of it, th e canal for the p itu ita ry vein leads on either side into the orbit, lying v entral and th en lateral,
or antero-lateral to the hypophysis.
The conditions in L epidosteus th u s all indicate th a t the myodome of Amia would arise if certain
of th e recti muscles of th e former should force an entrance into th e cranial cavity, and then into
th e saccular space, traversing, in this process, th e foramen and th e n th e canal for th e p itu ita ry vein.
The recti muscles, as already stated, already have their origins in th e immediate vicinity of th e foramen
for th e vein, and one head of th e rectus externus has, in my 80 mm specimen, already acquired a very
considerably intracranial extension. If this head of this muscle were to continue its progress, a t the
same time increasing largely in size, it would, following th e vein, make its way b ackward between
the cartilaginous floor of th e cranial cavity and th e overlying membrane, would force th a t membrane
and th e overlying brain upward in th e cranial cavity, and might finally reach and break down the
anterior wall of th e cross-canal ; the myodome of Amia thus being produced. And this is, in principle,
th e manner in which Sagemehl suggests th a t the teleostean myodome did arise, although in the details
of his explanation he is wrong. The conditions in Lepidosteus might however represent a stage
in th e abortion of the conditions th a t preceded and led to the establishment of a myodome, those
pre-existing conditions being represented in Polypterus; and this seems to me th e proper interpretation
of th e facts, as will be later discussed.
But, whatever the origin of the myodome, there are in Lepidosteus, as there are in Amia and
Scorpaena, two floors in the pituitary region of the cranial cavity, the dorsal one of these two floors
being membranous to a different extent in each of these three fishes. The lateral walls of this part
of the skull are also double in each of these three fishes, one or the other of the two walls being
also membranous to a different extent in each of the fishes. In the space between the two floors
lies the myodome, either actual or potential, while in the space between the two lateral walls lies
the trigemino-facialis chamber. In Dactylopterus the conditions are modified and obscured by
the fact th a t the dorsal floor arises from or near the anterior edge of the ventral floor and is inclined
a t a marked angle to it.
According to Starks (’05), th e ventral floor of th e myodome of fishes „is th e tru e cranial b ase“ ,
the dorsal floor being simply a septum of secondary development; and th e conditions th a t I have
described in Amia would seem to favor this interpretation, the dorsal wall of the myodome chondri-
fying, in th a t fish, much later th a n th e ventral wall. B u t this condition is n o t invariable in fishes,
as Handrick’s (’01) descriptions of Argyropelecus show. In th a t fish the myodome is said to begin
beneath th e membranous pitu ita ry fossa; and from there to extend backward immediately beneath
th e cartilaginous floor of th e cranium until it reaches and ends against th e „anterior outer wall“ of
th e bulla th a t encloses th e sacculi and lagenae. The side walls and floor of this myodome are not
particularly described, b u t they are evidently formed by portions of th e hard, modified, connective-
tissue membrane th a t is said to largely cover th e roof and side walls of th e cartilaginous cranium,
and th e trigemino-facialis chamber of either side would seem to be in direct communication with the
myodome, as it is in Amia. The trigeminus ganglion is said to be an extracranial one, and to lie
in the upper comer of th e myodome. The ciliary ganglion is also said to lie in th e myodome, while the
sympathetic ganglion is said to lie on th e outside of th e side wall of the myodome, and the communis
and lateralis ganglia of the V—V II complex, which evidently form th è large so-called facialis ganglion,
to lie entirely within th e cartilaginous cranium. The cartilaginous wall of th e cranium is said to
be perforated, o n either side, by three separate foramina, one of which transmits the nervus oculor
motorius, a second the root of the trigeminus accompanied by a single tru n k from which arise the
ophthalmicus and buccalis lateralis, and a th ird th e truncus facialis accompanied by th e palatinus
facialis and nervus abducens. The root of th e trigeminus evidently includes th e radix profundi,
b u t whether th e ganglion of th a t nerve is intracranial or extracranial is not evident. I t is however
plainly evident th a t th e cartilaginous floor of the cranium, immediately posterior to the pituitary
fossa, is a prootic bridge, and th a t the side wall of th e cranium on either side of the pituitary region
represents the inner wall of th e trigemino-facialis chamber, the outer wall of th a t chamber, and also
th e ventral wall of th e myodome, being wholly of membrane.
I t is thus evident th a t no positive conclusion can be formed as to which one of the two floors
of this p a rt of the skull of fishes is the primary, and which the secondary one, u ntil it is first known
what pa rts of th e membranes here concerned belong to th e primordial membranous capsule of the
brain and what parts, if any, are developed independently, internal to th a t capsule, as protective
coverings to the brain. The myodome, as it actually exists, is however certainly a space in the dura
mater, as th a t membrane is defined by Sagemehl in his descriptions of Amia, and as both its inner
and its outer walls may in large p a rt chondrify or ossify as parts of the cranial wall, the myodome
is an intramural and not an intracranial space. This I have already stated in an earlier work (’97b,
p. 10), there speaking of th e myodome as an intracranial space, b u t qualifying this by saying th a t
it is „a space th a t certainly lies morphologically in, and not internal to, the membranous bounding
walls of th e primordial skull“ .
In th e labyrinth region also, both the ventral and the lateral walls of the skull of fishes are
pa rtly double, the sacculi of the membranous ear lying between the two ventral walls and the semicircular.
canals between the lateral walls. The inner one of these two walls is largely membranous
in teleosts and the bony ganoids, b u t it may p a rtly ossify as the mesial processes of the exoccipitals.
In elasmobranchs th e wall may be largely of cartilage.
In most prepared skulls thesè two walls of th e skull of fishes are not apparent, for their membranous
portions can only be preserved in careful preparations, and are almost always entirely
removed. In serial sections of embryos, also, particularly of early embryos, the membranous portions
of these walls are a p t not to be recognized as such. . Yet th e fact th a t they both exist must always
be carefully borne in mind, for it, alone, permits of a proper comparison of this region, not only of
fishes with one another b u t of fishes with higher vertebrates.
In the chondrocranium of embryos of Lacerta agilis the hypophysis lies, according to Gaupp
(’00, p. 470), in a hypophysial fenestra bounded laterally by the diverging hind ends of the trabeculae,
and posteriorly by a, transverse bar of parachordal cartilage, called by Gaupp th e crista sellaris. This
crista sellaris unites the anterior .ends of th e parachordal plates, and separates the interparachordal
fontanelle from the intertrabecular fontanelle, òr hypophysial fenestra; thus having approximately,
if not exactly, the position of the posterior clinoid bridge of P arker’s descriptions of Lepidosteus. The
carotis cerebralis, on either side, traverses, in early stages, th e postero-lateral corner of the hypophysial
fenestra, b u t in later stages it becomes entirely enclosed in cartilage, in an independent carotid
foramen. Slightly anterior to the hypophysial fenestra lies the subiculum infundibuli, a Y-shaped
cartilaginous plate th a t rises from th e hind edge of the interorbital septum. Anteriorly, this plate
of cartilage forms p a rt of the hind border of thè optic fenestra, while posteriorly it gives support
to the anterior border of the lobus infundibularis. The Y-shaped plate "of cartilage thus has strikingly
th e position of the basisphenoid cartilage of Scorpaena, and this latter, cartilage has been shown