The face and neck (see Fig. 10), are partly formed by the subcranial and branchial arches, which variously unite with each other and with the frontonasal process. In all these there are possibilities of non-union, and so we have various forms of clefts. The incomplete closure may arise by abnormal protrusion of the viscera preventing closure in front, the cause of such protrusion being in some cases dropsical accumulations, more especially in the case of the thorax and abdomen; but it may also be due to interference, by adhesion, or otherwise, with the amnion or allantois. The most extreme case is that in which the fronto-nasal and the superior maxillary processes are defective, and the face presents in its middle part a large opening, which to a greater or less extent replaces nose and upper jaw, recalling the embryonic condition shown in Fig. 10, and in some cases also involves orbits and lower jaw. This condition is called Aprosopus or Schisto-prosopus.

Less degrees of it are shown in Cleft palate and Harelip. The fronto-nasal process forms the central part of the upper lip and of the alveolar process of the upper jaw. Hence, in these parts, the line of union is on either side of the middle line, while in the palate it is mesial. The cleft in the lip and alveolar process is therefore lateral and that in the palate mesial. All degrees of non-union exist, from complete fissure of the palate, with cleft of the alveolus and lip on each side, to the slightest notch on one side of the upper lip.

Head of embryo.

Fig. 10. - Head of embryo. From Quain (after His.) f. n. p., fronto-nasal process, pr. glob., globular extremity of same. /. n. pr., lateral nasal prosess. to. x., maxillary process, to. n., mandibular arch.

These- defects date back to the third month of foetal life, at which period the closure ought to occur.