It is a small cone-shaped gland (8 mm in length, 4mm in breadth) attached by a hallow stalk to the roof of the dorsal surface of the diencephalon. Pineal gland secretes melatonin hormone. In frog, this hormone aggregates melanin pigments present in melanophores. Environmental lighting affects melatonin production. It has been seen that if a mammal or a bird is exposed to environmental lighting for a greater period of time, melatonin production and its release into the blood are retarded.
On the other hand, if the animals are kept in darkness for most of the time. Melatonin production is increased. If melatonin hormone is injected into the body, it leads to reduction in the weight of the gonad (testis and ovary). Melatonin has been found to inhibit the growth of testis and ovary.