The anterolateral portion (squama) arises from the mesenchyme at 8 weeks of embryogenesis while the petromastoid portion develops later from a cartilaginous center at 6 months of fetal development.
This passage describes the origins of two distinct anatomical structures in fetal development. The anterolateral portion (squama) develops early, originating from mesenchyme at 8 weeks gestation. The petromastoid portion develops later, arising from a cartilaginous center at 6 months of fetal development.