AUDITORY ORGAN, OLFACTORY ORGAN AND SENSE
ORGANS OF THE LATERAL LINE.
_Auditory Organs._
A great variety of organs, very widely distributed amongst aquatic
forms, and also found, though less universally, in land forms, are
usually classed together as auditory organs.
In the case of all aquatic forms, or of forms which have directly
inherited their auditory organs from aquatic forms, these organs are
built upon a common type; although in the majority of instances the
auditory organs of the several groups have no genetic relations. All
the organs have their origin in specialized portions of the epidermis.
Some of the cells of a special region become provided at their free
extremities with peculiar hairs, known as auditory hairs; while in
other cells concretions, known as otoliths, are formed, which appear
often to be sufficiently free to be acted upon by vibrations of the
surrounding medium, and to be so placed as to be able in their turn to
transmit their vibrations to the cells with auditory hairs[194]. The
auditory regions of the epidermis are usually shut off from the
surface in special sacks.
[194] The function of the otoliths is not always clear. There is
evidence to shew that they sometimes act as dampers.
The actual function of these organs is no doubt correctly described,
in the majority of instances, as being auditory; but it appears to me
very possible that in some cases their function may be to enable the
animals provided with them to detect the presence of other animals in
their neighbourhood, through the undulatory movements in the water,
caused by the swimming of the latter.
Auditory organs with the above characters, sometimes freely open to
the external medium, but more often closed, are found in various
Coelenterata, Vermes and Crustacea, and universally or all but
universally in the Mollusca and Vertebrata.
In many terrestrial Insects a different type of auditory organ has
been met with, consisting of a portion of the integument modified to
form a tympanum or drum, and supported at its edge by a chitinous
ring. The vibrations set up in the membranous tympanum stimulate
terminal nerve organs at the ends of chitinous processes, placed in a
cavity bounded externally by the tympanic membrane.
The tympanum of Amphibia and Amniota is an accessory organ added, in
terrestrial Vertebrata, to an organ of hearing primitively adapted to
an aquatic mode of life; and it is interesting to notice the presence
of a more or less similar membrane in the two great groups of
terrestrial forms, _i.e._ terrestrial Vertebrata and Insecta.
Nothing is known with reference to the mode of development or
evolution of the tympanic type of auditory organ found in Insects,
and, except in the case of Vertebrates, but little is known with
reference to the development of what may be called the vesicular type
of auditory organ found in aquatic forms. Some very interesting facts
with reference to the evolution of such organs have however been
brought to light by the brothers Hertwig in their investigations on
the Coelenterata; and I propose to commence my account of the
development of the auditory organs in the animal kingdom by a short
statement of the results of their researches.
Coelenterata. Three distinct types of auditory organ have been
recognised in the Medusæ; two of them resulting from the
differentiation of a tentacle-like organ, and one from ectoderm cells
on the under surface of the velum. We may commence with the latter as
the simplest. It is found in the Medusæ known as the Vesiculata. The
least differentiated form of this organ, so far discovered, is present
in Mitrotrocha, Tiaropsis and other genera. It has the form of an open
pit; and a series of such organs are situated along the attached edge
of the velum with their apertures directed downwards. The majority of
the cells lining the outer, _i.e._ peripheral side of the pit, contain
an otolith, while a row of the cells on the inner, _i.e._ central
side, are modified as auditory cells. The auditory cells are somewhat
strap-shaped, their inner ends being continuous with the fibres of the
lower nerve-ring, and their free ends being provided with bent
auditory hairs, which lie in contact with the convex surfaces of the
cells containing the otoliths.
[FIG. 297. AUDITORY VESICLE OF PHIALIDIUM AFTER TREATMENT WITH
DILUTE OSMIC ACID. (From Lankester; after O. and R. Hertwig.)
_d^1._ epithelium of the upper surface of the velum; _d^2._
epithelium of the under surface of the velum; _r._ circular canal at
the edge of the velum; _nr^1._ upper nerve-ring; _h._ auditory
cells; _hh._ auditory hairs; _np._ nervous cushion formed of a
prolongation of the lower nerve-ring. Close to the nerve-ring is
seen a cell, shewn as black, containing an otolith.]
By the conversion of such open pits into closed sacks a more
complicated type of auditory organ, which is present in many of the
Vesiculata, viz. Æquorea, Octorchis, Phialidium, &c., is produced. A
closed vesicle of this type is shewn in fig. 297. Such organs form
projections on the upper surface of the velum. They are covered by a
layer of the epithelium (_d^1_) of the upper surface of the velum, but
the lining of the vesicle (_d^2_) is derived from what was originally
part of the epithelium of the lower surface of the velum, homologous
with that lining the open pits in the type already described. The
general arrangement of the cells lining such vesicles is the same as
that of the cells lining the open pits.
A second type of auditory organ, found in the Trachymedusæ, appears in
its simplest condition as a modified tentacle. It is formed of a basal
portion, covered by auditory cells with long stiff auditory hairs,
supporting at its apex a club-shaped body, attached to it by a
delicate stalk. An endodermal axis is continued through the whole
structure, and in one or more of the endoderm cells of the club-shaped
body otoliths are always present. The tails of the auditory cells are
directly continued into the upper nerve-ring.
In more complicated forms of this organ the tentacle becomes enclosed
in a kind of cup, by a wall-like upgrowth of the surrounding parts
(fig. 298); and in some forms, _e.g._ Geryonia, by the closure of the
cup, the whole structure takes the form of a completely closed
vesicle, in the cavity of which the original tentacle forms an
otolith-bearing projection.
[FIG. 298. AUDITORY ORGAN OF RHOPALONEMA. (From Lankester; after O.
and R. Hertwig.)
The organ consists of a modified tentacle (_hk_) with auditory cells
and concretions, partially enclosed in a cup.]
The auditory organs found in the Acraspedote Medusæ approach in many
respects to the type of organ found in the Trachymedusæ. They consist
of tentacular organs placed in grooves on the under surface of the
disc. They have a swollen extremity, and are provided with an
endodermal axis for half the length of which there is a diverticulum
of the gastrovascular canal system. The terminal portion of the
endoderm is solid, and contains calcareous concretions. The ectodermal
cells at the base of these organs have the form of auditory cells.
Mollusca. Auditory vesicles are found in almost all Mollusca on the
ventral side of the body in close juxtaposition to the pedal ganglia.
Except possibly in some Cephalopods, these vesicles are closed. They
are provided with free otoliths, supported by the cilia of the walls
of the sack, but in addition some of the cells of the sack are
provided with stiff auditory hairs.
In many forms these sacks have been observed to originate by an
invagination of the epiblast of the foot (_Paludina_, _Nassa_,
_Heteropoda_, _Limax_, _Clio_, Cephalopoda and Lamellibranchiata). In
other instances (some Pteropods, Lymnæus, &c.) they appear, by a
secondary modification in the development, to originate by a
differentiation of a solid mass of epiblast.
