EMBRYOLOGY AND EVOLUTION 



vertebral column. In all vertebrates the right and the left 

 side of the pharynx — the cavity behind the mouth and nose 

 — are pierced by a series of gill slits, and between these slits 

 run arterial arches, which in fishes and water-breathing 

 amphibians bring blood to the gills to be aerated; but in 

 all air-breathing vertebrates true gills are lacking, though 

 the arterial arches are still present in the embryo, and they 

 undergo characteristic modifications in different classes of 

 vertebrates. This very brief and general statement of the 

 course of embryonic development applies to all vertebrates, 

 man included. 



In all animals development begins with the fertilized egg, 

 which contains none of the structures of the developed ani- 

 mal, though every egg does contain specific kinds of proto- 

 plasm, which differ not only in different phyla and species 

 but even in different individuals of the same species. These 

 different kinds of protoplasm in the germ cells constitute 

 the material basis of inherited differences in mature organ- 

 isms, and the more unlike animals are in their adult state 

 the more unlike is this germinal protoplasm of the egg cells 

 from which they develop. In all organisms the egg contains 

 hereditary units by whose combinations and transformations 

 the characters of the cleavage, blastula, gastrula, embryo, 

 and adult are formed. Development consists in such com- 

 binations and transformations, and just as the chemical com- 

 bination of hydrogen and oxygen to form water gives rise 

 to a substance with new qualities which were not present 

 in the elements that entered into the combination, so new 

 structures and functions develop out of germ cells — struc- 

 tures and functions that were not present as such in the cells. 

 This is "creative synthesis," or "creative evolution," and it is 

 a phenomenon that is found in all nature. 



The development of functions goes hand in hand with the 



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