Prologue: Why Study Vertebrates? 



CUNG ANLAGE 

 PHARYNGEAL POUCHES 

 MYELENCEPHALON 



METENCEPH 



Fig. 4. A diagram of a 1.2 mm. human embryo showing five aortic arches. Why 

 does the human embryo have six aortic arches of which only three persist in the 

 adult, unless man's ancestors had six functional arches? (After His. Courtesy, 

 Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



SOMITE 2 SOM 3 

 I y i VII 

 ffl 



MIDGUT HINDGUJ 



SOMITE A ', 



SOMITE I HEART C<- 



PERICARDIAL CAVITY 



NEURENTERIC CANAL' 



Fig. 5. A 7 mm. Squalus embryo viewed as a cleared specimen from the left 

 side. The yolk-sac has been mostly removed. Two gill-chambers are open. Develop- 

 ing cranial nerves are indicated by Roman numerals. (Courtesy, Neal and Rand: 

 "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



theories which have exerted powerful influence on human thought have 

 been derived largely from the relative fullness of our knowledge of the 

 vertebrates. 



Facts of vertebrate comparative anatomy provided important build- 

 ing material for the foundations of the theory of evolution as it began 

 to acquire definite form in the latter part of the eighteenth century. 

 Later the theory was strengthened and elaborated by the results of 

 comparative study of the embryonic development of vertebrates, as 

 illustrated by the following instance. At an early stage in the develop- 

 ment of the embryo of a reptile, bird, or mammal (Fig. 4), the anterior 

 region of the digestive tube produces paired lateral pouches similar to 

 those which, in a fish embryo (Fig. 5), become gill-chambers. Blood- 

 vessels and other structures in this region of the embryo of a land verte- 

 brate are distinctly fishlike in their anatomic relations. In later de- 

 velopment this region undergoes a profound metamorphosis and loses 



