Development of the Third Day 187 



the superficial epithelium once more be- 

 comes continuous. 



The nose. The olfactory organs begin, in 

 the early part of the third day, as two 

 thickenings of epithelium on the under 

 side of the fore part of the head. These 

 thickened patches soon become pushed in 

 to form pits, the olfactory pits, and the 

 olfactory nerves very early fuse with the 

 inner walls of the pits. The olfactory pits 

 are formed in the same way as are the 

 auditory pits and the pits that form the lens 

 vesicles of the eyes, but while the lens and 

 auditory pits become closed completely, the 

 olfactory pits remain permanently open to 

 the exterior as the external nares or nostrils 

 (Fig. 64, OK). 



The epithelial lining of the olfactory pit 

 becomes folded and wrinkled to form the 

 sensory epithelium of the nose. The pos- 

 terior nares, or the opening of the nose into 

 the back part of the mouth, is a distinct 

 formation, and appears, about the begin- 

 ning of the fourth day, as a groove leading 

 from the nasal pit to the outer and ante- 

 rior angle of the stomodseum. This groove, 

 lying between the fronto-nasal process (the 



