Primary polyembryony:
Primary polyembryony refers to the process in which multiple embryos are formed from a single zygote or an earlier stage of development by cleavage.

Cleavage:
Cleavage is a series of rapid cell divisions that occur after fertilization. During cleavage, the zygote undergoes multiple rounds of mitosis, resulting in the formation of smaller cells called blastomeres. These blastomeres continue to divide and form a solid ball of cells called a morula.

Formation of multiple embryos:
In some organisms, such as certain insects, reptiles, and plants, the cleavage process can give rise to multiple embryos. This phenomenon is known as cleavage polyembryony.

Process of cleavage polyembryony:
1. In cleavage polyembryony, the zygote undergoes cleavage but instead of forming a single embryo, it divides into two or more units.
2. Each unit formed during cleavage retains the ability to develop into a complete individual.
3. These units are called embryonic rudiments or embryos.
4. The embryos formed through cleavage polyembryony can be identical or genetically different depending on the species.
5. In some cases, the embryos may remain connected to each other and share a common placenta, while in others, they may separate and develop independently.

Difference between primary and secondary polyembryony:
Primary polyembryony refers to the formation of multiple embryos from a single zygote or an earlier stage of development. On the other hand, secondary polyembryony involves the formation of multiple embryos from cells of the maternal tissue surrounding the embryo.

Spontaneous polyembryony:
Spontaneous polyembryony is a term used to describe the formation of multiple embryos in certain plants without any external intervention or manipulation.

In conclusion, the cleavage of the zygote or an earlier stage of development into two or more units to form multiple embryos is known as cleavage polyembryony. This process is different from both primary and secondary polyembryony. Cleavage polyembryony can occur naturally in certain organisms, leading to the development of genetically identical or different embryos.