DEFECTIVE NEURULATION IN FROG EMBRYOS EXPOSED TO DILUTE SEA-WATER

Embryos of Microhyla ornata were exposed to dilute sea water from earl y gastrula stage onwards for a period of 48 h. The process of neurulat ion was studied in control and treated embryos by using optical and sc anning electron microscopy. In the treated embryos, the neural folds f ormed normally in the initial period of exposure and subsequently appr oached each other. However, they failed to fuse mid-dorsally in the ce phalic region, In the posterior half of the embryos, the neural folds fused to form the neural tube. Irrespective of this, the treated embry os continued differentiation of the brain, as was evident from the dev elopment of the eyes. Failure of ectodermal cells to cover the neural cells may be related to the dramatic surface modifications induced due to high concentration of cations like Na+.