286 THE DEVELOPMENT OF THE CHICK 



the lumen, the superior and middle from the olfactory division 

 proper, and the inferior from the vestibulum; on the middle 

 turbinal, however, the sensory epithelium gradually flattens out 

 to the indifferent type. The middle turbinal appears first in 

 the ventral part of the olfactory division, about the beginning 

 of the fifth day, and the superior somewhat later, immediately 

 above the former, the two being separated by a deep groove 

 (Fig. 165). The vestibular turbinal arises still later, and is well 

 formed on the eighth day. 



Fig. 166 shows a reconstruction of the nasal cavity, seen from 

 the lateral side, of an embryo of about seven days. It is a re- 

 construction of the epithelium, and thus practically a mold of the 

 cavity; therefore projections into the cavity appear as depressions 

 in the model, and the grooves and outgrowths of the external 

 wall as projections. The superior turbinal has an oval shape with 

 the long axis in an apical direction; it is bounded by a fairly deep 

 depression, the elevated margin of the model, from the lower end 

 of which the supra-orbital sinus (S. s'o.) passes off ventrally and 

 externally. The deep depression immediately below the superior 

 turbinal lodges the median turbinal. A fairly long passage leads 

 off from its neighborhood to the choanae and a shorter one, the 

 vestibulum, to the external nares. The depression in the wall of 

 the vestibulum is caused by the vestibular or inferior turbinal. 

 The palatine and maxillary sinuses are not yet formed. 



The external nares are closed during the greater part of the 

 period of incubation by apposition of their walls. The form 

 and dimensions of the nasal cavities change greatly during incu- 

 bation, owing to shifting in the original positions of the turbinals, 

 outgrowth of the facial region, and development of sinuses. The 

 details are not very well investigated, and an examination of 

 them would lead too far. 



There has been a good deal of discussion as to the existence 

 of an organ of Jacobson in the nose of birds; it has usually been 

 assumed that it is entirely absent even in the embryo. Others 

 have identified the ducts of nasal glands as a modification of this 

 organ. Recently, however, Cohn has described a slight evagi- 

 nation in the median wall of the primary olfactory pit, that 

 agrees precisely in its form and relationship with the first rudi- 

 ment of the organ of Jacobson in reptiles. Although it persists 

 only from the stage of about 5.3 mm. to about the stage of 



