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

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(490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut  
 
(490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut  
Acad., Vol. iv. 1877.  
+
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
 
female Salmon and the male and female Eel the generative
 
products are carried to the exterior by abdominal pores. It is
 
possible that this may represent a primitive condition, though it
 
 
 
 
 
 
 
704
 
 
 
 
 
 
 
GANOIDEI.
 
 
 
 
 
 
 
is more probably a case of degeneration, as is indicated by the
 
presence of ducts in the male Salmon and in forms nearly allied
 
to the Salmonidae.
 
 
 
The coexistence of abdominal pores and generative ducts in
 
Mormyrus appears to me to demonstrate that the generative
 
ducts in Teleostei cannot be derived from the coalescence of the
 
investment of the generative organs with the abdominal pores.
 
 
 
Ganoidei. The true excretory gland of the adult Ganoidei
 
resembles on the whole that of Teleostei, consisting of an
 
elongated band on each side the mesonephros an anterior
 
dilatation of which probably represents the pronephros.
 
 
 
There is in both sexes a Mullerian duct, provided, except
 
in Lepidosteus, with an abdominal funnel, which is however
 
situated relatively very far back in the abdominal cavity. The
 
Mullerian ducts appear to serve as generative canals in both sexes.
 
In Lepidosteus they are continuous with the investment of the
 
generative glands, and thus a relation between the generative ducts
 
and glands, very similar to that in Teleostei, is brought about.
 
 
 
Posteriorly the Mullerian ducts and the ducts of the mesonephros remain united. The common duct so formed on each
 
side is clearly the primitive segmental duct. It receives the
 
secretion of a certain number of the posterior mesonephric
 
tubules, and usually unites with its fellow to form a kind of
 
bladder, opening by a single
 
pore into the cloaca, behind
 
the anus. The duct which
 
receives the secretion of the
 
anterior mesonephric tubules
 
is the true mesonephric or
 
Wolffian duct.
 
 
 
The development of the
 
excretory system, which has
 
been partially worked out in
 
Acipenscr and Lepidosteus 1 ,
 
is on the whole very similar
 
to that in the Teleostei. The
 
first portion of the system to
 
 
 
 
 
 
 
 
 
FIG. 396. SECTION THROUGH THE
 
TRUNK OF A LEPIDOSTEUS EMBRYO ON
 
THE SIXTH DAY AFTER IMPREGNATION.
 
 
 
me. medullary cord ; ms. mesoblast ; sg.
 
segmental duct ; ch. notochord ; .r. subnotochordal rod; hy. hypoblast.
 
 
 
 
 
 
 
1 Acipenser has been investigated by Fiirbringer, Salensky, Sedgwick, and also
 
by myself, and Lepidosteus by W. N. Parker and myself.
 
 
 
 
 
 
 
EXCRETORY ORGANS.
 
 
 
 
 
 
 
705
 
 
 
 
 
 
 
be formed is the segmental duct. In Lepidosteus this duct is
 
formed as a groove-like invagination of the somatic peritoneal
 
epithelium, precisely as in Teleostei, and shortly afterwards
 
forms a duct lying between the mesoblast and the epiblast
 
(fig. 396, sg}. In Acipenser (Salensky) however it is formed as
 
 
 
 
 
 
 
 
 
FIG. 397. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN ACIPENSER
 
 
 
EMBRYO. (After Salensky.)
 
 
 
Rf. medullary groove ; Alp. medullary plate ; Wg. segmental duct ; Ch. notochord ; En. hypoblast ; Sgp. mesoblastic somite ; Sp. parietal part of mesoblastic
 
plate.
 
 
 
a solid ridge of the somatic mesoblast, as in Petromyzon and
 
Elasmobranchii (fig. 397, Wg).
 
 
 
In both forms the ducts unite behind with the cloaca, and a
 
pronephros of the Teleostean type appears to be developed.
 
This gland is provided with but one 1 peritoneal opening, which
 
together with the glomerulus belonging to it becomes encapsuled
 
in a special section of the body cavity. The opening of the
 
pronephros of Acipenser into this cavity is shewn in fig. ^<^>,pr.n.
 
At this early stage of Acipenser (larva of 5 mm.) I could find
 
no glomerulus.
 
 
 
The mesonephros is formed some distance behind, and some
 
time after the pronephros, both in Acipenser and Lepidosteus,
 
so that in the larvae of both these genera the pronephros is for
 
a considerable period the only excretory organ. In Lepidosteus
 
especially the development of the mesonephros occurs very
 
late.
 
 
 
The development of the mesonephros has not been worked
 
out in Lepidosteus, but in Acipenser the anterior segmental
 
tubes become first established as (I believe) solid cords of cells,
 
attached at one extremity to the peritoneal epithelium on each
 
 
 
1 I have not fully proved this point, but have never found more than one
 
opening.
 
 
 
 
 
 
 
B. III.
 
 
 
 
 
 
 
45
 
 
 
 
 
 
 
GANOIDEI.
 
 
 
 
 
 
 
side of the insertion of the mesentery, and extending upwards
 
and outwards round the segmental duct 1 . The posterior segmental tubes arise later than the anterior, and (as far as can be
 
determined from the sections in my possession) they are formed
 
independently of the peritoneal epithelium, on the dorsal side
 
of the segmental duct.
 
 
 
In later stages (larvae of 7 10 mm.) the anterior segmental
 
tubes gradually lose their attachment to the peritoneal epithelium. The extremity near the peritoneal epithelium forms a
 
Malpighian body, and the other end unites with the segmental
 
duct. At a still later stage wide peritoneal funnels are es
 
 
 
 
 
sjy.c
 
 
 
 
 
 
 
mjo
 
 
 
 
 
 
 
pr.n
 
 
 
 
 
 
 
 
 
FIG. 398. TRANSVERSE SECTION THROUGH THE REGION OF THE STOMACH OF A
 
 
 
LARVA OF ACIPENSER 5 MM. IN LENGTH.
 
