The morula is an early stage in the development of an embryo. After the sperm and egg unite to form the zygote within the fallopian tube, the cells of the zygote continue to replicate through the process known asmitosis. Thecilia lining the fallopian tube continue to beat, moving the developing mass of cells along towards the uterus. When development reaches the 16-cell stage, it is referred to as a morula. This morula is the form which reaches the uterus. The morula is a solid ball of cells, with no fluid or hollow interior. The change in the interior of the ball of cells occurs during the next stage of development, called the blastocyst stage.