194
GEORGE SZÉKELY
In their experiment to trace histologically the termination and pathway
of regenerating optic fibers, the authors used the Bodian Protargol
technique. In our laboratory, several attempts have been made to show
the termination of optic fibers with various silver techniques, including
the Bodian method in amphibia, but we failed to trace the fine fibers
leaving the optic tract. Attardi and Sperry's pictures appear to show the
same failure. They could demonstrate only a "layer with retinal fibers,"
but the terminations could not be seen. Although they claimed to demonstrate a highly selective reconnection of the regenerating optic fibers,
their result is not convincing enough beyond the statement that fibers
of nasal or temporal origin tend to grow back to the caudal or rostral
part of the tectum by a preferential selection of their pathway within
the optic tract.
In the following paragraph a short description will be given about
the histology of the amphibian optic tectum, mainly for the purpose of
showing the exquisite complexity of its structure. The description is a
short review of the Ramon y Cajal (1911) and Herrick (1925) pioneer
studies on this problem, complemented with a few details found in the
course of investigation still in progress in our laboratory. The tectum
has a stratified structure and can be divided into two main strata. The
inner stratum consists of eight alternating layers of cells and plexiform
sheets, and it may be called "stratum granulosum." The outer stratum
contains fibers of different origin and a few scattered cells which form
two thin layers in Anura. This may be called "stratum moleculare."
Cells in the stratum granulosum have a pear-shaped body with several
basal dendrites arborizing richly after a short run in one of the plexiform
sheets, and extend a long "apical" dendrite up to the surface. The axon
originates from the apical dendrite. Several axon collaterals can be
traced to the plexiform sheets. There are no reliable data as to which
of these axons contributes to an efferent pathway, but it is very probable
that the majority of them accomplish interconnections among cells of
different layers and between the two sides of the tectum. The optic
afférents occupy the superficial zone of the molecular stratum in successively deeper sublayers. The optic fibers end by a dense meshwork of
richly arborizing axon terminals covering an area of about 200 to 300 μ
according to P. Ramon y Cajal's
1 picture in the frog. We have found
1 Note added in proof: P. Ramon y Cajal's picture is published in Ramon y
Cajal's (1911) book cited above. Recently, a series of Golgi studies on the frog's
optic tectum have been completed in our laboratory. The findings revealed that
there are no such forms of optic terminals as represented in P. Ramon y Cajal's picture.
Instead, we found moderately arborizing axon terminals covering a bell-shaped area,
10-20 μ in extent in the most superficial layer and 80-100/1 in deeper layers.
GEORGE SZÉKELY
In their experiment to trace histologically the termination and pathway
of regenerating optic fibers, the authors used the Bodian Protargol
technique. In our laboratory, several attempts have been made to show
the termination of optic fibers with various silver techniques, including
the Bodian method in amphibia, but we failed to trace the fine fibers
leaving the optic tract. Attardi and Sperry's pictures appear to show the
same failure. They could demonstrate only a "layer with retinal fibers,"
but the terminations could not be seen. Although they claimed to demonstrate a highly selective reconnection of the regenerating optic fibers,
their result is not convincing enough beyond the statement that fibers
of nasal or temporal origin tend to grow back to the caudal or rostral
part of the tectum by a preferential selection of their pathway within
the optic tract.
In the following paragraph a short description will be given about
the histology of the amphibian optic tectum, mainly for the purpose of
showing the exquisite complexity of its structure. The description is a
short review of the Ramon y Cajal (1911) and Herrick (1925) pioneer
studies on this problem, complemented with a few details found in the
course of investigation still in progress in our laboratory. The tectum
has a stratified structure and can be divided into two main strata. The
inner stratum consists of eight alternating layers of cells and plexiform
sheets, and it may be called "stratum granulosum." The outer stratum
contains fibers of different origin and a few scattered cells which form
two thin layers in Anura. This may be called "stratum moleculare."
Cells in the stratum granulosum have a pear-shaped body with several
basal dendrites arborizing richly after a short run in one of the plexiform
sheets, and extend a long "apical" dendrite up to the surface. The axon
originates from the apical dendrite. Several axon collaterals can be
traced to the plexiform sheets. There are no reliable data as to which
of these axons contributes to an efferent pathway, but it is very probable
that the majority of them accomplish interconnections among cells of
different layers and between the two sides of the tectum. The optic
afférents occupy the superficial zone of the molecular stratum in successively deeper sublayers. The optic fibers end by a dense meshwork of
richly arborizing axon terminals covering an area of about 200 to 300 μ
according to P. Ramon y Cajal's
1 picture in the frog. We have found
1 Note added in proof: P. Ramon y Cajal's picture is published in Ramon y
Cajal's (1911) book cited above. Recently, a series of Golgi studies on the frog's
optic tectum have been completed in our laboratory. The findings revealed that
there are no such forms of optic terminals as represented in P. Ramon y Cajal's picture.
Instead, we found moderately arborizing axon terminals covering a bell-shaped area,
10-20 μ in extent in the most superficial layer and 80-100/1 in deeper layers.