 
 
st. epithelium of stomach ; yk. yolk ; ch. notochord, below which is a subnotochordal rod; pr.n. pronephros ; ao. aorta; mf. muscle-plate formed of large cells,
 
the outer parts of which are differentiated into contractile fibres ; sp.c. spinal cord ;
 
b.c. body cavity.
 
 
 
tablished, for at any rate a considerable number of the tubes,
 
leading from the body cavity to the Malpighian bodies. These
 
 
 
1 Whether the segmental tubes are formed as ingrowths of the peritoneal
 
epithelium, or in situ, could not be determined.
 
 
 
 
 
 
 
EXCRETORY ORGANS. 707
 
 
 
funnels have been noticed by Furbringer, Salensky and myself,
 
but their mode of development has not, so far as I know, been
 
made out. The funnels appear to be no longer present in the
 
adult. The development of the Mullerian ducts has not been
 
worked out.
 
 
 
Dipnoi. The excretory system of the Dipnoi is only known in the
 
adult, but though in some respects intermediate in character between that of
 
the Ganoidei and Amphibia, it resembles that of the Ganoidei in the
 
important feature of the Mullerian ducts serving as genital ducts in both
 
sexes.
 
 
 
Amphibia. In Amphibia (Gotte, Furbringer) the development of the excretory system commences, as in Teleostei, by
 
the formation of the segmental duct from a groove formed by a
 
fold of the somatic layer of the peritoneal epithelium, near the
 
dorsal border of the body cavity (fig. 399, u). The anterior end
 
of the groove is placed immediately behind the branchial
 
region. Its posterior part soon becomes converted into a canal
 
by a constriction which commences a short way from the front
 
end of the groove, and thence extends backwards. This canal
 
at first ends blindly close to the cloaca, into which however it
 
soon opens.
 
 
 
The anterior open part of the groove in front of the constriction (fig. 399, n] becomes differentiated into a longitudinal
 
duct, which remains in open communication with the body
 
cavity by two (many Urodela) three (many Anura) or four
 
(Cceciliidae) canals. This constitutes the dorsal part of the
 
pronephros. The ventral part of the gland is formed from the
 
section of the duct immediately behind the longitudinal canal.
 
This part grows in length, and, assuming an S-shaped curvature,
 
becomes placed on the ventral side of the first formed part of
 
the pronephros. By continuous growth in a limited space the
 
convolutions of the canal of the pronephros become more numerous, and the complexity of the gland is further increased by the
 
outgrowth of blindly ending diverticula.
 
 
 
At the root of the mesentery, opposite the peritoneal openings
 
of the pronephros, a longitudinal fold, lined by peritoneal epithelium, and attached by a narrow band of tissue, makes its
 
appearance. It soon becomes highly vascular, and constitutes a
 
glomerulus homologous with that in Petromyzon and Teleostei.
 
 
 
452
 
 
 
 
 
 
 
AMPHIBIA.
 
 
 
 
 
 
 
a*'
 
 
 
 
 
 
 
The section of the body cavity which contains the openings
 
of the pronephros and the glomerulus,
 
becomes dilated, and then temporarily
 
shut off from the remainder. At a
 
later period it forms a special though
 
not completely isolated compartment.
 
For a long time the pronephros and
 
its duct form the only excretory organs
 
of larval Amphibia. Eventually however the formation of the mesonephros
 
commences, and is followed by the
 
atrophy of the pronephros. The mesonephros is composed, as in other
 
types, of a series of segmental tubes,
 
but these, except in Cceciliidae, no
 
longer correspond in number with the
 
myotomes, but are in all instances
 
more numerous. Moreover, in the
 
posterior part of the mesonephros in
 
the Urodeles, and through the whole
 
length of the gland in other types,
 
secondary and tertiary segmental tubes
 
are formed in addition to the primary
 
tubes.
 
 
 
 
 
 
 
 
 
FIG. 399. TRANSVERSE SECTION THROUGH A VERY YOUNG
 
TADPOLE OF BOMBINATOR AT
 
THE LEVEL OF THE ANTERIOR
 
END OF THE YOLK-SACK. (After
 
 
 
Gotte.)
 
 
 
a. fold of epiblast continuous
 
with the dorsal fin; is", neural
 
cord; m. lateral muscle; as 1 .
 
outer layer of muscle-plate; s.
 
lateral plate of mesoblast ; b.
 
mesentery ; u. open end of the
 
segmental duct, which forms the
 
pronephros ; f. alimentary tract ;
 
f. ventral diverticulum which
 
becomes the liver; e. junction of
 
yolk cells and hypoblast cells ;
 
d. yolk cells.
 
 
 
 
 
 
 
The development of the mesonephros
 
commences in Salamandra (Fiirbringer) with
 
the formation of a series of solid cords, which
 
in the anterior myotomes spring from the
 
peritoneal epithelium on the inner side of the
 
segmental duct, but posteriorly arise independently of this epithelium in the adjoining
 
mesoblast. Sedgwick informs me that in the
 
 
 
Frog the segmental tubes are throughout developed in the mesoblast, independently of the peritoneal epithelium. These cords next become detached
 
from the peritoneal epithelium (in so far as they are primitively united to it),
 
and after first assuming a vesicular form, grow out into coiled tubes, with a
 
median limb the blind end of which assists in forming a Malpighian body,
 
and a lateral limb which comes in contact with and opens into the segmental
 
duct, and an intermediate portion connecting the two. At the junction of
 
the median with the intermediate portion, and therefore at the neck of the
 
Malpighian body, a canal grows out in a ventral direction, which meets the
 
 
 
 
 
 
 
EXCRETORY ORGANS. 709
 
 
 
peritoneal epithelium, and then develops a funnel-shaped opening into the
 
body cavity, which subsequently becomes ciliated. In this way the peritoneal
 
funnels which are present in the adult are established.
 
