550 TEXT-BOOK OF EMBRYOLOGY. 



nasal sacs are not at first in communication with the oral cavity, but lie above, 

 and are separated from it by a plate of tissue which gradually becomes thinned 

 out along the deeper part of the sacs to form the bucco-nasal membrane (Hoch- 

 stetter). Later (in embryos of 15 mm.), the bucco-nasal membrane ruptures 

 and the deep ends of the sacs thus come to open into the mouth cavity, the 

 openings being known as the primitive choanen. In front of the primitive 

 choanen, the nasal passages (formerly the nasal sacs) are separated from 

 the mouth cavity by a plate of tissue, known as the primitive palate (Fig. 471). 

 The latter is produced by the fusion of the maxillary process with the lateral 

 and medial nasal processes (see p. 121), the outer nares thus being somewhat 

 separated from the border of the mouth. 



The further separation of the nasal passages from the o/al cavity has been 

 described in connection with the development of the mouth (p. 286) and the 



Lateral nasal process 



Outer nasal opening 



Maxillary process 



Eye 



Primitive choanen 



Palatine process 



FIG. 471. From a model of the anterior part of the head of a 15 mm. human embryo. The lower 

 jaws (mandibular processes) have been removed. Peter, 



development of the palatine processes of the maxillae. It may be repeated 

 briefly, however, that from each maxillary process a horizontal extension grows 

 across between the oral and nasal cavities until it meets and fuses with its fellow 

 of the opposite side and with the nasal septum in the medial line, thus forming 

 the palate which is continuous with the primitive palate mentioned above. 

 (See Figs. 140 and 47 2.) In this way the nasal cavities or chambers become 

 separated from the oral cavity, but remain in communication with the pharyn- 

 geal cavity through the posterior nares. 



The nasal cavities increase enormously in size and the epithelial surface in 

 extent, owing to (i) the formation of the palate alluded to above, (2) the develop- 

 ment of the nasal concha which has been described on page 161, and (3) the 

 development of accessory cavities maxillary, frontal and sphenoidal sinuses, 

 which represent evaginations, so to speak, from the nasal cavities. 



Probably correlated with the above-mentioned increase in extent of the 

 nasal chambers is the fact that in lung-breathing Vertebrates the chambers 



