Dipnoi.
 +
Its curvature is rather that of a screw than of a simple S. The truncus
 +
arteriosus lies to the left, and is continued into the ventricle which lies
 +
ventrally and more to the right, and this again into the dorsally placed
 +
auricular section.
 +
 +
After the heart has reached the piscine stage, the auricular section
 +
(Bombinator) becomes prolonged into a right and left auricular appendage^
 +
A septum next grows from the roof of the auricular portion of the heart
 +
 +
1 Vide Gegenbaur, "Zur vergleich. Anat. d. Herzens." Jenaische Zeit., Vol. n.
 +
1866, and for recent important observations, J. E. V. Boas, "Ueb. Herz u. Arterienbogenbei Ceratodenu. Protopterus," and " Ueber d. Conus arter. b. Butirinus, etc.,"
 +
Morphol. Jahrb., Vol. VI. 1880.
 +
 +
2 Boas holds that the longitudinal septum is formed by the coalescence of a row of
 +
longitudinal valves, but this is opposed to Lankester's statements, "On the hearts of
 +
Ceratodus, Protopterus and Chimaera, etc. Zool. Trans. Vol. x. 1879.
 +
 +
 +
 +
THE VASCULAR SYSTEM. 639
 +
 +
 +
 +
obliquely backwards and towards the left, and divides it in two chambers ;
 +
the right one of which remains continuous with the sinus venosus, while
 +
the left one is completely shut off from the sinus, though it soon enters
 +
into communication with the newly established pulmonary veins. The
 +
truncus arteriosus 1 is divided into a posterior conus arteriosus (pylangium)
 +
and an anterior bulbus (synangium). The former is provided with a
 +
proximal row of valves at its ventricular end, and a distal row at its anterior
 +
end near the bulbus. It is also provided with a longitudinal septum, which
 +
is no doubt homologous with the septum in the conus arteriosus of the
 +
Dipnoi. The bulbus is well developed in many Urodela, but hardly exists
 +
in the Anura.
 +
 +
In the Amniota further changes take place in the heart,
 +
resulting in the abortion of the distal rows of valves of the conus
 +
arteriosus 2 , and in the splitting up of the whole truncus arteriosus
 +
into three vessels in Reptilia, and two in Birds and Mammals,
 +
each opening into the ventricular section of the heart, and
 +
provided with a special set of valves at its commencement. In
 +
Birds and Mammals the ventricle becomes moreover completely
 +
divided into two chambers, each communicating with one of the
 +
divisions of the primitive truncus, known in the higher types
 +
as the systemic and pulmonary aortae. The character of the
 +
development of the heart in the Amniota will be best understood
 +
from a description of what takes place in the Chick.
 +
 +
In Birds the originally straight heart (fig. 109) soon becomes doubled up
 +
upon itself. The ventricular portion becomes placed on the ventral and
 +
right side, while the auricular section is dorsal and to the left. The two
 +
parts are separated from each other by a slight constriction known as the
 +
canalis auricularis. Anteriorly the ventricular cavity is continued into the
 +
truncus, and the venous or auricular portion of the heart is similarly connected behind with the sinus venosus. The auricular appendages grow out
 +
from the auricle at a very early period. The general appearance of the
 +
heart, as seen from the ventral side on the fourth day, is shewn in fig. 360.
 +
Although the external divisions of the heart are well marked even before
 +
this stage, it is not till the end of the third day that the internal partitions
 +
become apparent ; and, contrary to what might have been anticipated from
 +
the evolution of these parts in the lower types, the ventricular septum is the
 +
first to be established.
 +
 +
1 For a good description of the adult heart vide Huxley, Article "Amphibia," in
 +
the Encyclopedia Britannic a.
 +
 +
2 It is just possible that the reverse may be true, vide note on p. 640. If however,
 +
as is most probable, the statement in the text is correct, the valves at the mouth of
 +
the ventricle in Teleostei are not homologous with those of the Amniota ; the former
 +
being the distal rov/ of the valves of the conus, the latter the proximal.
 +
 +
 +
 +
 +
640 THE HEART OF AVES.
 +
 +
It commences on the third day as a crescentic ridge or fold springing
 +
from the convex or ventral side of the rounded ventricular portion of the
 +
heart, and on the fourth day grows rapidly across the ventricular cavity
 +
towards the concave or dorsal side. It thus forms an incomplete longitudinal partition, extending from the canalis auricularis to the commencement
 +
of the truncus arteriosus, and dividing the twisted ventricular tube into
 +
two somewhat curved canals, one more
 +
to the left and above, the other to
 +
 +
the right and below. These commu- A ^) ) CA
 +
 +
nicate with each other, above the free
 +
edge of the partition, along its whole
 +
length.
 +
 +
Externally the ventricular portion
 +
as yet shews no division into two parts.
 +
 +
By the fifth day the venous end of
 +
the heart, though still lying somewhat
 +
to the left and above, is placed as far FIG. 360. HEART OF A CHICK ON
 +
 +
forwards as the arterial end, the whole THE FOURTH DAY OF INCUBATION
 +
 +
VIEWED FROM THE VENTRAL SURFACE.
 +
 +
organ appearing to be drawn together.
 +
 +
The ventricular septum is complete. L ?.- lef t a , uricular appendage; C.A.
 +
 +
, e .. , . , , canahs auricularis ; v. ventricle ; b. trun
 +
The apex of the ventricles becomes cus arteriosus.
 +
 +
more and more pointed. In the auricular portion a small longitudinal fold appears as the rudiment of the
 +
auricular septum, while in the canalis auricularis, which is now at its greatest
 +
length, there is also to be seen a commencement of the valvular structures
 +
tending to separate the cavity of the auricles from those of the ventricles.
 +
 +
About the io6th hour, a septum begins to make its appearance in the
 +
truncus arteriosus in the form of a longitudinal fold, which according to
 +
Tonge (No. 495) starts at the end of the truncus furthest removed from the
 +
heart. It takes origin from the wall of the truncus between the fourth and
 +
fifth pairs of arches, and grows downwards in such a manner as to divide the
 +
truncus into two channels, one of which leads from the heart to the third and
 +
fourth pairs of arches, and the other to the fifth pair. Its course downwards
 +
is not straight but spiral, and thus the two channels into which it divides
 +
the truncus arteriosus wind spirally the one round the other.
 +
 +
At the time when the septum is first formed, the opening of the truncus
 +
arteriosus into the ventricles is narrow or slit-like, apparently in order to
 +
prevent the flow of the blood back into the heart. Soon after the appearance
 +
of the septum, however, semilunar valves (Tonge, No. 495) are developed
 +
from the wall of that portion of the truncus which lies between the free edge
 +
of the septum and the cavity of the ventricles 1 .
 +
 +
1 If Tonge is correct in his statement that the semilunar valves develop at some
 +
distance from the mouth of the ventricle, it would seem possible that the portion of
 +
the truncus between them and the ventricle ought to be regarded as the embryonic
 +
conus arteriosus, and that the distal row of valves of the conus (and not the proximal
 +
as suggested above, p. 639) has been preserved in the higher types.
 +
 +
 +
 +
THE VASCULAR SYSTEM.
 +
 +
 +
 +
641
 +
 +
 +
 +
The ventral and the dorsal pairs of valves are the first to appear : the
 +
former as two small solid prominences separated from each other by a
 +
narrow groove ; the latter as a single ridge, in the centre of which is a
 +
prominence indicating the point where the ridge will subsequently become
 +
divided into two. The outer valves appear opposite each other, at a
 +
considerably later period.
 +
 +
As the septum grows downwards towards the heart, it finally reaches
 +
the position of these valves. One of its edges then passes between the two
 +
ventral valves, and the other unites with the prominence on the dorsal
 +
valve-ridge. At the same time the growth of all the parts causes the valves
 +
to appear to approach the heart, and thus to be placed quite at the top
 +
of the ventricular cavities. The free edge of the septum of the truncus now
 +
 +
A. B.
 +
 +
 +
 +
 +
 +
FlG. 361. TWO VIEWS OF THE HEART OF A CHICK UPON THE FIFTH DAY
 +
 +
OF INCUBATION.
 +
 +
A. from the ventral, B. from the dorsal side.
 +
 +
La. left auricular appendage; r.a. right auricular appendage ; r.v. right ventricle;
 +
l.v. left ventricle; b. truncus arteriosus.
 +
 +
fuses with the ventricular septum, and thus the division of the truncus into
 +
two separate channels, each provided with three valves, and each communicating with a separate side of the heart, is complete ; the position of
 +
the valves not being very different from that in the adult heart.
 +
 +
That division of the truncus which opens into the fifth pair of arches is
 +
the one which communicates with the right ventricle, while that which
 +
opens into the third and fourth pairs communicates with the left ventricle.
 +
The former becomes the pulmonary artery, the latter the commencement of
 +
the systemic aorta.
 +
 +
The external constriction actually dividing the truncus into two vessels
 +
does not begin to appear till the septum has extended some way back
 +
towards the heart.
 +
 +
The semilunar valves become pocketed at a period considerably later
 +
than their first formation (from the H7th to the,i65th hour) in the order of
 +
their appearance.
 +
 +
At the end of the sixth day, and even on the fifth day (figs. 361 and 362),
 +
the appearance of the heart itself, without reference to the vessels which
 +
come from it, is not very dissimilar from that of the adult. The original
 +
 +
 +
 +
B. III.
 +
 +
 +
 +
4 1
 +
 +
 +
 +
642
 +
 +
 +
 +
THE HEART OF MAMMALIA.
 +
 +
 +
 +
r.a
 +
 +
 +
 +
 +
l.v
 +
 +
 +
 +
FIG. 362. HEART OF A
 +
CHICK UPON THE SIXTH DAY
 +
OF INCUBATION, FROM THE
 +
VENTRAL SURFACE.
 +
 +
La. left auricular appendage ;
 +
r,a. right auricular appendage ;
 +
r.v. right ventricle ; l.v. left ventricle ; b. truncus arteriosus.
 +
 +
 +
 +
protuberance to the right now forms the apex of the ventricles, and the
 +
two auricular appendages are placed at the anterior extremity of the heart.
 +
The most noticeable difference (in the ventral
 +
view) is the still externally undivided condition of the truncus arteriosus.
 +
 +
The subsequent changes which the heart
 +
undergoes are concerned more with its internal structure than with its external shape.
 +
Indeed, during the next three days, viz. the
 +
eighth, ninth, and tenth, the external form of
 +
the heart remains nearly unaltered.
 +
 +
In the auricular portion, however, the
 +
septum which commenced on the fifth day
 +
becomes now more conspicuous. It is placed
 +
vertically, and arises from the ventral wall ;
 +
commencing at the canalis auricularis and
 +
proceeding towards the opening into the
 +
sinus venosus.
 +
 +
This latter structure gradually becomes
 +
reduced so as to become a special appendage
 +
of the right auricle. The inferior vena cava
 +
 +
enters the sinus obliquely from the right, so that its blood has a tendency to
 +
flow towards the left auricle of the heart, which is at this time the larger of
 +
the two.
 +
 +
The valves between the ventricles and auricles are now well developed,
 +
and it is about this time that the division of the truncus arteriosus into the
 +
aorta and pulmonary artery becomes visible from the exterior.
 +
 +
By the eleventh to the thirteenth day the right auricle has become as
 +
large as the left, and the auricular septum much more complete, though
 +
there is still a small opening, the foramen ovale, by which the two cavities
 +
communicate with each other.
 +
 +
The most important feature in which the development of the Reptilian
 +
heart differs from that of Birds is the division of the truncus into three
 +
vessels, instead of two. The three vessels remain bound up in a common
 +
sheath, and appear externally as a single trunk. The vessel not represented
 +
in Birds is that which is continued into the left aortic arch.
 +
 +
In Mammals the early stages in the development of the heart present no
 +
important points of difference from those of Aves. The septa in the truncus,
 +
in the ventricular, and in the auricular cavities are formed, so far as
 +
is known, in the same way and at the same relative periods in both groups.
 +
In the embryo Man, the Rabbit, and other Mammals the division of
 +
the ventricles is made apparent externally by a deep cleft, which, though
 +
evanescent in these forms, is permanent in the Dugong.
 +
 +
The attachment of the auriculo-ventricular valves to the wall of the
 +
ventricle, and the similar attachment of the left auriculo-ventricular valves
 +
in Birds, have been especially studied by Gegenbaur and Bernays (No. 492),
 +
 +
 +
 +
ARTERIAL SYSTEM. 643
 +
 +
 +
 +
and deserve to be noticed. In the primitive state the ventricular walls
 +
have throughout a spongy character ; and the auriculo-ventricular valves are
 +
simple membranous projections like the auriculo-ventricular valves of Fishes.
 +
Soon however the spongy muscular tissue of both the ventricular and
 +
auricular walls, which at first pass uninterruptedly the one into the other,
 +
grows into the bases of the valves, which thus become in the main muscular
 +
projections of the walls of the heart. As the wall of the ventricle thickens,
 +
the muscular trabeculas, connected at one end with the valves, remain at the
 +
other end united with the ventricular wall, and form special bands passing
 +
between the two. The valves on the other hand lose their muscular
 +
attachment to the auricular walls. This is the condition permanent in
 +
Ornithorhynchus. In higher Mammalia the ends of the muscular bands
 +
inserted into the valves become fibrous, from the development of intermuscular connective tissue, and the atrophy of the muscular elements.
 +
The fibrous parts now form the chordae tendinea?, and the muscular the
 +
musculi papillares.
 +
 +
The sinus venosus in Mammals becomes completely merged into the
 +
right auricle, and the systemic division of the truncus arteriosus is apparently not homologous with that in Birds.
 +
 +
In the embryos of all the Craniata the heart is situated very
 +
far forwards in the region of the head. This position is retained
 +
in Pisces. In Amphibia the heart is moved further back, while
 +
in all the Amniota it gradually shifts its position first of all into
 +
the region of the neck and finally passes completely within the
 +
thoracic cavity. The steps in the change of position may be
 +
gathered from figs. 109, in, and 118.
 +
 +
BIBLIOGRAPHY of the Heart.
 +
 +
(492) A. C. Bernays. " Entwicklungsgeschichte d. Atrioventricularklappen."
 +
Morphol. Jahrbuch,^o\. II. 1876.
 +
 +
(493) E. Gasser. " Ueber d. Entstehung d. Herzens beim Hiihn." Archiv f.
 +
mikr. Anat., Vol. xiv.
 +
 +
(494) A. Thomson. "On the development of the vascular system of the foetus
 +
of Vertebrated Animals." Edinb. New Phil. Journal, Vol. ix. 1830 and 1831.
 +
 +
(495) M. Tonge. "Observations on the development of the semilunar valves
 +
of the aorta and pulmonary artery of the heart of the Chick." Phil. Trans. CLIX.
 +
1869.
 +
 +
Vide also Von Baer (291), Rathke (300), Hensen (182), Kolliker (298), Gotte (296),
 +
and Balfour (292).
 +
 +
Arterial System.
 +
 +
In the embryos of Vertebrata the arterial system consists of
 +
a forward continuation of the truncus arteriosus, on the ventral
 +
 +
41 2
 +
 +
 +
 +
644
 +
 +
 +
 +
ARTERIES OF PISCES.
 +
 +
 +
 +
side of the throat (figs. 363, abr, and 364, a), which, with a few
 +
exceptions to be noticed below, divides into as many branches on
 +
each side as there are visceral arches. These branches, after
 +
traversing the visceral arches, unite on the dorsal side of the
 +
throat into a common trunk on each side. This trunk (figs. 363
 +
and 364) after giving off one (or more) vessels to the head (c and
 +
c] turns backv/ards, and bends in towards the middle line, close
 +
to its fellow, immediately below the notochord (figs. 21 and 116)
 +
and runs backwards in this situation towards the end of the tail.
 +
The two parallel trunks below the notochord fuse very early into
 +
a single trunk, the dorsal aorta (figs. 363, ad, and 364, a"}.
 +
 +
 +
 +
 +
ttbr v "a,
 +
 +
FIG. 363. DIAGRAMMATIC VIEW OF THE HEAD OF AN EMBRYO TELEOSTEAN,
 +
WITH THE PRIMITIVE VASCULAR TRUNKS. (From Gegenbaur.)
 +
 +
a. auricle ; v. ventricle ; abr. branchial artery ; c'. carotid ; ad. dorsal aorta ;
 +
s. branchial clefts; sv. sinus venosus; dc. ductus Cuvieri; n. nasal pit
 +
 +
There is given off from each collecting trunk from the visceral
 +
arches, or from the commencement of the dorsal aorta, a subclavian
 +
artery to each of the anterior limbs ; from near the anterior end
 +
of the dorsal aorta a vitelline artery (or before the dorsal aortae
 +
have united a pair of arteries fig. 125, R of A and L of A) to the
 +
yolk-sack, which subsequently becomes the main visceral artery 1 ;
 +
and from the dorsal aorta opposite the hind limbs one (or two)
 +
arteries on each side the iliac arteries to the hind limbs ; from
 +
these arteries the allantoic arteries are given off in the higher
 +
types, which remain as the hypogastric arteries after the
 +
disappearance of the allantois.
 +
 +
The primitive arrangement of the arterial trunks is with a
 +
few modifications retained in Fishes. With the development of
 +
the gills the vessels to the arches become divided into two parts
 +
connected by a capillary system in the gill folds, viz. into the
 +
 +
1 In Mammalia the superior inesenteric artery arises from the vitelline artery,
 +
which may probably be regarded as a primitive crclinco-mescnteric artery.
 +
 +
 +
 +
ARTERIAL SYSTEM.
 +
 +
 +
 +
branchial arteries bringing the blood to the gills from the truncus
 +
arteriosus, and the branchial veins transporting it to the dorsal
 +
aorta. The branchial vessels to those arches which do not bear
 +
gills, either wholly or partially atrophy; thus in Elasmobranchii
 +
the mandibular trunk, which is fully developed in the embryo
 +
(fig. 193, \av}, atrophies, except for a small remnant bringing
 +
blood to the rudimentary gill of the spiracle from the branchial
 +
vein of the hyoid arch. In Ganoids the mandibular artery
 +
atrophies, but the hyoid is usually preserved. In Teleostei both
 +
mandibular 1 and hyoid arteries are absent in the adult, except
 +
that there is usually left a rudiment of the hyoid, supplying the
 +
pseudobranch, which is similar to the rudiment of the mandibular
 +
artery in Elasmobranchii. In Dipnoi the mandibular artery
 +
atrophies, but the hyoid is sometimes preserved (Protopterus),
 +
and sometimes lost.
 +
 +
In Fishes provided with a well developed air-bladder this
 +
organ receives arteries, which arise sometimes from the dorsal
 +
aorta, sometimes from the caeliac arteries, and sometimes from
 +
the dorsal section of the last (fourth) branchial trunk. The
 +
latter origin is found in Polypterus and Amia, and seems to have
 +
been inherited by the Dipnoi where the air-bladder forms a true
 +
lung.
 +
 +
The pulmonary artery of all the air-breathing Vertebrata is derived from the pulmonary artery of the
 +
Dipnoi.
 +
 +
In all the types above Fishes considerable changes are
 +
effected in the primitive arrangement of the arteries in the
 +
visceral arches.
 +
 +
In Amphibia the piscine condition is most nearly retained 2 .
 +
The mandibular artery is never developed, and the hyoid artery
 +
is imperfect, being only connected with the cephalic vessels and
 +
never directly joining the dorsal aorta. It is moreover developed
 +
later than the arteries of the true branchial arches behind. The
 +
subclavian arteries spring from the common trunks which unite
 +
to form the dorsal aorta.