According to Fol the otocysts in Gasteropods are formed by cells of
the wall of the auditory sacks; and the same appears to hold good for
Cephalopoda (Grenacher)[195] shewing that free otoliths have in these
instances originated from otoliths originally placed in cells.
[195] For the somewhat complicated details as to the development
of the auditory sacks of Cephalopods I must refer the reader to
Vol. II., pp. 278, 279, and to Grenacher (Vol. I., No. 280).
Crustacea. In the decapodous Crustacea organs, which have been
experimentally proved to be true organs of hearing, are usually
present on the basal joint of the anterior antennæ. They may have
(Hensen, No. 384) the form either of closed or of open sacks, lined by
an invagination of the epidermis. They are provided with chitinous
auditory hairs and free otoliths. In the case of the open sacks the
otoliths appear to be simply stones transported into the interior of
the sacks, but in the closed sacks the otoliths, though free, are no
doubt developed within the sacks.
The Schizopods, which, as mentioned in the last chapter, are
remarkable as containing a genus (Euphausia) with abnormally situated
eyes, distinguish themselves again with reference to their auditory
organs, in that another genus (Mysis) is characterized by the presence
of a pair of auditory sacks in the inner plates of the tail. These
sacks have curved auditory hairs supporting an otolith at their
extremity.
The development of the auditory organs in the Crustacea has not been
investigated.
The Vertebrata. The Cephalochorda are without organs of hearing, and
the auditory organ of the Urochorda is constructed on a special type
of its own. The primitive auditory organs of the true Vertebrata have
the same fundamental characters as those of the majority of aquatic
invertebrate forms. They consist of a vesicle, formed by the
invagination of a patch of epiblast, and usually shut off from the
exterior, but occasionally (Elasmobranchii) remaining open. The walls
of this vesicle are always much complicated and otoliths of various
forms are present in its cavity. To this vesicle accessory structures,
derived from the walls of the hyomandibular cleft, are added in the
majority of terrestrial Vertebrata.
The development of the true auditory vesicle will be considered
separately from that of the accessory structures derived from the
hyomandibular cleft.
[FIG. 299. SECTION THROUGH THE HEAD OF AN ELASMOBRANCH EMBRYO, AT
THE LEVEL OF THE AUDITORY INVOLUTION.
_aup._ auditory pit; _aun._ ganglion of auditory nerve; _iv.v._ roof
of fourth ventricle; _a.c.v._ anterior cardinal vein; _aa._ aorta;
_I.aa._ aortic trunk of mandibular arch; _pp._ head cavity of
mandibular arch; _Ivc._ alimentary pouch which will form the first
visceral cleft; _Th._ rudiment of thyroid body].
In all Vertebrata the development of the auditory vesicle commences
with the formation of a thickened patch of epiblast, at the side of
the hind-brain, on the level of the second visceral cleft. This patch
soon becomes invaginated in the form of a pit (fig. 299, _aup_), to
the inner side of which the ganglion of the auditory nerve (_aun_),
which as shewn in a previous chapter is primitively a branch of the
seventh nerve, closely applies itself.
In those Vertebrata (viz. Teleostei, Lepidosteus and Amphibia) in
which the epiblast is early divided into a nervous and epidermic
stratum, the auditory pit arises as an invagination of the nervous
stratum only, and the mouth of the auditory pit is always closed (fig.
300) by the epidermic stratum of the skin. Since the opening of the
pit is retained through life in Elasmobranchii the closed form of pit
in the above forms is clearly secondary.
In Teleostei the auditory pit arises as a solid invagination of the
epiblast.
The mouth of the auditory vesicle gradually narrows, and in most forms
soon becomes closed, though in Elasmobranchii it remains permanently
open. In any case the vesicle is gradually removed from the surface,
remaining connected with it by an elongated duct, either opening on
the dorsal aspect of the head (Elasmobranchii), or ending blindly
close beneath the skin.
[FIG. 300. SECTION THROUGH THE HEAD OF A LEPIDOSTEUS EMBRYO ON THE
SIXTH DAY AFTER IMPREGNATION.
_au.v._ auditory vesicle; _au.n._ auditory nerve; _ch._ notochord;
_hy._ hypoblast.]
[FIG. 301. SECTION THROUGH THE HIND-BRAIN OF A CHICK AT THE END OF
THE THIRD DAY OF INCUBATION.
_IV._ fourth ventricle. The section shews the very thin roof and
thicker sides of the ventricle. _Ch._ notochord; _CV._ anterior
cardinal vein; _CC._ involuted auditory vesicle (_CC_ points to the
end which will form the cochlear canal); _RL._ recessus labyrinthi
(remains of passage connecting the vesicle with the exterior); _hy._
hypoblast lining the alimentary canal; _AO._, _AO.A._ aorta, and
aortic arch.]
In all Vertebrata the auditory vesicle undergoes further changes of a
complicated kind. In the Cyclostomata these changes are less
complicated than in other forms, though whether this is due to
degeneration, or to the retention of a primitive state of the auditory
organ, is not known. In the Lamprey the auditory vesicle is formed in
the usual way by an invagination of the epiblast, which soon becomes
vesicular, and for a considerable period retains a simple character.
As pointed out by Max Schultze, a number of otoliths appears in the
vesicle during larval life, and, although such otoliths are stated by
J. Müller to be absent both in the full-grown Ammocoete and in the
adult, they have since been found by Ketel (No. 387). The formation of
the two semicircular canals has not been investigated.
In all the higher Vertebrates the changes of the auditory sacks are
more complicated. The ventral end of the sack is produced into a short
process (fig. 301, _CC_); while at the dorsal end there is the
canal-like prolongation of the lumen of the sack (_RL_), derived from
the duct which primitively opened to the exterior, and which in most
cases persists as a blind diverticulum of the auditory sack, known as
the recessus labyrinthi or aqueductus vestibuli. The parts thus
indicated give rise to the whole of the membranous labyrinth of the
ear. The main body of the vesicle becomes the utriculus and
semicircular canals, while the ventral process forms the sacculus
hemisphericus and cochlear canal.
The growth of these parts has been most fully studied in Mammalia,
where they reach their greatest complexity, and it will be convenient
to describe their development in this group, pointing out how they
present, during some of the stages in their growth, a form permanently
retained in lower types.
The auditory vesicle in Mammalia is at first nearly spherical, and is
imbedded in the mesoblast at the side of the hind-brain. It soon
becomes triangular in section, with the apex of the triangle pointing
inwards and downwards. This apex gradually elongates to form the
rudiment of the cochlear canal and sacculus hemisphericus (fig. 302,
_CC_). At the same time the recessus labyrinthi (_R.L_) becomes
distinctly marked, and the outer wall of the main body of the vesicle
grows out into two protuberances, which form the rudiments of the
vertical semicircular canals (_V.B_). In the lower forms (fig. 305)
the cochlear process of the vestibule hardly reaches a higher stage of
development than that found at this stage in Mammalia.
