This experiment was carried out to investigate the developmental potential of bisected morulae and blastocysts, and the isolated blastomeres from 4-and 8-cell embryos in mouse. The results obtained were summarized as follows: l. The rates of blastomeres survived manipulation were 83.4 and 76.7%, in 4-and 8-cell embryos, respectively. 2. The rates of half embryos survived bisection were 77.7 and 75.3%. in morulae and blastocysts, respectively. 3. The rates of development of isolated blastorneres from 4-and 8-cell embryos were 24.7 and 10,4% respectively. 4. The rates of development of the bisected morulae and blastocysts were 67.7 and 71.9% respectively.