Recent studies have identified pathways in the developing visual system which, as development proceeds, either disappear completely or become diminished in their area of origin or termination. In this proposal, we will investigate how some of these transient projections develop and the mechanisms whereby they are maintained or lost. Attention will be directed in part to the development of the corpus callosum of rats and tree shrews. We are interested to know how the projection becomes focussed in development at the 17/18(a) border and why after manipulations such as optic nerve or tract section, the pathway shows an abnormal distribution. It is hoped furthermore that by concentrating on the development of the 17/18 border region, it will be possible to approach the important issue of how the heterogeneities characteristic of the mature cortex are determined in development. Finally we hope by placing transplanted tissue along the course of pathways which normally disappear to maintain them, addiding credence to the view that the presence of an appropriate target region is an important requirement for preserving a projection. From these experiments, we hope to gain insight into factors which influence how the precise connections, forming the substrates of normal visual function, are established. In addition we hope to define certain conditions under which the normal developmental process may be disturbed.