The parts of the auditory labyrinth thus established soon increase in
distinctness (fig. 303); the cochlear canal (_CC_) becomes longer and
curved; its inner and concave surface being lined by a thick layer of
columnar epiblast. The recessus labyrinthi also increases in length,
and just below the point where the bulgings to form the vertical
semicircular canals are situated, there is formed a fresh protuberance
for the horizontal semicircular canal. At the same time the central
parts of the walls of the flat bulgings of the vertical canals grow
together, obliterating this part of the lumen, but leaving a canal
round the periphery; and, on the absorption of their central parts,
each of the original simple bulgings of the wall of the vesicle
becomes converted into a true semicircular canal, opening at its two
extremities into the auditory vesicle. The vertical canals are first
established and then the horizontal canal.
[FIG. 302. TRANSVERSE SECTION OF THE HEAD OF A FOETAL SHEEP (16 MM.
IN LENGTH) IN THE REGION OF THE HIND-BRAIN. (After Böttcher.)
_HB._ the hind-brain.
The section is somewhat oblique, hence while on the right side the
connections of the recessus vestibuli _R.L._, and of the commencing
vertical semicircular canal _V.B._, and of the ductus cochlearis
_CC._, with the cavity of the primary otic vesicle are seen; on the
left side, only the extreme end of the ductus cochlearis _CC_, and
of the semicircular canal _V.B._ are shewn.
Lying close to the inner side of the otic vesicle is seen the
cochlear ganglion _GC_; on the left side the auditory nerve _G_ and
its connection _N_ with the hind-brain are also shewn.
Below the otic vesicle on either side lies the jugular vein.]
Shortly after the formation of the rudiment of the horizontal
semicircular canal a slight protuberance becomes apparent on the inner
commencement of the cochlear canal. A constriction arises on each side
of the protuberance, converting it into a prominent hemispherical
projection, the sacculus hemisphericus (fig. 304, _S.R_).
[FIG. 303. SECTION OF THE HEAD OF A FOETAL SHEEP 20 MM. IN LENGTH.
(After Böttcher.)
_R.V._ recessus labyrinthi; _V.B._ vertical semicircular canal;
_H.B._ horizontal semicircular canal; _C.C._ cochlear canal; _G._
cochlear ganglion.]
The constrictions are so deep that the sacculus is only connected with
the cochlear canal on the one hand, and with the general cavity of the
auditory vesicle on the other, by, in each case, a narrow though short
canal.
The former of these canals (fig. 304, _b_) is known as the canalis
reuniens. At this stage we may call the remaining cavity of the
original otic vesicle, into which all the above parts open, the
utriculus.
Soon after the formation of the sacculus hemisphericus, the cochlear
canal and the semicircular canals become invested with cartilage. The
recessus labyrinthi remains however still enclosed in undifferentiated
mesoblast.
Between the cartilage and the parts which it surrounds there remains a
certain amount of indifferent connective tissue, which is more
abundant around the cochlear canal than around the semicircular
canals.
As soon as they have acquired a distinct connective-tissue coat, the
semicircular canals begin to be dilated at one of their terminations
to form the ampullæ. At about the same time a constriction appears
opposite the mouth of the recessus labyrinthi, which causes its
opening to be divided into two branches--one towards the utriculus and
the other towards the sacculus hemisphericus; and the relations of the
parts become so altered that communication between the sacculus and
utriculus can only take place through the mouth of the recessus
labyrinthi (fig. 305).
When the cochlear canal has come to consist of two and a half coils,
the thickened epithelium which lines the lower surface of the canal
forms a double ridge from which the organ of Corti is subsequently
developed. Above the ridge there appears a delicate cuticular
membrane, the membrane of Corti or membrana tectoria.
The epithelial walls of the utricle, the recessus labyrinthi, the
semicircular canals, and the cochlear canal constitute together the
highly complicated product of the original auditory vesicle. The whole
structure forms a closed cavity, the various parts of which are in
free communication. In the adult the fluid present in this cavity is
known as the endolymph.
In the mesoblast lying between these parts and the cartilage, which at
this period envelopes them, lymphatic spaces become established, which
are partially developed in the Sauropsida, but become in Mammals very
important structures.
They consist in Mammals partly of a space surrounding the utricle and
semicircular canals, and partly of two very definite channels, which
largely embrace between them the cochlear canal. The latter channels
form the scala vestibuli on the upper side of the cochlear canal and
the scala tympani on the lower. The scala vestibuli is in free
communication with the lymphatic cavity surrounding the vestibule, and
opens at the apex of the cochlea into the scala tympani. The latter
ends blindly at the fenestra rotunda.
The fluid contained in the two scalæ, and in the remaining lymphatic
cavities of the auditory labyrinth, is known as perilymph.
[FIG. 304. SECTION THROUGH THE INTERNAL EAR OF AN EMBRYONIC SHEEP 28
MM. IN LENGTH. (After Böttcher.)
_D.M._ dura mater; _R.V._ recessus labyrinthi; _H.V.B._ posterior
vertical semicircular canal; _U._ utriculus; _H.B._ horizontal
semicircular canal; _b._ canalis reuniens; _a._ constriction by
means of which the sacculus hemisphericus _S.R._ is formed; _f._
narrowed opening between sacculus hemisphericus and utriculus;
_C.C._ cochlea; _C.C´._ lumen of cochlea; _K.K._ cartilaginous
capsule of cochlea; _K.B._ basilar plate; _Ch._ notochord.]
The cavities just spoken of are formed by an absorption of parts of
the embryonic mucous tissue between the perichondrium and the walls of
the membranous labyrinth.
The scala vestibuli is formed before the scala tympani, and both scalæ
begin to be developed at the basal end of the cochlea: the cavity of
each is continually being carried forwards towards the apex of the
cochlear canal by a progressive absorption of the mesoblast. At first
both scalæ are somewhat narrow, but they soon increase in size and
distinctness.
The cochlear canal, which is often known as the scala media of the
cochlea, becomes compressed on the formation of the scalæ so as to be
triangular in section, with the base of the triangle outwards. This
base is only separated from the surrounding cartilage by a narrow
strip of firm mesoblast, which becomes the stria vascularis, etc. At
the angle opposite the base the canal is joined to the cartilage by a
narrow isthmus of firm material, which contains nerves and vessels.
This isthmus subsequently forms the lamina spiralis, separating the
scala vestibuli from the scala tympani.
