THE ORGANS OP THE OUTER GERM-LAYER. 519 



2. At a later stage the pits are united with the angle of the oral 

 cavity by means of the nasal grooves. 



3. The inner and outer margins of the olfactory pits and the nasal 

 grooves project out as ridges and constitute the inner and outer nasal 



4. By fusion of the margins of the nasal grooves the organ of 

 smell is converted into two nasal passages, which open out on the 

 frontal process by means of the external nares and on the roof of 

 the primitive oral cavity a little back of the upper lip by means of 

 the internal nares. 



5. The internal nares afterwards become fissure-like and move 

 nearer together, owing to the nasal septum becoming thinner and 

 growing downward somewhat into the primitive oral cavity. 



6. The upper part of the primitive oral cavity shares in the forma 

 tion of the organ of smell and serves for the increase of its re- 

 spiratory region, since horizontal ridges (the palatal processes) grow 

 inward from the maxillary processes toward the lower margin of the 

 nasal septum, with which they fuse, and produce the hard and soft 



7. In the organ of smell a further enlargement of the spaces 

 serving for respiratory purposes is produced by 



(a) The formation of folds of its mucous membrane, by which 



the turbinated processes arise ; 



(b) Evaginations of its mucous membrane into the adjacent 



parts of the cartilaginous and bony cephalic skeleton 

 (formation of the " cells " in the cribriform plate, the 

 frontal and sphenoidal sinuses, and the antrum of 

 HIGHMORE). 



8. In human embryos there is early formed in the olfactory pit 

 a special depression of the outer germ-layer as fundament of the 

 organ of JACOBSON, which receives a special branch of the olfactory 

 nerve. 



9. JACOBSON'S organ comes to lie at the base of the nasal septum 

 remote from the olfactory region. 



10. The ducts of STENSON in many Mammals and the canales 

 incisivi in Man are preserved as remnants of the so-called palatal 

 fissures the original fissure-like communications between nasal 

 cavities arid secondary oral cavity. 



