GROWING UP - 123 



cells as different as nerve cells, skin cells, bone 



cells, blood cells, and many others. 

 The young of the higher plants and animals have a long, 

 difficult, and wonderful road to travel before they come to 

 be like their parents. It is "natural" for them to do it, 

 and we fully expect it to happen, but it is a marvel, none 

 the less. 



4. Direct Developmient. The tgg of no one of the 

 higher organisms looks in the least like the parent. If we 

 take the chick, for example, the egg when fertilized has 

 no resemblance to the chicken. Yet when the chicken 

 hatches, in twenty-one days, the young chicken looks very 

 much like the adult. It still needs to grow, and to 

 differentiate a few things, like feathers, etc., before it is 

 just like the parent. Now if we go back in the egg to the 

 eighteenth or nineteenth day it still looked like a chick, 

 but a little less so. By studying it thus we can find that 

 it came very gradually to take on its likeness to the 

 parent. There- are no sudden or sharp changes in its 

 development. We call this sort of development direct. 

 This is a very common method of development. It is 

 found in mammals, in birds, in reptiles, in most fishes, and 

 in many lower forms. 



5. Metamorphosis. Not all animals, however, develop 

 in this way. We have already seen in the frog that the 

 thing that hatches out of the egg does not look at all like 

 a frog. It must make more changes after hatching than 

 the chick does. This tadpole, which looks more like a fish 

 than like a frog, may grow as a tadpole and live an 

 independent life for quite a period before it begins the 

 changes that make it appear as a frog. We see it as three 

 different things: egg, tadpole, and frog. This indirect 

 development is called a metamorphosis. It differs from 

 the direct development in this: in a metamorphosis the 

 organism becomes like the adult by passing through one 

 or more somewhat sharp or sudden changes or stages. 



