92
JAMES CLARKE FENWICK
it is a specialized functional organ. Its location on the top of the head
orients the pineal organ directly toward the primary source of light and
thereby effectively eliminates the pineal body as a receptor of a visual
field. But this orientation renders the pineal well suited for detecting
changes in light intensity or for acting as a dosimeter of incident radiation. In other words, the pineal of fishes probably never was, in the
true sense, an eye. Nevertheless, numerous investigators have shown
that the pineal organ of the fishes does have a light receptive function.
The problem then is not the presence or absence of a function, but
rather, a question of the precise function or functions.
Although several of the current hypotheses are not mutually exclusive,
they can, for present purposes, be divided into two categories-one
emphasizing primarily a sensory role and the other a secretory function.
Of the sensory theories, the following are the most credible: (1) The
pineal body is a photosensory structure, ( 2 ) it acts as a baro- or chemoreceptor for the cerebrospinal fluid (CSF), or, ( 3 ) it performs the function of a mediator in the olfactory induced response to exohormones.
Conversely, ( 4 ) the pineal may be primarily a gland of external secretion related to the chemical composition of the CSF or the metabolism
of the brain tissue, or (5) it may be a gland of internal secretion with
an endocrine function.
It is not proposed here to review all of the literature concerning these
various hypotheses but to provide a framework into which the accumulated knowledge of pineal function in the fishes can be channeled.
111. ONTOGENY AND STRUCTURE OF THE PINEAL ORGAN
The description of pineal ontogeny, morphology, and histology
given here may be amplified from detailed accounts and bibliographies
by Hill (1891, 1894), Terry (1910), Tilney and Warren (1919), Holmgren (195813, 1959a,b), Oksche ( 1965), Van de Kamer ( 1965), and
Rudeberg ( 1966).
A. Ontogeny
Much of the polemic surrounding the pineal organ of the fishes
originates from the prevailing ignorance of its embryological origin.
This has given rise to inconsistent nomenclature and a failure to distinguish clearly the pineal body from other epithalamic outgrowths.
In vertebrates, two approximately middorsal evaginations develop
JAMES CLARKE FENWICK
it is a specialized functional organ. Its location on the top of the head
orients the pineal organ directly toward the primary source of light and
thereby effectively eliminates the pineal body as a receptor of a visual
field. But this orientation renders the pineal well suited for detecting
changes in light intensity or for acting as a dosimeter of incident radiation. In other words, the pineal of fishes probably never was, in the
true sense, an eye. Nevertheless, numerous investigators have shown
that the pineal organ of the fishes does have a light receptive function.
The problem then is not the presence or absence of a function, but
rather, a question of the precise function or functions.
Although several of the current hypotheses are not mutually exclusive,
they can, for present purposes, be divided into two categories-one
emphasizing primarily a sensory role and the other a secretory function.
Of the sensory theories, the following are the most credible: (1) The
pineal body is a photosensory structure, ( 2 ) it acts as a baro- or chemoreceptor for the cerebrospinal fluid (CSF), or, ( 3 ) it performs the function of a mediator in the olfactory induced response to exohormones.
Conversely, ( 4 ) the pineal may be primarily a gland of external secretion related to the chemical composition of the CSF or the metabolism
of the brain tissue, or (5) it may be a gland of internal secretion with
an endocrine function.
It is not proposed here to review all of the literature concerning these
various hypotheses but to provide a framework into which the accumulated knowledge of pineal function in the fishes can be channeled.
111. ONTOGENY AND STRUCTURE OF THE PINEAL ORGAN
The description of pineal ontogeny, morphology, and histology
given here may be amplified from detailed accounts and bibliographies
by Hill (1891, 1894), Terry (1910), Tilney and Warren (1919), Holmgren (195813, 1959a,b), Oksche ( 1965), Van de Kamer ( 1965), and
Rudeberg ( 1966).
A. Ontogeny
Much of the polemic surrounding the pineal organ of the fishes
originates from the prevailing ignorance of its embryological origin.
This has given rise to inconsistent nomenclature and a failure to distinguish clearly the pineal body from other epithalamic outgrowths.
In vertebrates, two approximately middorsal evaginations develop