 
 
The median and lateral sections of the segmental tubes become highly
 
convoluted, and the separate tubes soon come into such close proximity that
 
their primitive distinctness is lost.
 
 
 
The first fully developed segmental tube is formed in Salamandra maculata in about the sixth myotome behind the pronephros. But in the region
 
between the two structures rudimentary segmental tubes are developed.
 
 
 
The number of primary segmental tubes in the separate myotomes of
 
Salamandra is as follows :
 
 
 
In the 6th myotome (i.e. the first with a true
 
 
 
segmental tube) 12 segmental tubes
 
 
 
yth roth myotome 23
 
 
 
IIth ... 34
 
 
 
I2th 3 4 or 4 5
 
 
 
I3th y> 45
 
 
 
1 3th i6th 56
 
 
 
It thus appears that the segmental tubes are not only more numerous than
 
the myotomes, but that the number in each myotome increases from before
 
backwards. In the case of Salamandra there are formed in the region of
 
the posterior (10 16) myotomes secondary, tertiary, etc. segmental tubes out
 
of independent solid cords, which arise in the mesoblast dorsally to the tubes
 
already established.
 
 
 
The secondary segmental tubes appear to develop out of these cords
 
exactly in the same way as the primary ones, except that they do not join the
 
segmental duct directly, but unite with the primary segmental tubes shortly
 
before the junction of the latter with the segmental duct. In this way compound segmental tubes are established with a common collecting tube, but
 
with numerous Malpighian bodies and ciliated peritoneal openings. The
 
difference in the mode of origin of these compound tubes and of those in
 
Elasmobranchii is very striking.
 
 
 
The later stages in the development of the segmental tubes have not been
 
studied in the other Amphibian types.
 
 
 
In Cceciliidas the earliest stages are not known, but the tubes present in
 
the adult (Spengel) a truly segmental arrangement, and in the young each of
 
them is single, and provided with only a single peritoneal funnel. In the
 
adult however many of the segmental organs become compound, and may
 
have as many as twenty funnels, etc. Both simple and compound segmental
 
tubes occur in all parts of the mesonephros, and are arranged in no definite
 
order.
 
 
 
In the Anura (Spengel) all the segmental tubes are compound, and an
 
enormous number of peritoneal funnels are present on the ventral surface,
 
but it has not yet been definitely determined into what part of the segmental
 
tubes they open.
 
 
 
 
 
 
 
710 AMPHIBIA.
 
 
 
 
 
 
 
Before dealing with the further changes of the Wolffian body
 
it is necessary to return to the segmental duct, which, at the
 
time when the pronephros is undergoing atrophy, becomes split
 
into a dorsal Wolffian and ventral Mullerian duct. The process
 
in Salamandra (Fiirbringer) has much the same character as in
 
Elasmobranchii, the Mullerian duct being formed by the gradual
 
separation, from before backwards, of a solid row of cells from
 
the ventral side of the segmental duct, the remainder of the duct
 
constituting the Wolffian duct. During the formation of the
 
Mullerian duct its anterior part becomes hollow, and attaching
 
itself in front to the peritoneal epithelium acquires an opening
 
into the body cavity. The process of hollowing is continued
 
backwards pari passu with the splitting of the segmental duct.
 
In the female the process is continued till the Mullerian duct
 
opens, close to the Wolffian duct, into the cloaca. In the male
 
the duct usually ends blindly. It is important to notice that
 
the abdominal opening of the Mullerian duct in the Amphibia
 
(Salamandra) is a formation independent of the pronephros, and
 
placed slightly behind it ; and that the undivided anterior part
 
of the segmental duct (with the pronephros) is not, as in Elasmobranchii, united with the Mullerian duct, but remains connected
 
with the Wolffian duct.
 
 
 
The development of the Mullerian duct has not been satisfactorily
 
studied in other forms besides Salamandra. In Cceciliidae its abdominal
 
opening is on a level with the anterior end of the Wolffian body. In other
 
forms it is usually placed very far forwards, close to the root of the lungs
 
(except in Proteus and Batrachoseps, where it is placed somewhat further
 
back), and some distance in front of the Wolffian body.
 
 
 
The Mullerian duct is always well developed in the female, and serves as
 
oviduct. In the male it does not (except possibly in Alytes) assist in the
 
transportation of the genital products, and is always more or less rudimentary, and in Anura may be completely absent.
 
 
 
After the formation of the Mullerian duct, the Wolffian duct
 
remains as the excretory channel for the Wolffian body, and, till
 
the atrophy of the pronephros, for this gland also. Its anterior
 
section, in front of the Wolffian body, undergoes a more or less
 
complete atrophy.
 
 
 
The further changes of the excretory system concern (i) the
 
junction in the male of the anterior part of the Wolffian body
 
with the testis ; (2) certain changes in the collecting tubes of the
 
 
 
 
 
 
 
EXCRETORY ORGANS.
 
 
 
 
 
 
 
711
 
 
 
 
 
 
 
posterior part of the mesonephros. The first of these processes
 
results in the division of the Wolffian body into a sexual and a
 
non-sexual part, and in Salamandra and other Urodeles the
 
division corresponds with the distribution of the simple and
 
compound segmental tubes.
 
