The development of the flat bones of the skull primarily occurs through intramembranous ossification.

In this process, mesenchymal stem cells within a membrane-like layer differentiate directly into osteoblasts, which then begin to lay down bone matrix. Unlike endochondral ossification, which involves a cartilage intermediate, intramembranous ossification bypasses the cartilage stage altogether. This type of bone formation is essential for producing the flat bones of the skull, the mandible, and the clavicles.

Key points:
- Intramembranous ossification involves direct differentiation of mesenchymal cells into bone without a cartilage model.
- It is responsible for the formation of flat bones of the skull.
- Endochondral ossification, on the other hand, requires a cartilage precursor and forms most other bones, such as long bones.
- Perichondral ossification is related to the development of the bone collar around cartilage models and is a part of endochondral ossification.
- Metaplastic ossification refers to the formation of bone in unusual locations, such as in certain soft tissues, and is not involved in normal skull bone development.

Reference: Gray's Anatomy, 41st Edition, Volume 1, Chapter: The Skeleton/Development of the Skeleton, Pages 390-395