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 (8. s’0.) 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 choanz 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 
