216 THE DEVELOPMENT OF THE CHICK 
forward to form the tip of the upper jaw. The two mandibular 
arches have also fused in the middle line and begin to project 
forward to form the lower jaw. This projection of upper and 
lower jaw causes a great increase in the depth of the oral cavity 
(Fig. 147). 
The upper jaw is thus composed of three independent parts: 
viz., the median part formed from the naso-frontal process and 
Fic. 126. — Head of an embryo of about 5 days 
from the oral surface. (N. L. 8 mm.) 
ch. F., Choroid fissure. E. L., Eye-lid (nie- 
titating membrane). ex. nar., External nares. 
1. Gr., Lachrymal groove. Other abbreviations 
as before. 
the two lateral parts formed from the maxillary processes. The 
former becomes the intermaxillary and the latter the maxillary 
region. 
Il. Empryronic MEMBRANES 
General. The extension of the blastoderm over the surface 
of the yolk goes on very rapidly up to the end of the fourth day 
of incubation (Fig. 33), at which time there is left a small cir- 
cumseribed area of uncovered yolk, that may be called the 
umbilicus of the yolk-sac, which remains uncovered for a long 
time. Its final closure is associated with the formation of the 
albumen-sac. 
