Introduction

Abstract

As the neural tube develops it is modified locally along the neuraxis. These regional differences reflect the future organization of the CNS. In the fetal rat the anterior part of the neural tube is closed on the tenth day of gestation (E10), and develops into the forebrain (Witschi 1962). Subsequently, the telencephalon expands and forms the hemispheres. The latter, enclosing the fluid-filled ventricles, are joined in the dorsal midline by a layer of cells which is called the telencephalon medium or the telencephalic roof plate. In developing mammals, the roof-plate consists of two parts: an anterior, thicker portion, the area or lamina terminalis, and a posterior, thinner portion, the area or lamina choroidea (Bailey 1916; Warren 1917). The transition point between the two portions is called the angulus terminalis (Hines 1922). The area choroidea extends in a posterior direction from the angulus terminalis to the velum transversum, which separates the telencephalon from the diencephalon.