 +
 +
In the Urodela there are developed, in addition to the hyoid,
 +
 +
1 The mandibular artery is stated by Gotte never to be developed in Teleostei, but
 +
is distinctly figured in Lereboullet (No. 71).
 +
 +
2 In my account of the Amphibia, Gotte (No. 296) has been followed.
 +
 +
 +
 +
646 ARTERIES OF THE AMNIOTA.
 +
 +
four branchial arteries. The three foremost of these at first
 +
supply gills, and in the Perennibranchiate forms continue to do
 +
so through life. The fourth does not supply a gill, and very
 +
early gives off, as in the Dipnoi, a pulmonary branch.
 +
 +
The hyoid artery soon sends forward a lingual artery from its
 +
ventral end, and is at first continued to the carotid which grows
 +
forward from the dorsal part of the first branchial vessel.
 +
 +
In the Caducibranchiata, where the gills atrophy, the following
 +
changes take place. The remnant of the hyoid is continued
 +
entirely into the lingual artery. The first branchial is mainly
 +
continued into the carotid and other cephalic branches, but a
 +
narrow remnant of the trunk, which originally connected it with
 +
the dorsal aorta, remains, forming what is known as a ductus
 +
Botalli. A rete mirabile on its course is the remnant of the
 +
original gill.
 +
 +
The second and third branchial arches are continued as
 +
simple trunks into the dorsal aorta, and the blood from the fourth
 +
arch mainly passes to the lungs, but a narrow ductus Botalli still
 +
connects this arch with the dorsal aorta.
 +
 +
In the Anura the same number of arches is present in the
 +
embryo as in the Urodela, all four branchial arteries supplying
 +
branchiae, but the arrangement of the two posterior trunks is
 +
different from that in the Urodela. The third arch becomes at a
 +
very early period continued into a pulmonary vessel, a relativelynarrow branch connecting it with the second arch. The fourth
 +
arch joins the pulmonary branch of the third. At the metamorphosis the hyoid artery loses its connection with the carotid, and
 +
the only part of it which persists is the root of the lingual artery.
 +
The first branchial artery ceases to join the dorsal aorta, and
 +
forms the root of the carotid : the so-called carotid gland placed
 +
on its course is the remnant of the gill supplied by it before the
 +
metamorphosis.
 +
 +
The second artery forms a root of the dorsal aorta. The
 +
third, as in all the Amniota, now supplies the lungs, and also
 +
sends off a cutaneous branch. The fourth disappears. The
 +
connection of the pulmonary artery with both the third and
 +
fourth branchial arches in the embryo appears to me clearly to
 +
indicate that this artery was primitively derived from the fonrtli
 +
arc/i as in the Urodela, and that its permanent connection
 +
 +
 +
 +
ARTERIAL SYSTEM.
 +
 +
 +
 +
647
 +
 +
 +
 +
with the third arch in the Anura and in all the Amniota is
 +
secondary.
 +
 +
In the Amniota the metamorphosis of the arteries is in all
 +
cases very similar. Five arches, viz. the mandibular, hyoid, and
 +
three branchial arches are always developed (fig. 364), but, owing
 +
to the absence of branchiae,
 +
never function as branchial arteries. Of these the main parts of
 +
the first two, connecting the truncus arteriosus with the collecting
 +
trunk into which the arterial
 +
arches fall, always disappear, usually before the complete development of the arteries in the posterior arches.
 +
 +
The anterior part of the collecting trunk into which these
 +
vessels fall is not obliterated
 +
when they disappear, but is on
 +
the contrary continued forwards
 +
as a vessel supplying the brain,
 +
homologous with that found in
 +
Fishes. It constitutes the internal
 +
carotid. Similarly the anterior
 +
part of the trunk from which the mandibular and hyoid arteries
 +
sprang is continued forwards as a small vessel 1 , which at first
 +
passes to the oral region and constitutes in Reptiles the lingual
 +
artery, homologous with the lingual artery of the Amphibia ; but
 +
in Birds and Mammals becomes more important, and is then
 +
known as the external carotid (fig. 125). By these changes the
 +
roots of the external and internal carotids spring respectively
 +
from the ventral and dorsal ends of the primitive third artery,
 +
i.e. the artery of the first branchial arch (fig. 365, c and c'} ; and
 +
thus this arterial arch persists in all types as the common carotid,
 +
 +
 +
 +
 +
FIG. 364. DIAGRAM OF THE ARRANGEMENT OF THE ARTERIAL
 +
ARCHES IN AN EMBRYO OF ONE OF THE
 +
 +
AMNIOTA. (From Gegenbaur ; after
 +
RATHKE.)
 +
 +
a. ventral aorta; a", dorsal aorta;
 +
' 2 > 3> 4> 5- arterial arches ; c. carotid
 +
artery.
 +
 +
 +
 +
1 His (No. 232) describes in Man two ventral continuations of the truncus arteriosus, one derived from the mandibular artery, forming the external maxillary artery,
 +
and one from the hyoid artery, forming the lingual artery. The vessel from which
 +
they spring is the external carotid. These observations of His will very probably be
 +
found to hold true for other types.
 +
 +
 +
 +
6 4 8
 +
 +
 +
 +
ARTERIAL ARCHES OF THE AMNIOTA.
 +
 +
 +
 +
and the basal part of the internal carotid. The trunk connecting
 +
the third arterial arch with the system of the dorsal aorta persists
 +
in some Reptiles (Lacertilia, fig. 366 A) as a ductus Botalli, but
 +
is lost in the remaining Reptiles and in Birds and Mammals (fig.
 +
366 B, C, D). It disappears earliest in Mammals (fig. 365 C),
 +
later in Birds (fig. 365 B), and still later in the majority of
 +
Reptiles.
 +
 +
The fourth arch always continues to give rise, as in the Anura,
 +
to the system of the dorsal aorta.
 +
 +
In all Reptiles it persists on both sides (fig. 366 A and B),
 +
but with the division of the truncus arteriosus into three vessels
 +
 +
 +
 +
 +
ad
 +
 +
 +
 +
FIG. 365. DEVELOPMENT OF THE GREAT ARTERIAL TRUNKS IN THE EMBRYOS
 +
OF A. A LIZARD ; B. THE COMMON FOWL; C. THE PIG. (From Gegenbaur; after
 +
Rathke.)
 +
 +
The first two arches have disappeared in all three. In A and B the last three are
 +
still complete, but in C the last two are alone complete.
 +
 +
/. pulmonary artery springing from the fifth arch, but still connected with the
 +
system of the dorsal aorta by a ductus Botalli; c. external carotid; <'. internal
 +
carotid; ad. dorsal aorta; a. auricle; v. ventricle; n. nasal pit; m, rudiment of
 +
fore-limb.
 +
 +
one of these, i.e. that opening furthest to the left side of the
 +
ventricle (e and d), is continuous with the right fourth arch, and
 +
also with the common carotid arteries (c) ; while a second
 +
springing from the right side of the ventricle is continuous with
 +
the left fourth arch (Ji and f). The right and left divisions of the
 +
fourth arch meet however on the dorsal side of the oesophagus to
 +
give origin to the dorsal aorta (g).
 +
 +
In Birds (fig. 366 C) the left fourth arch (h) loses its connection with the dorsal aorta, though the ventral part remains as
 +
 +
 +
 +
ARTERIAL SYSTEM.
 +
 +
 +
 +
649
 +
 +
 +
 +
the root of the left subclavian. The truncus arteriosus is moreover only divided into two parts, one of which is continuous
 +
with all the systemic arteries. Thus it comes about that in
 +
Birds the right fourth arch (e) alone gives rise to the dorsal
 +
aorta.
 +
 +
In Mammals (fig. 366 D) the truncus arteriosus is only
 +
divided into two, but the left fourth arch (>), instead of the right,
 +
is that continuous with the dorsal aorta, and the right fourth
 +
arch (/) is only continued into the right vertebral and right
 +
subclavian arteries.
 +
 +
The fifth arch always gives origin to the pulmonary artery
 +
(fig. 365, /) and is continuous with one of the divisions of the
 +
truncus arteriosus. In Lizards (fig. 366 A, i), Chelonians and
 +
Birds (fig. 366 C, i] and probably in Crocodilia, the right and
 +
left pulmonary arteries spring respectively from the right and
 +
left fifth arches, and during the greater part of embryonic life
 +
the parts of the fifth arches between the origins of the pulmonary
 +
arteries and the system of the dorsal aorta are preserved as
 +
ductus Botalli. These ductus Botalli persist for life in the
 +
Chelonia. In Ophidia (fig. 366 B, Ji) and Mammalia (fig.
 +
366 D, m) only one of the fifth arches gives origin to the two
 +
pulmonary arteries, viz. that on the right side in Ophidia, and
 +
the left in Mammalia.
 +
 +
The ductus Botalli of the fifth arch (known in Man as the
 +
ductus arteriosus) of the side on which the pulmonary arteries
 +
are formed, may remain (e.g. in Man) as a solid cord connecting
 +
the common stem of the pulmonary aorta with the systemic
 +
aorta.
 +
 +
The main history of the arterial arches in the Amniota has
 +
been sufficiently dealt with, and the diagram, fig. 366, copied
 +
from Rathke, shews at a glance the character of the metamorphosis these arches undergo in the different types. It merely
 +
remains for me to say a few words about the subclavian and
 +
vertebral arteries.
 +
 +
The subclavian arteries in Fishes usually spring from the
 +
trunks connecting the branchial veins with the dorsal aorta.
 +
This origin, which is also found in Amphibia, is typically found
 +
in the embryos of the Amniota. In the Lizards this origin
 +
persists through life, but both subclavians spring from the right
 +
 +
 +
 +
650
 +
 +
 +
 +
ARTERIAL ARCHES OF THE AMNIOTA.
 +
 +
 +
 +
side. In most other types the origin of the subclavians is
 +
carried upwards, so that they usually spring from a trunk
 +
common to them and the carotids (arteria anonyma) (Birds and
 +
some Mammals); or the left one, as in Man and some other
 +
Mammals, arises from the systemic aorta just beyond the
 +
carotids. Various further modifications in the origin of the
 +
subclavians of the same general nature are found in Mammalia,
 +
A 13
 +
 +
 +
 +
 +
 +
FIG. 366. DIAGRAMS ILLUSTRATING THE METAMORPHOSIS OF THE ARTERIAL
 +
 +
ARCHES IN A LlZARD A, A SNAKE B, A BlRD C AND A MAMMAL D. (From
 +
Mivart ; after Rathke.)
 +
 +
A. a. internal carotid; b. external carotid ; c. common carotid; d. ductus Botalli
 +
between the third and fourth arches ; e. right aortic trunk ; /. subclavian ; g. dorsal
 +
aorta; h. left aortic trunk; i. pulmonary artery; k. rudiment of ductus Botalli
 +
between the pulmonary artery and the system of the dorsal aorta.
 +
 +
B. a. internal carotid; b. external carotid; c. common carotid; d. right aortic
 +
trunk; e. vertebral artery;/, left aortic trunk of dorsal aorta; h. pulmonary artery ;
 +
i. ductus Botalli of pulmonary artery.
 +
 +
C. a. internal carotid ; b. external carotid ; c. common carotid ; d. systemic
 +
aorta; e. fourth arch of right side (root of dorsal aorta);/, right subclavian; g. dorsal
 +
aorta; h, left subclavian (fourth arch of left side); i. pulmonary artery; k. and /.
 +
right and left ductus Botalli of pulmonary arteries.
 +
 +
D. a. internal carotid; b. external carotid; c. common carotid; d. systemic aorta;
 +
c. fourth arch of left side (root of dorsal aorta);/ dorsal aorta; g. left vertebral
 +
artery; h. left subclavian artery; i. right subclavian (fourth arch of right side); k.
 +
right vertebral; /. continuation of right subclavian; in. pulmonary artery; n. ductus
 +
Botalli of pulmonary artery.
 +
 +
 +
 +
THE VENOUS SYSTEM.
 +
 +
 +
 +
6 5 I
 +
 +
 +
 +
but they need not be specified in detail. The vertebral arteries
 +
usually arise in close connection with the subclavians, but in
 +
Birds they arise from the common carotids.
 +
 +
BIBLIOGRAPHY of the Arterial System.
 +
 +
(496) H. Rathke. " Ueb. d. Entwick. d. Arterien vv. bei d. Saugethiere von
 +
d. Bogen d. Aorta ausgehen." Miiller's Archiv, 1843.
 +
 +
(-197) H. Rathke. " Untersuchungen lib. d. Aortenwurzeln d. Saurier."
 +
Denkschriften d. k. Akad. Wien, Vol. XIII. 1857.
 +
 +
Vide also His (No. 232) and general works on Vertebrate Embryology.
 +
 +
TJie Venous System,.
 +
 +
The venous system, as it is found in the embryos of Fishes,
 +
consists in its earliest condition of a single large trunk, which
 +
traverses the splanchnic mesoblast investing the part of the
 +
alimentary tract behind the heart. This trunk is directly continuous in front with the heart, and underlies the alimentary
 +
canal through both its praeanal and postanal sections. It is
 +
shewn in section in fig. 367, v, and may be called the subintestinal vein. This vein has been found in the embryos of
 +
Teleostei, Ganoidei, Elasmobranchii and Cyclostomata, and runs
 +
parallel to the dorsal aorta above, into which it is sometimes
 +
continued behind (Teleostei, Ganoidei, etc.).
 +
 +
In Elasmobranch embryos the subintestinal vein terminates,
 +
as may be gathered from sections (fig. 368, v.cau), shortly before
 +
the end of the tail. The same series of sections also shews that
 +
at the cloaca, where the gut enlarges and comes in contact with
 +
the skin, this vein bifurcates, the two branches uniting into a
 +
single vein both in front of and behind the cloaca.
 +
 +
In most Fishes the anterior part of this vein atrophies, the
 +
caudal section alone remaining, but the anterior section of it
 +
persists in the fold of the intestine in Petromyzon, and also
 +
remains in the spiral valve of some Elasmobranchii. In
 +
Amphioxus, moreover, it forms, as in the embryos of higher
 +
types, the main venous trunk, though even here it is usually
 +
broken up into two or three parallel vessels.
 +
 +
It no doubt represents one of the primitive longitudinal trunks of the
 +
vermiform ancestors of the Chordata. The heart and the branchial artery
 +
constitute a specially modified anterior continuation of this vein. The
 +
 +
 +
 +
652
 +
 +
 +
 +
THE SUBINTESTINAL VEIN.
 +
 +
 +
 +
-p.o
 +
 +
 +
 +
rp.r.
 +
 +
 +
 +
dilated portal sinus of Myxine is probably also part of it ; and if this is
 +
really rhythmically contractile 1 the fact would be interesting as shewing that
 +
this quality, which is now localised in the heart, was once probably common
 +
to the subintestinal vessel for its whole length.
 +
 +
On the development of the cardinal veins (to be described
 +
below) considerable changes are
 +
effected in the subintestinal vein.
 +
Its postanal section, which is known
 +
in the adult as the caudal vein,
 +
unites with the cardinal veins. On
 +
this junction being effected retrogressive changes take place in the
 +
praeanal section of the original subintestinal vessel. It breaks up in
 +
front into a number of smaller
 +
vessels, the most important of which
 +
is a special vein, which lies in the
 +
fold of the spiral valve, and which is
 +
more conspicuous in some Elasmobranchii than in Scyllium, in which
 +
the development of the vessel has
 +
been mainly studied. The lesser of
 +
the two branches connecting it
 +
round the cloaca with the caudal
 +
vein first vanishes, and then the
 +
larger ; and the two posterior cardinals are left as the sole forward
 +
continuations of the caudal vein.
 +
The latter then becomes prolonged
 +
forwards, so that the two cardinals
 +
open into it some little distance in
 +
front of the hind end of the kidneys.
 +
By these changes, and by the disappearance of the postanal section of the gut, the caudal vein is
 +
made to appear as a supraintestinal and not, as it really is, a
 +
subintestinal vessel.
 +
 +
From the subintestinal vein there is given off a branch which
 +
supplies the yolk-sack. This leaves the subintestinal vein close
 +
 +
1 J. Miiller holds that this sack is not rhythmically contractile.
 +
 +
 +
 +
 +
FIG. 367. SECTION THROUGH
 +
THE TRUNK OF A SCYLLIUM
 +
EMBRYO SLIGHTLY YOUNGER
 +
 +
THAN 28 F.
 +
 +
sp.c. spinal canal; W. white
 +
matter of spinal cord ; pr. posterior nerve-roots; ch. notochord ;
 +
x. sub-notochordal rod ; ao. aorta ;
 +
mp. muscle plate; ;;//'. inner layer
 +
of muscle-plate already converted
 +
into muscles; Vr. rudiment of
 +
vertebral body; st. segmental
 +
tube ; sd. segmental duct ; sp.v.
 +
spiral valve; v. subintestinal vein ;
 +
p.o. primitive generative cells.
 +
 +
 +
 +
THE VENOUS SYSTEM.
 +
 +
 +
 +
653
 +
 +
 +
 +
to the liver. The liver, on its development, embraces the
 +
subintestinal vein, which then breaks up into a capillary system
 +
in the liver, the main part of its blood coming at this period
 +
from the yolk-sack.
 +
 +
The portal system is thus established from the subintestinal
 +
vein ; but is eventually joined by the various visceral, and sometimes by the genital, veins as they become successively developed.
 +
 +
The blood from the liver is brought back to the sinus venosus by veins known as the hepatic veins, which, like the hepatic
 +
capillary system, are derivatives of the subintestinal vessel.
 +
 +
There join the portal system in Myxinoids and many
 +
Teleostei a number of veins from the anterior abdominal walls,
 +
representing a commencement of the anterior abdominal or
 +
epigastric vein of higher types 1 .
 +
 +
In the higher Vertebrates the original subintestinal vessel never attains a
 +
full development, even in the embryo. It is represented by (i) the ductus
 +
 +
 +
 +
 +
FIG. 368. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL
 +
OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F.
 +
 +
A. is the posterior section.
 +
 +
nc. neural canal; al. post-anal gut; alv. caudal vesicle of post-anal gut; x.
 +
subnotochordal rod; mp. muscle-plate; c/i. notochord; cl.al. cloaca; ao. aorta;
 +
v.cait. caudal vein.
 +
 +
1 Stannius, Vergleich. Anat., p. 251.
 +
 +
 +
 +
654
 +
 +
 +
 +
THE CARDINAL VEINS.
 +
 +
 +
 +
venosus, which, like the true subintestinal vein, gives origin (in the Amniota)
 +
to the vitelline veins to the yolk-sack, and (2) by the caudal vein. Whether
 +
the partial atrophy of the subintestinal vessel was primitively caused by the
 +
development of the cardinal veins, or for some other reason, it is at any rate
 +
a fact that in all existing Fishes the cardinal veins form the main venous
 +
channels of the trunk.
 +
 +
Their later development than the subintestinal vessel as well as their
 +
absence in Amphioxus, probably indicate that they became evolved, at any
 +
rate in their present form, within the Vertebrate phylum.
 +
 +
The embryonic condition of the venous system, with a single
 +
large subintestinal vein is, as has been stated, always modified
 +
by the development of a paired system of vessels, known as the
 +
cardinal veins, which bring to the heart the greater part of the
 +
blood from the trunk.
 +
 +
The cardinal veins appear in Fishes as four paired longitudinal trunks (figs. 363 and 369), two anterior (/) and two
 +
posterior (c). They unite into two transverse trunks on either
 +
side, known as the ductus Cuvieri (dc), which fall into the sinus
 +
venosus, passing from the body wall to the sinus by a lateral
 +
mesentery of the heart already spoken of (p. 627, fig. 352). The
 +
anterior pair, known as the anterior cardinal or jugular veins,
 +
bring to the heart the blood from the head and neck. They
 +
are placed one on each side above the level
 +
of the branchial arches (fig. 299, a.cv). The
 +
posterior cardinal veins lie immediately dorsal to the mesonephros (Wolfifian body), and
 +
are mainly supplied by the blood from this
 +
organ and from the walls of the body (fig.
