Intramembranous ossification begins with differentiation of cells.

Intramembranous ossification begins with differentiation of mesenchymal  cells.

Explanation:

Fibrous membranes are involved in the development of the bones. The process through which this occurs is termed as the intramembranous ossification.

The cells of the mesenchyma begin to split. These cells differentiate into specialized cells each performing different functions. The following bones are formed by the process of intramembranous ossification:

explanation:

the fossil record extends back to a dog-like ancestor some 55 million years ago, which gave rise to the first horse-like species 55 to 42 million years ago in the genus eohippus.