28o 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 



Fig. 67.— (After Duval.) Embryo of sixty-eight hours, in, forebrain, to become 

 optic lobes ; B2, mid brain ; H3, hind brain, to divide into cerebellum and medulla 

 oblongata ; ch, cerebral hemispheres ; E, eye ; ea, internal ear ; g, gill arches, with 

 clefts between ; h, heart ; n, notochord, to pass into backbone ; ml, nasal pit. 



the heart, and do not set to work till respiration begins 

 with the hatching of the bird. The system of breath- 