 +
275, c.a.v). In many forms (Cyclostomata,
 +
Elasmobranchii and many Teleostei) they
 +
unite posteriorly with the caudal veins in
 +
the manner already described, and in a large
 +
number of instances the connecting branch
 +
between the two systems, in its passage through
 +
the mesonephros, breaks up into a capillary
 +
network, and so gives rise to a renal portal
 +
system.
 +
 +
The vein from the anterior pair of fins
 +
(subclavian) usually unites with the anterior
 +
jugular vein.
 +
 +
 +
 +
 +
j
 +
 +
 +
 +
FIG. 369. DIAGRAM OF THE PAIRED VENOUS SYSTEM
 +
 +
OF A FISH. (From
 +
Gegenbaur. )
 +
 +
j. jugular vein
 +
(anterior cardinal
 +
vein) ; c. posterior
 +
cardinal vein; //. hepatic veins ; sv. sinus
 +
venosus ; dc. ductus
 +
Cuvieri.
 +
 +
 +
 +
THE VENOUS SYSTEM. 655
 +
 +
The venous system of the Amphibia and Amniota always
 +
differs from that of Fishes in the presence of a new vessel, the
 +
vena cava inferior, which replaces the posterior cardinal veins;
 +
the latter only being present, in their piscine form, during
 +
embryonic life. It further differs from that of all Fishes, except
 +
the Dipnoi, in the presence of pulmonary veins bringing back
 +
the blood directly from the lungs.
 +
 +
In the embryos of all the higher forms the general characters
 +
of the venous system are at first the same as in Fishes, but with
 +
the development of the vena cava inferior the front sections of
 +
the posterior cardinal veins atrophy, and the ductus Cuvieri,
 +
remaining solely connected with the anterior cardinals and their
 +
derivatives, constitute the superior venae cavae. The inferior
 +
cava receives the hepatic veins.
 +
 +
Apart from the non-development of the subintestinal vein
 +
the visceral section of the venous system is very similar to that
 +
in Fishes.
 +
 +
The further changes in the venous system must be dealt
 +
with separately for each group.
 +
 +
Amphibia. In Amphibia (Gotte, No. 296) the anterior and posterior
 +
cardinal veins arise as in Pisces. From the former the internal jugular vein
 +
arises as a branch ; the external jugular constituting the main stem. The
 +
subclavian with its large cutaneous branch also springs from the system of
 +
the anterior cardinal. The common trunk formed by the junction of these
 +
three veins falls into the ductus Cuvieri.
 +
 +
The posterior cardinal veins occupy the same position as in Pisces, and
 +
unite behind with the caudal veins, which Gotte has shewn to be originally
 +
situated below the post-anal gut. The iliac veins unite with the posterior
 +
cardinal veins, where the latter fall into the caudal vein. The original
 +
piscine condition of the veins is not long retained. It is first of all disturbed
 +
by the development of the anterior part of the important unpaired venous
 +
trunk which forms in the adult the vena cava inferior. This is developed
 +
independently, but unites behind with the right posterior cardinal. From
 +
this point backwards the two cardinal veins coalesce for some distance, to
 +
give rise to the posterior section of the vena cava inferior, situated between
 +
the kidneys 1 . The anterior sections of the cardinal veins subsequently
 +
atrophy. The posterior part of the cardinal veins, from their junction with
 +
the vena cava inferior to the caudal veins, forms a rhomboidal figure. The
 +
iliac vein joins the outer angle of this figure, and is thus in direct communication with the inferior vena cava, but it is also connected with a longitu
 +
1 This statement of Gotte's is opposed to that of Rathke for the Amniota, and
 +
cannot be considered as completely established.
 +
 +
 +
 +
656 VEINS OF THE SNAKE.
 +
 +
dinal vessel on the outer border of the kidneys, which receives transverse
 +
vertebral veins and transmits their blood to the kidneys, thus forming a
 +
renal portal system. The anterior limbs of the rhomboid formed by the
 +
cardinal veins soon atrophy, so that the blood from the hind limbs can only
 +
pass to the inferior vena cava through the renal portal system. The
 +
posterior parts of the two cardinal veins (uniting in the Urodela directly
 +
with the unpaired caudal vein) still persist. The iliac veins also become
 +
directly connected with a new vein, the anterior abdominal vein, which
 +
has meanwhile become developed. Thus the iliac veins become united
 +
with the system of the vena cava inferior through the vena renalis advehens
 +
on the outer border of the kidney, and with the anterior abdominal veins by
 +
the epigastric veins.
 +
 +
The visceral venous system begins with the development of two vitelline
 +
veins, which at first join the sinus venosus directly. They soon become
 +
enveloped in the liver, where they break up into a capillary system, which
 +
is also joined by the other veins from the viscera. The hepatic system has
 +
in fact the same relations as in Fishes. Into this system the anterior
 +
abdominal vein also pours itself in the adult. This vein is originally
 +
formed of two vessels, which at first fall directly into the sinus venosus,
 +
uniting close to their opening into the sinus with a vein from the truncus
 +
arteriosus. They become prolonged backwards, and after receiving the
 +
epigastric veins above mentioned from the iliac veins, and also veins from
 +
the allantoic bladder, unite behind into a single vessel. Anteriorly the
 +
right vein atrophies and the left continues forward the unpaired posterior
 +
section.
 +
 +
A secondary connection becomes established between the anterior abdominal vein and the portal system ; so that the blood originally transported
 +
by the former vein to the heart becomes diverted so as to fall into the liver.
 +
A remnant of the primitive connection is still retained in the adult in the
 +
form of a small vein, the so-called vena bulbi posterior, which brings the
 +
blood from the walls of the truncus arteriosus directly into the anterior
 +
abdominal vein.
 +
 +
The pulmonary veins grow directly from the heart to the lungs.
 +
 +
For our knowledge of the development of the venous system of the
 +
Amniota we are mainly indebted to Rathke.
 +
 +
Reptilia. As an example of the Reptilia the Snake may be selected,
 +
its venous system having been fully worked out by Rathke in his important
 +
memoir on its development (No. 300).
 +
 +
The anterior (external jugular) and posterior cardinal veins are formed in
 +
the embryo as in all other types (fig. 370, vj and vc] ; and the anterior
 +
cardinal, after giving rise to the anterior vertebral and to the cephalic veins,
 +
persists with but slight modifications in the adult ; while the two ductus
 +
Cuvieri constitute the superior venos cavas.
 +
 +
The two posterior cardinals unite behind with the caudal veins. They
 +
are placed in the usual situation on the dorsal and outer border of the
 +
kidneys.
 +
 +
 +
 +
THE VENOUS SYSTEM.
 +
 +
 +
 +
657
 +
 +
 +
 +
 +
U
 +
FIG. 370. ANTERIOR
 +
PORTION OF THE VENOUS
 +
SYSTEM OF AN EMBRYONIC
 +
SNAKE. (From Gegenbaur;
 +
after Rathke.)
 +
 +
vc. posterior cardinal
 +
vein; vj. jugular vein; DC.
 +
ductus Cuvieri ; vu. allantoic vein ; v. ventricle ; ba.
 +
truncus arteriosus ; a. visceral clefts ; /. auditory
 +
vesicle.
 +
 +
 +
 +
With the development of the vena cava inferior, to be described below,
 +
the blood from the kidneys becomes mainly
 +
transported by this vessel to the heart ; and the
 +
section of the posterior cardinals opening into
 +
the ductus Cuvieri gradually atrophies, their
 +
posterior parts remaining however on the outer
 +
border of the kidneys as the vena? renales
 +
advehentes 1 .
 +
 +
While the front part of the posterior cardinal
 +
veins is undergoing atrophy, the intercostal veins,
 +
which originally poured their blood into the
 +
posterior cardinal veins, become also connected
 +
with two longitudinal veins the posterior vertebral veins which are homologous with the
 +
azygos and hemiazygos veins of Man ; and bear
 +
the same relation to the anterior vertebral veins
 +
that the anterior and posterior cardinals do to
 +
each other.
 +
 +
These veins are at first connected by trans
 +
verse anastomoses with the posterior cardinals,
 +
but, on the disappearance of the front part of the
 +
latter, the whole of the blood from the intercostal veins falls into the
 +
posterior vertebral veins. They are united in front with the anterior vertebral veins, and the common trunk of the two veins on each side falls into
 +
the jugular vein.
 +
 +
The posterior vertebral veins are at first symmetrical, but after becoming
 +
connected by transverse anastomoses, the right becomes the more important
 +
of the two.
 +
 +
The vena cava inferior, though considerably later in its development
 +
than the cardinals, arises fairly early. It constitutes in front an unpaired
 +
trunk, at first very small, opening into the right allantoic vein, close to the
 +
heart. Posteriorly it is continuous with two veins placed on the inner
 +
border of the kidneys 2 .
 +
 +
The vena cava inferior passes through the dorsal part of the liver, and in
 +
doing so receives the hepatic veins.
 +
 +
The portal system is at first constituted by the vitelline vein, which is
 +
directly continuous with the venous end of the heart, and at first receives
 +
the two ductus Cuvieri, but at a later period unites with the left ductus.
 +
 +
1 Rathke's account of the vena renalis advehens is thus entirely opposed to that
 +
which Gotte gives for the Frog, but my own observations on the Lizard incline me to
 +
accept Rathke's statements, for the Amniota at any rate.
 +
 +
2 The vena cava inferior does not according to Rathke's account unite behind with
 +
the posterior cardinal veins, as it is stated by Gotte to do in the Anura. Gb'tte
 +
questions the accuracy of Rathke's statements on this head, but my own observations
 +
are entirely in favour of Rathke's observations, and lend no support whatever to
 +
Gotte's views.
 +
 +
 +
 +
B. III.
 +
 +
 +
 +
658 VEINS OF THE CHICK.
 +
 +
It soon receives a mesenteric vein bringing the blood from the viscera,
 +
which is small at first but rapidly increases in importance.
 +
 +
The common trunk of the vitelline and mesenteric veins, which may be
 +
called the portal vein, becomes early enveloped by the liver, and gives off
 +
branches to this organ, the blood from which passes by the hepatic veins
 +
to the vena cava inferior. As the branches in the liver become more
 +
important, less and less blood is directly transported to the heart, and finally
 +
the part of the original vitelline vein in front of the liver is absorbed, and the
 +
whole of the blood from the portal system passes from the liver into the
 +
vena cava inferior.
 +
 +
The last section of the venous system to be dealt with is that of the
 +
anterior abdominal vein. There are originally, as in the Anura, two veins
 +
belonging to this system, which owing to the precocious development of the
 +
bladder to form the allantois, constitute the allantoic veins (fig. 370, vu}.
 +
 +
These veins, running along the anterior abdominal wall, are formed
 +
somewhat later than the vitelline vein, and fall into the two ductus Cuvieri.
 +
They unite with two epigastric veins (homologous with those in the Anura),
 +
which connect them with the system of the posterior cardinal veins. The
 +
left of the two eventually atrophies, so that there is formed an unpaired
 +
allantoic vein. This vein at first receives the vena cava inferior close to the
 +
heart, but eventually the junction of the two takes place in the region of the
 +
liver, and finally the anterior abdominal vein (as it comes to be after the
 +
atrophy of the allantois) joins the portal system and breaks up into capillaries
 +
in the liver 1 .
 +
 +
In Lizards the iliac veins join the posterior cardinals, and so pour part of
 +
their blood into the kidneys ; they also become connected by the epigastric
 +
veins with the system of the anterior abdominal or allantoic vein. The
 +
subclavian veins join the system of the superior venae cavas.
 +
 +
The venous system of Birds and Mammals differs in two important
 +
points from that of Reptilia and Amphibia. Firstly the anterior abdominal
 +
vein is only a foetal vessel, forming during foetal life the allantoic vein ;
 +
and secondly a direct connection is established between the vena cava
 +
inferior and the veins of the hind limbs and posterior parts of the cardinal
 +
veins, so that there is no renal portal system.
 +
 +
Aves. The Chick may be taken to illustrate the development of the
 +
venous system in Birds.
 +
 +
On the third day, nearly the whole of the venous blood from the body
 +
of the embryo is carried back to the heart by two main venous trunks,
 +
the anterior (fig. 125, S.Ca.V) and posterior (V.Ca) cardinal veins, joining on
 +
each side to form the short transverse ductus Cuvieri (DC), both of which
 +
unite with the sinus venosus close to the heart. As the head and neck
 +
continue to enlarge, and the wings become developed, the single anterior
 +
 +
1 The junction between the portal system and the anterior abdominal vein is
 +
apparently denied by Rathke (No. 300, p. 173), hut this must he an error on
 +
his part.
 +
 +
 +
 +
THE VENOUS SYSTEM.
 +
 +
 +
 +
659
 +
 +
 +
 +
 +
V.C.L
 +
 +
 +
 +
cardinal or jugular vein (fig. 371, /), of each side, is joined by two new
 +
veins : the vertebral vein, bringing back blood from the head and neck, and
 +
the subclavian vein from the wing (W\
 +
 +
On the third day the posterior cardinal veins are the only veins which
 +
return the blood from the hinder part of the body of the embryo.
 +
 +
About the fourth or fifth day, however, the vena cava inferior (fig. 371,
 +
V.C.L) makes its appearance. This, starting
 +
from the sinus venosus not far from the heart,
 +
is on the fifth day a short trunk running backward in the middle line below the aorta, and
 +
speedily losing itself in the tissues of the
 +
Wolffian bodies. When the true kidneys are
 +
formed it also receives blood from them, and
 +
thenceforward enlarging rapidly becomes the
 +
channel by which the greater part of the blood
 +
from the hinder part of the body finds its way
 +
to the heart. In proportion as the vena cava
 +
inferior increases in size, the posterior cardinal
 +
veins diminish.
 +
 +
The blood originally coming to them from
 +
the posterior part of the spinal cord and trunk
 +
is transported into two posterior vertebral veins,
 +
similar to those in Reptilia, which are however
 +
placed dorsally to the heads of the ribs, and
 +
join the anterior vertebral veins. With their
 +
appearance the anterior parts of the posterior
 +
cardinals disappear. The blood from the hind
 +
limbs becomes transported directly through the
 +
kidney into the vena cava inferior, without
 +
forming a renal portal system 1 .
 +
 +
On the third day the course of the vessels from the yolk-sack is very
 +
simple. The two vitelline veins, of which the right is already the smaller,
 +
form the ductus venosus, from which, as it passes through the liver on its
 +
way to the heart, are given off the two sets of vena advehentes and vena
 +
revehentes (fig. 371).
 +
 +
With the appearance of the allantois on the fourth day, a new feature is
 +
introduced. From the ductus venosus there is given off a vein which
 +
quickly divides into two branches. These, running along the ventral walls
 +
of the body from which they receive some amount of blood, pass to the
 +
allantois. They are the allantoic veins (fig. 371, U] homologous with the
 +
anterior abdominal vein of the lower types. They unite in front to form a
 +
single vein, which becomes, by reason of the rapid growth of the allantois,
 +
very long. The right branch soon diminishes in size and finally disappears.
 +
Meanwhile the left on reaching the allantois bifurcates ; and, its two
 +
 +
 +
 +
FIG. 371. DIAGRAM OF
 +
THE VENOUS CIRCULATION
 +
IN THE CHICK AT THE COMMENCEMENT OF THE FIFTH
 +
 +
DAY.
 +
 +
H. heart ; d. c. ductus Cuvieri. Into the ductus Cuvieri
 +
of each side fall/, the jugular
 +
vein, W. the vein from the
 +
wing, and c. the inferior cardinal vein ; S. V. sinus venosus ;
 +
Of. vitelline vein ; U. allantoic vein, which at this stage
 +
gives off branches to the bodywalls ; V.C.l. inferior vena
 +
cava ; /. liver.
 +
 +
 +
 +
The mode in which this is effected requires further investigation.
 +
 +
42 2
 +
 +
 +
 +
66o
 +
 +
 +
 +
VEINS OF THE CHICK.
 +
 +
 +
 +
 +
branches becoming large and conspicuous, there still appear to be two
 +
main allantoic veins. At its first appearance the allantoic vein seems to be
 +
but a small branch of the vitelline, but as the allantois grows rapidly,
 +
and the yolk-sack dwindles, this state of things is reversed, and the less conspicuous vitelline appears as a branch of the larger allantoic vein.
 +
 +
On the third day the blood returning from the walls of the intestine is
 +
insignificant in amount. As however the
 +
intestine becomes more and more developed, it acquires a distinct venous system,
 +
and its blood is returned by veins which
 +
form a trunk, the mesenteric vein (fig. 372,
 +
M") falling into the vitelline vein at its
 +
junction with the allantoic vein.
 +
 +
These three great veins, in fact, form a
 +
large common trunk, which enters at once
 +
into the liver, and which we may now call
 +
the portal vein (fig. 372, P. V}. This, at its
 +
entrance into the liver, partly breaks up
 +
into the vena advehentes, and partly continues as the ductus venosus (D.V}
 +
straight through the liver, emerging from
 +
which it joins the vena cava inferior. Before
 +
the establishment of the vena cava inferior,
 +
the venas revehentes, carrying back the
 +
blood which circulates through the hepatic
 +
capillaries, join the ductus venosus close to
 +
its exit from the liver. By the time however that the vena cava has become a large
 +
and important vessel it is found that the
 +
venae revehentes, or as we may now call
 +
them the hepatic veins, have shifted their
 +
embouchment, and now fall directly into
 +
that vein, the ductus venosus making a separate junction rather higher up (fig. 372).
 +
 +
This state of things continues with but slight changes till near the end
 +
of incubation, when the chick begins to breathe the air in the air-chamber
 +
of the shell, and respiration is no longer carried on by the allantois. Blood
 +
then ceases to flow along the allantoic vessels ; they become obliterated.
 +
The vitelline vein, which as the yolk becomes gradually absorbed proportionately diminishes in size and importance, comes to appear as a mere
 +
branch of the portal vein. The ductus venosus becomes obliterated ; and
 +
hence the whole of the blood coming through the portal vein flows into the
 +
substance of the liver, and so by the hepatic veins into the vena cava.
 +
 +
Although the allantoic (anterior abdominal) vein is obliterated in the
 +
adult, there is nevertheless established an anastomosis between the portal
 +
system and the veins bringing the blood from the limbs to the vena cava
 +
 +
 +
 +
FIG. 372. DIAGRAM OF THE
 +
VENOUS CIRCULATION IN THE
 +
CHICK DURING THE LATER DAYS
 +
OF INCUBATION.
 +
 +
H. heart ; V.S.R. right vena
 +
cava superior; V.S.L. left vena cava
 +
superior. The two venas cavrc
 +
superiores are the original 'ductus
 +
Cuvieri,' they open into the sinus
 +
venosus. J. jugular vein; Su.V.
 +
anterior vertebral vein ; In. V. inferior vertebral vein ; W. subclavian; V.C.I, vena cava inferior;
 +
D. V. ductus venosus ; P. V. portal
 +
vein ; M. mesenteric vein bringing
 +
blood from the intestines into the
 +
portal vein ; O.f. vitelline vein ; U.
 +
allantoic vein. The three last mentioned veins unite together to form
 +
the portal vein ; /. liver.
 +
 +
 +
 +
THE VENOUS SYSTEM.
