192
GEORGE SZÉKELY
lobe and those from the middle part of the eye terminated in the caudal
part of the lobe to occupy the place of nasal fibers. Each hemiretina
extended in this fashion over the whole tectum, having common areas
located as mirror images in each half of the visual field for each electrode position on the lobe.
Before interpreting the result, the possibility of a malformation of the
optic lobe should be excluded. It is known that after removal of the eye
the corresponding half of the optic center becomes atrophied (Kollros,
1953) ; in a species of newt with small eyes, replacement of the small eye
by a large one taken from another species gives rise to hypertrophy of
the corresponding tectum (Harrison, 1929; Twitty, 1932). Thus, it
could be argued that the rostral half of the optic lobe, being invaded
only by nasal fibers from a double-nasal eye, had atrophied because of
the lack of temporal fibers, and the caudal part hypertrophied because
of the double-nasal innervation. If this were so, then there would be
nothing peculiar in the result. This possibility was made unlikely by
study of the magnification factor and the ipsilateral projection (Gaze
et al, 1965). By the retinotectal magnification factor is meant the
number of microns of tectum representing 1° of retina measured radially
from the optic axis (Daniel and Whitteridge, 1961). The magnification
in the case of compound eyes was about twice as big as in normal toads
along the anteroposterior axis of the retina, but was normal along the
dorsoventral axis. This increase of the magnification factor in one axis
only makes it very improbable that a hypertrophy could account for
the extended projection of both half-retinas upon the entire surface of
the tectum. An ipsilateral projection from the anterosuperior part of the
visual field was found by Gaze and Jacobson (1962) in the frog. This
was retinotopically organized, but in a reversed order from the contralateral projection. Evoked potentials from the ipsilateral projection could
be abolished by localized lesions of the corresponding tectal points in
the contralateral projection. This suggests that the normal organization
of the ipsilateral projection depends on an intact contralateral lobe, and
that distortions in the ipsilateral projection indicate defective development of the contralateral lobe. The finding of a normally organized
ipsilateral projection sugests that each hemiretina of compound eyes
projects over an intact optic lobe.
The result shows that fibers from the nasal or temporal pole of the
eye always manage to find their proper place in the optic lobe, whereas
fibers from the middle part of the eye grow either to the anterior or to
the posterior part of the lobe. This finding, together with the two
former experiments, shows that retinal fibers can establish connections
with any part of the tectum under certain experimental conditions. The
GEORGE SZÉKELY
lobe and those from the middle part of the eye terminated in the caudal
part of the lobe to occupy the place of nasal fibers. Each hemiretina
extended in this fashion over the whole tectum, having common areas
located as mirror images in each half of the visual field for each electrode position on the lobe.
Before interpreting the result, the possibility of a malformation of the
optic lobe should be excluded. It is known that after removal of the eye
the corresponding half of the optic center becomes atrophied (Kollros,
1953) ; in a species of newt with small eyes, replacement of the small eye
by a large one taken from another species gives rise to hypertrophy of
the corresponding tectum (Harrison, 1929; Twitty, 1932). Thus, it
could be argued that the rostral half of the optic lobe, being invaded
only by nasal fibers from a double-nasal eye, had atrophied because of
the lack of temporal fibers, and the caudal part hypertrophied because
of the double-nasal innervation. If this were so, then there would be
nothing peculiar in the result. This possibility was made unlikely by
study of the magnification factor and the ipsilateral projection (Gaze
et al, 1965). By the retinotectal magnification factor is meant the
number of microns of tectum representing 1° of retina measured radially
from the optic axis (Daniel and Whitteridge, 1961). The magnification
in the case of compound eyes was about twice as big as in normal toads
along the anteroposterior axis of the retina, but was normal along the
dorsoventral axis. This increase of the magnification factor in one axis
only makes it very improbable that a hypertrophy could account for
the extended projection of both half-retinas upon the entire surface of
the tectum. An ipsilateral projection from the anterosuperior part of the
visual field was found by Gaze and Jacobson (1962) in the frog. This
was retinotopically organized, but in a reversed order from the contralateral projection. Evoked potentials from the ipsilateral projection could
be abolished by localized lesions of the corresponding tectal points in
the contralateral projection. This suggests that the normal organization
of the ipsilateral projection depends on an intact contralateral lobe, and
that distortions in the ipsilateral projection indicate defective development of the contralateral lobe. The finding of a normally organized
ipsilateral projection sugests that each hemiretina of compound eyes
projects over an intact optic lobe.
The result shows that fibers from the nasal or temporal pole of the
eye always manage to find their proper place in the optic lobe, whereas
fibers from the middle part of the eye grow either to the anterior or to
the posterior part of the lobe. This finding, together with the two
former experiments, shows that retinal fibers can establish connections
with any part of the tectum under certain experimental conditions. The