Localization and pattern of the mitotic activity in the central nervous system of developing oviparous and viviparous teleosts.

The subject of this work is an analysis of the mitotic activity in the developing spinal cord and rhombencephalon of the oviparous teleost Jordanella floridae and the viviparous teleost Gambusia affinis (Cyprinodontoidei). In Gambusia there is considerable and long lasting mitotic activity that decreases gradually and is still present 20 days after birth. In Jordanella the mitotic activity is concentrated in the early stages of development, it then decreases drastically and practically disappears even before hatching and reappears 20 days later. These differences are probably due to the variations in speed and pattern of development in the two teleosts. As regard the localization of mitoses, it emerged that migration towards the lumen may be prevented by physiological conditions related to normal development. It emerged moreover, that migration towards the lumen is not a generalised phenomenon in development of the central nervous system since it is related, in the spinal cord of teleosts at least, to the modality and speed of embryo development.