 +
 +
 +
 +
66l
 +
 +
 +
 +
inferior, in that the caudal vein and posterior pelvic veins open into a
 +
vessel, known as the coccygeo-mesenteric vein, which joins the portal
 +
vein ; while at the same time the posterior pelvic veins are connected with
 +
the common iliac veins by a vessel which unites with them close to their
 +
junction with the coccygeo-mesenteric vein.
 +
 +
Mammalia. In Mammals the same venous trunks are developed in
 +
the embryo as in other types (fig. 373 A). The anterior cardinals or
 +
external jugulars form the primitive veins of the anterior part of the body,
 +
and the internal jugulars and anterior vertebrals are subsequently formed.
 +
The subclavians (fig. 373 A, j), developed on the formation of the anterior
 +
limbs, also pour their blood into these primitive trunks. In the lower
 +
Mammalia (Monotremata, Marsupialia, Insectivora, some Rodentia, etc.,
 +
the two ductus Cuvieri remain as the two superior venae cavae, but more
 +
usually an anastomosis arises between the right and left innominate veins,
 +
and eventually the whole of the blood of the left superior cava is carried to
 +
the right side, and there is left only a single superior cava (fig. 373 B and C).
 +
 +
 +
 +
 +
 +
F IG - 373- DIAGRAM OF THE DEVELOPMENT OF THE PAIRED VENOUS SYSTEM OF
 +
 +
MAMMALS (MAN). (From Gegenbaur.)
 +
 +
j. jugular vein ; cs. vena cava superior; s. subclavian veins; c. posterior cardinal
 +
vein ; v. vertebral vein ; az. azygos vein ; cor. coronary vein.
 +
 +
A. Stage in which the cardinal veins have already disappeared. Their position
 +
is indicated by dotted lines.
 +
 +
B. Later stage when the blood from the left jugular vein is carried into the right
 +
to form the single vena cava superior ; a remnant of the left superior cava being however still left.
 +
 +
C. Stage after the left vertebral vein has disappeared; the right vertebral
 +
remaining as the azygos vein. The coronary vein remains as the last remnant of the
 +
left superior vena cava.
 +
 +
A small rudiment of the left superior cava remains however as the sinus
 +
coronartus and receives the coronary vein from the heart (figs. 373 C,
 +
cor and 374, cs).
 +
 +
The posterior cardinal veins form at first the only veins receiving the
 +
 +
 +
 +
662
 +
 +
 +
 +
THE VEINS OF MAMMALIA.
 +
 +
 +
 +
blood from the posterior part of the trunk and kidneys ; and on the
 +
development of the hind limbs receive the blood from them also.
 +
 +
As in the types already described
 +
an unpaired vena cava inferior becomes
 +
eventually developed, and gradually
 +
carries off a larger and larger portion
 +
of the blood originally returned by the
 +
posterior cardinals. It unites with the
 +
common stem of the allantoic and
 +
vitelline veins in front of the liver.
 +
 +
At a later period a pair of trunks
 +
is established bringing the blood from
 +
the posterior part of the cardinal veins
 +
and the crural veins directly into the
 +
vena cava inferior (fig. 374, il}. These
 +
vessels, whose development has not
 +
been adequately investigated, form the
 +
common iliac veins, while the posterior
 +
ends of the cardinal veins which join
 +
them become the hypogastric veins (fig.
 +
374, hy). Owing to the development of
 +
the common iliac veins there is no renal
 +
portal system like that of the Reptilia
 +
and Amphibia.
 +
 +
Posterior vertebral veins, similar to
 +
those of Reptilia and Birds, are established in connection with the intercostal
 +
and lumbar veins, and unite anteriorly
 +
with the front part of the posterior
 +
 +
 +
 +
 +
FIG. 374. DIAGRAM OF THE CHIEF
 +
 +
VENOUS TRUNKS OF MAN. (From
 +
Gegenbaur.)
 +
 +
cs. vena cava superior ; s. subclavian vein ; ji. internal jugular ; je.
 +
external jugular ; az. azygos vein ; ha.
 +
hemiazygos vein ; c. clotted line shewing previous position of cardinal veins ;
 +
ci. vena cava inferior ; r. renal veins ;
 +
il. iliac ; hy. hypogastric veins ; h.
 +
hepatic veins.
 +
 +
The dotted lines shew the position
 +
of embryonic vessels aborted in the
 +
adult.
 +
 +
 +
 +
cardinal veins (fig. 373 A) 1 .
 +
 +
On the formation of the posterior vertebral veins, and as the inferior
 +
vena cava becomes more important, the middle part of the posterior cardinals becomes completely aborted (fig. 374, f), the anterior and posterior
 +
parts still persisting, the former as the continuations of the posterior
 +
vertebrals into the anterior vena cava (az\ the latter as the hypogastric veins
 +
(Ay).
 +
 +
Though in a few Mammalia both the posterior vertebrals persist, a
 +
transverse connection is usually established between them, and the one (the
 +
right) becoming the more important constitutes the azygos vein (fig. 374, az),
 +
the persisting part of the left forming the hemiazygos vein (ha}.
 +
 +
The remainder of the venous system is formed in the embryo of the
 +
vitelline and allantoic veins, the former being eventually joined by the
 +
mesenteric vein so as to constitute the portal vein.
 +
 +
1 Rathke, as mentioned above, holds that in the Snake the front part of the
 +
posterior cardinals completely aborts. Further investigations are required to shew
 +
whether there really is a difference between Mammalia and Reptilia in this matter.
 +
 +
 +
 +
 +
 +
 +
THE VENOUS SYSTEM. 663
 +
 +
The vitelline vein is the first part of this system established, and divides
 +
near the heart into two veins bringing back the blood from the yolk-sack
 +
(umbilical vesicle). The right vein soon however aborts.
 +
 +
The allantoic (anterior abdominal) veins are originally paired. They
 +
are developed very early, and at first course along the still widely open
 +
somatic walls of the body, and fall into the single vitelline trunk in front.
 +
The right allantoic vein disappears before long, and the common trunk
 +
formed by the junction of the vitelline and allantoic veins becomes considerably elongated. This trunk is soon enveloped by the liver.
 +
 +
The succeeding changes have been somewhat differently described by
 +
Kolliker and Rathke. According to the former the common trunk of the
 +
allantoic and vitelline veins in its passage through the liver gives off
 +
branches to the liver, and also receives branches from this organ near its
 +
anterior exit. The main trunk is however never completely aborted, as in
 +
the embryos of other types, but remains as the ductus venosus Arantii.
 +
 +
With the development of the placenta the allantoic vein becomes the
 +
main source of the ductus venosus, and the vitelline or portal vein, as it may
 +
perhaps be now conveniently called, ceases to join it directly, but falls into
 +
one of its branches in the liver.
 +
 +
The vena cava inferior joins the continuation of the ductus venosus in
 +
front of the liver, and, as it becomes more important, it receives directly
 +
the hepatic veins which originally brought back blood into the ductus
 +
venosus. The ductus venosus becomes moreover merely a small branch of
 +
the vena cava.
 +
 +
At the close of foetal life the allantoic vein becomes obliterated up to its
 +
place of entrance into the liver ; the ductus venosus becomes a solid cord
 +
the so-called round ligament and the whole of the venous blood is brought
 +
to the liver by the portal vein 1 .
 +
 +
Owing to the allantoic (anterior abdominal) vein having merely a fcetal
 +
existence an anastomosis between the iliac veins and the portal system by
 +
means of the anterior abdominal vein is not established.
 +
 +
 +
 +
BIBLIOGRAPHY of the Venous System.
 +
 +
(498) J. Marshall. "On the development of the great anterior veins." Phil.
 +
Trans., 1859.
 +
 +
(499) H. Rathke. " Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Saugethieren." MeckeVs Archiv, 1830.
 +
 +
(500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbelthiere." Bericht. Jib. d. natttrh. Seminar, d. Univ. Konigsberg, 1838.
 +
 +
Vide also Von Baer (No. 291), Gotte (No. 296), Kolliker (No. 298), and Rathke
 +
(Nos. 299, 300, and 301).
 +
 +
1 According to Rathke the original trunk connecting the allantoic vein directly
 +
with the heart through the liver is aborted, and the ductus venosus Arantii is a
 +
secondary connection established in the latter part of foetal life.
 +
 +
 +
 +
664 LYMPHATIC SYSTEM.
 +
 +
 +
 +
Lymphatic System.
 +
 +
The lymphatic system arises from spaces in the general parenchyma of
 +
the body, independent in their origin of the true body cavity, though communicating both with this cavity and with the vascular system.
 +
 +
In all the true Vertebrata certain parts of the system form definite trunks
 +
communicating with the venous system ; and in the higher types the walls of
 +
the main lymphatic trunks become quite distinct.
 +
 +
But little is known with reference to the ontogeny of the lymphatic vessels,
 +
but they originate late in larval life, and have at first the form of simple
 +
intercellular spaces.
 +
 +
The lymphatic glands appear to originate from lymphatic plexuses, the
 +
cells of which produce lymph corpuscles. It is only in Birds and Mammals,
 +
and especially in the latter, that the lymphatic glands form definite structures.
 +
 +
The Spleen. The spleen, from its structure, must be classed with the
 +
lymphatic glands, though it has definite relations to the vascular system.
 +
It is developed in the mesoblast of the mesogastrium, usually about the
 +
same time and in close connection with the pancreas.
 +
 +
According to Miiller and Peremeschko the mass of mesoblast which
 +
forms the spleen becomes early separated by a groove on the one side from
 +
the pancreas and on the other from the mesentery. Some of its cells
 +
become elongated, and send out processes which uniting with like processes
 +
from other cells form the trabecular system. From the remainder of the
 +
tissue are derived the cells of the spleen pulp, which frequently contain more
 +
than one nucleus. Especial accumulations of these cells take place at a
 +
later period to form the so-called Malpighian corpuscles of the spleen.
 +
 +
BIBLIOGRAPHY of Spleen.
 +
 +
(501) W. Miiller. "The Spleen." Strieker's Histology.
 +
 +
(502) Peremeschko. " Ueb. d. Entwick. d. Milz." Sitz. d. Wuti. Akad.
 +
Wiss., Vol. LVI. 1867.
 +
 +
Suprarenal ^bodies.
 +
 +
In Elasmobranch Fishes two distinct sets of structures are found, both of
 +
which have been called suprarenal bodies. As shewn in the sequel both of
 +
these structures probably unite in the higher types to form the suprarenal
 +
bodies.
 +
 +
One of them consists of a series of paired bodies, situated on the
 +
branches of the dorsal aorta, segmentally arranged, and forming a chain
 +
extending from close behind the heart to the hinder end of the body cavity.
 +
Each body is formed of a series of lobes, and exhibits a well-marked
 +
distinction into a cortical layer of columnar cells, and a medullary substance
 +
formed of irregular polygonal cells. As first shewn by Leydig, they are
 +
 +
 +
 +
SUPRARENAL BODIES. 665
 +
 +
closely connected with the sympathetic ganglia, and usually contain numerous
 +
ganglion cells distributed amongst the proper cells of the body.
 +
 +
The second body consists of an unpaired column of cells placed between
 +
the dorsal aorta and unpaired caudal vein, and bounded on each side by the
 +
posterior parts of the kidney. I propose to call it the interrenal body.
 +
In front it overlaps the paired suprarenal bodies, but does not unite with
 +
them. It is formed of a series of well-marked lobules, etc. In the fresh
 +
state Leydig (No. 506) finds that "fat molecules form the chief mass of the
 +
body, and one finds freely imbedded in them clear vesicular nuclei." As
 +
may easily be made out from hardened specimens it is invested by a tunica
 +
propria, which gives off septa dividing it into well-marked areas filled with
 +
polygonal cells. These cells constitute the true parenchyma of the body.
 +
By the ordinary methods of hardening, the oil globules, with which they are
 +
filled in the fresh state, completely disappear.
 +
 +
The paired suprarenal bodies (Balfour, No. 292, pp. 242 244) are developed from the sympathetic ganglia. These ganglia, shewn in an early
 +
stage in fig. 380, sy.g, become gradually divided into a ganglionic part and a
 +
glandular part. The former constitutes the sympathetic ganglia of the adult ;
 +
the latter the true paired suprarenal bodies. The interrenal body is however
 +
developed (Balfour, No. 292, pp. 245 247) from indifferent mesoblast cells
 +
between the two kidneys, in the same situation as in the adult.
 +
 +
The development of the suprarenal bodies in the Amniota has been most
 +
fully studied by Braun (No. 503) in the Reptilia.
 +
 +
In Lacertilia they consist of a pair of elongated yellowish bodies, placed
 +
between the vena renalis revehens and the generative glands.
 +
 +
They are formed of two constituents, viz. (i) masses of brown cells placed
 +
on the dorsal side of the organ, which stain deeply with chromic acid, like
 +
certain of the cells of the suprarenals of Mammalia, and (2) irregular cords,
 +
in part provided with a lumen, filled with fat-like globules l , amongst which
 +
are nuclei. On treatment with chromic acid the fat globules disappear, and
 +
the cords break up into bodies resembling columnar cells.
 +
 +
The dorsal masses of brown cells are developed from the sympathetic
 +
ganglia in the same way as the paired suprarenal bodies of the Elasmobranchii, while the cords filled with fat-like globules are formed of indifferent
 +
mesoblast cells as a thickening in the lateral walls of the inferior vena cava,
 +
and the cardinal veins continuous with it. The observations of Brunn (No.
 +
504) on the Chick, and Kolliker (No. 298, pp. 953955) n the Mammal,
 +
add but little to those of Braun. They shew that the greater part of the
 +
gland (the cortical substance) in these two types is derived from the mesoblast,
 +
and that the glands are closely connected with sympathetic ganglia ; while
 +
Kolliker also states that the posterior part of the organ is unpaired in the
 +
embryo rabbit of 1 6 or 17 days.
 +
 +
The structure and development of what I have called the interrenal body
 +
 +
1 These globules are not formed of a true fatty substance, and this is also probably
 +
true for the similar globules of the interrenal bodies of Elasmobranchii.
 +
 +
 +
 +
666 SUPRARENAL BODIES.
 +
 +
in Elasmobranchii so closely correspond with that of the mesoblastic part of
 +
the suprarenal bodies of the Reptilia, that I have very little hesitation in
 +
regarding them as homologous 1 ; while the paired bodies in Elasmobranchii,
 +
derived from the sympathetic ganglia, clearly correspond with the part of the
 +
suprarenals of Reptilia having a similar origin ; although the anterior parts
 +
of the paired suprarenal bodies of Fishes have clearly become aborted in the
 +
higher types.
 +
 +
In Elasmobranch Fishes we thus have (i) a series of paired
 +
bodies, derived from the sympathetic ganglia, and (2) an unpaired body of mesoblastic origin. In the Amniota these bodies
 +
unite to form the compound suprarenal bodies, the two constituents of which remain, however, distinct in their development.
 +
The mesoblastic constituent appears to form the cortical part of
 +
the adult suprarenal body, and the nervous constituent the
 +
medullary part.
 +
 +
BIBLIOGRAPHY of the Suprarenal bodies,
 +
 +
(503) M. Braun. "Bau u. Entwick. d. Nebennieren bei Reptilien. " Arbeit,
 +
a. d. zool.-zoot. Institut Wurzlttrg, Vol. V. 1879.
 +
 +
(504) A. v. Brunn. "Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick.
 +
d. Nebennieren." Archiv f. mikr. Anat., Vol. VIII. 1872.
 +
 +
(505) Fr. Leydig. Untersiich. iib. Fische u. fieptilten. Berlin, 1853.
 +
 +
(506) Fr. Leydig. Rochen u. Haie. Leipzig, 1852.
 +
 +
Vide also F. M. Balfour (No. 292), Kolliker (No. 298), Remak (No. 302), etc.
 +
 +
1 The fact of the organ being unpaired in Elasmobranchii and paired in the
 +
Amniota is of no importance, as is shewn by the fact that part of the organ is unpaired
 +
in the Rabbit.
 +
 +
 +
 +
CHAPTER XXII.
 +
 +
 +
 +
THE MUSCULAR SYSTEM.
 +
 +
 +
 +
 +
IN all the Ccelenterata, except the Ctenophora, the contractile elements of the body wall consist of filiform processes of
 +
ectodermal or entodermal epithelial cells (figs. 375 and 376 B).
 +
The elements provided with these processes, which were first
 +
discovered by Kleinenberg, are known as myo-epithelial
 +
cells. Their contractile parts may either be striated (fig. 376)
 +
or non-striated (fig. 375). In some
 +
instances the epithelial part of the
 +
cell may nearly abort, its nucleus
 +
alone remaining (fig. 376 A) ; and
 +
in this way a layer of muscles lying
 +
completely below the surface may
 +
be established.
 +
 +
There is embryological evidence
 +
of the derivation of the voluntary
 +
muscular system of a large number of types from myo-epithelial
 +
cells of this kind. The more important of these groups are the
 +
Chaetopoda, the Gephyrea, the Chaetognatha, the Nematoda, and
 +
the Vertebrata 1 .
 +
 +
While there is clear evidence that the muscular system of a
 +
large number of types is composed of cells which had their
 +
origin in myo-epithelial cells, the mode of evolution of the
 +
 +
1 If recent statements of Metschnikoff are to be trusted, the Echinodermata must
 +
be added to these groups. The amoeboid cells stated in the first volume of this
 +
treatise to form the muscles in this group, on the authority of Selenka, give rise,
 +
according to Metschnikoff, only to the cutis, while the same naturalist states the
 +
epithelial cells of the vasoperitoneal vesicles are provided with muscular tails.
 +
 +
 +
 +
FIG. 375. MYO-EPITHELIAL
 +
CELLS OF HYDRA. (From Gegenbaur ; after Kleinenberg.)
 +
 +
m. contractile fibres.
 +
 +
 +
 +
668 THE MUSCULAR FIBRES.
 +
 +
muscular system of other types is still very obscure. The
 +
muscles may arise in the embryo from amoeboid or indifferent
 +
cells, and the Hertwigs 1 hold that in many of these instances the
 +
muscles have also phylogenetically taken their origin from
 +
indifferent connective-tissue cells. The subject is however beset
 +
with very serious difficulties, and to discuss it here would carry
 +
me too far into the region of pure histology.
 +
 +
The voluntary muscular system of the CJiordata.
 +
 +
The muscular fibres. The muscular elements of the
 +
Chordata undoubtedly belong to the myo-epithelial type. The
 +
embryonic muscle-cells are at first simple epithelial cells, but
 +
 +
 +
 +
 +
FIG. 376. MUSCLE-CELLS OF LIZZIA KOLLIKERI. (From Lankester ; after
 +
O. and R. Hertwig.)
 +
 +
A. Muscle-cell from the circular fibres of the subumbrella.
 +
 +
B. Myo-epithelial cells from the base of a tentacle.
 +
 +
soon become spindle-shaped : part of their protoplasm becomes
 +
differentiated into longitudinally placed striated muscular fibrils,
 +
while part, enclosing the nucleus, remains indifferent, and constitutes the epithelial element of the cells. The muscular
 +
fibrils are either placed at one side of the epithelial part of the
 +
cell, or in other instances (the Lamprey, the Newt, the Sturgeon,
 +
the Rabbit) surround it. The latter arrangement is shewn for
 +
the Sturgeon in fig. 57.
 +
 +
The number of the fibrils of each cell gradually increases,
 +
and the protoplasm diminishes, so that eventually only the
 +
nucleus, or nuclei resulting from its division, are left. The
 +
products of each cell probably give rise, in conjunction with a
 +
further division of the nucleus, to a primitive bundle, which,
 +
 +
1 O. and R. Hertwig, Die Calomthcorie. Jena, 1881.
 +
 +
 +
 +
THE MUSCULAR SYSTEM.
