2O4 THE MEANING OF EVOLUTION 



tities of fat. It will be remembered that upon breaking 

 a hen's egg and dropping it into a bowl, the yolk 

 holds together because it is enclosed in a delicate sac. 

 As the yolk falls into the bowl there floats to the 

 top of it a lighter yellow spot as big as the end of a 

 lead pencil. This is all of the egg which thus far 

 represents the chick itself. All the rest is nourish- 

 ment. This disk already consists of three reasonably 

 distinguishable layers of cells, which grow rapidly 

 different from each other. They spread and bend 

 and twist, forming the young chick and a set of or- 

 gans which serve for its protection and maintenance 

 during its embryonic life. Within a few days these 

 accessory organs will have formed distinctly. Within 

 the upper half of the yolk will be found the small 

 developing chick, which for the first thirty-six hours 

 of its development passes through a stage not unlike 

 the fish, or the earlier steps of the turtle. Within 

 a few days it becomes clearly evident that this crea- 

 ture is to be a bird, though it is much longer before 

 it is clearly a chick. 



This embryo is so soft that it is almost like curd 

 in thickened milk, and could be very easily destroyed 

 were it not for a protective device which Nature has 

 employed. It seems necessary that it should be pro- 

 tected with the utmost care. The matter will be 

 better understood if we recall a common experience. 



