Products of Cleavage (Morula and blastula)
In several cases, the blastomeres in early cleaving stages tend to presume spherical shape like that of the egg earlier than cleavage. Even though their mutual pressure (resulting from limited available space in the egg envelopes) flattens the surface of blastomeres in contact with each other, the free surfaces of each blastomere stay spherical. The result of it is that after some cleavage divisions have taken place the embryo has a shape that resembles mulberry. Due to this superficial resemblance this stage of embryonic development of many animals has been considered to as Morula (Latin for mulberry).
As cleavage divisions carry on the subsequent arrangement of the blastomeres in the morula may change in different groups of animals. In some the blastomeres are packed together with no any space among them, or a small cavity appears but is soon obliterated. In both cases the result is the creation of a solid blastula called stereoblastula example some flatworms, annelids, molluscs and coelenterates. In such type of blastulae some of the blastomeres lie externally and others in the interior. But, in several animals the cavity appearing among the blastomeres persists and may enlarge. This cavity is termed as blastocoel. As the cleavage progresses, the adhesion of the blastomeres to one other increases and they arrange themselves as an epithelial layer around the blastocoel. This stage of embryonic development is termed as blastula and such blastula is called coeloblastula (hollow blastula). The layer of blastomeres is considered to as blastoderm.