1. Properties of Botryllus schlosseri which give it outstanding promise for studies in developmental genetics are reviewed. 2. Laboratory culture procedures, in vitro fertilization, and a method for raising embryos in vitro are described. Controlled successions of complete life cycles can now be achieved in any laboratory. 3. Experiments involving colony fusion, subsequent vascular budding, and the analysis of color patterns in resultant systems suggest that cells of the simple vessel walls govern the morphology of the regenerated zooids. 4. Results of some preliminary genetic crosses are reported.