280 THE STRUCTURE AND LIFE OF BIRDS cHap. 
incubation, but not through its gill arches, nor through 
the lungs, which are not for some time connected with 
Fic. 67.—(After Duval.) Embryo of sixty-eight hours, 81, forebrain, to become 
optic lobes; B2, mid brain; B3, hind brain, to divide into cerebellum and medulla 
ablongatas cH, cerebral hemispheres ; E, eye; HA, internal ear ; G, gill arches, with 
clefts between ; , heart ; nN, notochord, to pass into backbone; NL, nasal pit. 
the heart, and do not set to work till respiration begins 
with the hatching of the bird. The system of breath- 