 
 
Since the development of the canals connecting the testes with
 
the sexual part of the Wolffian body has not been in all points
 
satisfactorily elucidated, it will be convenient to commence with a
 
description of the adult arrangement of the parts (fig. 400 B). In
 
most instances a non-segmental system of canals the vasa effcrentia (ve) coming from the testis, fall into a canal known as the
 
longitudinal canal of the Wolffian body, from which there pass off
 
transverse canals, which fall into, and are equal in number to, the
 
primary Malpighian bodies of the sexual part of the gland. The
 
spermatozoa, brought to the Malpighian bodies, are thence transported along the segmental tubes to the Wolffian duct, and so to
 
the exterior. The system of canals connecting the testis with
 
the Malpighian bodies is known as the testicular network. The
 
number of segmental tubes connected with the testis varies
 
very greatly. In Siredon there are as many as from 30 32
 
(Spengel).
 
 
 
The longitudinal canal of the Wolffian body is in rare instances
 
(Spelerpes, etc.) absent, where the sexual part of the Wolffian body is
 
slightly developed. In the Urodela the testes are united with the anterior
 
part of the Wolffian body. In the Cceciliidas the junction takes place in an
 
homologous part of the Wolffian body, but, owing to the development of the
 
anterior segmental tubes, which are rudimentary in the Urodela, it is
 
situated some way behind the front end. Amongst the Anura the connection
 
of the testis with the tubules of the Wolffian body is subject to considerable
 
variations. In Bufo cinereus the normal Urodele type is preserved, and in
 
Bombinator the same arrangement is found in a rudimentary condition, in
 
that there are transverse trunks from the longitudinal canal of the Wolffian
 
body, which end blindly, while the semen is carried into the Wolffian
 
duct by canals in front of the Wolffian body. In Alytes and Discoglossus
 
the semen is carried away by a similar direct continuation of the longitudinal canal in front of the Wolffian body, but there are no rudimentary transverse canals passing into the Wolffian body, as in Bombinator. In Rana the transverse ducts which pass off from the longitudinal
 
canal of the Wolffian body, after dilating to form (?) rudimentary Malpighian
 
bodies, enter directly into the collecting tubes near their opening into the
 
Wolffian duct.
 
 
 
 
 
 
 
712 AMPHIBIA.
 
 
 
 
 
 
 
In most Urodeles the peritoneal openings connected with the primary
 
generative Malpighian bodies atrophy, but in Spelerpes they persist. In
 
the Cceciliidie they also remain in the adult state.
 
 
 
With reference to the development of these parts little is
 
known except that the testicular network grows out from the
 
primary Malpighian bodies, and becomes united with the testis.
 
Embryological evidence, as well as the fact of the persistence of
 
the peritoneal funnels of the generative region in the adults
 
of some forms, proves that the testicular network is not developed
 
from the peritoneal funnels.
 
 
 
Rudiments of the testicular network are found in the female Cceciliidae
 
and in the females of many Urodela (Salamandra, Triton). These rudiments may in their fullest development consist of a longitudinal canal and
 
of transverse canals passing from this to the Malpighian bodies, together
 
with some branches passing into the mesovarium.
 
 
 
Amongst the Urodela the collecting tubes of the hinder non-sexual part
 
of the Wolffian body, which probably represents a rudimentary metanephros,
 
undergo in the male sex a change similar to that which they usually undergo
 
in Elasmobranchii. Their points of junction with the Wolffian duct are
 
carried back to the hindermost end of the duct (fig. 400 B), and the collecting
 
tubes themselves unite together into one or more short ducts (ureters) before
 
joining the Wolffian duct.
 
 
 
In Batrachoseps only the first collecting tube becomes split off in
 
this way ; and it forms a single elongated ureter which receives all the
 
collecting tubes of the posterior segmental tubes. In the female and in
 
the male of Proteus, Menobranchus, and Siren the collecting tubes retain
 
their primitive transverse course and open laterally into the Wolffian duct.
 
In rare cases (Ellipsoglossus, Spengel} the ureters open directly into the
 
cloaca.
 
 
 
The urinary bladder of the Amphibia is an outgrowth of the
 
ventral wall of the cloacal section of the alimentary tract, and is
 
homologous with the allantois of the amniotic Vertebrata.
 
 
 
The subjoined diagram (fig. 400) of the urogenital system of
 
Triton illustrates the more important points of the preceding
 
description.
 
 
 
In the female (A) the following parts are present :
 
 
 
(1) The Mullerian duct or oviduct (od) derived from the
 
splitting of the segmental duct.
 
 
 
(2) The Wolffian duct (sug) constituting the portion of the
 
segmental duct left after the formation of the Mullerian duct.
 
 
 
(3) The mesonephros (r), divided into an anterior sexual part
 
 
 
 
 
 
 
EXCRETORY ORGANS.
 
 
 
 
 
 
 
7'3
 
 
 
 
 
 
 
connected with a rudimentary testicular network, and a posterior
 
part. The collecting tubes from both
 
parts fall transversely into the Wolffian duct.
 
 
 
(4) The ovary (ov).
 
 
 
(5) The rudimentary testicular
 
network.
 
 
 
In the male (B) the following
 
parts are present :
 
 
 
(1) The functionless though fairly
 
developed Miillerian duct (;).
 
 
 
(2) The Wolffian duct (sug).
 
 
 
(3) The mesonephros (r) divided
 
into a true sexual part, through the
 
segmental tubes of which the semen
 
passes, and a non-sexual part. The
 
collecting tubes of the latter do not
 
enter the Wolffian duct directly, but
 
bend obliquely backwards and only
 
fall into it close to its cloacal aperture, after uniting to form one or two
 
primary tubes (ureters).
 
