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- 

 dermic 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. THE 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 stomodceum, which communicates 

 only secondarily with the entodermal portion; similarly the last 

 portion, external to the cloaca, arises from an ectodermal pit, 

 the proctodceum, which communicates only secondarily with the 

 entodermal part. We shall thus have to consider the origin of 

 the stomodseum and the proctodseum in connection with the 

 alimentary tract. 



