A blastocyst is an early-stage embryo that resembles a fluid-filled ball, with cells forming the outer wall and an inner cell mass like a bunch of grapes attached to it on the inside.
Stem Cells in Skeletal Tissue Engineering: Technologies and Models. Langhans MT, Yu S and Tuan RS. Curr Stem Cell Res Ther. 2016; 11(6): 453–474.