 
 
(4) The testicular network (ve)
 
consisting of (i) transverse ducts
 
from the testes, falling into (2) the
 
longitudinal canal of the Wolffian
 
body, from which (3) transverse canals are again given off to the Malpighian bodies.
 
 
 
Amniota. The amniotic Vertebrata agree, so far as is known, very
 
closely amongst themselves in the
 
formation of the urinogenital system.
 
 
 
The most characteristic feature of the system is the full
 
development of a metanephros, which constitutes the functional
 
kidney on the atrophy of the mesonephros or Wolffian body,
 
which is a purely embryonic organ. The first part of the
 
system to develop is a duct, which is usually spoken of as the
 
Wolffian duct, but which is really the homologue of the seg
 
 
 
 
 
 
 
FIG. 400. DIAGRAM OF THE
 
URINOGENITAL SYSTEM OF TRITON. (From Gegenbaur ; after
 
Spengel.)
 
 
 
A. Female. B. Male.
 
r. mesonephros, on the surface
 
of which numerous peritoneal funnels are visible ; sug. mesonephric
 
or Wolffian duct; od. oviduct
 
(Miillerian duct); in. Miillerian
 
duct of male ; ve. vasa efferentia of
 
testis ; t. testis ; ov. ovary ; up.
 
urinogenital pore.
 
 
 
 
 
 
 
714 AMNIOTA.
 
 
 
 
 
 
 
mental duct. It apparently develops in all the Amniota nearly
 
on the Elasmobranch type, as a solid rod, primarily derived
 
from the somatic mesoblast of the intermediate cell mass (fig.
 
401 W.d}\
 
 
 
The first trace of it is visible in an embryo Chick with eight
 
somites, as a ridge projecting from the intermediate cell mass towards the epiblast in the region of the seventh somite. In the
 
course of further development it continues to constitute such a
 
ridge as far as the eleventh somite (Sedgwick), but from this
 
point it grows backwards in the space between the epiblast and
 
mesoblast In an embryo with fourteen somites a small lumen
 
has appeared in its middle part and in front it is connected with
 
rudimentary Wolffian tubules, which develop in continuity with
 
it (Sedgwick). In the succeeding stages the lumen of the duct
 
gradually extends backwards and forwards, and the duct itself
 
also passes inwards relatively to the epiblast (fig. 402). Its hindend elongates till it comes into connection with, and opens into,
 
the cloacal section of the hind-gut' 2 .
 
 
 
It might have been anticipated that, as in the lower types,
 
the anterior end of the segmental duct would either open into
 
the body cavity, or come into connection with a pronephros.
 
Neither of these occurrences takes place, though in some types
 
(the Fowl) a structure, which is probably the rudiment of a
 
pronephros, is developed ; it does not however appear till a later
 
stage, and is then unconnected with the segmental duct. The
 
next part of the system to appear is the mesonephros or
 
Wolffian body.
 
 
 
This is formed in all Amniota as a series of segmental tubes,
 
which in Lacertilia (Braun) correspond with the myotomes, but
 
in Birds and Mammalia are more numerous.
 
 
 
In Reptilia (Braun, No. 542), the mesonephric tubes develop as segmentally-arranged masses on the inner side of the Wolffian duct, and
 
appear to be at first united with the peritoneal epithelium. Each mass soon
 
becomes an oval vesicle, probably opening for a very short period into the
 
 
 
1 Dansky and Kostenitsch (No. 543) describe the Wolffian duct in the Chick as
 
developing from a groove opening to the peritoneal cavity, which subsequently
 
becomes constricted into a duct. I have never met with specimens such as those
 
figured by these authors.
 
 
 
2 The foremost extremity of the segmental duct presents, according to Gasser,
 
curious irregularities and an anterior completely isolated portion is often present.
 
 
 
 
 
 
 
EXCRETORY ORGANS.
 
 
 
 
 
 
 
715
 
 
 
 
 
 
 
peritoneal cavity by a peritoneal funnel. The vesicles become very early
 
detached from the peritoneal epithelium, and lateral outgrowths from them
 
give rise to the main parts of the segmental tubes, which soon unite with the
 
segmental duct.
 
 
 
In Birds the development of the segmental tubes is more complicated 1 .
 
 
 
The tubules of the Wolffian body are derived from the intermediate cell
 
mass, shewn in fig. 401, between the upper end of the body cavity and the
 
 
 
 
 
 
 
g.o.
 
 
 
 
 
 
 
 
 
FIG. 401. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN
 
 
 
EMBRYO CHICK OF 45 HOURS.
 
 
 
M.c. medullary canal ; P.v. mesoblastic somite ; W.d. Wolffian duct which is in
 
contact with the intermediate cell mass ; So. somatopleure ; S.p. splanchnopleure ;
 
p.p. pleuroperitoneal cavity ; ch. notochord ; op. boundary of area opaca; v. bloodvessel.
 
 
 
muscle-plate. In the Chick the mode of development of this mass into the
 
segmental tubules is different in the regions in front of and behind about the
 
sixteenth segment. In front of about the sixteenth segment the intermediate
 
cell mass becomes detached from the peritoneal epithelium at certain points,
 
remaining attached to it at other points, there being several such to each
 
segment. The parts of the intermediate cell mass attached to the peritoneal
 
epithelium become converted into S-shaped cords (fig. 402, st] which soon
 
unite with the segmental duct (wd}. Into the commencement of each
 
of these cords the lumen of the body cavity is for a short distance
 
prolonged, so that this part constitutes a rudimentary peritoneal funnel.
 
