Here's why:

* Flexibility: Cartilage is a flexible and strong tissue, allowing the embryo to grow and move within the womb.

* Ossification: As the embryo develops, much of the cartilage will be gradually replaced by bone in a process called ossification. This process continues after birth.

While cartilage is the primary component, there are also small amounts of bone present in the embryo's skeleton, particularly in the skull and clavicles. This is because these bones begin to ossify early in development.