. The development of the chick; an introduction to embryology . ^pits and dividing the openings in two parts, one within the oralcavity, which becomes the internal nares or choanae, and onewithout, which becomes the external nares or nostrils. Duringthe same time ihe whole naso-frontal process begins to project 216 THE DEVELOPMExXT OF THE CHICK forward to form the ti]) of the u])i)er jaw. The two mandibulararches have also fused in the middle line and begin to projectforward to form the lower jaw. This projection of upper andlower jaw causes a great increase in the depth of the oral cavity(Fig
. The development of the chick; an introduction to embryology . ^pits and dividing the openings in two parts, one within the oralcavity, which becomes the internal nares or choanae, and onewithout, which becomes the external nares or nostrils. Duringthe same time ihe whole naso-frontal process begins to project 216 THE DEVELOPMExXT OF THE CHICK forward to form the ti]) of the u])i)er jaw. The two mandibulararches have also fused in the middle line and begin to projectforward to form the lower jaw. This projection of upper andlower 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. Fig. 126. — Head of an embryo of about 5 daysfrom the oral surface. (N. L. 8 mm.)ch. F., Choroid fissure. E. L., Eye-hd (nic-titating membrane), ex. nar., External Gr., Lachrymal groove. Other abbreviationsas before. the two lateral parts formed from the maxillary processes. Theformer becomes the intermaxillary and the latter the maxillary region. II. Embryonic Membranes General. The extension of the blastoderm over the surfaceof the yolk goes on very rapidly \ip to the end of the fourtli dayof incubation (Fig. 33). at which time there is left a small cir-cumscribed area of uncovered yolk, that may be called theumbilicus of the yolk-sac. which remains uncovered for a longtime. Its final closure is associated witii the foi-mation of thealbumen-sac. EMBRYO AND EMBRYONIC MEMBRANES 217 The splitting of the mesoblast is never complete; but on thecontrary the undivided margin begins to thicken after the fourthday, and gradually forms a ring of connective tissue that surroundsthe
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