 
 
1 Correct figures of the early stages of these structures were first given by
 
Kolliker, but the correct interpretation of them and the first satisfactory account of
 
the development of the excretory organs of Birds was given by Sedgwick (No. 549).
 
 
 
 
 
 
 
716
 
 
 
 
 
 
 
AMNIOTA.
 
 
 
 
 
 
 
In the Duck the attachment of the intermediate cell mass to the peritoneal
 
epithelium is prolonged further back than in the Chick.
 
 
 
In the foremost segmental tubes, which never reach a very complete
 
development, the peritoneal funnels widen considerably, while at the same
 
time they acquire a distinct lumen. The section of the tube adjoining
 
the wide peritoneal funnel becomes partially invaginated by the formation of
 
a glomerulus, and this glomerulus soon grows to such an extent as to project
 
through the peritoneal funnel, the neck of which it completely fills, into the
 
body cavity (fig. 403, gl). There is thus formed a series of free peritoneal
 
glomeruli belonging to the anterior Wolfnan tubuli 1 . These tubuli become
 
however early aborted.
 
 
 
In the case of the remaining tubules developed from the S-shaped cords
 
the attachment to the peritoneal epithelium is very soon lost. The cords
 
acquire a lumen, and open into the segmental duct. Their blind extremities
 
constitute the rudiments of Malpighian bodies.
 
 
 
 
 
 
 
am
 
 
 
 
 
 
 
 
 
FIG. 402. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO WITH
 
 
 
ABOUT TWENTY-FOUR MESOBLASTIC SOMITES.
 
 
 
am. amnion ; so. somatopleure ; sp. splanchnopleure ; ivd. Wolffian duct ; st. segmental tube; ca.v. cardinal vein; m.s. muscle-plate; sp.g. spinal ganglion; sp.c.
 
spinal cord ; ch. notochord ; ao. aorta ; hy. hypoblast.
 
 
 
1 These external glomeruli were originally mistaken by me (No. 539) for the
 
glomeralus of the pronephros, from their resemblance to the glomerulus of the
 
Amphibian pronephros. Their true meaning was made out by Sedgwick (No.
 
550).
 
 
 
 
 
 
 
EXCRETORY ORGANS.
 
 
 
 
 
 
 
717
 
 
 
 
 
 
 
In the posterior part of the Wolffian body of the Chick the intermediate
 
cell mass becomes very early detached from the peritoneal epithelium, and
 
at a considerably later period breaks up into oval vesicles similar to those of
 
the Reptilia, which form the rudiments of the segmental tubes.
 
 
 
Secondary and tertiary segmental tubules are formed in the Chick, on the
 
dorsal side of the primary tubules,
 
as direct differentiations of the mesoblast. They open independently into
 
the Wolffian duct.
 
 
 
In Mammalia the segmental tubules (Egli) are formed as solid masses
 
in the same situation as in Birds and
 
Reptiles. It is not known whether
 
they are united with the peritoneal
 
epithelium. They soon become oval
 
vesicles, which develop into complete
 
tubules in the manner already indicated.
 
 
 
 
 
 
 
 
 
After the establishment of
 
the Wolffian body there is formed
 
in both sexes in all the Amniota
 
a duct, which in the female
 
becomes the oviduct, but which
 
is functionless and disappears
 
more or less completely in the
 
male. This duct, in spite of certain peculiarities in its development, is without doubt homologous with the Mullerian duct of
 
 
 
 
 
 
 
FIG. 403. SECTION THROUGH THE
 
EXTERNAL GLOMERULUS OF ONE OF
 
THE ANTERIOR SEGMENTAL TUBES OF
 
AN EMBRYO CHICK OF ABOUT IOO H.
 
 
 
gl. glomerulus ; ge. peritoneal epithelium ; Wd. Wolffian duct ; ao.
 
aorta ; me. mesentery. The segmental
 
tube, and the connection between the
 
external and internal parts of the glomerulus are not shewn in this figure.
 
 
 
 
 
 
 
 
 
FIG. 404. SECTIONS SHEWING TWO OF THE PERITONEAL INVAGINATIONS WHICH
 
GIVE RISE TO THE ANTERIOR PART OF THE MULLERIAN DUCT (PRONEPHROS).
 
(After Balfour and Sedgwick. )
 
 
 
A is the nth section of the series.
 
B i 5th
 
 
 
C i8th ,, ,,
 
 
 
gri. second groove ; gr$. third groove ; ri. second ridge ; wit. Wolffian duct.
 
 
 
 
 
 
 
7 i8
 
 
 
 
 
 
 
AMNIOTA.
 
 
 
 
 
 
 
the Ichthyopsida. In connection with its anterior extremity
 
certain structures have been found in the Fowl, which are
 
probably, on grounds to be hereafter stated, homologous with
 
the pronephros (Balfour and Sedgwick).
 
 
 
The pronephros, as I shall call it, consists of a slightly
 
convoluted longitudinal canal with three or more peritoneal
 
openings. In the earliest condition, it consists of three successive
 
open involutions of the peritoneal epithelium, connected together
 
by more or less well-defined ridge-like thickenings of the
 
epithelium. It takes its origin from the layer of thickened
 
peritoneal epithelium situated near the dorsal angle of the body
 
cavity, and is situated some considerable distance behind the
 
front end of the Wolfifian duct.
 