The scala vestibuli lies on the upper border of the cochlear canal,
and is separated from it by a very thin layer of mesoblast, bordered
on the cochlear aspect by flat epiblast cells. This membrane is called
the membrane of Reissner. The scala tympani is separated from the
cochlear canal by a thicker sheet of mesoblast, called the basilar
membrane, which supports the organ of Corti and the epithelium
adjoining it. The upper extremity of the cochlear canal ends in a
blind extremity called the cupola, to which the two scalæ do not for
some time extend. This condition is permanent in Birds, where the
cupola is represented by a structure known as the lagena (fig. 305,
II. _L_). Subsequently the two scalæ join at the extremity of the
cochlear canal; the point of the cupola still however remains in
contact with the bone, which has now replaced the cartilage, but at a
still later period the scala vestibuli, growing further round,
separates the cupola from the adjoining osseous tissue.
The ossification around the internal ear is at first confined to the
cartilage, but afterwards extends into the thick periosteum between
the cartilage and the internal ear, and thus eventually makes its way
into the lamina spiralis, etc.
The organ of Corti. In Mammalia there is formed from the epithelium of
the cochlear canal a very remarkable organ known as the organ of
Corti, the development of which is of sufficient importance to merit a
brief description. A short account of this organ in the adult state
may facilitate the understanding of its development.
[FIG. 305. DIAGRAMS OF THE MEMBRANOUS LABYRINTH. (From Gegenbaur.)
I. Fish. II. Bird. III. Mammal.
_U._ utriculus; _S._ sacculus; _US._ utriculus and sacculus; _Cr._
canalis reuniens; _R._ recessus labyrinthi; _UC._ commencement of
cochlea; _C._ cochlear canal; _L._ lagena; _K._ cupola at apex of
cochlear canal; _V._ cæecal sack of the vestibulum of the cochlear
canal.]
The cochlear canal is bounded by three walls, the outer one being the
osseous wall of the cochlea. The membrane of Reissner bounds it
towards the scala vestibuli, and the basilar membrane towards the
scala tympani. This membrane stretches from the margin of the lamina
spiralis to the ligamentum spirale; the latter being merely an
expanded portion of the connective tissue lining the osseous cochlea.
The lamina spiralis is produced into two lips, called respectively the
labium tympanicum and labium vestibulare; it is to the former and
longer of these that the basilar membrane is attached. At the margin
of the junction of the labium tympanicum with the basilar membrane the
former is perforated for the passage of the nervous fibres, and this
region is called the habenula perforata.
The labium vestibulare, so called from its position, is shorter than
the labium tympanicum and is raised above into numerous blunt teeth.
Partly springing out from the labium vestibulare, and passing from
near the inner attachment of the membrane of Reissner towards the
outer wall of the cochlea, is an elastic membrane, the membrana
tectoria. Resting on the basilar membrane is the organ of Corti.
Considering for the moment that a transverse section of the cochlear
canal only one cell deep is being dealt with, the organ of Corti will
be found to consist of a central part composed of two peculiarly
shaped rods widely separated below, but in contact above. These are
the rods or fibres of Corti. On their outer side, _i.e._ on the side
towards the osseous wall of the canal, is a reticulate membrane which
passes from the inner rod of Corti towards the osseous wall of the
canal. With their upper extremities fixed in that membrane, and their
lower resting on the basilar membrane are three (four in man) cells
with auditory hairs known as the outer 'hair cells,' which alternate
with three other cells known as Deiters' cells. Between these and the
outer attachment of the basilar membrane is a series of cells
gradually diminishing in height in passing outwards. On the inner side
of the rods of Corti is one hair cell, and then a number of peculiarly
modified cells which fill up the space between the two lips of the
lamina spiralis.
It will not be necessary to say much in reference to the development
of the labium tympanicum and the labium vestibulare.
The labium vestibulare is formed by a growth of the connective tissue
which fuses with and passes up between the epithelial cells. The
epithelial cells which line its upper (vestibular) border become
modified, and remain as its teeth.
The labium tympanicum is formed by the coalescence of the connective
tissue layer separating the scala tympani from the cochlear canal with
part of the connective tissue of the lamina spiralis. At first these
two layers are separate, and the nerve fibres to the organ of Corti
pass between them. Subsequently however they coalesce, and the region
where they are penetrated by the nervous fibres becomes the habenula
perforata.
The organ of Corti itself is derived from the epiblast cells lining
the cochlear canal, and consists in the first instance of two
epithelial ridges or projections. The larger of them forms the cells
on the inner side of the organ of Corti, and the smaller the rods of
Corti together with the inner and outer hair cells and Deiters' cells.
At first both these ridges are composed of simple elongated epithelial
cells one row deep. The smaller ridge is the first to shew any change.
The cells adjoining the larger ridge acquire auditory hairs at their
free extremities, and form the row of inner hair cells; the next row
of cells acquires a broad attachment to the basilar membrane, and
gives origin to the inner and outer rods of Corti.
Outside the latter come several rows of cells adhering together so as
to form a compact mass which is quadrilateral in section. This mass is
composed of three upper cells with nuclei at the same level, which
form the outer hair cells, each of them ending above in auditory
hairs, and three lower cells which form the cells of Deiters. Beyond
this the cells gradually pass into ordinary cubical epithelial cells.
As just mentioned, the cells of the second row, resting with their
broad bases on the basilar membrane, give rise to the rods of Corti.
The breadth of the bases of these cells rapidly increases, and
important changes take place in the structure of the cells themselves.
The nucleus of each cell divides; so that there come to be two nuclei
or sometimes three which lie close together near the base of the cell.
Outside the nuclei on each side a fibrous cuticular band appears. The
two bands pass from the base of the cell to its apex, and there meet
though widely separated below. The remaining contents of the cell,
between the two fibrous bands, become granular, and are soon to a
great extent absorbed; leaving at first a round, and then a triangular
space between the two fibres. The two nuclei, surrounded by a small
amount of granular matter, come to lie, each at one of the angles
between the fibrous bands and the basilar membrane.
The two fibrous bands become, by changes which need not be described
in detail, converted into the rods of Corti--each of their upper ends
growing outwards into the processes which the adult rods possess.
Each pair of rods of Corti is thus (Böttcher) to be considered as the
product of one cell; and the nuclei embedded in the granular mass
between them are merely the remains of the two nuclei formed by the
division of the original nucleus of that cell[196]. The larger ridge
is for the most part not permanent, and from being the most
conspicuous part of the organ of Corti comes to be far less important
than the smaller ridge. Its cells undergo a partial degeneration; so
that the epithelium in the hollow between the two lips of the lamina
spiralis, which is derived from the larger ridge, comes to be composed
of a single row of short and broad cells. In the immediate
neighbourhood however of the inner hair cell, one or two of the cells
derived from the larger ridge are very much elongated.
[196] It is not clear from Böttcher's description how it comes
about that the inner rods of Corti are more numerous than the
outer.
The membrana reticularis is a cuticular structure derived from the
parts to which it is attached.
