70 



EMBRYOLOGY 





maxillary and nasal processes in the roof of the stomodeum the primitive palate 

 (Fig. 49) is formed, and the olfactory pits extend backward above it. The pos- 

 terior end of each pit is closed by an epithelial membrane, the bucco-nasal membrane, 

 formed by the apposition of the nasal and stomodeal epithelium. By the rupture 

 of these membranes the primitive choanse or openings between the olfactory pits 

 and the stomodeum are established. The floor of the nasal cavity is completed 

 by the development of a pair of shelf-like palatine processes which extend medial- 

 ward from the maxillary processes (Figs. 50 and 51); these coalesce with each 

 other in the middle line, and constitute the entire palate, except a small part in 

 front which is formed by the premaxillary bones. Two apertures persist for a time 

 between the palatine processes and the premaxillse and represent the permanent 

 channels which in the lower animals connect the nose and mouth. The union of 

 the parts which form the palate commences in front, the premaxillary and palatine 

 processes joining in the eighth week, while the region of the future hard palate 



Lateral part of. ^ 



nasal capsule 



Inferior concha 



Inferior meatus 



Vomeronasal vj^:'at- 

 cartilage 



Palatine process 



Cartilage of 

 nasal septum 



Vomeronasal 

 organ of Jacobson 



Inferior meatus 



Cavity of mouth 



FIG. 51. Frontal section of nasal cavities of a human embryo 28 mm. long. (Kollmann.) 



is completed by the ninth, and that of the soft palate by the eleventh week. By 

 the completion of the palate the permanent choanse are formed and are situated a 

 considerable distance behind the primitive choanse. The deformity known as 

 cleft palate results from a non-union of the palatine processes, and that of hare- 

 lip through a non-union of the maxillary and globular processes (see page 199). 

 The nasal cavity becomes divided by a vertical septum, which extends downward 

 and backward from the medial nasal process and nasal laminae, and unites below 

 with the palatine processes. Into this septum a plate of cartilage extends from 

 the under aspect of the ethmoid plate of the chodrocranium. The anterior part 

 of this cartilaginous plate persists as the septal cartilage of the nose and the medial 

 crus of the alar cartilage, but the posterior and upper parts are replaced by the 

 vomer and perpendicular plate of the ethmoid. On either side of the nasal septum, 

 at its lower and anterior part, the ectoderm is invaginated to form a blind pouch 

 or diverticulum, which extends backward and upward into the nasal septum and 

 is supported by a curved plate of cartilage. These pouches form the rudiments of 