 
 
In a slightly later stage the ridges connecting the grooves
 
become partially constricted off from the peritoneal epithelium,
 
 
 
 
 
 
 
 
 
FIG. 405. SECTION OF THE WOLFFIAN BODY DEVELOPING PRONEPHROS AND
 
GENITAL GLAND OF THE FOURTH DAY. (After Waldeyer.) Magnified 160 times.
 
m. mesentery; Z. somatopleure ; a', portion of the germinal epithelium from
 
which the involution (2) to form the pronephros (anterior part of Miillerian duct) takes
 
place; a. thickened portion of the germinal epithelium in which the primitive
 
germinal cells C and o are lying ; E. modified mesoblast which will form the stroma
 
of the ovary ; WK. Wolffian body ; y. Wolffian duct.
 
 
 
 
 
 
 
EXCRETORY ORGANS. 719
 
 
 
and develop a lumen. The condition of the structure at this
 
stage is illustrated by fig. 404, representing three transverse
 
sections through two grooves, and through the ridge connecting
 
them.
 
 
 
The pronephros may in fact now be described as a slightly
 
convoluted duct, opening into the body cavity by three groovelike apertures, and continuous behind with the rudiment of the
 
true Miillerian duct.
 
 
 
The stage just described is that of the fullest development
 
of the pronephros. In it, as in all the previous stages, there
 
appear to be only three main openings into the body cavity ; but
 
in some sections there are indications of the possible presence of
 
one or two additional rudimentary grooves.
 
 
 
In an embryo not very much older than the one last
 
described the pronephros atrophies as such, its two posterior
 
openings vanishing, and its anterior opening remaining as the
 
permanent opening of the Miillerian duct.
 
 
 
The pronephros is an extremely transitory structure, and its
 
development and atrophy are completed between the QOth and
 
i2Oth hours of incubation.
 
 
 
The position of the pronephros in relation to the Wolffian
 
body is shewn in fig. 405, which probably passes through a
 
region between two of the peritoneal openings. As long as the
 
pronephros persists, the Mullerian duct consists merely of a very
 
 
 
 
 
 
 
 
 
FlG. 406. TWO SECTIONS SHEWING THE JUNCTION OF THE TERMINAL SOLID
 
PORTION OF THE MtJLLERIAN DUCT WITH THE WOLFFIAN DUCT. (After Balfour
 
 
 
and Sedgwick.)
 
 
 
In A the terminal portion of the duct is quite distinct ; in B it has united with the
 
walls of the Wolffian duct.
 
 
 
md. Mullerian duct ; Wd. Wolffian duct.
 
 
 
 
 
 
 
72O AMNIOTA.
 
 
 
 
 
 
 
small rudiment, continuous with the hindermost of the three
 
peritoneal openings, and its solid extremity appears to unite
 
with the walls of the Wolffian duct.
 
 
 
After the atrophy of the pronephros, the Miillerian duct
 
commences to grow rapidly, and for the first part of its course it
 
appears to be split off as a solid rod from the outer or ventral
 
wall of the Wolffian duct (fig. 406). Into this rod the lumen,
 
present in its front part, subsequently extends. Its mode of
 
development in front is thus precisely similar to that of the
 
Miillerian duct in Elasmobranchii and Amphibia.
 
 
 
This mode of development only occurs however in the
 
anterior part of the duct. In the posterior part of its course its
 
growing point lies in a bay formed by the outer walls of the
 
Wolffian duct, but does not become definitely attached to that
 
duct. It seems however possible that, although not actually
 
split off from the walls of the Wolrfian duct, it may grow backwards from cells derived from that duct.
 
 
 
The Miillerian duct finally reaches the cloaca though it does
 
not in the female for a long time open into it, and in the male
 
never does so.
 
 
 
The mode of growth of the Miillerian duct in the posterior part of its
 
course will best be understood from the following description quoted from
 
the paper by Sedgwick and myself.
 
 
 
"A few sections before its termination the Miillerian duct appears as a
 
well-defined oval duct lying in contact with the wall of the Wolffian duct on
 
the one hand and the germinal epithelium on the other. Gradually, however,
 
as we pass backwards, the Miillerian duct dilates ; the external wall of the
 
Wolffian duct adjoining it becomes greatly thickened and pushed in in its
 
middle part, so as almost to touch the opposite wall of the duct, and so form
 
a bay in which the Miillerian duct lies. As soon as the Miillerian duct has
 
come to lie in this bay its walls lose their previous distinctness of outline,
 
and the cells composing them assume a curious vacuolated appearance. No
 
well-defined line of separation can any longer be traced between the walls of
 
the Wolffian duct and those of the Miillerian, but between the two is a
 
narrow clear space traversed by an irregular network of fibres, in some of
 
the meshes of which nuclei are present.
 
 
 
The Miillerian duct may be traced in this condition for a considerable
 
number of sections, the peculiar features above described becoming more
 
and more marked as its termination is approached. It continues to dilate
 
and attains a maximum size in the section or so before it disappears. A
 
lumen may be observed in it up to its very end, but is usually irregular in
 
outline and frequently traversed by strands of protoplasm. The Miillerian
 
 
 
 
 
 
 
EXCRETORY ORGANS. 721
 
 
 
duct finally terminates quite suddenly, and in the section immediately
 
behind its termination the Wolffian duct assumes its normal appearance,
 
and the part of its outer wall on the level of the Miillerian duct conies into
 
contact with the germinal epithelium."
 
 
 
Before describing the development of the Mullerian duct in other
 
Amniotic types it will be well to say a few words as to the identifications
 
above adopted. The identification of the duct, usually called the Wolffian
 
duct, with the segmental duct (exclusive of the pronephros) appears to be
 
morphologically justified for the following reasons : (i) that it gives rise to
 
part of the Mullerian duct as well as to the duct of the Wolffian body ;
 
behaving in this respect precisely as does the segmental duct of Elasmobranchii and Amphibia. (2) That it serves as the duct for the Wolffian
 
body, before the Mullerian duct originates from it. (3) That it develops in a
 
manner strikingly similar to that of the segmental duct of various lower
 
forms.
 
