190
GEORGE S Ζ É Κ E L Y
spread over such a wide area as half the tectum and that later the inappropriate arborizations should disappear. The striking observation that
the recovery of an organized projection occurred first in the anteroposterior, and only later in the dorsoventral axis of the retina, is consistent with the finding of separate embryonic determination of the two
axes (Székely, 1954). This could suggest that two different gradients
are acting in the establishment both of functional specificity in the
embryo and of orderly regeneration in the adult. Although the assumption of "gradients" can be useful in describing what happens in an
experiment, it is not very helpful in finding an explanation, since nothing
is known about the nature of the gradients. Another possible assumption,
that fibers from the temporal and nasal pole of the retina grew quicker
and reached the tectum sooner, does not have convincing support. There
is, however, one more aspect worth considering; namely, a normal
projection developed in advanced stages of regeneration. This could
possibly mean that the more fibers reached the tectum the better was the
organization of the projection. It would be tempting to consider the
possible significance of the number of ingrowing fibers in the establishment of an orderly projection pattern on a functional basis, but our
scanty knowledge of the visual system does not allow elaborate speculation here.
Finally, it may be noted that a normal projection was only found in
twelve out of sixty-seven cases, in contrast with the behavioral experiments of other authors, in which complete visual recovery was reported
in nearly all the animals. Unfortunately, Gaze and Jacobson did not
make behavioral tests before recording. It would, however, be interesting
to know how visual tests match electrophysiological findings.
c. Projection of Compound Eyes. Double-nasal compound eyes were
made in Xenopus embryos by replacing the excised temporal half of the
eye primordium by a nasal half taken from another embryo. Similar
operations were done to form double-temporal compound eyes. The
retinotectal projection of such eyes was electrophysiologically mapped
in adult toads (Gaze et al., 1963). In the normal toad, as in other
amphibia, fibers of nasal origin project to the caudal part of the lobe,
and fibers of temporal origin to the rostral part of the lobe. In the case
of double-nasal retinas, fibers from the nasal and temporal (originally
nasal) poles projected to the caudal half of the lobe, whereas fibers
from the middle part of the eye occupied the rostral part of the lobe,
i.e., the place where normally the temporal fibers terminate (Fig. 2). A
similar projection was found in the case of double-temporal retinas, but
now the fibers from the poles projected to the rostral part of the optic
GEORGE S Ζ É Κ E L Y
spread over such a wide area as half the tectum and that later the inappropriate arborizations should disappear. The striking observation that
the recovery of an organized projection occurred first in the anteroposterior, and only later in the dorsoventral axis of the retina, is consistent with the finding of separate embryonic determination of the two
axes (Székely, 1954). This could suggest that two different gradients
are acting in the establishment both of functional specificity in the
embryo and of orderly regeneration in the adult. Although the assumption of "gradients" can be useful in describing what happens in an
experiment, it is not very helpful in finding an explanation, since nothing
is known about the nature of the gradients. Another possible assumption,
that fibers from the temporal and nasal pole of the retina grew quicker
and reached the tectum sooner, does not have convincing support. There
is, however, one more aspect worth considering; namely, a normal
projection developed in advanced stages of regeneration. This could
possibly mean that the more fibers reached the tectum the better was the
organization of the projection. It would be tempting to consider the
possible significance of the number of ingrowing fibers in the establishment of an orderly projection pattern on a functional basis, but our
scanty knowledge of the visual system does not allow elaborate speculation here.
Finally, it may be noted that a normal projection was only found in
twelve out of sixty-seven cases, in contrast with the behavioral experiments of other authors, in which complete visual recovery was reported
in nearly all the animals. Unfortunately, Gaze and Jacobson did not
make behavioral tests before recording. It would, however, be interesting
to know how visual tests match electrophysiological findings.
c. Projection of Compound Eyes. Double-nasal compound eyes were
made in Xenopus embryos by replacing the excised temporal half of the
eye primordium by a nasal half taken from another embryo. Similar
operations were done to form double-temporal compound eyes. The
retinotectal projection of such eyes was electrophysiologically mapped
in adult toads (Gaze et al., 1963). In the normal toad, as in other
amphibia, fibers of nasal origin project to the caudal part of the lobe,
and fibers of temporal origin to the rostral part of the lobe. In the case
of double-nasal retinas, fibers from the nasal and temporal (originally
nasal) poles projected to the caudal half of the lobe, whereas fibers
from the middle part of the eye occupied the rostral part of the lobe,
i.e., the place where normally the temporal fibers terminate (Fig. 2). A
similar projection was found in the case of double-temporal retinas, but
now the fibers from the poles projected to the rostral part of the optic