 +
 +
 +
 +
669
 +
 +
 +
 +
t>r
 +
 +
 +
 +
 +
except in Amphioxus, Petromyzon, etc., is surrounded by a
 +
special investment of sarcolemma.
 +
 +
The voluntary muscular system. For the purposes of
 +
description the muscular system of the Vertebrata may conveniently be divided into two sections, viz. that of the head and
 +
that of the trunk. The main part, if
 +
not the whole, of the muscular system
 +
of the trunk is derived from certain
 +
structures, known as the muscle-plates,
 +
which take their origin from part of
 +
the primitive mesoblastic somites.
 +
 +
It has already been stated (pp.
 +
292 ^296) that the mesoblastic somites
 +
are derived from the dorsal segmented
 +
part of the primitive mesoblastic plates.
 +
Since the history of these bodies is
 +
presented in its simplest form in Elasmobranchii it will be convenient to
 +
commence with this group. Each
 +
somite is composed of two layers a
 +
somatic and a splanchnic both formed
 +
of a single row of columnar cells.
 +
Between these two layers is a cavity,
 +
which is at first directly continuous
 +
with the general body cavity, of which
 +
indeed it merely forms a specialised
 +
part (fig. 377). Before long the cavity
 +
becomes however completely constricted off from the permanent body cavity.
 +
 +
Very early (fig. 377) the inner or splanchnic wall of the
 +
somites loses its simple constitution, owing to the middle part of
 +
it undergoing peculiar changes. The meaning of the changes is
 +
at once shewn by longitudinal horizontal sections, which prove
 +
(% 378) that the cells in this situation (mp') have become
 +
extended in a longitudinal direction, and, in fact, form typical
 +
spindle-shaped embryonic muscle-cells, each with a large
 +
nucleus. Every muscle-cell extends for the whole length of a
 +
somite. The inner layer of each somite, immediately within
 +
the muscle-band just described, begins to proliferate, and produce
 +
 +
 +
 +
FIG. 377. TRANSVERSE
 +
SECTION THROUGH THETRUNK
 +
OF AN EMBRYO SLIGHTLY
 +
OLDER THAN FIG. 28 E.
 +
 +
nc. neural canal ; pr. posterior root of spinal nerve ; x.
 +
subnotochordal rod ; ao. aorta ;
 +
sc. somatic mesoblast ; sf>.
 +
splanchnic mesoblast ; mp.
 +
muscle-plate ; mp', portion of
 +
muscle-plate converted into
 +
muscle ; Vr. portion of the
 +
vertebral plate which will give
 +
rise to the vertebral bodies ; al.
 +
alimentary tract.
 +
 +
 +
 +
THE MUSCLE-PLATES.
 +
 +
 +
 +
a mass of cells, placed between the muscles and the notochord
 +
( Vr\ These cells form the commencing vertebral bodies, and
 +
have at first (fig. 378) the same segmentation as the somites
 +
from which they sprang.
 +
 +
After the separation of the vertebral bodies from the somites
 +
the remaining parts of the somites may be called muscle-plates ;
 +
since they become directly converted into the whole voluntary
 +
muscular system of the trunk (fig. 379, mp}.
 +
 +
According to the statements of Bambeke and Go'tte, the Amphibians
 +
present some noticeable peculiarities in the development of their muscular
 +
system, in that such distinct muscle-plates as those of other vertebrate types
 +
are not developed. Each side-plate of mesoblast is divided into a somatic
 +
and a splanchnic layer, continuous throughout the vertebral and parietal
 +
portions of the plate. The vertebral portions (somites) of the plates soon
 +
become separated from the parietal, and form independent masses of cells
 +
constituted of two layers, which were originally continuous with the
 +
somatic and splanchnic layers of the parietal plates (fig. 79). The outer or
 +
somatic layer of the vertebral plates is formed of a single row of cells, but
 +
the inner or splanchnic layer is made up of a kernel of cells on the side of
 +
the somatic layer and an inner layer. The kernel of the splanchnic layer
 +
and the outer or somatic layer together correspond to a muscle- plate of other
 +
Vertebrata, and exhibit a similar segmentation.
 +
 +
Osseous Fishes are stated to agree with Amphibians in the development
 +
of their somites and muscular
 +
system 1 , but further observations
 +
on this point are required.
 +
 +
In Birds the horizontal splitting of the mesoblast extends at
 +
first to the dorsal summit of the
 +
mesoblastic plates, but after the
 +
isolation of the somites the split
 +
between the somatic and splanchnic layers becomes to a large extent obliterated, though in the anterior somites it appears in part
 +
to persist. The somites on the
 +
second day, as seen in a transverse section (fig. 115, P.?'.), are
 +
somewhat quadrilateral in form
 +
but broader than they are deep.
 +
 +
Each at that time consists of
 +
a somewhat thick cortex of radi
 +
 +
 +
 +
FlG. 378. HORIZONTALSECTION THROUGH
 +
THE TRUNK OF AN EMBRYO OF SCYLL1UM
 +
CONSIDERABLY YOUNGER THAN 28 F.
 +
 +
 +
 +
The section is taken at the level of the
 +
notochord, and shews the separation of the
 +
cells to form the vertebral bodies from the
 +
muscle-plates.
 +
 +
ch. notochord ; ep. epiblast ; Vr, rudiment
 +
of vertebral body ; mp. muscle- plate ; mp' .
 +
portion of muscle-plate already differentiated
 +
into longitudinal muscles.
 +
 +
 +
 +
1 Ehrlich, " Ueber den peripher. Theil d. Urwirbel." Archiv f. mikr. Anal.,
 +
Vol. XI.
 +
 +
 +
 +
THE MUSCULAR SYSTEM. 671
 +
 +
ating rather granular columnar cells, enclosing a small kernel of spherical
 +
cells. They are not, as may be seen in the above figure, completely
 +
separated from the ventral (or lateral as they are at this period) parts of the
 +
mesoblastic plate, and the dorsal and outer layer of the cortex of the
 +
somites is continuous with the somatic layer of mesoblast, the remainder of
 +
the cortex, with the central kernel, being continuous with the splanchnic
 +
layer. Towards the end of the second and beginning of the third day the
 +
upper and outer layer of the cortex, together probably with some of the
 +
central cells of the kernel, becomes separated off as a muscle-plate (fig. 1 16).
 +
The muscle-plate when formed (fig. 117) is found to consist of two layers,
 +
an inner and an outer, which enclose between them an almost obliterated
 +
central cavity ; and no sooner is the muscle-plate formed than the middle
 +
portion of the inner layer becomes converted into longitudinal muscles.
 +
The avian muscle-plates have, in fact, precisely the same constitution as
 +
those of Elasmobranchii. The central space is clearly a remnant of the
 +
vertebral portion of the body cavity, which, though it wholly or partially
 +
disappears in a previous stage, reappears again on the formation of the
 +
muscle-plate.
 +
 +
The remainder of the somite, after the formation of the muscle-plate,
 +
is of very considerable bulk ; the cells of the cortex belonging to it lose
 +
their distinctive characters, and the major part of it becomes the vertebral
 +
rudiment.
 +
 +
In Mammalia the history appears to be generally the same as in Elasmobranchii. The split which gives rise to the body cavity is continued to
 +
the dorsal summit of the mesoblastic plates, and the dorsal portions of the
 +
plates with their contained cavities become divided into somites, and are
 +
then separated off from the ventral. The later development of the somites
 +
has not been worked out with the requisite care, but it would seem that they
 +
form somewhat cubical bodies in which all trace of the primitive slit is lost.
 +
The further development resembles that in Birds.
 +
 +
The first changes of the mesoblastic somites and the formation of the muscle-plates do not, according to existing statements,
 +
take place on quite the same type throughout the Vertebrata,
 +
yet the comparison which has been instituted between Elasmobranchs and other Vertebrates appears to prove that there are
 +
important common features in their development, which may be
 +
regarded as primitive, and as having been inherited from the
 +
ancestors of Vertebrates. These features are (i) the extension
 +
of the body cavity into the vertebral plates, and subsequent
 +
enclosure of this cavity between the two layers of the muscleplates ; (2) the primitive division of the vertebral plate into an
 +
outer (somatic) and an inner (splanchnic) layer, and the formation
 +
of a large part of the voluntary muscular system out of the inner
 +
 +
 +
 +
THE MUSCLE-PLATES.
 +
 +
 +
 +
sp.c
 +
 +
 +
 +
layer, which in all cases is converted into muscles earlier than
 +
the outer layer.
 +
 +
The conversion of the muscle-plates into muscles. It
 +
 +
will be convenient to commence this subject with a description
 +
of the changes which take place in
 +
such a simple type as that of the
 +
Elasmobranchii.
 +
 +
At the time when the muscleplates have become independent
 +
structures they form flat two-layered
 +
oblong bodies enclosing a slit-like
 +
central cavity (fig. 379, mp). The
 +
outer or somatic wall is formed of
 +
simple epithelial -like cells. The
 +
inner or splanchnic wall has however a somewhat complicated structure. It is composed dorsally and
 +
ventrally of a columnar epithelium,
 +
but in its middle portion of the
 +
muscle-cells previously spoken of.
 +
Between these and the central cavity
 +
of the plates the epithelium forming
 +
the remainder of the layer commences to insert itself; so that between the first-formed muscle and
 +
the cavity of the muscle-plate there
 +
appears a thin layer of cells, not
 +
however continuous throughout.
 +
 +
When first formed the muscleplates, as viewed from the exterior,
 +
have nearly straight edges ; soon
 +
however they become bent in the middle, so that the edges have
 +
an obtusely angular form, the apex of the angle being directed
 +
forwards. They are so arranged that the anterior edge of the
 +
one plate fits into the posterior edge of the one in front. In the
 +
lines of junction between the plates layers of connective-tissue
 +
cells appear, which form the commencements of the intermuscular
 +
septa.
 +
 +
The growth of the plates is very rapid, and their upper ends
 +
 +
 +
 +
 +
FIG. 379. SECTION THROUGH
 +
THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN
 +
 +
28 F.
 +
 +
sp.c. spinal canal ; W. white
 +
matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ;
 +
x. sub-notochordal rod ; ao. aorta ;
 +
mp. muscle-plate; mp' . inner layer
 +
of muscle-plate already converted
 +
into muscles ; Vr. rudiment of
 +
vertebral body ; si. segmental
 +
tube ; sd. segmental duct ; sp.v.
 +
spiral valve ; z/. subintestinal vein ;
 +
P.O. primitive generative cells.
 +
 +
 +
 +
THE MUSCULAR SYSTEM. 673
 +
 +
soon extend to the summit of the neural canal, and their lower
 +
ones nearly meet in the median ventral line. The original band
 +
of muscles, whose growth at first is very slow, now increases
 +
with great rapidity, and forms the nucleus of the whole voluntary muscular system (fig. 380, mp'). It extends upwards and
 +
downwards by the continuous conversion of fresh cells of the
 +
splanchnic layer into muscle-cells. At the same time it grows
 +
rapidly in thickness by the addition of fresh spindle-shaped
 +
muscle-cells from the somatic layer as well as by the division of
 +
the already existing cells.
 +
 +
Thus both layers of the muscle-plate are concerned in forming
 +
the great longitudinal lateral muscles, though the splanchnic layer
 +
is converted into muscles very much sooner than the somatic 1 .
 +
 +
Each muscle-plate is at first a continuous structure, extending
 +
from the dorsal to the ventral surface, but after a time it becomes
 +
divided by a layer of connective tissue, which becomes developed
 +
nearly on a level with the lateral line, into a dorso-lateral and
 +
a ventro-lateral section. The ends of the muscle-plates
 +
continue for a long time to be formed of undifferentiated
 +
columnar cells. The complicated outlines of the inter-muscular
 +
septa become gradually established during the later stages of
 +
development, causing the well-known appearances of the muscles
 +
in transverse sections, which require no special notice here.
 +
 +
The muscles of the limbs. The limb muscles are formed
 +
in Elasmobranchii, coincidently with the cartilaginous skeleton,
 +
as two bands of longitudinal fibres on the dorsal and ventral
 +
surfaces of the limbs (fig. 346). The cells, from which these
 +
muscles originate, are derived from the muscle-plates. When
 +
the ends of the muscle-plates reach the level of the limbs they
 +
bend outwards and enter the tissue of the limbs (fig. 380).
 +
Small portions of several muscle-plates (m.pl) come in this way
 +
to be situated within the limbs, and are very soon segmented
 +
off from the remainder of the muscle-plates. The portions of
 +
the muscle-plates thus introduced soon lose their original dis
 +
1 The brothers Hertwig have recently maintained that only the inner layer of the
 +
muscle-plates is converted into muscles. In the Elasmobranchs it is easy to demonstrate the incorrectness of this view, and in Acipenser (vide fig. 57, mp) the two layers
 +
of the muscle-plate retain their original relations after the cells of both of them have
 +
become converted into muscles.
 +
 +
B. in. 43
 +
 +
 +
 +
674
 +
 +
 +
 +
THE MUSCLE-PLATES.
 +
 +
 +
 +
3,-n,
 +
 +
 +
 +
 +
FIG. 380. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK
 +
OF AN EMBRYO OF SCYLLIUM SLIGHTLY OLDER THAN FIG. 29 B.
 +
 +
The section is diagrammatic in so far that the anterior nerve-roots have been
 +
inserted for the whole length ; whereas they join the spinal cord half-way between
 +
two posterior roots.
 +
 +
sp.c. spinal cord; sp.g. ganglion of posterior root; ar. anterior root; dn. dorsally
 +
directed nerve springing from posterior root; nip. muscle-plate; mp'. part of muscleplate already converted into muscles; vi.pl. part of muscle-plate which gives rise to
 +
the muscles of the limbs; /. nervus lateralis; ao. aorta; ch. notochord; sy.g. sympathetic ganglion; ca.v. cardinal vein; sp.n. spinal nerve; sd. segmental (archinephric)
 +
duct; st. segmental tube; du. duodenum; pan. pancreas; hp.d. point of junction of
 +
hepatic duct with duodenum ; umc. umbilical canal.
 +
 +
 +
 +
THE MUSCULAR SYSTEM. 675
 +
 +
tinctness. There can however be but little doubt that they
 +
supply the tissue for the muscles of the limbs. The muscleplates themselves, after giving off buds to the limbs, grow
 +
downwards, and soon cease to shew any trace of having given
 +
off these buds.
 +
 +
In addition to the longitudinal muscles of the trunk just described,
 +
which are generally characteristic of Fishes, there is found in Amphioxus a
 +
peculiar transverse abdominal muscle, extending from the mouth to the
 +
abdominal pore, the origin of which has not been made out.
 +
 +
It has already been shewn that in all the higher Vertebrata
 +
muscle-plates appear, which closely resemble those in Elasmobranchii; so that all the higher Vertebrata pass through, with
 +
reference to their muscular system, a fish- like stage. The
 +
middle portion of the inner layers of their muscle-plates becomes, as in Elasmobranchii, converted into muscles at a very
 +
early period, and the outer layer for a long time remains formed
 +
of indifferent cells. That these muscle-plates give rise to the
 +
main muscular system of the trunk, at any rate to the episkeletal
 +
muscles of Huxley, is practically certain, but the details of the
 +
process have not been made out.
 +
 +
In the Perennibranchiata the fish-like arrangement of muscles is retained through life in the tail and in the dorso-lateral parts of the trunk.
 +
In the tail of the Amniotic Vertebrata the primitive arrangement is also
 +
more or less retained, and the same holds good for the dorso-lateral trunk
 +
muscles of the Lacertilia. In the other Amniota and the Anura the
 +
dorso-lateral muscles have become divided up into a series of separate
 +
muscles, which are arranged in two main layers. It is probable that the
 +
intercostal muscles belong to the same group as the dorso-lateral muscles.
 +
 +
The abdominal muscles of the trunk, even in the lowest Amphibia,
 +
exhibit a division into several layers. The recti abdominis are the least
 +
altered part of this system, and usually retain indications of the primitive
 +
inter-muscular septa, which in many Amphibia and Lacertilia are also
 +
to some extent preserved in the other abdominal muscles.
 +
 +
In the Amniotic Vertebrates there is formed underneath the vertebral
 +
column and the transverse processes a system of muscles, forming part
 +
of the hyposkeletal system of Huxley, and called by Gegenbaur the subvertebral muscles. The development of this system has not been worked
 +
out, but on the whole I am inclined to believe that it is derived from
 +
the muscle-plates. Kolliker, Huxley and other embryologists believe
 +
however that these muscles are independent of the muscle-plates in their
 +
origin.
 +
 +
432
 +
 +
 +
 +
676 THE HEAD-CAVITIES.
 +
 +
 +
 +
Whether the muscle of the diaphragm is to be placed in the same
 +
category as the hyposkeletal muscles has not been made out.
 +
 +
It is probable that the cutaneous muscles of the trunk are derived
 +
from the cells given off from the muscle-plates. Kolliker however believes
 +
that they have an independent origin.
 +
 +
The limb-muscles, both extrinsic and intrinsic, as may be concluded
 +
from their development in Elasmobranchii, are derived from the muscleplates. Kleinenberg found in Lacertilia a growth of the muscle-plates
 +
into the limbs, and in Amphibia Gotte finds that the outer layer of the
 +
muscle-plates gives rise to the muscles of the limbs.
 +
 +
In the higher Vertebrata on the other hand the entrance of the muscleplates into the limbs has not been made out (Kolliker). It seems therefore
 +
probable that by an embryological modification, of which instances are so
 +
frequent, the cells which give rise to the muscles of the limbs in the higher
 +
Vertebrata can no longer be traced into a direct connection with the muscleplates.
 +
 +
TJte Somites and muscular system of the head.
 +
 +
The extension of the somites to the anterior end of the body
 +
in Amphioxus clearly proves that somites, similar to those of
 +
the trunk, were originally present in a region, which in the
 +
higher Vertebrata has become differentiated into the head. In
 +
the adult condition no true Vertebrate exhibits indications of
 +
such somites, but in the embryos of several of the lower Vertebrata structures have been found, which are probably equivalent
 +
to the somites of the trunk : they have been frequently alluded
 +
to in the previous chapters of this volume. These structures
 +
have been most fully worked out in Elasmobranchii.
 +
 +
The mesoblast in Elasmobranch embryos becomes first split
 +
into somatic and splanchnic layers in the region of the head ;
 +
and between these layers there are formed two cavities, one on
 +
each side, which end in front opposite the blind anterior extremity of the alimentary canal ; and are continuous behind
 +
with the general body-cavity (fig. 20 A, vp}. I propose calling
 +
them the head-cavities. The cavities of the two sides have
 +
no communication with each other.
 +
 +
Coincidently with the formation of an outgrowth from the
 +
throat to form the first visceral cleft, the head-cavity on each
 +
side becomes divided into a section in front of the cleft and a
 +
section behind the cleft ; and at a later period it becomes, owing
 +
to the formation of a second cleft, divided into three sections :
 +
 +
 +
 +
THE MUSCULAR SYSTEM.
 +
 +
 +
 +
677
 +
 +
 +
 +
vn~.
 +
 +
 +
 +
 +
(i) a section in front of the first or hyomandibular cleft; (2) a
 +
section in the hyoid arch between the hyomandibular cleft and
 +
the hyobranchial or first branchial cleft ; (3) a section behind
 +
the first branchial cleft.
 +
 +
The front section of the head-cavity grows forward, and soon
 +
becomes divided, without the intervention of a visceral cleft, into
 +
an anterior and posterior division.
 +
The anterior lies close to the eye,
 +
and in front of the commencing
 +
mouth involution. The posterior
 +
part lies completely within the mandibular arch.
