ORGANIC EVOLUTION 301 
days of incubation, gill-slits, or openings into the throat, 
like the gill-openings of lower fishes. These organs belong 
primarily to water life, and are not of direct use to the chick. 
\ vi 
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Fic. 109.—The Gill-clefts of a Shark (upper fig.) Compared with 
Those of the Embryonic Chick (to the left) and Rabbit. 
The heart and the blood-vessels at this stage are also of the 
fish-like type, but this condition does not last long; the gill- 
slits, or gill-clefts, fade away within a few days, and the 
