morula, solid mass of blastomeres resulting from a number of cleavages of a zygote, or fertilized egg. Its name derives from its resemblance to a mulberry (Latin: morum). A morula is usually produced in those species the eggs of which contain little yolk and, consequently, undergo complete cleavage. Those blastomeres on the surface of the morula give rise to extra-embryonic parts of the embryo. The cells of the interior, the inner cell mass, develop into the embryo proper.
In humans, the morula is composed of 60 or more cells. As the number of cells in a morula increases, the zygote develops in a blastocyst, a hollow bubblelike structure, which eventually becomes implanted in the uterine lining.