 +
 +
As the rudiments of the successive visceral clefts are formed, the
 +
posterior part of the head-cavity becomes divided into successive sections, there being one section for
 +
each arch. Thus the whole headcavity becomes on each side divided
 +
into (i) a premandibular section ; (2)
 +
a mandibular section (vide fig. 29 A,
 +
PP] > (3) a hyoid section ; (4) sections
 +
in each of the branchial arches.
 +
 +
The first of these divisions forms
 +
a space of a considerable size, with
 +
epithelial walls of somewhat short
 +
columnar cells (fig. 381, ipp}. It is
 +
situated close to the eye, and presents a rounded or sometimes a
 +
triangular figure in section. The
 +
two halves of the cavity are prolonged ventralwards, and meet below
 +
the base of the fore-brain. The
 +
connection between them appears to last for a considerable time.
 +
These two cavities are the only parts of the body-cavity within
 +
the head which unite ventrally. The section of the head-cavity
 +
just described is so similar to the remaining sections that it
 +
must be considered as serially homologous with them.
 +
 +
The next division of the head-cavity, which from its position
 +
 +
 +
 +
FIG. 381. TRANSVERSE SECTION THROUGH THE FRONT PART
 +
OF THE HEAD OF A YOUNG PRISTIURUS EMBRYO.
 +
 +
The section, owing to the cranial flexure, cuts both the foreand the hind-brain. It shews the
 +
premandibular and mandibular
 +
head-cavities ipp and ipp, etc.
 +
The section is moreover somewhat
 +
oblique from side to side.
 +
 +
fb. fore-brain ; /. lens of eye ;
 +
m. mouth ; pt. upper end of mouth,
 +
forming pituitary involution; lao.
 +
mandibular aortic arch; ipp. and
 +
ipp. first and second head-cavities;
 +
\vc. first visceral cleft; V. fifth
 +
nerve ; aim. auditory nerve ; VII.
 +
seventh nerve ; aa. dorsal aorta ;
 +
acv. anterior cardinal vein ; ch,
 +
notochord.
 +
 +
 +
 +
678 THE HEAD-CAVITIES.
 +
 +
may be called the mandibular cavity, presents a spatulate shape,
 +
being dilated dorsally, and produced ventrally into a long thin
 +
process parallel to the hyomandibular gill-cleft (fig. 20, pp}.
 +
Like the previous space it is lined by a short columnar epithelium.
 +
 +
The mandibular aortic arch is situated close to its inner side
 +
(fig. 381, 2pp). After becoming separated from the lower part
 +
(Marshall), the upper part of the cavity atrophies about the time
 +
of the appearance of the external gills. Its lower part also
 +
becomes much narrowed, but its walls of columnar cells persist.
 +
The outer or somatic wall becomes very thin indeed, the
 +
splanchnic wall, on the other hand, thickens and forms a layer
 +
of several rows of elongated cells. In each of the remaining
 +
arches there is a segment of the original body-cavity fundamentally similar to that in the mandibular arch (fig. 382). A dorsal
 +
dilated portion appears, however, to be present in the third or
 +
hyoid section alone (fig. 20), and even
 +
there disappears very soon, after being
 +
segmented off from the lower part
 +
(Marshall). The cavities in the posterior parts of the head become much
 +
reduced like those in its anterior part,
 +
though at rather a later period. FlG . 382 . HORIZONTAL
 +
 +
It has been shewn that the divi- SECTION THROUGH THE PENULTIMATE VISCERAL ARCH OF
 +
 +
sions of the body-cavity in the head, AN EMBRYO OF PRISTIURUS.
 +
with the exception of the anterior, e p. epiblast; vc. pouch of
 +
early become atrophied, not so how- hypoblast which will form the
 +
 +
walls of a visceral cleit ; //.
 +
CVer their walls. The cells forming segment of body-cavity in vis
 +
the walls both of the dorsal and ven- ceral arch ; aa ' aortic arch '
 +
tral sections of these cavities become elongated, and finally
 +
become converted into muscles. Their exact history has not
 +
been followed in its details, but they almost unquestionably
 +
become the musculus contrictor superficialis and musculus interbranchialis 1 ; and probably also musculus levator mandibuli and
 +
other muscles of the front part of the head.
 +
 +
The anterior cavity close to the eye remains unaltered much
 +
longer than the remaining cavities.
 +
 +
1 Vide Vetter, " Die Kiemen und Kiefermusculatur d. Fische." Jenaische Zcltschrift, Vol. vn.
 +
 +
 +
 +
 +
THE MUSCULAR SYSTEM.
 +
 +
 +
 +
679
 +
 +
 +
 +
Its further history is very interesting. In my original account
 +
of this cavity (No. 292, p. 208) I stated my belief that its walls
 +
gave rise to the eye-muscles, and the history of this process has
 +
been to some extent worked out by Marshall in his important
 +
memoir (No. 509).
 +
 +
Marshall finds that the ventral portion of this cavity, where
 +
its two halves meet, becomes separated from the remainder.
 +
The eventual fate of this part has not however been followed.
 +
Each dorsal section acquires a cup-like form, investing the
 +
posterior and inner surface of the eye. The cells of its outer
 +
wall subsequently give rise to three sets of muscles. The middle
 +
of these, partly also derived from the inner walls of the cup,
 +
becomes the rectus internus of the eye, the dorsal set forms the
 +
rectus superior, and the ventral the rectus inferior. The obliquus
 +
inferior appears also to be in part developed from the walls of
 +
this cavity.
 +
 +
Marshall brings evidence to shew that the rectus externus (as
 +
might be anticipated from its nerve supply) has no connection
 +
with the walls of the premandibular head-cavity, and finds that
 +
it arises close to the position originally occupied by the second
 +
and third cavities. Marshall has not satisfactorily made out the
 +
mode of development of the obliquus superior.
 +
 +
The walls of the cavities, whose history has just been recorded, have definite relations with the cranial nerves, an account
 +
of which has already been given at p. 461.
 +
 +
Head-cavities, in the main similar to those of Elasmobranchii, have been found in the embryo of Petromyzon (fig. 45,
 +
/ic\ the Newt (Osborn and Scott), and various Reptilia (Parker).
 +
 +
BIBLIOGRAPHY.
 +
 +
(507) G.M.Humphry. " Muscles in Vertebrate Animals." Journ. of Anat.
 +
and Phys., Vol. vi. 1872.
 +
 +
(508) J. Miiller. " Vergleichende Anatomic d. Myxinoiden. Part I. Osteologie
 +
u. Myologie." Akad. Wiss., Berlin, 1834.
 +
 +
(509) A. M. Marshall. "On the head cavities and associated nerves of
 +
Elasmobranchs." Quart. J. of Micr. Science, Vol. xxi. 1881.
 +
 +
(510) A. Schneider. " Anat. u. Entwick. d. Muskelsystems d. Wirbelthierc."
 +
Silz. d. Oberhessischen Gesellschaft, 1873.
 +
 +
(511) A. Schneider. Beitrdge z. vergleich. Anat. . Entwick. d. Wirbelthiere.
 +
Berlin, 1879.
 +
 +
Vide 2^0 Gotte (No. 296), Kolliker (N o. 298), Balfour (No. 292), Huxley, etc.
 +
 +
 +
 +
CHAPTER XXIII.
 +
 +
 +
 +
EXCRETORY ORGANS.
 +
 +
 +
 +
EXCRETORY organs consist of coiled or branched and often
 +
ciliated tubes, with an excretory pore opening on the outer surface
 +
of the body, and as a rule an internal ciliated orifice placed in the
 +
body-cavity. In forms provided with a true vascular system,
 +
there is a special development of capillaries around the glandular
 +
part of the excretory organs. In many instances the glandular
 +
cells of the organs are filled with concretions of uric acid or some
 +
similar product of nitrogenous waste.
 +
 +
There is a very great morphological and physiological similarity between almost all the forms of excretory organ found in
 +
the animal kingdom, but although there is not a little to be said
 +
for holding all these organs to be derived from some common
 +
prototype, the attempt to establish definite homologies between
 +
them is beset with very great difficulties.
 +
 +
Platyelminthes. Throughout the whole of the Platyelminthes these organs are constructed on a well-defined type, and
 +
in the Rotifera excretory organs of a similar form to those of the
 +
Platyelminthes are also present.
 +
 +
These organs (Fraipont, No. 513) are more or less distinctly
 +
paired, and consist of a system of wide canals, often united into a
 +
network, which open on the one hand into a pair of large tubes
 +
leading to the exterior, and on the other into fine canals which
 +
terminate by ciliated openings, either in spaces between the
 +
connective-tissue cells (Platyelminthes), or in the body-cavity
 +
(Rotifera). The fine canals open directly into the larger ones,
 +
without first uniting into canals of an intermediate size.
 +
 +
 +
 +
EXCRETORY ORGANS.
 +
 +
 +
 +
68 1
 +
 +
 +
 +
The two large tubes open to the exterior, either by means of
 +
a median posteriorly placed contractile vesicle, or by a pair of
 +
vesicles, which have a ventral and anterior position. The former
 +
type is characteristic of the majority of the Trematoda, Cestoda.
 +
and Rotifera, and the latter of the Nemertea and some Trematoda.
 +
In the Turbellaria the position of the external openings of the
 +
system is variable, and in a few Cestoda (Wagner) there are
 +
lateral openings on each of the successive proglottides, in addition
 +
to the terminal openings. The mode of development of these
 +
organs is unfortunately not known.
 +
 +
Mollusca. In the Mollusca there are usually present two
 +
independent pairs of excretory organs one found in a certain
 +
number of forms during early larval life only 1 , and the other
 +
always present in the adult.
 +
 +
The larval excretory organ has been found in the pulmonate
 +
Gasteropoda (Gegenbaur, Fol 2 , Rabl), in Teredo (Hatschek), and
 +
possibly also in Paludina. It is placed in the anterior region of
 +
the body, and opens ventrally on each side, a short way behind
 +
the velum. It is purely a larval organ, disappearing before the
 +
close of the veliger stage. In the aquatic Pulmonata, where it is
 +
best developed, it consists on each side of a V-shaped tube, with
 +
a dorsally-placed apex, containing an enlargement of the lumen.
 +
There is a ciliated cephalic limb, lined by cells with concretions,
 +
and terminating by an internal opening near the eye, and a nonciliated pedal limb opening to the exterior 3 .
 +
 +
Two irreconcilable views are held as to the development of
 +
this system. Rabl (Vol. II. No. 268) and Hatschek hold that it
 +
is developed in the mesoblast ; and Rabl states that in Planorbis
 +
it is formed from the anterior mesoblast cells of the mesoblastic
 +
bands. A special mesoblast cell on each side elongates into two
 +
processes, the commencing limbs of the future organ. A lumen
 +
is developed in this cell, which is continued into each limb, while
 +
 +
1 I leave out of consideration an external renal organ found in many marine
 +
Gasteropod larvte, vide Vol. II. p. 280.
 +
 +
2 H. Fol, "Etudes sur le devel. d. Mollusques. " Mem. Hi. Archiv d. Zool.
 +
exfJr. et gener., Vol. VIII.
 +
 +
3 The careful observations of Fol seem to me nearly conclusive in favour of this
 +
limb having an external opening, and the statement to the reverse effect on p. 280 of
 +
Vol. ii. of this treatise, made on the authority of Rabl and Biitschli, must probably be
 +
corrected.
 +
 +
 +
 +
682 POLYZOA.
 +
 +
the continuations of the two limbs are formed by perforated
 +
mesoblast cells.
 +
 +
According to Fol these organs originate in aquatic Pulmonata
 +
as a pair of invaginations of the epiblast, slightly behind the
 +
mouth. Each invagination grows in a dorsal direction, and after
 +
a time suddenly bends on itself, and grows ventralwards and
 +
forwards. It thus acquires its V-shaped form.
 +
 +
In the terrestrial Pulmonata the provisional excretory organs
 +
are, according to Fol, formed as epiblastic invaginations, in the
 +
same way as those in the aquatic Pulmonata, but have the form
 +
of simple non-ciliated sacks, without internal openings.
 +
 +
The permanent renal organ of the Mollusca consists typically
 +
of a pair of tubes, although in the majority of the Gasteropoda
 +
one of the two tubes is not developed. It is placed considerably
 +
behind the provisional renal organ.
 +
 +
Each tube, in its most typical form, opens by a ciliated funnel
 +
into the pericardial cavity, and has its external opening at the
 +
side of the foot. The pericardial funnel leads into a glandular
 +
section of the organ, the lining cells of which are filled with
 +
concretions. This section is followed by a ciliated section, from
 +
which a narrow duct leads to the exterior.
 +
 +
As to the development of this organ the same divergence of
 +
opinion exists as in the case of the provisional renal organ.
 +
 +
Rabl's careful observations on Planorbis (Vol. II. No. 268) tend
 +
to shew that it is developed from a mass of mesoblast cells, near
 +
the end of the intestine. The mass becomes hollow, and,
 +
attaching itself to the epiblast on the left side of the anus,
 +
acquires an opening to the exterior. Its internal opening is not
 +
established till after the formation of the heart. Fol gives an
 +
equally precise account, but states that the first rudiment of the
 +
organ arises as a solid mass of epiblast cells. Lankester finds
 +
that this organ is developed as a paired invagination of the.
 +
epiblast in Pisidium, and Bobretzky also derives it from the
 +
epiblast in marine Prosobranchiata. In Cephalopoda on the
 +
other hand Bobretzky's observations (I conclude this from his
 +
figures) indicate that the excretory sacks of the renal organs are
 +
derived from the mesoblast.
 +
 +
Polyzoa. Simple excretory organs, consisting of a pair of
 +
ciliated canals, opening between the mouth and the anus, have
 +
 +
 +
 +
EXCRETORY ORGAN>.
 +
 +
 +
 +
68 3
 +
 +
 +
 +
been found by Hatschek and Joliet in the Entoproctous Polyzoa,
 +
and are developed, according to Hatschek, by whom they were
 +
first found in the larva, from the mesoblast
 +
 +
Brachiopoda. One or rarely two (Rhynchonella) pairs of
 +
canals, with both peritoneal and external openings, are found in
 +
the Brachiopoda. They undoubtedly serve as genital ducts, but
 +
from their structure are clearly of the same nature as the
 +
excretory organs of the Chaetopoda described below. Their
 +
development has not been worked out.
 +
 +
Chaetopoda. Two forms of excretory organ have been met
 +
with in the Chaetopoda. The one form is universally or nearly
 +
universally present in the adult, and typically consists of a pair
 +
of coiled tubes repeated in every segment. Each tube has an
 +
internal opening, placed as a rule in the segment in front of that
 +
in which the greater part of the organ and the external opening
 +
are situated.
 +
 +
There are great variations in the structure of these organs,
 +
which cannot be dealt with here. It may be noted however that
 +
the internal opening may be absent, and that there may be
 +
several internal openings for each organ (Polynoe). In the
 +
Capitellidae moreover several pairs of excretory tubes have been
 +
shewn by Eisig (No. 512) to be present in each of the posterior
 +
segments.
 +
 +
The second form of excretory organ has as yet only been
 +
found in the larva of Polygordius, and will be more conveniently
 +
dealt with in connection with the development of the excretory
 +
system of this form.
 +
 +
There is still considerable doubt as to the mode of formation
 +
of the excretory tubes of the Chaetopoda. Kowalevsky (No. 277),
 +
from his observations on the Oligochasta, holds that they develop
 +
as outgrowths of the epithelial layer covering the posterior side
 +
of the dissepiments, and secondarily become connected with the
 +
epidermis.
 +
 +
Hatschek finds that in Criodrilus they arise from a continuous
 +
linear thickening of the somatic mesoblast, immediately beneath
 +
the epidermis, and dorsal to the ventral band of longitudinal
 +
muscles. They break up into S-shaped cords, the anterior end
 +
of each of which is situated in front of a dissepiment, and is
 +
formed at first of a single large cell, while the posterior part is
 +
 +
 +
 +
684 CHvETOPODA.
 +
 +
 +
 +
continued into the segment behind. The cords are covered by
 +
a peritoneal lining, which still envelopes them, when in the
 +
succeeding stage they are carried into the body-cavity. They
 +
subsequently become hollow, and their hinder ends acquire
 +
openings to the exterior. The formation of their internal
 +
openings has not been followed.
 +
 +
Kleinenberg is inclined to believe that the excretory tubes
 +
take their origin from the epiblast, but states that he has not
 +
satisfactorily worked out their development.
 +
 +
The observations of Risig (No. 512) on the Capitellidae
 +
support Kowalevsky's view that the excretory tubes originate
 +
from the lining of the peritoneal cavity.
 +
 +
Hatschek (No. 514) has given a very interesting account of
 +
the development of the excretory system in Polygordius.
 +
 +
The excretory system begins to be formed, while the larva is
 +
still in the trochospere stage (fig. 383, npli), and consists of a
 +
provisional excretory organ, which is placed in front of the future
 +
segmented part of the body, and occupies a position very
 +
similar to that of the provisional excretory organ found in some Molluscan
 +
larvae (vide p. 68 1).
 +
 +
Hatschek, with some shew of reason, holds that the provisional excretory organs of Polygordius are homologous with those of the Mollusca.
 +
 +
In its earliest stage the provisional
 +
excretory organ of Polygordius consists of a pair of simple ciliated tubes, FIG. 383. POLYOORDIUS
 +
 +
, . , r 11-1 LARVA. (After Hatschek.)
 +
 +
each with an anterior funnel-like open- m _ moulh . ^ supraKBSO .
 +
 +
ing situated in the midst of the meSO- phageal ganglion ; nph. nephri11 11 . , dion ; ine.p. mesoblastic band;
 +
 +
blast cells, and a posterior external an _ anus 5 oL stomach .
 +
opening. The latter is placed immediately in front of what afterwards becomes the segmented region
 +
of the embryo. While the larva is still unsegmented, a second
 +
internal opening is formed for each tube (fig. 383, np/i) and the
 +
two openings so formed may eventually become divided into
 +
five (fig. 384 A), all communicating by a single pore with the
 +
exterior.
 +
 +
When the posterior region of the embryo becomes segmented,
 +
 +
 +
 +
 +
EXCRETORY ORGANS.
 +
 +
 +
 +
685
 +
 +
 +
 +
paired excretory organs are formed in each of the posterior
 +
segments, but the account of their development, as given by
 +
Hatschek, is so remarkable that I do not think it can be
 +
definitely accepted without further confirmation.
 +
 +
From the point of junction of the two main branches of the
 +
larval kidney there grows backwards (fig. 384 B), to the hind
 +
end of the first segment, a very delicate tube, only indicated by
 +
its ciliated lumen, its walls not being differentiated. Near the
 +
front end of this tube a funnel, leading into the larval body
 +
cavity of the head, is formed, and subsequently the posterior end
 +
of the tube acquires an external opening, and the tube distinct
 +
walls. The communication with the provisional excretory organ
 +
is then lost, and thus the excretory tube of the first segment is
 +
established.
 +
 +
The excretory tubes in the second and succeeding segments
 +
are formed in the same way as in the first, i.e. by the continuation of the lumen of the hind end of the excretory tube from
 +
the preceding segment, and the subsequent separation of this
 +
part as a separate tube.
 +
 +
The tube may be continued with a sinuous course through
 +
 +
 +
 +
 +
 +
A
 +
A
 +
 +
A
 +
+
 +
 +
A.
 +
 +
 +
 +
Y
 +
 +
Y
 +
Y
 +
Y
 +
Y
 +
 +
 +
 +
J)
 +
 +
 +
 +
FIG. 384. DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE EXCRETORY
 +
SYSTEM OF POLYGORDIUS. (After Hatschek.)
