Morula

The morula (Latin for "mulberry") is a transitional stage between the cleavage (day 2–3) and blastocyst (day 5–6) stages of embryo development. By day 4, the embryo has undergone compaction — its cells flatten and tighten together — forming a solid sphere of 16–32 blastomeres. This compaction is necessary for the next step: cavitation, when fluid accumulates inside to create the blastocyst cavity.

In IVF laboratories, embryologists observe morula compaction as a positive sign that the embryo is progressing normally. Embryos that fail to compact or cavitate typically arrest and are not transferred. Morula-stage transfers (day 4) are rarely performed today, as extended culture to day 5–6 allows better embryo selection.