ORGANS OF SPECIAL SENSE 285 
nerve grow into its ventral wall exclusively, between its epithelial 
cells, which gradually become disarranged and irregular. Thus 
the ventral wall becomes increasingly thick and the lumen excen- 
tric. By the sixth day the lumen appears in cross-section as a 
narrow lenticular space with an epithelial roof, above the large 
optic nerve. Soon after, the lumen disappears entirely; no trace 
of its former existence is to be found on the eighth day. 
Il. THE DEVELOPMENT OF THE OLFACTORY ORGAN 
The origin of the olfactory pit, external and internal nares, and 
olfactory nerve, has already been considered (pp. 169,215, and 263). 
Before the formation of the internal and external nares, not only 
has the entire olfactory epithelium become invaginated, but, owing 
to the elevation of internal and external nasal processes, the pit 
has become so deepened that the margin of the olfactory epithe- 
lium proper now lies a considerable distance within the cavity. 
That part of the nasal cavity thus lined with indifferent epithelium 
is known as the olfactory vestibule. After the fusion of the 
internal nasal process with the external nasal and maxillary 
processes, the cavity deepens still more. 
The choane lie at first just within the oral cavity, but the 
palatine processes of the maxillary process, growing inwards 
across the primitive oral cavity (pp. 298, 299), unite on the sixth 
or seventh day at their anterior ends with the internal nasal 
processes, and thus cut off an upper division of the primitive 
oral cavity at its anterior end from the remainder; in this way 
the internal openings of the nasal cavities into the oral cavity 
are carried back of the primitive choan; they are henceforward 
known as the secondary choane. Further growth of the palatine 
processes brings them nearly together in the middle line along 
the remainder of their length, about the eleventh day; but fusion 
does not take place, the birds possessing a split palate. Thus 
the superior division of the primitive oral cavity is added to the 
respiratory part of the nasal passages. 
The nasal cavity is further elaborated between the fourth 
and eighth days by ingrowths from the lateral wall (turbinals) 
and by the formation of the supraorbital sinus as an evagination 
that grows outwards above the orbit. Three turbinals are formed 
in the nasal cavities, viz., the superior, middle, and inferior tur- 
binals. These arise as folds of the lateral wall projecting into 