 +
 +
several segments without a distinct wall. The external and
 +
internal openings of the permanent excretory tubes are thus
 +
secondarily acquired. The internal openings communicate with
 +
the permanent body-cavity. The development of the perma
 +
 +
 +
686 GEPHYREA.
 +
 +
 +
 +
nent excretory tubes is diagrammatically represented in fig.
 +
384 C and D.
 +
 +
The provisional excretory organ atrophies during larval life.
 +
 +
If Hatschek's account of the development of the excretory system of
 +
Polygordius is correct, it is clear that important secondary modifications
 +
must have taken place in it, because his description implies that there sprouts
 +
from the anterior excretory organ, while it has its own external opening, a
 +
posterior duct, which does not communicate either with the exterior or with
 +
the body-cavity! Such a duct could have no function. It is intelligible
 +
either (i) that the anterior excretory organ should lead into a longitudinal
 +
duct, opening posteriorly ; that then a series of secondary openings into the
 +
body-cavity should attach themselves to this, that for each internal opening
 +
an external should subsequently arise, and the whole break up into separate
 +
tubes ; or (2) that behind an anterior provisional excretory organ a series of
 +
secondary independent segmental tubes should be formed. But from Hatschek's account neither of these modes of evolution can be deduced.
 +
 +
Gephyrea. The Gephyrea may have three forms of excretory organs, two of which are found in the adult, and one,
 +
similar in position and sometimes also in structure, to the
 +
provisional excretory organ of Polygordius, has so far only been
 +
found in the larvae of Echiurus and Bonellia.
 +
 +
In all the Gephyrea the so-called 'brown tubes' are
 +
apparently homologous with the segmented excretory tubes of
 +
Chaetopods. Their main function appears to be the transportation of the generative products to the exterior. There is but a
 +
single highly modified tube in Bonellia, forming the oviduct and
 +
uterus ; a pair of tubes in the Gephyrea inermia, and two or
 +
three pairs in most Gephyrea armata, except Bonellia. Their
 +
development has not been studied.
 +
 +
In the Gephyrea armata there is always present a pair of
 +
posteriorly placed excretory organs, opening in the adult into
 +
the anal extremity of the alimentary tract, and provided with
 +
numerous ciliated peritoneal funnels. These organs were stated
 +
by Spengel to arise in Bonellia as outgrowths of the gut ; but in
 +
Echinrus Hatschek (No. 515) finds that they are developed from
 +
the somatic mesoblast of the terminal part of the trunk. They
 +
soon become hollow, and after attaching themselves to the
 +
epiblast on each side of the anus, acquire external openings.
 +
They are not at first provided with peritoneal funnels, but these
 +
parts of the organs become developed from a ring of cells at
 +
 +
 +
 +
EXCRETORY ORGANS.
 +
 +
 +
 +
687
 +
 +
 +
 +
their inner extremities ; and there is at first but a single funnel
 +
for each vesicle. The mode of increase of the funnels has not
 +
been observed, nor has it been made out how the organs themselves become attached to the hind-gut.
 +
 +
The provisional excretory organ of Echiurus is developed at
 +
an early larval stage, and is functional during the whole of
 +
larval life. It at first forms a ciliated tube on each side, placed
 +
in front of that part of the larva which becomes the trunk of the
 +
adult. It opens to the exterior by a fine pore on the ventral
 +
side, immediately in front of one of the mesoblastic bands, and
 +
appears to be formed of perforated cells. It terminates internally in a slight swelling, which represents the normal internal
 +
ciliated funnel. The primitively simple excretory organ becomes
 +
eventually highly complex by the formation of numerous
 +
branches, each ending in a slightly swollen extremity. These
 +
branches, in the later larval stages, actually form a network, and
 +
the inner end of each main branch divides into a bunch of fine
 +
tubes. The whole organ resembles in many respects the excretory organ of the Platyelminthes.
 +
 +
In the larva of Bonellia Spengel has described a pair of
 +
provisional excretory tubes, opening near the anterior end of
 +
the body, which are probably homologous with the provisional
 +
excretory organs of Echiurus (vide Vol. II., fig. 162 C, se).
 +
 +
Discophora. As in many of the types already spoken of,
 +
permanent and provisional excretory organs may be present in
 +
the Discophora. The former are usually segmentally arranged,
 +
and resemble in many respects the excretory tubes of the
 +
Chaetopoda. They may either be provided with a peritoneal
 +
funnel (Nephelis, Clepsine) or have no internal opening
 +
(Hirudo).
 +
 +
Bourne 1 has shewn that the cells surrounding the main duct
 +
in the medicinal Leech are perforated by a very remarkable
 +
network of ductules, and the structure of these organs in the
 +
Leech is so peculiar that it is permissible to state with due reserve
 +
their homology with the excretory organs of the Chaetopoda.
 +
 +
The excretory tubes of Clepsine are held by Whitman to be
 +
developed in the mesoblast.
 +
 +
1 "On the Structure of the Nephridia of the Medicinal Leech." Quart. J. of
 +
Micr. Science, Vol. XX. 1880.
 +
 +
 +
 +
688 ARTHROPODA.
 +
 +
 +
 +
There are found in the embryos of Nephelis and Hirudo
 +
certain remarkable provisional excretory organs the origin and
 +
history of which are not yet fully made out. In Nephelis they
 +
appear as one (according to Robin), or (according to Biitschli)
 +
as two successive pairs of convoluted tubes on the dorsal side of
 +
the embryo, which are stated by the latter author to develop
 +
from the scattered mesoblast cells underneath the skin. At
 +
their fullest development they extend, according to Robin, from
 +
close to the head to near the ventral sucker. Each of them is
 +
U-shaped, with the open end of the U forwards, each limb of the
 +
U being formed by two tubes united in front. No external
 +
opening has been clearly made out. Fiirbringer is inclined from
 +
his own researches to believe that they open laterally. They
 +
contain a clear fluid.
 +
 +
In Hirudo, Leuckart has described three similar pairs of
 +
organs, the structure of which he has fully elucidated. They
 +
are situated in the posterior part of the body, and each of them
 +
commences with an enlargement, from which a convoluted tube
 +
is continued for some distance backwards; the tube then turns
 +
forwards again, and after bending again upon itself opens to the
 +
exterior. The anterior part is broken up into a kind of
 +
labyrinthic network.
 +
 +
The provisional excretory organs of the Leeches cannot be
 +
identified with the anterior provisional organs of Polygordius
 +
and Echiurus.
 +
 +
Arthropoda. Amongst the Arthropoda Peripatus is the
 +
only form with excretory organs of the type of the segmental
 +
excretory organs of the Chsetopoda 1 .
 +
 +
These organs are placed at the bases of the feet, in the
 +
lateral divisions of the body-cavity, shut off from the main
 +
median division of the body-cavity by longitudinal septa of
 +
transverse muscles.
 +
 +
Each fully developed organ consists of three parts :
 +
 +
(i) A dilated vesicle opening externally at the base of a
 +
foot. (2) A coiled glandular tube connected with this, and
 +
subdivided again into several minor divisions. (3) A short
 +
terminal portion opening at one extremity into the coiled tube
 +
 +
1 Vide F. M. Balfour, " On some points in the Anatomy of Peripatus Capensis."
 +
Quart. J, of Micr. Science, Vol. XIX. 1879.
 +
 +
 +
 +
EXCRETORY ORGANS. 689
 +
 +
 +
 +
and at the other, as I believe, into the body cavity. This
 +
section becomes very conspicuous, in stained preparations, by
 +
the intensity with which the nuclei of its walls absorb the
 +
colouring matter.
 +
 +
In the majority of the Tracheata the excretory organs have
 +
the form of the so-called Malpighian tubes, which always (vide
 +
Vol. II.) originate as a pair of outgrowths of the epiblastic
 +
proctodaeum. From their mode of development they admit of
 +
comparison with the anal vesicles of the Gephyrea, though in
 +
the present state of our knowledge this comparison must be
 +
regarded as somewhat hypothetical.
 +
 +
The antennary and shell-glands of the Crustacea, and
 +
possibly also the so-called dorsal organ of various Crustacean
 +
larvae appear to be excretory, and the two former have been
 +
regarded by Claus and Grobben as belonging to the same
 +
system as the segmental excretory tubes of the Chaetopoda.
 +
 +
Nematoda. Paired excretory tubes, running for the whole
 +
length of the body in the so-called lateral line, and opening in
 +
front by a common ventral pore, are present in the Nematoda.
 +
They do not appear to communicate with the body cavity, and
 +
their development has not been studied.
 +
 +
Very little is known with reference either to the structure or
 +
development of excretory organs in the Echinodermata and the
 +
other Invertebrate types of which no mention has been so far
 +
made in this Chapter.
 +
 +
Excretory organs and generative ducts of the Craniata.
 +
 +
Although it would be convenient to separate, if possible, the
 +
history of the excretory organs from that of the generative
 +
ducts, yet these parts are so closely related in the Vertebrata, in
 +
some cases the same duct having at once a generative and a
 +
urinary function, that it is not possible to do so.
 +
 +
The excretory organs of the Vertebrata consist of three
 +
distinct glandular bodies and of their ducts. These are (i) a
 +
small glandular body, usually with one or more ciliated funnels
 +
opening into the body cavity, near the opening of which there
 +
projects into the body cavity a vascular glomerulus. It is
 +
situated very far forwards, and is usually known as the head
 +
44
 +
 +
 +
 +
690 ELASMOBRANCHII.
 +
 +
 +
 +
kidney, though it may perhaps be more suitably called, adopting
 +
Lankester's nomenclature, the pronepliros. Its duct, which forms
 +
the basis for the generative and urinary ducts, will be called the
 +
segmented duct.
 +
 +
(2) The Wolffian body, which may be also called the
 +
mesonepJiros. It consists of a series of, at first, segmentally
 +
(with a few exceptions) arranged glandular canals (segmental
 +
tubes) primitively opening at one extremity by funnel-shaped
 +
apertures into the body cavity, and at the other into the
 +
segmental duct. This duct becomes in many forms divided
 +
longitudinally into two parts, one of which then remains
 +
attached to the segmental tubes and forms the Wolffian or
 +
mesonepJiric duct, while the other is known as the Milllerian
 +
dnct.
 +
 +
(3) The kidney proper or metanephros. This organ is only
 +
found in a completely differentiated form in the amniotic Vertebrata. Its duct is an outgrowth from the Wolrfian duct.
 +
 +
The above parts do not coexist in full activity in any living
 +
adult member of the Vertebrata, though all of them are found
 +
together in certain embryos. They are so intimately connected
 +
that they cannot be satisfactorily dealt with separately.
 +
 +
Elasmobranchii. The excretory system of the Elasmobranchii is by no means the most primitive known, but at the
 +
same time it forms a convenient starting point for studying the
 +
modifications of the system in other groups. The most remarkable peculiarity it presents is the absence of a pronephros.
 +
The development of the Elasmobranch excretory system has
 +
been mainly studied by Semper and myself.
 +
 +
The first trace of the system makes its appearance as a knob
 +
of mesoblast, springing from the intermediate cell-mass near the
 +
level of the hind end of the heart (fig. 385 K,pd). This knob is
 +
the rudiment of the abdominal opening of the segmental duct,
 +
and from it there grows backwards to the level of the anus a
 +
solid column of cells, which constitutes the rudiment of the
 +
segmental duct itself (fig. 385 B, pd). The knob projects
 +
towards the epiblast, and the column connected with it lies
 +
between the mesoblast and epiblast. The knob and column do
 +
not long remain solid, but the former acquires an opening into
 +
the body cavity (fig. 421, sd) continuous with a lumen, which
 +
 +
 +
 +
EXCRETORY ORGANS.
 +
 +
 +
 +
691
 +
 +
 +
 +
makes its appearance in the column (fig. 386, sd). The knob
 +
forms the only structure which can be regarded as a rudiment of
 +
the pronephros.
 +
 +
 +
 +
spn
 +
 +
 +
 +
spn
 +
 +
 +
 +
 +
FlG. 385. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL
 +
 +
CLEFTS.
 +
 +
The sections illustrate the development of the segmental duct (pd) or primitive
 +
duct of the pronephros. In A (the anterior of the two sections) this appears as a
 +
solid knob (pd) projecting towards the epiblast. In B is seen a section of the column
 +
which has grown backwards from the knob in A.
 +
 +
spn. rudiment of a spinal nerve; me. medullary canal; ch. notochord; X. subnotochordal rod; mp. muscle-plate; mp' . specially developed portion of muscle-plate;
 +
ao. dorsal aorta ; pd. segmental duct ; so. somatopleure ; sp. splanchnopleure ; //.
 +
body cavity; ep. epiblast; al. alimentary canal.
 +
 +
While the lumen is gradually being formed, the segmental
 +
tubes of the mesonephros become established. They appear to
 +
arise as differentiations of the parts of the primitive lateral plates
 +
of mesoblast, placed between the dorsal end of the body cavity
 +
and the muscle-plate (fig. 386, st) 1 , which are usually known as
 +
the intermediate cell-masses.
 +
 +
The lumen of the segmental tubes, though at first very small,
 +
soon becomes of a considerable size. It appears to be established
 +
in the position of the section of the body cavity in the intermediate cell-mass, which at first unites the part of the body
 +
cavity in the muscle-plates with the permanent body cavity.
 +
The lumen of each tube opens at its lower end into the dorsal
 +
part of the body cavity (fig. 386, st}, and each tube curls obliquely
 +
 +
1 In my original account of the development I held these tubes to be invaginations
 +
of the peritoneal epithelium. Sedgwick (No. 549) was led to doubt the accuracy of
 +
my original statement from his investigations on the chick ; and from a re-examination of my specimens he arrived at the results stated above, and which I am now
 +
myself inclined to adopt.
 +
 +
442
 +
 +
 +
 +
692
 +
 +
 +
 +
ELASMOBRANCHII.
 +
 +
 +
 +
sp.c
 +
 +
 +
 +
 +
backwards round the inner and dorsal side of the segmental
 +
duct, near which it at first ends blindly.
 +
 +
One segmental tube makes its
 +
appearance for each somite (fig. 265),
 +
commencing with that immediately
 +
behind the abdominal opening of the
 +
segmental duct, the last tube being
 +
situated a few segments behind the
 +
anus. Soon after their formation
 +
the blind ends of the segmental tubes
 +
come in contact with, and open into
 +
the segmental duct, and each of them
 +
becomes divided into four parts.
 +
These are (i) a section carrying the
 +
peritoneal opening, known as the
 +
peritoneal funnel, (2) a dilated vesicle
 +
into which this opens, (3) a coiled
 +
tubulus proceeding from (2), and
 +
terminating in (4) a wider portion
 +
opening into the segmental duct. At
 +
the same time, or shortly before this,
 +
each segmental duct unites with and
 +
opens into one of the horns of the
 +
cloaca, and also retires from its
 +
primitive position between the epiblast and mesoblast, and assumes a
 +
position close to the epithelium lining
 +
the body cavity (fig. 380, sd}. The
 +
general features of the excretory
 +
organs at this period are diagrammatically represented in the
 +
woodcut (fig. 387). In this fig. pd is the segmental duct and
 +
o its abdominal opening; s.t points to the segmental tubes,
 +
the finer details of whose structure are not represented in the
 +
diagram. The mesonephros thus forms at this period an elongated gland composed of a series of isolated coiled tubes, one
 +
extremity of each of which opens into the body cavity, and the
 +
other into the segmental duct, which forms the only duct of the
 +
system, and communicates at its front end with the body cavity,
 +
and behind with the cloaca.
 +
 +
 +
 +
FIG. 386. SECTION THROUGH
 +
THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN
 +
 +
28 F.
 +
 +
sp.c. spinal canal; W. white
 +
matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ;
 +
x. sub-notochordal rod ; ao. aorta ;
 +
nip, muscle-plate ; nip', inner layer
 +
of muscle-plate already converted
 +
into muscles ; Vr, rudiment of
 +
vertebral body ; st. segmental
 +
tube; sd. segmental duct; sp.v.
 +
spiral valve ; v. subintestinal vein ;
 +
p.o. primitive generative cells.
 +
 +
 +
 +
EXCRETORY ORGANS. 693
 +
 +
 +
 +
The next important change concerns the segmental duct,
 +
which becomes longitudinally split into two complete ducts in
 +
the female, and one complete duct and parts of a second duct in
 +
the male. The manner in which this takes place is diagrammatically represented in fig. 387 by the clear line x, and in
 +
transverse section in figs. 388 and 389. The resulting ducts are
 +
(i) the Wolffian duct or mesonephric duct (wd\ dorsally, which
 +
remains continuous with the excretory tubules of the mesonephros, and ventrally (2) the oviduct or Miillerian duct in the
 +
female, and the rudiments of this duct in the male. In the
 +
 +
 +
 +
 +
 +
FIG. 387. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN
 +
 +
ELASMOBRANCH EMBRYO.
 +
 +
pd. segmental duct. It opens at o into the body cavity and at its other extremity
 +
into the cloaca; x. line along which the division appears which separates the segmental
 +
duct into the Wolffian duct above and the Miillerian duct below; s.t. segmental
 +
tubes. They open at one end into the body cavity, and at the other into the segmental duct.
 +
 +
female the formation of these ducts takes place (fig. 389) by a
 +
nearly solid rod of cells being gradually split off from the
 +
ventral side of all but the foremost part of the original segmental
 +
duct. This nearly solid cord is the Miillerian duct (pd}. A
 +
very small portion of the lumen of the original segmental duct
 +
is perhaps continued into it, but in any case it very soon acquires
 +
a wide lumen (fig. 389 A). The anterior part of the segmental
 +
duct is not divided, but remains continuous with the Mullerian
 +
duct, of which its anterior pore forms the permanent peritoneal
 +
opening 1 (fig. 387). The remainder of the segmental duct (after
 +
the loss of its anterior section, and the part split off from its
 +
ventral side) forms the Wolffian duct. The process of formation
 +
of these ducts in the male differs from that in the female chiefly
 +
 +
1 Five or six segmental tubes belong to the region of the undivided anterior part
 +
of the segmental duct, which forms the front end of the Mullerian duct ; but they appear to atrophy very early, without acquiring a definite attachment to the segmental
 +
duct.
 +
 +
 +
 +
694
 +
 +
 +
 +
ELASMOBRANCHIL
 +
 +
 +
 +
in the fact of the anterior undivided part of the segmental duct,
 +
which forms the front end of the Miillerian duct, being shorter,
 +
 +
 +
 +
 +
trd/
 +
 +
 +
 +
 +
FIG. 389. FOUR SECTIONS
 +
THROUGH THE ANTERIOR
 +
I'ART OF THE SEGMENTAL
 +
DUCT OF A FEMALE EMBRYO
 +
OF SCYLLIUM CANICULA.
 +
 +
The figure shews how the
 +
segmental duct becomes split
 +
into the Wolffian or mesonephric duct above, and Miillerian duct or oviduct below.
 +
 +
wd. Wolffian or mesonephric duct; od. Miillerian
 +
duct or oviduct ; sd. segmental duct.
 +
 +
 +
 +
FIG. 388. DIAGRAMMATIC REPRESENTATION OF A TRANSVERSE SECTION OF A
 +
 +
SCYLLIUM EMBRYO ILLUSTRATING THE
 +
FORMATION OF THE WOLFFIAN AND MlJLLERIAN DUCTS BY THE LONGITUDINAL
 +
SPLITTING OF THE SEGMENTAL DUCT.