_Accessory structures connected with the organ of hearing in
Terrestrial Vertebrata._
In all the Amphibia, Sauropsida and Mammalia, except the Urodela and a
few Anura and Reptilia, the first visceral or hyomandibular cleft
enters into intimate relations with the organs of hearing, and from it
and the adjoining parts are formed the tympanic cavity, the Eustachian
tube, the tympanic membrane and the meatus auditorius externus. The
tympanic membrane serves to receive from the air the sound vibrations,
which are communicated to fluids contained in the true auditory
labyrinth by one ossicle or by a chain of auditory ossicles.
The addition to the organ of hearing of a tympanic membrane to receive
aerial sound vibrations is an interesting case of the adaptation of a
structure, originally required for hearing in water, to serve for
hearing in air; and as already pointed out, the similarity of this
membrane to the tympanic membrane of some Insects is also striking.
There is much that is obscure with reference to the actual development
of the above parts of the ear, which has moreover only been carefully
studied in Birds and Mammals.
The Eustachian tube and tympanic cavity seem to be derived from the
inner part of the first visceral or hyomandibular cleft, the external
opening of which becomes soon obliterated. Kölliker holds that the
tympanic cavity is simply a dorsally and posteriorly directed
outgrowth of the median part of the inner section of this cleft; while
Moldenhauer (No. 392) holds, if I understand him rightly, that it is
formed as an outgrowth of a cavity called by him the sulcus
tubo-tympanicus, derived from the inner aperture of the first visceral
cleft together with the groove of the pharynx into which it opens; and
Moldenhauer is of opinion that the greater part of the original cleft
atrophies.
The meatus auditorius externus is formed at the region of a shallow
depression where the closure of the first visceral cleft takes place.
It is in part formed by the tissue surrounding this depression growing
up in the form of a wall, and Moldenhauer believes that this is the
whole process. Kölliker states however that the blind end of the
meatus becomes actually pushed in towards the tympanic cavity.
The tympanic membrane is derived from the tissue which separates the
meatus auditorius externus from the tympanic cavity. This tissue is
obviously constituted of an hypoblastic epithelium on its inner
aspect, an epiblastic epithelium on its outer aspect, and a layer of
mesoblast between them, and these three layers give rise to the three
layers of which this membrane is formed in the adult. During the
greater part of foetal life it is relatively very thick, and presents
a structure bearing but little resemblance to that in the adult.
A proliferation of the connective tissue-cells in the vicinity of the
tympanic cavity causes in Mammalia the complete or nearly complete
obliteration of the cavity during foetal life.
The tympanic cavity is bounded on its inner aspect by the osseous
investment of the internal ear, but at one point, known as the
fenestra ovalis, the bone is deficient in the Amphibia, Sauropsida and
Mammalia, and its place is taken by a membrane; while in Mammalia and
Sauropsida a second opening, the fenestra rotunda, is also present.
These two fenestræ appear early, but whether they are formed by an
absorption of the cartilage, or by the nonchondrification of a small
area, is not certainly known. The upper of the two, or fenestra
ovalis, contains the base of a bone, known in the Sauropsida and
Amphibia as the columella. The main part of the columella is formed of
a stalk which is held by Parker to be derived from part of the
skeleton of the visceral arches, but its nature is discussed in
connection with the skeleton, while the base, forming the stapes,
appears to be derived from the wall of the periotic cartilage.
In all Amphibia and Sauropsida with a tympanic cavity, the stalk of
the columella extends to the tympanic membrane; its outer end becoming
imbedded in this membrane, and serving to transmit the vibrations of
the membrane to the fluid in the internal ear. In Mammalia there is a
stapes not directly attached to the tympanic membrane by a stalk, and
two additional auditory ossicles, derived from parts of the skeleton
of the visceral arches, are placed between the stapes and the tympanic
membrane. These ossicles are known as the malleus and incus, and the
chain of the three ossicles replaces physiologically the single
ossicle of the lower forms.
These ossicles are at first imbedded in the connective tissue in the
neighbourhood of the tympanic cavity, but on the full development of
this cavity, become apparently placed within it; though really
enveloped in the mucous membrane lining it.
The fenestra ovalis is in immediate contiguity with the walls of the
utricle, while the fenestra rotunda adjoins the scala tympani.
Hunt (No. 391) holds, from his investigations on the embryology of the
pig, that "the Eustachian tube is an involution of the pharyngeal
mucous membrane;" and that "the meatus is an involution of the
integument" while "the drum is formed by the Eustachian tube
overlapping the extremity of the meatus." Urbantschitsch also holds
that the first visceral cleft has nothing to do with the formation of
the tympanic cavity and Eustachian tube, and that these parts are
derived from lateral outgrowths of the oral cavity.
The evolution of the accessory parts of the ear would be very
difficult to explain on Darwinian principles if the views of Hunt and
Urbantschitsch were correct; and the accepted doctrine, originally
proposed by Huschke (No. 389), according to which these structures
have originated by a 'change of function' of the parts of the first
visceral cleft, may fairly be held till more conclusive evidence has
been brought against it than has yet been done.
[FIG. 306. LARVA OF ASCIDIA MENTULA. (From Gegenbaur; after
Kupffer.) Only the anterior part of the tail is represented.
_N´._ anterior swelling of neural tube; _N._ anterior swelling of
spinal portion of neural tube; _n._ hinder part of neural tube;
_ch._ notochord; _K._ branchial region of alimentary tract; _d._
oesophageal and gastric region of alimentary tract; _O._ eye; _a._
otolith; _o._ mouth; _s._ papilla for attachment.]
Tunicata. The auditory organ of the Tunicata (fig. 306) is placed on
the under surface of the anterior vesicle of the brain. It consists of
two parts (1) a prominence of the cells of the floor of the brain
forming a crista acustica, and (2) an otolith projecting into the
cavity of the brain, and attached to the crista by delicate hairs.
The crista acustica is formed of very delicate cylindrical cells, and
in its most projecting part is placed a vesicle with clear contents.
The otolith is an oval body with its dorsal half pigmented, and its
ventral half clear and highly refractive. It is balanced on the
highest point of the crista.
The crista acustica would seem to be developed from the cells of the
lower part of the front vesicle of the brain. The otolith however is
developed from a single cell on the dorsal and right side of the
brain. This cell commences to project into the cavity of the brain and
its free end becomes pigmented. It gradually grows inwards till it
forms a spherical prominence in the cavity of the brain, to the wall
of which it is attached by a stalk. At the same time it travels round
the right side of the vesicle of the brain (in a way not fully
explained) till it reaches the summit of the crista, which has become
in the meantime established.
The auditory organ of the simple Ascidians can hardly be brought into
relation with that of the other Chordata, and has most probably been
evolved within the Tunicate phylum.
BIBLIOGRAPHY.
_Invertebrata_.
(384) V. Hensen. "Studien üb. d. Gehörorgan d. Decapoden." _Zeit. f.
wiss. Zool._, Vol. XIII. 1863.