 
 
With reference to the pronephros it is obvious that the organ identified
 
as such is in many respects similar to the pronephros of the Amphibia.
 
Both consist of a somewhat convoluted longitudinal canal, with a certain
 
number of peritoneal openings ;
 
 
 
The main difficulties in the homology are :
 
 
 
(1) the fact that the pronephros in the Bird is not united with the
 
segmental duct ;
 
 
 
(2) the fact that it is situated behind the front end of the Wolffian body.
 
It is to be remembered in connection with the first of these difficulties
 
 
 
that in the formation of the Mullerian duct in Elasmobranchii the anterior
 
undivided extremity of the primitive segmental duct, with the peritoneal
 
opening, which probably represents the pronephros, is attached to the
 
Mullerian duct, and not to the Wolffian duct ; though in Amphibia the
 
reverse is the case. To explain the discontinuity of the pronephros with the
 
segmental duct it is only necessary to suppose that the segmental duct and
 
pronephros, which in the Ichthyopsida develop as a single formation,
 
develop in the Bird as two independent structures a far from extravagant
 
supposition, considering that the pronephros in the Bird is undoubtedly
 
quite functionless.
 
 
 
With reference to the posterior position of the pronephros it is only
 
necessary to remark that a change in position might easily take place after
 
the acquirement of an independent development, and that the shifting is
 
probably correlated with a shifting of the abdominal opening of the
 
Mullerian duct.
 
 
 
The pronephros has only been observed in Birds, and is very
 
possibly not developed in other Amniota. The Mullerian duct
 
is also usually stated to develop as a groove of the peritoneal
 
epithelium, shewn in the Lizard in fig. 354, md., which is continued backward as a primitively solid rod in the space between
 
B. ill. 46
 
 
 
 
 
 
 
722
 
 
 
 
 
 
 
AM N IOTA.
 
 
 
 
 
 
 
the Wolffian duct and peritoneal epithelium, without becoming
 
attached to the Wolffian duct.
 
 
 
On the formation of the Miillerian duct, the duct of the
 
mesonephros becomes the true mesonephric or Wolffian duct.
 
 
 
After these changes have taken place a new organ of great
 
importance makes its appearance. This organ is the permanent
 
kidney, or metanephros.
 
 
 
Metanephros. The mode of development of the metanephros has as yet only been satisfactorily elucidated in the Chick
 
(Sedgwick, No. 549). The ureter and the collecting tubes of
 
the kidney are developed from a dorsal outgrowth of the hinder
 
part of the Wolffian duct. The outgrowth from the Wolffian
 
duct grows forwards, and extends along the outer side of a mass
 
of mesoblastic tissue which lies mainly behind, but somewhat
 
overlaps the dorsal aspect of the Wolffian body.
 
 
 
This mass of mesoblastic cells may be called the metanephric blastema. Sedgwick, of the accuracy of whose
 
account I have satisfied myself, has shewn that in the Chick it is
 
derived from the intermediate cell mass of the region of about
 
the thirty-first to the thirty-fourth somite. It is at first continuous with, and indistinguishable in structure from, the portion
 
of the intermediate cell mass of the region immediately in front
 
of it, which breaks up into Wolffian tubules. The metanephric
 
blastema remains however quite passive during the formation of
 
the Wolffian tubules in the adjoining blastema ; and on the
 
formation of the ureter breaks off from the Wolffian body in
 
front, and, growing forwards and dorsalwards, places itself on
 
the inner side of the ureter in the position just described.
 
 
 
In the subsequent development of the kidney collecting tubes
 
grow out from the ureter, and become continuous with masses of
 
cells of the metanephric blastema, which then differentiate themselves into the kidney tubules.
 
 
 
The process just described appears to me to prove that the
 
kidney of the A mniota is a specially differentiated posterior section
 
of the primitive mesonephros.
 
 
 
According to the view of Remak and Kolliker the outgrowths from the
 
ureter give rise to the whole of the tubuli uriniferi and the capsules of the
 
Malpighian bodies, the mesoblast around them forming blood-vessels, etc.
 
On the other hand some observers (Kupffer, Bornhaupt, Braun) maintain, in
 
 
 
 
 
 
 
EXCRETORY ORGANS. 723
 
 
 
 
 
 
 
accordance with the account given above, that the outgrowths of the ureter
 
form only the collecting tubes, and that the secreting tubuli, etc. are formed
 
in situ in the adjacent mesoblast.
 
 
 
Braun (No. 542) has arrived at the conclusion that in the Lacertilia the
 
tissue, out of which the tubuli of the metanephros are formed, is derived
 
from irregular solid ingrowths of the peritoneal epithelium, in a region
 
behind the Wolffian body, but in a position corresponding to that in which
 
the segmental tubes take their origin. These ingrowths, after separating
 
from the peritoneal epithelium, unite together to form a cord into which the
 
ureter sends the lateral outgrowths already described. These outgrowths
 
unite with secreting tubuli and Malpighian bodies, formed in situ. In
 
Lacertilia the blastema of the kidney extends into a postanal region.
 
Braun's account of the origin of the metanephric blastema does not appear
 
to me to be satisfactorily demonstrated.
 
 
 
The ureter does not long remain attached to the Wolffian
 
duct, but its opening is gradually carried back, till (in the Chick
 
between the 6th and 8th day) it opens independently into the
 
cloaca.
 
 
 
Of the further changes in the excretory system the most important is the atrophy of the greater part of the Wolffian body,