ANATOMY

Detailed explanation-1: -The primitive mouth is now the stomodeum (or stomatodeum), which initially appeared as a shallow depression in the embryonic surface ectoderm at the cephalic end before the fourth week (see Figures 4-1 and 4-2). At this time, the stomodeum is limited in depth by the oropharyngeal membrane.

Detailed explanation-2: -The mesenchymal primordia of the face form about the stomodeum (primitive mouth) and include the frontonasal prominence, maxillary prominences, and the mandibular prominences.

Detailed explanation-3: -Because the stomodeum primarily forms on the ventral side of the head, is rather deep, and the foregut lining reaches the anterior notochord dorsally, the oral membrane has a large extent (Fig.

Detailed explanation-4: -Initially, stomodeal cavity and nasal pits are separated by an oronasal membrane. Subse-quently, the oronasal membrane degenerates and oral and nasal cavities communicate freely. Eventu-ally a secondary palate develops, shifting oral-nasal communication caudally into the pharynx.

Detailed explanation-5: -The stomodeum is lined by ectoderm, and is separated from the anterior end of the fore-gut by the buccopharyngeal membrane.