28o 



ELEMENTARY BIOLOGY 



329. Transformation. When we compare a chick as it comes 

 out of the shell with the contents of the eggshell before hatch- 

 ing begins, we cannot conceive that the little speck on the side 

 of the yolk has become the chick, with its many organs and 

 its many kinds of cells with their many peculiar functions. 

 And yet all that comes out of the eggshell must have been 

 there at the beginning of the incubation period. We are so 

 familiar, however, with the fact that chicks come from eggs, 



that we are content to ac- 

 cept the changes that go 

 on inside, on the supposi- 

 tion that, since they are 

 so gradual, everything is 

 possible. 



A study of the develop- 

 ment of insects will give 

 us an idea of how sharply 

 limited the stages in an 

 individual's life may be. 

 When a locust or a cock- 

 roach comes out of the 

 egg, it is very much like the parent, except that it is very 

 small and lacks wings (Fig. 115, I{b^, dj). By a series of 

 violtings the animal advances not only in the matter of size 

 but in the development of the wings and other organs. 



When the egs; of a moth or of a butterflv hatches out, the 

 young animal that emerges is not at all like the parent ; it 

 looks more like a worm (Fig. 117, b, b). It has no wings; 

 its mouth has biting jaws that work sideways ; its coloring 

 is different. Indeed, if we did not know that it came from 

 the egg of a butterfly, or that it would in time become a 

 butterfly itself, we should never suspect, from its appearance, 

 that it had anything to do with butterflies. We may well 

 believe that during all the months of outward inaction some- 

 thing was going on inside the case of the pupa, just as 



Fig. 116. Molting cicada 



In many jointed-legged animals (Arthropoda) 

 the growth takes place at intervals between 

 molts. The hard outer skeleton breaks open 

 and the soft-skinned animal crawls out. After a 

 while the shell hardens and growth stops again 



