Our work is concerned with the elucidation of basic principles in the development of higher organisms. The emphasis is on understanding the processes by which cells become committed, i.e., determined, to a particular developmental pathway and the processes that order various determinations in space, i.e., pattern formation. The advantages of applying genetic methodology make Drosophila an organism of choice as a model system for investigating these fundamental processes. We are continuing to investigate the developmental and cellular effects of two temperature-sensitive mutations which cause a broad range of anomalies, including eye defects, abortive gastrulation and other congenital abnormalities. One of the mutations, shibire ts, also causes a reversible paralysis which may be caused by the loss of synaptic vesicles at the neuromuscular junction. We are trying to establish cell lines deficient for the shibire and Notch loci in vitro. The fact that cells can grow in the absence of the shi or Notch gene products but do not maintain their developmental commitments indicates that we are dealing with a level of cell regulation particular to multicellular organisms. The results promise to increase our understanding of basic processes applicable to development and neoplastic growth.