The term 'KIND OF SAC' can be answered by the 8-letter word 'AMNIOTIC'. An amniotic sac is a fluid-filled membrane that surrounds and protects a developing embryo in the uterus. It is filled with amniotic fluid, which helps to cushion the fetus and protect it from injury. The amniotic sac also helps to regulate the temperature of the fetus and provide nutrients. The amniotic sac is an essential part of the development of a healthy baby.