A temperature sensitive (Ts) maternal-effect mutation has recently been discovered in the axolotl (Ambystoma mexicanum). At 25 degrees the eggs of mutant females cleave normally but stop developing in late blastula stage. At 10 degrees the eggs develop into larvae and eventually adults. At intermediate temperatures some eggs are arrested in blastula and gastrula stages; the remainder develop into abnormal post-neurula embryos. The temperature sensitive period in the development of mutant eggs appears to occur remarkably early - probably between cleavages 3 and 6. Research on this mutant, proposed in this application, includes: 1) A descriptive study of mutant embryos developing at permissive and restrictive temperatures, the purpose of this study being to determine exactly the stages of developent at which the maternal effect is expressed, and to define the alterations from normal patterns of morphogenesis exhibited by the mutants. 2) The cytology of the mutant embryos. We are particularly interested in the structure of the mitotic apparatus and the behavior of chromosomes during and following the temperature sensitive period. The reason for this interest is that there are indications that chromosome behavior during mitosis may be grossly abnormal in mutants developing at 25 degrees. 3) Nuclear transfers between mutant and normal embryos and eggs, to determine whether nuclei undergo heritable changes in response to mutant (or normal) cytoplasm during the temperature sensitive period.