 +
 +
me. medullary canal; mp. muscle-plate;
 +
ch. notochord; ao. aorta; cav. cardinal
 +
vein; st. segmental tube. On the left side
 +
the section passes through the opening of
 +
a segmental tube into the body cavity. On
 +
the right this opening is represented by
 +
dotted lines, and the opening of the segmental tube into the Wolffian duct has
 +
been cut through; iv.d. Wolffian duct;
 +
m.d. Miillerian duct. The section is taken
 +
through the point where the segmental
 +
duct and Wolffian duct have just become
 +
separate; gr. the germinal ridge with the
 +
thickened germinal epithelium ; /. liver ;
 +
i. intestine with spiral valve.
 +
 +
and in the column of cells with which it is continuous being
 +
from the first incomplete.
 +
 +
The segmental tubes of the mesonephros undergo further
 +
important changes. The vesicle at the termination of each peritoneal funnel sends a bud forwards towards the preceding
 +
tubulus, which joins the fourth section of it close to the opening
 +
 +
 +
 +
EXCRETORY ORGANS.
 +
 +
 +
 +
695
 +
 +
 +
 +
 +
into the Wolffian duct (fig. 390, px). The remainder of the
 +
vesicle becomes converted
 +
into a Malpighian body (mg}.
 +
 +
By the first of these changes 10^-4 M @W>f
 +
a tube is established connecting each pair of segments
 +
of the mesonephros, and
 +
though this tube is in part
 +
aborted (or only represented
 +
by a fibrous band) in the
 +
anterior part of the excretory
 +
organs in the adult, and most
 +
probably in the hinder part,
 +
yet it seems almost certain
 +
that the secondary and tertiary Malpighian bodies of
 +
the majority of segments are
 +
developed from its persisting
 +
blind end. Each of these
 +
 +
 +
 +
FIG. 390. LONGITUDINAL VERTICAL
 +
SECTION THROUGH PART OF THE MESONEPHROS OF AN EMBRYO OF SCYLLIUM.
 +
 +
The figure contains two examples of the
 +
budding of the vesicle of a segmental tube
 +
(which forms a Malpighian body in its own
 +
segment) to unite with the tubulus in the
 +
preceding segment close to its opening into
 +
the Wolffian (mesonephric) duct.
 +
 +
ge. epithelium of body-cavity; st. peritoneal funnel of segmental tube with its
 +
peritoneal opening; mg. Malpighian body;
 +
px. bud from Malphigian body uniting with
 +
preceding segment.
 +
 +
 +
 +
secondary and tertiary Malpighian bodies is connected with a
 +
convoluted tubulus (fig. 391, a.mg), which is also developed from
 +
the tube connecting each pair of segmental tubes, and therefore
 +
falls into the primary tubulus close to its junction with the
 +
 +
 +
 +
st.c
 +
 +
 +
 +
 +
w.d
 +
 +
 +
 +
FIG. 391. THREE SEGMENTS OF THE ANTERIOR PART OF THE MESONEPHROS OF A
 +
NEARLY RIPE EMBRYO OF SCYLLIUM CANICULA AS A TRANSPARENT OBJECT.
 +
The figure shews a fibrous band passing from the primary to the secondary Malpighian bodies in two segments, which is the remains of the outgrowth from the
 +
primary Malpighian body.
 +
 +
sf.o. peritoneal funnel; p. ing. primary Malpighian body; a.mg. accessory Malpighian body; w.d. mesonephric (Wolffian) duct.
 +
 +
 +
 +
696 ELASMOBRANCI1II.
 +
 +
 +
 +
segmental duct. Owing to the formation of the accessory tubuli
 +
the segments of the mesonephros acquire a compound character.
 +
 +
The third section of each tubulus becomes by continuous
 +
growth, especially in the hinder segments, very bulky and
 +
convoluted.
 +
 +
The general character of a slightly developed segment of
 +
the mesonephros at its full growth may be gathered from fig.
 +
391. It commences with (i) a peritoneal opening, somewhat
 +
oval in form (st.d) and leading directly into (2) a narrow tube,
 +
the segmental tube, which takes a more or less oblique course
 +
backwards, and, passing superficially to the Wolffian duct (w.d},
 +
opens into (3) a Malpighian body (p.mg) at the anterior extremity of an isolated coil of glandular tubuli. This coil forms
 +
the third section of each segment, and starts from the Malpighian body. It consists of a considerable number of rather
 +
definite convolutions, and after uniting with tubuli from one,
 +
two, or more (according to the size of the segment) accessory
 +
Malpighian bodies (a.mg) smaller than the one into which the
 +
segmental tube falls, eventually opens by (4) a narrowish
 +
collecting tube into the Wolffian duct at the posterior end of
 +
the segment. Each segment is probably completely isolated
 +
from the adjoining segments, and never has more than one
 +
peritoneal funnel and one communication with the Wolffian duct.
 +
 +
Up to this time there has been no distinction between the
 +
anterior and posterior tubuli of the mesonephros, which alike
 +
open into the Wolffian duct. The collecting tubes of a considerable number of the hindermost tubuli (ten or eleven in
 +
Scyllium canicula), either in some species elongate, overlap,
 +
while at the same time their openings travel backward so that
 +
they eventually open by apertures (not usually so numerous as
 +
the separate tubes), on nearly the same level, into the hindermost section of the Wolffian duct in the female, or into the
 +
urinogenital cloaca, formed by the coalesced terminal parts of
 +
the Wolffian ducts, in the male; or in other species become
 +
modified, by a peculiar process of splitting from the Wolnian
 +
duct, so as to pour their secretion into a single duct on each
 +
side, which opens in a position corresponding with the numerous
 +
ducts of the other species (fig. 392). In both cases the modified
 +
posterior kidney-segments are probably equivalent to the per
 +
 +
 +
EXCRETORY ORGANS. 697
 +
 +
 +
 +
manent kidney or metanephros of the amniotic Vertebrates, and
 +
for this reason the numerous collecting tubes or single collecting
 +
tube, as the case may be, will be spoken of as ureters. The
 +
anterior tubuli of the primitive excretory organ retain their early
 +
relation to the Wolffian duct, and form the permanent Wolffian
 +
body or mesonephros.
 +
 +
The originally separate terminal extremities of the Wolffian
 +
ducts always coalesce, and form a urinal cloaca, opening by a
 +
single aperture, situated at the extremity of the median papilla
 +
behind the anus. Some of the peritoneal openings of the segmental tubes in Scyllium, or in other cases all the openings,
 +
become obliterated.
 +
 +
In the male the anterior segmental tubes undergo remarkable modifications, and become connected with the testes.
 +
Branches appear to grow from the first three or four or more of
 +
them (though probably not from their peritoneal openings),
 +
which pass to the base of the testis, and there uniting into a
 +
longitudinal canal, form a network, and receive the secretion of
 +
the testicular ampullae (fig. 393, nf). These ducts, the vasa
 +
efferent ia, carry the semen to the Wolffian body, but before
 +
opening into the tubuli of this body they unite into a canal
 +
known as the longitudinal canal of the Wolffian body (l.c\ from
 +
which pass off ducts equal in number to the vasa efferentia,
 +
each of which normally ends in a Malpighian corpuscle. From
 +
the Malpighian corpuscles so connected there spring the convoluted tubuli, forming the generative segments of the Wolffian
 +
body, along which the semen is conveyed to the Wolffian duct
 +
(v.d). The Wolffian duct itself becomes much contorted and
 +
acts as vas deferens.
 +
 +
Figs. 392 and 393 are diagrammatic representations of the
 +
chief constituents of the adult urinogenital organs in the two
 +
sexes. In the adult female (fig. 392), there are present the
 +
following parts :
 +
 +
(1) The oviduct or Mullerian duct (m.d) split off from the
 +
segmental duct of the kidneys. Each oviduct opens at its
 +
anterior extremity into the body cavity, and behind the two
 +
oviducts have independent communications with the general
 +
cloaca.
 +
 +
(2) The mesonephric ducts (w.d), the other product of the
 +
 +
 +
 +
698
 +
 +
 +
 +
ELASMOBRANCHII.
 +
 +
 +
 +
segmental ducts of the kidneys. They end in front by becoming continuous with the tubulus of the anterior persisting
 +
segment of the mesonephros on each side, and unite behind to
 +
 +
 +
 +
 +
FIG. 392. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS
 +
 +
IN AN ADULT FEMALE ELASMOBRANCH.
 +
 +
m.d. Miillerian duct; w.d. Wolffian duct; s.t. segmental tubes; five of them are
 +
represented with openings into the body cavity, the posterior segmental tubes form
 +
the mesonephros ; ov. ovary.
 +
 +
open by a common papilla into the cloaca. The mesonephric
 +
duct receives the secretion of the anterior tubuli of the primitive
 +
mesonephros.
 +
 +
(3) The ureter which carries off the secretion of the kidney
 +
proper or metanephros. It is represented in my diagram in its
 +
most rare and differentiated condition as a single duct connected
 +
with the posterior segmental tubes.
 +
 +
(4) The segmental tubes (.$-./) some of which retain their
 +
 +
 +
 +
-S.t:
 +
 +
 +
 +
 +
FIG. 393. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS
 +
 +
IN AN ADULT MALE ELASMOBRANCH.
 +
 +
m.d. rudiment of Miillerian duct; w.d. Wolffian duct, marked vd in front and
 +
serving as vas deferens; s.t. segmental tubes; two of them are represented with openings into the body cavity; d. ureter; /. testis; nt. canal at the base of the testis;
 +
VE, vasa efferentia; Ic. longitudinal canal of the Wolffian body.
 +
 +
 +
 +
EXCRETORY ORGANS. 699
 +
 +
 +
 +
original openings into the body cavity, and others are without
 +
them. They are divided into two groups, an anterior forming
 +
the mesonephros or Wolffian body, which pours its secretion
 +
into the Wolffian duct ; and a posterior group forming a gland
 +
which is probably equivalent to the kidney proper of amniotic
 +
Craniata, and is connected with the ureter.
 +
 +
In the male the following parts are present (fig. 393):
 +
 +
(1) The Mlillerian duct (m.d], consisting of a small rudiment attached to the liver, representing the foremost end of the
 +
oviduct of the female.
 +
 +
(2) The mesonephric duct (w.d] which precisely corresponds
 +
to the mesonephric duct of the female, but, in addition to
 +
serving as the duct of the Wolffian body, also acts as a vas
 +
deferens (vd}. In the adult male its foremost part has a very
 +
tortuous course.
 +
 +
(3) The ureter (d\ which has the same fundamental constitution as in the female.
 +
 +
(4) The segmental tubes (s.t). The posterior tubes have
 +
the same arrangement in both sexes, but in the male modifications take place in connection with the anterior tubes to fit them
 +
to act as transporters of the semen.
 +
 +
Connected with the anterior tubes there are present (i) the
 +
vasa efferentia (VE], united on the one hand with (2) the
 +
central canal in the base of the testis (/), and on the other with
 +
the longitudinal canal of the Wolffian body (/<?). From the
 +
latter are seen passing off the successive tubuli of the anterior
 +
segments of the Wolffian body, in connection with which Malpighian bodies are typically present, though not represented in
 +
my diagram.
 +
 +
Apart from the absence of the pronephros the points which
 +
deserve notice in the Elasmobranch excretory system are (i)
 +
The splitting of the segmental duct into Wolffian (mesonephric)
 +
and Mullerian ducts. (2) The connection of the former with
 +
the mesonephros, and of the latter with the abdominal opening
 +
of the segmental duct which represents the pronephros of other
 +
types. (3) The fact that the Mullerian duct serves as oviduct,
 +
and the Wolffian duct as vas deferens. (4) The differentiation
 +
of a posterior section of the mesonephros into a special gland
 +
foreshadowing the metanephros of the Amniota.
 +
 +
 +
 +
/OO CYCLOSTOMATA.
 +
 +
 +
 +
Cyclostomata. The development of the excretory system
 +
amongst the Cyclostomata has only been studied in Petromyzon
 +
(Miiller, Furbringer, and Scott).
 +
 +
The first part of the system developed is the segmental duct.
 +
It appears in the embryo of about 14 days (Scott) as a solid
 +
cord of cells, differentiated from the somatic mesoblast near the
 +
dorsal end of the body cavity. This cord is at first placed
 +
immediately below the epiblast, and grows backwards by a
 +
continuous process of differentiation of fresh mesoblast cells. It
 +
soon acquires a lumen, and joins the cloacal section of the
 +
alimentary tract before the close of foetal life. Before this
 +
communication is established, the front end of the duct sends a
 +
process towards the body cavity, the blind end of which acquires
 +
a ciliated opening into the latter. A series of about four or five
 +
successively formed outgrowths from the duct, one behind the
 +
other, give rise to as many ciliated funnels opening into the body
 +
cavity, and each communicating by a more or less elongated
 +
tube with the segmental duct. These funnels, which have a
 +
metameric arrangement, constitute the pronephros, the whole
 +
of which is situated in the pericardial region of the body
 +
cavity.
 +
 +
On the inner side of the peritoneal openings of each pronephros there is formed a vascular glomerulus, projecting into
 +
the body cavity, and covered by peritoneal epithelium. For a
 +
considerable period the pronephros constitutes the sole functional part of the excretory system.
 +
 +
A mesonephros is formed (Furbringer) relatively late in
 +
larval life, as a segmentally arranged series of solid cords,
 +
derived from the peritoneal epithelium. These cords constitute
 +
the rudiments of the segmental tubes. They are present for a
 +
considerable portion of the body cavity, extending backwards
 +
from a point shortly behind the pronephros. They soon separate
 +
from the peritoneal epithelium, become hollowed out into canals,
 +
and join the segmental duct. At their blind extremity (that
 +
originally connected with the peritoneal epithelium) a Malpighian
 +
body is formed.
 +
 +
The pronephros is only a provisional excretory organ, the
 +
atrophy of which commences during larval life, and is nearly
 +
completed when the Ammoccete has reached 180 mm. in length.
 +
 +
 +
 +
EXCRETORY ORGANS. 70 1
 +
 +
Further changes take place in connection with the excretory
 +
system on the conversion of the Ammoccete into the adult.
 +
 +
The segmental ducts in the adult fall into a common urinogenital cloaca, which opens on a papilla behind the anus. This
 +
cloaca also communicates by two apertures (abdominal pores)
 +
with the body cavity. The generative products are carried into
 +
the cloaca by these pores ; so that their transportation outwards
 +
is not performed by any part of the primitive urinary system.
 +
The urinogenital cloaca is formed by the separation of the portion
 +
of the primitive cloaca containing the openings of the segmental
 +
ducts from that connected with the alimentary tract.
 +
 +
The mesonephros of the Ammoccete undergoes at the metamorphosis complete atrophy, and is physiologically replaced by
 +
a posterior series of segmental tubes, opening into the hindermost portion of the segmental duct (Schneider).
 +
 +
In Myxine the excretory system consists (i) of a highly developed pronephros with a bunch of ciliated peritoneal funnels opening into the pericardial section of the body cavity. The coiled and branched tubes of which
 +
the pronephros is composed open on the ventral side of the anterior portion
 +
of the segmental duct, which in old individuals is cut off from the posterior
 +
section of the duct. On the dorsal side of the portion of the segmental duct
 +
belonging to the pronephros there are present a small number of diverticula,
 +
terminating in glomeruli : they are probably to be regarded as anterior
 +
segmental tubes. (2) Of a mesonephros, which commences a considerable
 +
distance behind the pronephros, and is formed of straight extremely simple
 +
segmental tubes opening into the segmental duct (fig. 385).
 +
 +
The excretory system of Myxine clearly retains the characters of the
 +
system as it exists in the larva of Petromyzon.
 +
 +
Teleostei. In most Teleostei the pronephros and mesonephros coexist through life, and their products are carried off by
 +
a duct, the nature of which is somewhat doubtful, but which is
 +
probably homologous with the mesonephric duct of other types.
 +
 +
The system commences in the embryo (Rosenberg, Oellacher,
 +
Gotte, Furbringer) with the formation of a groove-like fold of the
 +
somatic layer of peritoneal epithelium, which becomes gradually
 +
constricted into a canal; the process of constriction commencing
 +
in the middle and extending in both directions. The canal does
 +
not however close anteriorly, but remains open to the body
 +
cavity, thus giving rise to a funnel equivalent to the pronephric
 +
funnels of Petromyzon and Myxine. On the inner side of this
 +
 +
 +
 +
702
 +
 +
 +
 +
TELEOSTEI.
 +
 +
 +
 +
funnel there is formed a glomerulus, projecting into the body
 +
 +
cavity ; and at the same time that
 +
 +
this is being formed the anterior end
 +
 +
of the canal becomes elongated and
 +
 +
convoluted. The above structures
 +
 +
constitute a pronephros, while the
 +
 +
posterior part of the primitive canal
 +
 +
forms the segmental duct.
 +
 +
The portion of the body cavity
 +
with the glomerulus and peritoneal
 +
funnel of the pronephros (fig. 395,
 +
po) soon becomes completely isolated from the remainder, so as to
 +
form a closed cavity (gl). The
 +
development of the mesonephros
 +
does not take place till long after
 +
that of the pronephros. The segmental tubes which form it are
 +
stated by Fiirbringer to arise from
 +
solid ingrowths of peritoneal epithelium, developed successively from
 +
before backwards, but Sedgwick
 +
informs me that they arise as differentiations of the mesoblastic cells
 +
near the peritoneal epithelium. They
 +
soon become hollow, and unite with
 +
the segmental duct. Malpighian
 +
bodies are developed on their median
 +
portions. They grow very greatly
 +
in length, and become much convoluted, but the details of this
 +
process have not been followed out.
 +
 +
The foremost segmental tubes are situated close behind the
 +
pronephros, while the hindermost are in many cases developed
 +
in the post-anal continuations of the body cavity. The pronephros appears to form the swollen cephalic portion of the kidney
 +
of the adult, and the mesonephros the remainder ; the so-called
 +
caudal portion, where present, being derived (?) from the postanal segmental tubes.
 +
 +
In some cases the cephalic portion of the kidneys is absent
 +
 +
 +
 +
 +
FIG. 394. PORTIONS OF THE
 +
MESONEPHROS OF MYXINE. (From
 +
Gegenbaur; after J. Miiller.)
 +
 +
a. segmental duct ; b. segmental tube; c. glomerulus ; d. afferent,
 +
e. efferent artery.
 +
 +
B represents a portion of A
 +
highly magnified.
 +
 +
 +
 +
EXCRETORY ORGANS. 703
 +
 +
 +
 +
in the adult, which probably implies the atrophy of the pronephros ; in other instances the cephalic portion of the kidneys is
 +
the only part developed. Its relation to the embryonic proncphros requires however further elucidation.
 +
 +
In the adult the ducts in the lower part of the kidneys lie as
 +
a rule on their outer borders, and almost invariably open into a
 +
 +
 +
 +
 +
pr
 +
 +
 +
 +
FIG. 395. SECTION THROUGH THE PRONEPHROS OF A TROUT AND ADJACENT
 +
PARTS TEN DAYS BEFORE HATCHING.
 +
 +
pr.n. pronephros ; po. opening of pronephros into the isolated portion of the body
 +
cavity containing the glomerulus ; gl. glomerulus ; ao. aorta ; ch. notochord ; x.
 +
subnotochordal rod ; al. alimentary tract.
 +
 +
urinary bladder, which usually opens in its turn on the urinogenital papilla immediately behind the genital pore, but in a few
 +
instances there is a common urinogenital pore.
 +
 +
In most Osseous Fish there are true generative ducts continuous with the investment of the generative organs. It
 +
appears to me most probable, from the analogy of Lepidostcus,
 +
to be described in the next section, that these ducts are split off
 +
from the primitive segmental duct, and correspond with the
 +
Miillerian ducts of Elasmobranchii, etc. ; though on this point
 +
we have at present no positive embryological evidence (vide
 +
general considerations at the end of the Chapter). In the