(385) O. and R. Hertwig. _Das Nervensystem u. d. Sinnesorgane d.
Medusen._ Leipzig, 1878.
_Vertebrata._
(386) A. Boettcher. "Bau u. Entwicklung d. Schnecke." _Denkschriften
d. kaiserl. Leop. Carol. Akad. d. Wissenschaft._, Vol. XXXV.
(387) C. Hasse. _Die vergleich. Morphologie u. Histologie d. häutigen
Gehörorgane d. Wirbelthiere._ Leipzig, 1873.
(388) V. Hensen. "Zur Morphologie d. Schnecke." _Zeit. f. wiss.
Zool._, Vol. XIII. 1863.
(389) E. Huschke. "Ueb. d. erste Bildungsgeschichte d. Auges u. Ohres
beim bebrüteten Küchlein." _Isis von Oken_, 1831, and Meckel's
_Archiv_, Vol. VI.
(390) Reissner. _De Auris internæ formatione. Inaug. Diss._ Dorpat,
1851.
_Accessory parts of Vertebrate Ear._
(391) David Hunt. "A comparative sketch of the development of the ear
and eye in the Pig." _Transactions of the International Otological
Congress_, 1876.
(392) W. Moldenhauer. "Zur Entwick. d. mittleren u. äusseren Ohres."
_Morphol. Jahrbuch_, Vol. III. 1877.
(393) V. Urbantschitsch. "Ueb. d. erste Anlage d. Mittelohres u. d.
Trommelfelles." _Mittheil. a. d. embryol. Instit. Wien_, Heft I. 1877.
_Olfactory organ._
Amongst the Invertebrata numerous sense organs have been described
under the title of olfactory organs. In aquatic animals they often
have the form of ciliated pits or grooves, while in the Insects and
Crustacea delicate hairs and other structures present on the antennæ
are usually believed to be organs of smell. Our knowledge of all these
organs is however so vague that it would not be profitable to deal
with them more fully in this place. Amongst the Chordata there are
usually well developed olfactory organs.
Amongst the Urochorda (Tunicata) it is still uncertain what organs (if
any) deserve this appellation. The organ on the dorsal side of the
opening of the respiratory pharynx may very possibly have an olfactory
function, but it is certainly not homologous with the olfactory pits
of the true Vertebrata, and as mentioned above (pp. 436 and 437), may
perhaps be homologous with the pituitary body.
[FIG. 307. VIEWS OF THE HEAD OF ELASMOBRANCH EMBRYOS AT TWO STAGES
AS TRANSPARENT OBJECTS.
A. Pristiurus embryo of the same stage as fig. 28 F.
B. Somewhat older Scyllium embryo.
_III._ third nerve; _V._ fifth nerve; _VII._ seventh nerve; _au.n._
auditory nerve; _gl._ glossopharyngeal nerve; _Vg._ vagus nerve; _fb._
fore-brain; _pn._ pineal gland; _mb._ mid-brain; _hb._ hind-brain;
_iv.v._ fourth ventricle; _cb._ cerebellum; _ol._ olfactory pit; _op._
eye; _au.V._ auditory vesicle; _m._ mesoblast at base of brain; _ch._
notochord; _ht._ heart; _Vc._ visceral clefts; _eg._ external gills;
_pp._ sections of body cavity in the head.]
In the Cephalochorda (Amphioxus) there is a shallow ciliated pit,
discovered by Kölliker, which is situated on the left side of the
head, and is closely connected with a special process of the front end
of the brain. It is most probably the homologue of the olfactory pits
of the true Vertebrata.
In the true Vertebrata the olfactory organ has usually the form of a
pair of pits, though in the Cyclostomata the organ is unpaired.
In all the Vertebrata with two olfactory pits these organs are formed
from a pair of thickened patches of the epiblast, on the under side of
the fore-brain, immediately in front of the mouth (fig. 307, _ol_).
Each thickened patch of epiblast soon becomes involuted as a pit (fig.
308, _N_), the lining cells of which become the olfactory or
Schneiderian epithelium. The surface of this epithelium is usually
much increased by various foldings, which in the Elasmobranchii arise
very early, and are bilaterally symmetrical, diverging on each side
like the barbs of a feather from the median line. They subsequently
become very pronounced (fig. 309), serving greatly to increase the
surface of the olfactory epithelium. At a very early stage the
olfactory nerve attaches itself to the olfactory epithelium.
In Petromyzon the olfactory organ arises as an _unpaired_ thickening
of the epiblast, which in the just hatched larva forms a shallow pit,
on the ventral side of the head, immediately in front of the mouth.
This pit rapidly deepens, and soon extends itself backwards nearly as
far as the infundibulum (fig. 310, _ol_). By the development of the
upper lip the opening of the olfactory pit is gradually carried to the
dorsal surface of the head, and becomes at the same time narrowed and
ciliated (fig. 47, _ol_). The whole organ forms an elongated sack, and
in later stages becomes nearly divided by a median fold into two
halves.
It is probable that the unpaired condition of the olfactory organ in
the Lamprey has arisen from the fusion of two pits into one; there is
however no evidence of this in the early development; but the division
of the sack into two halves by a median fold may be regarded as an
indication of such a paired character in the later stages.
In Myxine the olfactory organ communicates with the mouth through the
palate, but the meaning of this communication, which does not appear
to be of the same nature as the communication between the olfactory
pits and the mouth by the posterior nares in the higher types, is not
known.
The opening of the olfactory pit does not retain its embryonic
characters. In Elasmobranchii and Chimæra it becomes enclosed by a
wall of integument, often deficient on the side of the mouth, so that
there is formed a groove leading from the nasal pit towards the angle
of the mouth. This groove is usually constricted in the middle, and
the original single opening of the nasal sack thus becomes nearly
divided into two. In Teleostei and Ganoids the division of the nasal
opening into two parts becomes complete, but the ventral opening is
generally carried off some distance from the mouth, and placed, by the
growth of the snout, on the upper surface of the head (figs. 54 and
68). In all these instances it is probable that the dorsal opening of
the external nares, and the ventral opening with the posterior nares
of higher types. Thus the posterior nares would in fact seem to be
represented in all Fishes by a ventral part of the opening of the
original nasal pit which either adjoins the border of the mouth (many
Elasmobranchii) or is quite separate from the mouth (Teleostei and
Ganoidei). In the Dipnoi, Amphibia and all the higher types the oral
region becomes extended so as to enclose the posterior nares, and then
each nasal pit acquires two openings; viz. one outside the mouth, the
external nares, and one within the mouth, the internal or posterior
nares. In the Dipnoi the two nasal openings are very similar to those
in Ganoidei and Teleostei, but both are placed on the under surface of
the head, the inner one being within the mouth, and the external one
is so close to the outer border of the upper lip that it also has been
considered by some anatomists to lie within the mouth.
