Following fertilisation – the fusion of the ovum and sperm – the fertilised ovum begins to divide into progressively smaller cells (blastomeres); first into two cells, then four cells, eight cells and so on. At a particular stage of division - 16 to 32 cell stage in humans - the ball of dividing cells resembles a mulberry and is called the morula from the Latin term morus meaning mulberry.
The blastomeres of the morula adopt a more flattened shape; the inner side (basolateral region) and outer side (apical region) of these cells begin to differ in appearance. Flattening of blastomeres occurs during the process of compaction and the development of differences between the basolateral and apical regions of blastomeres takes place during the process of polarisation.
Reference:Nikas G, Ao A, Winston RM, Handyside AH: Compaction and Surface Polarity in the Human Embryo In Vitro. Biol Reprod, 1996 55(1):32-7. Go to reference
Morula - Mulberry