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THE POPULAR SCIENCE MONTHLY 



fertilized egg which contains none of the structures of the developed 

 animal, though it may exhibit the polarity and symmetry of the adult 

 and may also contain specific kinds of protoplasm which will give rise to 

 specific tissues or organs of the adult. From this egg cell arise by divi- 

 sion many cells which differ from one another more and more as develop- 

 ment proceeds, until finally the adult animal results. A specific type of 

 development is due to a specific organization of the germ cells with 

 which development begins, but the earlier differentiations of the egg are 

 relatively few and simple as compared with the bewildering complexities 

 of the adult, and the best way of understanding adult structures is to 

 trace them back in development to their simpler beginnings and to study 

 them in the process of becoming. 



7. Development of Functions. — The development of functions goes 

 hand in hand with the development of structures ; indeed function and 

 structure are merely different aspects of one and the same thing, namely 

 organization. All the general functions of living things are present in 







Fig. 16. A, human embryo of forty-two somites ; ages about twenty-one days. B, 

 embryo of about four weeks. C, still older embryo showing the beginnings of the for- 

 mation of digits. jF, embryo of about two months. (After Keibel.) 



