204 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. 
