Development.—About the middle of the first month of intra-uterine life the prosencephalon bends acutely forward over the end of the notochord and sends out from its base a series of processes, which ultimately blend to form the face ([Fig. 231]). These processes surround a stellate depression, the primitive buccal cavity or stomatodæum, from which the mouth and nasal cavities are developed. The buccal cavity is bounded above by the fronto-nasal process, which is divided by a fissure—the nasal cleft or olfactory pit—into a lateral nasal process, and a mesial nasal process, at the outer angle of which a spheroidal elevation appears—the globular process.
Fig. 231.—Head of human embryo about 29 days old, showing the division of the lower part of the mesial frontal process into the two globular processes, the intervention of the nasal clefts between the mesial and lateral nasal processes, and the approximation of the maxillary and lateral nasal processes, which, however, are separated by the nasal-orbital cleft. (After His.)
From the mesial nasal and globular processes the septum of the nose, the mesial segment of the premaxillary bone, and the middle portion of the upper lip are developed; while the lateral nasal process forms the roof of the nasal cavity, the ala nasi and adjacent portion of the cheek, and the lateral segment of the os incisivum or premaxillary bone. Each segment of the os incisivum carries one of the incisor teeth, and each of the mesial segments may contain in addition an accessory tooth. The nasal cleft ultimately becomes the anterior nares.
The primitive buccal cavity is bounded below by the mandibular arch, which contains Meckel's cartilage, and from which are developed the mandible, the lower lip, and the floor of the mouth.
From the lateral and back part of the mandibular arch springs the maxillary process, which grows upwards and blends with the lateral nasal process across the naso-orbital cleft—the deeper portion of which persists as the nasal duct. From the maxillary process are developed the cheeks, certain of the facial bones, the lateral portions of the upper lip, the soft and hard palate (with the exception of the os incisivum). The development of the face is completed about the end of the second month of intra-uterine life.
Hare-lip and Cleft Palate
Hare-lip is a congenital notch or fissure in the substance of the upper lip, and cleft palate a congenital defect in the roof of the mouth. Either of these conditions may exist alone, but they occur so frequently in combination that it is convenient to consider them together.
In hare-lip the cleft may be median or lateral, and it may or may not be associated with a cleft in the palate. The resemblance to the Y-shaped cleft in the upper lip of the hare, suggested by the name, is in most cases only superficial.