[FIG. 308. SIDE VIEW OF THE HEAD OF AN EMBRYO CHICK OF THE THIRD DAY
AS AN OPAQUE OBJECT. (Chromic acid preparation.)
_C.H._ cerebral hemispheres; _F.B._ vesicle of third ventricle;
_M.B._ mid-brain; _Cb._ cerebellum; _H.B._ medulla oblongata; _N._
nasal pit; _ot._ auditory vesicle in the stage of a pit with the
opening not yet closed up; _op._ optic vesicle, with _l._ lens and
_ch.f._ choroidal fissure.
_1 F._ The first visceral fold; above it is seen the superior
maxillary process.
_2, 3, 4 F._ Second, third and fourth visceral folds, with the
visceral clefts between them.]
In all the higher types the nasal pits have originally only a single
opening, and the ontogenetic process by which the posterior nasal
opening is formed has been studied in the Amniota and Amphibia.
Amongst the Amniota we may take the Chick as representing the process
in a very simple form. The general history of the process was first
made out by Kölliker.
[FIG. 309. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN
EMBRYO OF SCYLLIUM. (Modified from figures by Marshall and myself.)
_c.h._ cerebral hemispheres; _ol.v._ olfactory vesicle; _olf._
olfactory pit; _Sch._ Schneiderian folds; _I._ olfactory nerve. The
reference line has been accidentally taken through the nerve to the
brain.]
[FIG. 310. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA
OF PETROMYZON.
The larva had been hatched three days, and was 4.8 mm. in length.
The optic and auditory vesicles are supposed to be seen through the
tissues.
_c.h._ cerebral hemisphere; _th._ optic thalamus; _in._
infundibulum; _pn._ pineal gland; _mb._ mid-brain; _cb._ cerebellum;
_md._ medulla oblongata; _au.v._ auditory vesicle; _op._ optic
vesicle; _ol._ olfactory pit; _m._ mouth; _br.c._ branchial pouches;
_th._ thyroid involution; _v.ao._ ventral aorta; _ht._ ventricle of
heart; _ch._ notochord.]
The opening of the nasal pit becomes surrounded by a ridge except on
its oral side. The deficiency of this ridge on the side of the mouth
gives rise to a kind of shallow groove leading from the nasal pit to
the mouth. The ridge enveloping the opening of the nasal pit next
becomes prolonged along the sides of this groove, especially on its
inner one; and at the same time the superior maxillary process grows
forwards so as to bound the lower part of its outer side. The inner
and outer ridges, together with the superior maxillary process,
enclose a deep groove, connecting the original opening of the nasal
pit with the mouth. The process just described is illustrated by fig.
311 A, and it may be seen that the ridge on the inner side of the
groove forms the edge of the frontonasal process (_k_).
[FIG. 311. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS
OF INCUBATION. (From Huxley.)
_I^a._ cerebral vesicles; _a._ eye, in which the remains of the
choroid slit can still be seen in A; _g._ nasal pits; _k._
frontonasal process; _l._ superior maxillary process; 1. inferior
maxillary process or first visceral arch; 2. second visceral arch;
_x._ first visceral cleft.
In A the cavity of the mouth is seen enclosed by the frontonasal
process, the superior maxillary processes and the first pair of
visceral arches. At the back of it is seen the opening leading into
the throat. The nasal grooves leading from the nasal pits to the
mouth are already closed over.
In B the external opening of the mouth has become much constricted,
but it is still enclosed by the frontonasal process and superior
maxillary processes above, and by the inferior maxillary processes
(first pair of visceral arches) below.
The superior maxillary processes have united with the frontonasal
process, along nearly the whole length of the latter.]
On the sixth day (Born, 394) the sides of this groove unite together
in the middle, and convert it into a canal open at both ends--the
ventral openings of the canals of the two sides being placed just
within the border of the mouth, and forming the posterior nares; while
the external openings form the anterior nares. The upper part of the
canal, together with the original nasal pit, is alone lined by
olfactory epithelium; the remaining epithelium of the nasal cavity
being indifferent epiblastic epithelium. Further changes subsequently
take place in connection with the posterior nares, but these are
described in the section dealing with the mouth.
In Mammalia the general formation of the anterior and posterior nares
is the same as in Birds; but, as shewn by Dursy and Kölliker, an
outgrowth from the inner side of the canal between the two openings
arises at an early period; and becoming separate from the posterior
nares and provided with a special opening into the mouth, forms the
organ of Jacobson. The general relations of this organ when fully
formed are shewn in fig. 312.
In Lacertilia the formation of the posterior nares differs in some
particulars from that in Birds (Born). A groove is formed leading from
the primitive nasal pit to the mouth, bordered on its inner side by
the swollen edge of the frontonasal process, and on its outer by an
outer-nasal process; while the superior maxillary process does not
assist in bounding it. On the inner side of the narrowest part of this
groove there is formed a large lateral diverticulum, which is lined by
a continuation of the Schneiderian epithelium, and forms the rudiment
of Jacobson's organ. The nasal groove continues to grow in length, but
soon becomes converted into a canal by the junction of the outer-nasal
process with the frontonasal process. This canal is open at both ends:
at its dorsal end is placed the original opening of the nasal pit, and
its ventral opening is situated within the cavity of the mouth. The
latter forms the primitive posterior nares. The superior maxillary
process soon grows inwards on the under side of the posterior part of
the nasal passage, and assists in forming its under wall. This
ingrowth of the superior maxillary process is the rudiment of the hard
palate.
On the conversion of the nasal groove into a closed passage, the
opening of Jacobson's organ into the groove becomes concealed; and at
a later period Jacobson's organ becomes completely shut off from the
nasal cavity, and opens into the mouth at the front end of an
elongated groove leading back to the posterior nares.
[FIG. 312. SECTION THROUGH THE NASAL CAVITY AND JACOBSON'S ORGAN.
(From Gegenbaur.)
_sn._ septum nasi; _cn._ nasal cavity; _J._ Jacobson's organ; _d._
edge of upper jaw.]
In Amphibia the posterior nares are formed in a manner very different
from that of the Amniota. At an early stage a shallow groove is formed
leading from the nasal pit to the mouth; but this groove instead of
forming the posterior nares soon vanishes, and by the growth of the
front of the head the nasal pits are carried farther away from the
mouth.
The actual posterior nares are formed by a perforation in the palate,
opening into the blind end of the original nasal pit.
Considering that the various stages in the formation of the posterior
nares of the Amniota are so many repetitions of the adult states of
lower forms, it may probably be assumed that the mode of formation of
the posterior nares in Amphibia is secondary, as compared with that in
the Amniota.
A diverticulum of the front part of the nasal cavity of the Anura is
probably to be regarded as a rudimentary form of Jacobson's organ.
BIBLIOGRAPHY.
