2. THE PINEAL ORGAN
93
from the roof of the diencephalon. The rostra1 evagination represents
the anlage of the parapineal body (Hill, 1891, 1894; Friedrich-Freksa,
1932; U. Holmgren, 1959a) which is homologous with the parietal eye
of certain reptiles (U. Holmgren, 1965). The pineal organ develops
from the more posterior of the diencephalic evaginations (Rasquin,
1958). Therefore, the parapineal and pineal organs are two independent
structures and are not homologous (Tilney and Warren, 1919). The
paraphysis, which has occasionally been confused with the parapineal
(Rasquin, 1958), is morphologically part of the telencephalon and is
not part of the pineal area.
In fishes, the parapineal anlage usually shows some early ontogenetic
development (U. Holmgren, 1959a) but is generally absent or rudimentary in adults (Kelly, 1962; Van de Kamer, 1965), including the
primitive ganoid Amiu calva ( Kingsbury, 1897) and the dipnoan Protop
term annectens (U. Holmgren, 1959b). However, a parapineal body
resembling a parietal eye of lizards was described in at least one adult
teleost (Steyn and Webb, 1960). In other groups of fishes, a well-developed parapineal body with distinguishable photosensory-like cells
is found only among the nonmyxinoid cyclostomes ( Oksche, 1965).
However, the same author notes that the problem of the parapineal
organ is not resolved and should be completely reexamined.
B. Structure
1. TOPOGRAPHY AND GENERAL MORPHOLOGY
The pineal body, s e m stricto, is a conspicuous structure in most
adult fishes (U. Holmgren, 1959a); it is, however, rudimentary in
Syngnathus acus and Hippocampus spinosa ( StudniEka, 1896) and absent in Torpedo ocelluta, T . marmorata (Selachii), and the adult Myxine
glutinosa ( Oksche, 1965). Despite contradictov opinions, a pineal organ
is present in the tuna Thynnus thynnus (U. Holmgren, 1958a).
In basic form, the fish pineal organ consists of a hollow, variously
invaginated and well-vascularized structure lying dorsal to the diencephalon to which it is connected by a hollow narrow stalk. The pineal body
consists of an expanded distal vesicular area containing a central
lumen which connects with the third ventricle via the narrow proximal
stalk. Posteriomedially the stalk connects with the subcommissural organ
and the commissura posterior and makes contact with the commissura
habenulae on its anterior side before joining the dorsal sac epithelium
which comes to lie directly ventral to the distal vesicle of the pineal
body. With advancing age, the epithelium of the distal vesicle under-
93
from the roof of the diencephalon. The rostra1 evagination represents
the anlage of the parapineal body (Hill, 1891, 1894; Friedrich-Freksa,
1932; U. Holmgren, 1959a) which is homologous with the parietal eye
of certain reptiles (U. Holmgren, 1965). The pineal organ develops
from the more posterior of the diencephalic evaginations (Rasquin,
1958). Therefore, the parapineal and pineal organs are two independent
structures and are not homologous (Tilney and Warren, 1919). The
paraphysis, which has occasionally been confused with the parapineal
(Rasquin, 1958), is morphologically part of the telencephalon and is
not part of the pineal area.
In fishes, the parapineal anlage usually shows some early ontogenetic
development (U. Holmgren, 1959a) but is generally absent or rudimentary in adults (Kelly, 1962; Van de Kamer, 1965), including the
primitive ganoid Amiu calva ( Kingsbury, 1897) and the dipnoan Protop
term annectens (U. Holmgren, 1959b). However, a parapineal body
resembling a parietal eye of lizards was described in at least one adult
teleost (Steyn and Webb, 1960). In other groups of fishes, a well-developed parapineal body with distinguishable photosensory-like cells
is found only among the nonmyxinoid cyclostomes ( Oksche, 1965).
However, the same author notes that the problem of the parapineal
organ is not resolved and should be completely reexamined.
B. Structure
1. TOPOGRAPHY AND GENERAL MORPHOLOGY
The pineal body, s e m stricto, is a conspicuous structure in most
adult fishes (U. Holmgren, 1959a); it is, however, rudimentary in
Syngnathus acus and Hippocampus spinosa ( StudniEka, 1896) and absent in Torpedo ocelluta, T . marmorata (Selachii), and the adult Myxine
glutinosa ( Oksche, 1965). Despite contradictov opinions, a pineal organ
is present in the tuna Thynnus thynnus (U. Holmgren, 1958a).
In basic form, the fish pineal organ consists of a hollow, variously
invaginated and well-vascularized structure lying dorsal to the diencephalon to which it is connected by a hollow narrow stalk. The pineal body
consists of an expanded distal vesicular area containing a central
lumen which connects with the third ventricle via the narrow proximal
stalk. Posteriomedially the stalk connects with the subcommissural organ
and the commissura posterior and makes contact with the commissura
habenulae on its anterior side before joining the dorsal sac epithelium
which comes to lie directly ventral to the distal vesicle of the pineal
body. With advancing age, the epithelium of the distal vesicle under-