Morula: An embryo with 8-16 blastomeres, resembling a solid ball, formed in the fallopian tube.
Blastocyst: A ball of 200-300 cells with a fluid-filled cavity (blastocoel), formed from the morula.
Gastrula: The embryonic stage where cells differentiate into three germ layers (endoderm, ectoderm, mesoderm) and position themselves in the embryo.