214 THE DEVELOPMENT OF THE CHICK 
their supporting and skeletal structures and accessory parts: 
(a) the central nervous system, (6) the organs of special sense, 
and (ce) the visceral organs, mouth and pharynx. The origin of 
all these parts has been considered, and it is proposed to take 
Kia. 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 cur present 
purposes, except in the ease of the facial region. At four days 
this region appears as follows (Fig. 125): the mouth is a large, 
‘-defined opening, bounded behind by the mandibular arches, 