(394) G. Born. "Die Nasenhöhlen u. d. Thränennasengang d. amnioten
Wirbelthiere." Parts I. and II. _Morphiologisches Jahrbuch_, Bd. V.,
1879.
(395) A. Kölliker. "Ueber die Jacobson'schen Organe des Menschen."
_Festschrift f. Rienecker_, 1877.
(396) A. M. Marshall. "Morphology of the Vertebrate Olfactory Organ."
_Quart. Journ. of Micr. Science_, Vol. XIX., 1879.
_Sense organs of the lateral line._
Although I do not propose dealing with the general development of
various sense organs of the skin, there is one set of organs, viz.
that of the lateral line, which, both from its wide extension amongst
the Ichthyopsida and from the similarity of some of its parts to
certain organs found amongst the Chætopoda[197], has a great
morphological importance.
[197] The organs which resemble those of the lateral line are the
remarkable sense organs found by Eisig in the Capitellidæ
(_Mittheil. a. d. Zool. Station zu Neapel_, Vol. 1); but I
am not inclined to think that there is a true homology between
these organs and the lateral line of Vertebrata. It seems to me
probable that the segmentally arranged optic organs of
Polyophthalmus are a special modification of the more indifferent
sense organs of the Capitellidæ. The close affinity of these two
types of Chætopods is favourable to this view.
The organs of the lateral line consist as a rule of canals, partly
situated in the head, and partly in the trunk. These canals open at
intervals on the surface, and their walls contain a series of
nerve-endings. The branches of the canal in the head are innervated
for the most part by the fifth pair, and those of the trunk by the
nervus lateralis of the vagus nerve. There is typically but a single
canal in the trunk, the openings and nerve-endings of which are
segmentally arranged.
Two types of development of these organs have been found. One of these
is characteristic of Teleostei; the other of Elasmobranchii.
In just hatched Teleostei, Schulze (No. 402) found that instead of the
normal canals there was present a series of sense bulbs, projecting
freely on the surface and partly composed of cells with stiff hairs.
In most cases each bulb is enclosed in a delicate tube open at its
free extremity; while the bulbs correspond in number with the
myotomes. In some Teleostei (Gobius, Esox, etc.) such sense organs
persist through life; in most forms however each organ becomes covered
by a pair of lobes of the adjacent tissue, one formed above and the
other below it. The two lobes of each pair then unite and form a tube
open at both ends. The linear series of tubes so formed is the
commencement of the adult canal; while the primitive sense bulbs form
the sensory organs of the tubes. The adjacent tubes partially unite
into a continuous canal, but at their points of apposition pores are
left, which place the canal in communication with the exterior.
Besides these parts, I have found that there is present in the just
hatched Salmon a linear streak of modified epidermis on the level of
the lateral nerve, and from the analogy of the process described below
for Elasmobranchii it appears to me probable that these streaks play
some part in the formation of the canal of the lateral line.
In Elasmobranchii (Scyllium) the lateral line is formed as a linear
thickening of the mucous layer of the epidermis. This thickening is at
first very short, but gradually grows backwards, its hinder end
forming a kind of enlarged growing point. The lateral nerve is formed
shortly after the lateral line, and by the time that the lateral line
has reached the level of the anus the lateral nerve has grown back for
about two-thirds of that distance. The lateral nerve would seem to be
formed as a branch of the vagus, but is at first half enclosed in the
modified cells of the lateral line (fig. 275, _nl_)[198], though it
soon assumes a deeper position.
[198] Götte and Semper both hold that the lateral nerve, instead
of growing in a centrifugal manner like other nerves, is directly
derived from the epiblast of the lateral line. For the reasons
which prevent me accepting this view I must refer the reader to
my _Monograph on Elasmobranch Fishes_, pp. 141-146.
A permanent stage, more or less corresponding to the stage just
described in Elasmobranchii, is retained in Chimæra, and Echinorhinus
spinosus, where the lateral line has the form of an open groove
(Solger, No. 404).
The epidermic thickening, which forms the lateral line, is converted
into a canal, not as in Teleostei by the folding over of the sides,
but by the formation of a cavity between the mucous and epidermic
layers of the epiblast, and the subsequent enclosure of this cavity by
the modified cells of the mucous layer of the epiblast which
constitute the lateral line. The cavity first appears at the hind end
of the organ, and thence extends forwards.
After its conversion into a canal the lateral line gradually recedes
from the surface; remaining however connected with the epidermis at a
series of points corresponding with the segments, and at these points
perforations are eventually formed to constitute the segmental
apertures of the system.
The manner in which the lumen of the canal is formed in Elasmobranchs
bears the same relation to the ordinary process of conversion of a
groove into a canal that the formation of the auditory involution in
Amphibia does to the same process in Birds. In both Elasmobranchii and
Amphibia the mucous layer of the epiblast behaves exactly as does the
whole epiblast in the other types, but is shut off from the surface by
the passive epidermic layer of the epiblast.
The mucous canals of the head and the ampullæ are formed from the
mucous layer of the epidermis in a manner very similar to the lateral
line; but the nerves to them arise as simple branches of the fifth and
seventh nerves, which unite with them at a series of points, but do
not follow their course like the lateral nerve.
It is clear that the canal of the lateral line is secondary, as
compared with the open groove of Chimæra or the segmentally arranged
sense bulbs of young Teleostei; and it is also clear that the
phylogenetic mode of formation of the canal consisted in the closure
of a primitively open groove. The abbreviation of this process in
Elasmobranchii was probably acquired after the appearance of food-yolk
in the egg, and the consequent disappearance of a free larval stage.
While the above points are fairly obvious it does not seem easy to
decide _à priori_ whether a continuous sense groove or isolated sense
bulbs were the primitive structures from which the canals of the
lateral line took their origin. It is equally easy to picture the
evolution of the canal of the lateral line either from (1) a
continuous unsegmented sense line, certain points of which became
segmentally differentiated into special sense bulbs, while the whole
subsequently formed a groove and then a canal; or from (2) a series of
isolated sense bulbs, for each of which a protective groove was
developed; and from the linear fusion of which a continuous canal
became formed.
From the presence however of a linear streak of modified epidermis in
larval Teleostei, as well as in Elasmobranchii, it appears to me more
probable that a linear sense streak was the primitive structure from
which all the modifications of the lateral line took their origin, and
that the segmentally arranged sense bulbs of Teleostei are secondary
differentiations of this primitive structure.
The, at first sight remarkable, distribution of the vagus nerve to the
lateral line is probably to be explained in connection with the
evolution of this organ. As is indicated both by its innervation from
the vagus, as also from the region where it first becomes developed,
the lateral line was probably originally restricted to the anterior
part of the body. As it became prolonged backwards it naturally
carried with it the vagus nerve, and thus a sensory branch of this
nerve has come to innervate a region which is far beyond the limits of
its original distribution.
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