THE MARVEL OF GROWTH 1 13 



the marvelous structure that toddles about on its two feet and 

 begins to peck at bits of straw and kernels of grain. 



Other animals also have their rate for rapid growing. 

 But before going farther, four or five statements must be made. 



1 . Vertebrates are either oviparous, that is, developed from 

 an egg and hatched outside the body of the parent, or vivip- 

 arous, developed' within the body of the parent and born alive. 



2. All mammals, except the duckbills of Australia, are 

 viviparous. 



3. Whether it is oviparous or viviparous, any developing 

 creature is called an embryo until it is hatched or born, as 

 the case may be. 



4. Before birth oviparous animals receive their nourish- 

 ment from food which is stored within the eggshell, while 

 viviparous animals receive their prenatal nourishment from 

 the mother herself. It reaches them through a tubelike cord 

 which joins the embryo to its parent. At the outset this cord 

 is a mere thread, but it grows stronger and stouter as the cells 

 of the embryo multiply and as the developing creature grows 

 heavier and larger from day to day. 



5. It takes twenty-one days for a chick to develop in its 

 shell. A duck develops in four weeks, a guinea pig in twenty- 

 one days, rabbits and squirrels in thirty days, while for cats 

 it is sixty-five days, for dogs sixty-two, for a lion three months, 

 for a pig four months, for sheep and goats five months, for 

 a bear six months, for a cow nine months, for a human baby 

 nine months, for a whale ten months, for a giraffe fourteen 

 months, and for an elephant twenty-one months. 



Notice that in general the time needed for development 

 is in proportion to the size which the developing creature is 

 to attain in the end. Yet in every case the starting point is 

 always the same — no more than two germ cells for either 



