I am giving here a very simplified version of the various stages of embryonic development of human and several other animals. The purpose is to clarify the concept, give only the basic main points and to differentiate between the three stages.
Morula
1. This is a solid mass of cells (blastomeres).
2. It is formed 3 days after fertilisation
3. It has 16 cells. All cells are totipotent.
4. Its later stages have 32 cells and 64 cells.
5. The outer cells form extra-embryonic parts and the inner cells form the future embryo proper.
Blastula
1. This is formed 4-5 days after fertilisation.
2. This is a hollow sphere of cells with a cavity inside.
3. The cavity is called blastocoel.
4. Blastocoel has uterine fluid inside.
5. Its later stage is called blastocyst with inner mass and outer trophoblast.
Gastrula
1. It is an early multi-cellular embryo.
2. It is formed after implantation.
3. It is multi-layered. It has three germinal layers.
4. It has an outer cavity called blastocoel.
5. There is also a central cavity called archenteron.
