A morula is an embryo at the early stages of development that is made up of cells in a solid ball within the zona pellucida. This stage of development occurs in the first few days after conception. This stage is key to the further proper development of the fetus that is being created.
For an embryo to move into the morula stage, it has to have already developed into an eight- or 16-cell embryo. The number of cells will depend on the species. These multi-celled embryos are called blastomeres. The blastomeres begin to make tight junctions with each other, which causes them to deform. Once they are no longer round, they become a solid mulberry-shaped mass known as the morula. When the embryo changes shape, it is referred to as compaction.
The morula stage leads to the creation of junctional complexes between blastomeres. This gives the embryo an inside section and an outside. This separation leads to the formation of blastocysts, which gathers fluid and expands. This is the point where tests can reveal two different tissues in the embryo. The most important part of the two tissues is the inner cell mass. This mass is what will make up the embryo and fetus. The other tissue is the trophoblast, which contributes to the fetal membrane system.