142 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



inclosed space is in one chamber, there being no 

 separation between the mouth and nose cavities. 

 The formation of a palate, however, effects a separa- 

 tion between the two. The rudiments of the palate 

 appear as shelf-like projections from the inner or 

 oral surface of the upper jaw. A triangular piece 

 grows backward from the globular protuberance of 

 the nasofrontal process, which ultimately unites 

 with horizontal or palatal plates from the maxillary 

 arch. In the eighth week of embryonic life, union of 

 the palatal plates begins at their anterior extremi- 

 ties and proceeds backward. A deficiency in the 

 union constitutes the deformity of cleft palate. 

 Cleft palate is therefore, embryologically, a later 

 development than harelip. Either may occur with- 

 out the other, but they are usually found together. 

 The cleft of the palate usually turns to one side, 

 passing out between the cuspid and lateral incisor 

 teeth. A double cleft palate is Y-shaped, the 

 center piece in front containing the incisors, and 

 representing the anterior triangular piece of the 

 rudimentary palate, this piece having failed to unite 

 with the lateral palatine plates. This deficiency 

 may involve the hard or the soft palate, or it may 

 affect both, and even produce a cleft or bifid uvula. 



The completion of the palate definitely separates 

 the nasal chambers from the mouth, the only com- 

 munication between the two being through the 

 posterior nares. The permanent limitations of the 

 mouth are thus established from a cavity that 

 develops primarily as an ectodermal invagination. 

 The ectoderm invests not only the mouth proper, 



