Following the tearing of the oropharyngeal membrane in the young embryo, the ectoderm-covered stomodeum, which gets surrounded by the mandibular prominences, goes directly over into the endoderm-covered foregut. It is also called a primary oral cavity because at this time no nasal cavity yet exists. (For the forming of the exterior nose, see facial development).
Only through the formation of the nasal sacs are the primary palates (= floors of the two nasal sacs) formed that divide the primary oral cavity into oral and nasal cavities. The wall between the two nasal sacs is called the nasal septum.
Using the two interactive diagrams the relationships can be studied in detail.