DEVELOPMENT OF THE ORGANISM 



ancestor and diverged from one another owing to their adop- 

 tion of different habits; and, strangest of all, that backboned 

 animals, the class to which we ourselves belong, diverged 

 from the same root as the starfish, sea-urchins, and sea 

 cucumbers, which constitute the class Echinodermata. 



Comparative anatomy and systematic 2o51ogy take us only 

 a little way, for we have no reason to assume that the ances- 

 tral forms of animals have persisted unchanged to the present 

 day. The evidence from fossils is best, but fossils preserve 

 only the hard parts, and the earliest fossils thus far found 

 are already far advanced in evolution. But every animal 

 begins its development in the egg, which is a single cell, 

 comparable in structure to the lowest forms of life known 

 to us, and as it grows to the adult form it sketches in broad 

 outlines the whole story of its evolution. 



REFERENCES 



There are unfortunately no short, comprehensive books on 

 embryology, and the reader who wishes to pursue the subject fur- 

 ther must have recourse to large treatises, in which it is dealt with 

 in detail. For the development of the invertebrates we recommend 

 Textbook of Embryology (vol. 1), Invertebrates, by E. W. Mac- 

 Bride ; Macmillan & Co. For the development of the vertebrata we 

 recommend The Embryology of the Vertebrata (this book does not 

 include the frog) and The Frog, both by A. Milnes Marshall; for 

 The Embryology of the Chick, Frank R. Lillie. Text books of 

 embryology by Clement Heisler and by Bailey and Miller deal with 

 the embryology of man. 



[61] 



