The work deals with the description of and genetic control of development in Polysphondylium pallidum, a eucaryotic protozoan organism which exhibits integrated multicellular development. The experiments are of three sorts: 1. A more detailed description of changes in labeling of specific proteins during the aggregative phase of development will be made. This will utilize autoradiography of two-dimensional polyacrylamide gels. 2. The hypothesis that the order of the observed protein labeling changes is controlled by a chain reaction of causes (a "causal sequence") will be tested. Experiments here will utilize temperature sensitive developmental mutants, double mutant haploid strains derived from two different developmental mutants, and chemical agents which specifically affect development but not growth. 3. The regulation of particular labeling changes will be studied by genetic investigation of groups of mutants which appear to block a single step of the causal sequences. Parasexual techniques will be used to test complementation betweeen pairs of strains within such a group, and to assign them to linkage groups. The map positions of mutations on the same linkage group will be determimed by means of macrocyst crosses.