ORGANIC EVOLUTION 



363 



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. 



Fig. 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 



