Intramembranous ossification is a process of bone formation that produces flat bones such as the parietal bones. From what I gather, it forms inside the mesenchymal tissue and uses mesenchymal cells as stem cells to produce osteoblasts.
The part I don't fully understand is, what is mesenchymal tissue? I know it's in embryos, but where is it on the embryo and what shape does it have? Is it in the shape of a parietal bone already and then the bone just develops out of it?