<prompt> Stage of 16 cells isEmbryoMorulaZygoteBlastocyst  <response>: A morula is distinct from a blastocyst in that a morula 3-4 days post feilization is a 16-cell mass in a spherical shape whereas a blastocyst 4-5 days post feilization has a cavity inside the zona pellucida along with an inner cell mass 