216 THE DEVELOPMExXT 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. 148). 



The upper jaw is thus composed of three independent parts: 

 vdz., the median part formed from the naso-frontal process and 



■ — .-^.*>»** 



\ 



Fig. 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 (nic- 

 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. 



II. Embryonic 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- 

 cumscribed 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. 



