112 THE NEXT GENERATION 



became two ; the two became four ; the four, eight. Then 

 there were sixteen, thirty-two, sixty-four — the total number 

 doubling about once every hour. And this is the way every 

 embryo begins to grow. 



But, strange to say, although, when born, each different 

 kind of animal is to look so different from all others, this 

 difference does not show itself in the earliest stages of the 

 different embryos. On the contrary, among vertebrates, dif- 

 ferent embryos in their earliest stages look so much alike 

 that often even an experienced biologist is hardly able to dis- 

 tinguish fish from salamander or rabbit from man. 



For days and weeks, however, the multiplication of cells 

 goes on steadily, never ceasing, and even before birth the 

 time comes at last when each creature declares what it is by 

 its shape of body, legs, and head. 



In the meantime the animal must have nourishment if it 

 is to grow, and nature supplies it. Animals in eggshells get 

 their first food from the yolk and then from the white' of 

 egg that surrounds the yolk. But when the yolk supply is ex- 

 hausted, and when the white part is consumed by the cells as 

 they multiply, then it is that the expanding chick has to strike 

 for freedom and for a new environment. The time has come 

 when he must have more food and more room to grow in. 

 He finds neither the one nor the other within his eggshell. 



Twenty-one days after the hen begins to sit on her eggs, 

 or an incubator begins to warm them, some instinct within the 

 chick tells it to peck at its shell and make its way out into the 

 world. The instinct is obeyed, and out steps a damp little 

 creature, as perfect as a chick can ever be. Those three 

 weeks have been long enough for multiplying cells to turn 

 themselves into head and feet and claws, down and muscle, 

 brain and nervous system, and every other minutest part of 



