Caudal neurosecretory system and the neurohemal organ of Tor tor (Ham.).

The Caudal neurosecretory system and the neurohemal organ of a fresh water fish, Tor tor, is described. In this fish the neurosecretory system is well developed which extends upon an stretch of the last 6--7 caudal vertebrae. This region of the spinal cord is characterised by the presence of large Dahlgren cells. These cells send off long processes into a ventrally situated storage release centre, "the neurohemal organ" termed as "urophysis". In Tor tor the urophysis is a distinct, whitish, unpaired and ventral prominence of typical teleostean type. It remains located in a shallow concavity formed at the urostyle and communicates with the spinal cord broadly. Gross morphology of this system provides the evidence that the caudal spinal cord is composed of Dahlgren cells, glial cells, ependymal cells, blood capillaries, ordinary nerve fibres and the central with Reisnner's fibres. The Dahlgren cells are recognized to be of two types--small and large. These cells are polymorphic and usually multinucleated. Polymorphic and lobulated nuclei have also been encountered. Histologically the urophysis can be divided into two regions--an outer cortex and inner medulla. The cortex is composed of epedymal cells, glial cells, connective tissue and the blood capillaries while the medulla consists of the neurosecretory processes of the Dahlgren cells, connective tissue fibres, secretory droplets including large size herring bodies and abundantly present blood capillaries. A few of the herring bodies are also observed with vacuoles, a feature suggesting them to be in the functional state.