214 



THE DEVELOPMENT OF THE CHICK 



their supporting and skeletal structures and accessory parts: 

 (a) the central nervous system, {h) the organs of special sense, 

 and (c) the visceral organs, mouth and pharynx. The origin of 

 all these parts has been considered, and it is proposed to take 



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Fig. 124. — Embryo of 10 days and 2 hours x 5. (After Keibel and Abraham.) 



up here only the development of the external form of the head. 

 The preceding section gives an account sufficient for our present 

 purposes, except in the case of the facial region. At four days 

 this region appears as follows (Fig. 125) : the mouth is a large, 

 ill-defined opening, bounded behind by the mandibular arches, 



