112 



EARLY EMBRYOLOGY OF THE CHICK 



region by study of entire embryos. If the head and pharyngeal 

 region are cut from the trunk and viewed from the ventral 

 aspect the relations of the structures about the mouth are well 

 shown (Fig. 41). The mandibular arch forms the caudal 

 boundary of the oral depression. Arising on either side in 

 connection with the mandibular arch are paired elevations, the 

 maxillary processes, which grow mesiad and form the cephalo- 

 lateral boundaries of the mouth opening. The nasal pits 

 appear as shallow depressions in the ectoderm of the anterior 

 part of the head which overhangs the mouth region. Surround- 



epiphysif 



lateral telencephalic 

 vesicle 



hyoid arch 

 visceral arch III 



hyomandibular cleft 



visceral cleft II 

 visceral cleft III 



Fig. 41. — Drawing to show the external appearance of the structures in the ora 

 region of a four-day chick. Ventral aspect. 



ing each nasal pit is a U-shaped elevation with its limbs directed 

 toward the oral cavity. The lateral limb of the elevation is the 

 naso-lateral process, and the median limb is the naso-medial 

 process. As development proceeds the two naso-medial proces- 

 ses grow toward the mouth and meet the maxillary pro- 

 cesses which are growing in from either side. The fusion of 

 the two naso-medial processes with each other in the mid-line, 

 and the fusion of each of them laterally with the maxillary 

 process of its own side gives rise to the upper jaw (maxilla). 

 The fusion in the mid-line of the right and left components of 

 the mandibular arch gives rise to the lower jaw (mandible). 



The Appendage Buds. — Both the anterior and posterior ap- 

 pendage-buds have appeared in embryos of three days. They 



