170 THE DEVELOPMENT OF THE CHICK 
the anterior part of the head at the stage of 36 somites. During 
the displacement a depression appears in the center of each olfac- 
tory plate, and as this becomes deeper, the olfactory pits are 
formed (Figs. 99 and 117). At the stage of 36 somites each is 
a deep pit situated at the junction of the sides and ventral sur- 
face of the anterior portion of the head, with the wide mouth 
opening outwards and ventrally. 
The olfactory epithelium now becomes sharply differentiated 
from the ectoderm of the head, owing to the formation of a super- 
ficial layer of cells (teloderm, see p. 285) above the columnar cells 
in the ectoderm, but not in the region of the sensory epithelium, 
where the cells still form a single layer. In the center of the 
olfactory pit the epithelium is very much thickened owing to 
elongation of the cells, and the nuclei lie in five or six layers; 
there is a gradual thinning of the epithelium to the lips of the 
pit and then a sudden, but graduated, decrease to the general 
ectoderm. The line of junction of olfactory epithelium and 
indifferent ectoderm of the head is a little distance beyond the 
margin of the pit, as may be determined by the edge of the telo- 
dermice layer; in other words, all of the olfactory epithelium is 
not yet invaginated. 
It is probable that the invagination of the olfactory plates is 
due mostly, up to this time, to the processes of growth within 
the plates themselves, although there has been considerable 
accumulation of mesenchyme in this region. But the subsequent 
deepening of the pits appears to be due largely to the formation 
of processes around the mouths of the primary pits. (See 
Chap. IX.) 
V. Tue ALIMENTARY TRACT AND ITS APPENDAGES 
We have already learned that the main portion of the alimen- 
tary tract arises from the splanchnopleure; a portion of the mouth 
cavity is, however, lined with ectoderm and arises from an inde- 
pendent ectodermal pit, the stomodeum, which communicates 
only secondarily with the entodermal portion; similarly the last 
portion, external to the cloaca, arises from an ectodermal pit, 
the proctodeum, which communicates only secondarily with the 
entodermal part. We shall thus have to consider the origin of 
the stomodeum and the proctodeum in connection with the 
alimentary tract. 
