The outer upper part of the tip of the beak is provided with a short, stout, spike-like arrangement, called the _egg tooth_. On the twentieth day this part of the beak is forced against the wall of the egg and gradually breaks through the egg shell. The breathing by the lungs commences some time before hatching; this is evidenced by the chick within the shell giving chirping sounds. [Illustration: FIG. 83. _A._ Longitudinal section of chick, 4 days incubation. 1, Cephalic fold. 2, Caudal fold. 3, True amniotic cavity. 4, Epiblast. 5, Somatic mesoblast. 6, Visceral mesoblast. 7, Hypoblast. 8, Future anua still closed. 9. The allantoic vessicle. 10, The mesentery. 11, Intestine. 12 and 13, Yolk sac. 14, Cavity of true amnion. 15, The mouth. 16, Pleuroperitoneal cavity. 17, Fore gut. _B._ Membranes of the chick at the third day of incubation. 1, Membrane surrounding albumin. 2, Amnion. 3, Allantois. 4, Vitellicle. _C._ The fore part of an embryo chick at the second day. 1, Mesencephalon. 2, The eye. 3, Olfactory organ. 4, Mandibular arch. 5, Maxillary arch. 6, Maxillary arch. 7, Hyoidean arch. 8, Scapular arch. 9, Depression organ of hearing. 11, Process from encephalon for union with nerve of hearing. _D._ Primitive blood-vessels at second day of incubation (Owen). 1, Mesocephalon. 2, The right anterior longitudinal vein. 3, The eye. 4, The mandibular arch. 5, The hyoidean arch. 6, Scapular arch. 7, The same. 8, The ventricular portion of the heart. 9, The right transverse vitelline vein. 10, Posterior longitudinal vitelline vein. 11, Vertebral vein. 12, Tributaries of same. 14, The auricle. 16, Aortic bulb. _E._, A transverse section of chick embryo, 29 hours incubation. 1, Neural canal. 2, Neural crest. 3, Somatopleure. 4, Splanchnopleure. 5, Omphalomesenteric vein. 6, Aorta. 7, Notocord. 8, Pleuroperitoneal cavity. _F._ Chick embryo at the ninth day of incubation. 1, Allantois. 2, Amnion. 3, Air cell at large end of the egg. 4, Egg shell. 5, Outer shell membrane. 6, Inner shell membrane. 7, Yolk sac. 8, Albumin (Bradley). ]