Divided into two lobes. Homologue to C-cells of mammal thyroid. Found that inactive testes in winter could be activated by exposure to long daylength. Eyes are not essential for photoperiodic response. Eyeless birds respond to changing photoperiod in the same fashion as untreated birds.
Response appears to be mediated by hypothalamic receptors.
In some manner, gonadotropins from anterior pituitary are blocked. Prolactin may be involved. Short days of winter somehow act to reset photostimulatory phase. Circadian Cycles in photosensitivity relative to the amount of daylength present may also be important.