524 PATTEN— COOPERATION AS A FACTOR IN EVOLUTION. 



sitional stage between the typical arthropod and typical vertebrate 

 jaws. 



pynx.< . 



Fig. 6. Development of the jaws of the frog. The figures show the 

 presence of three pairs of appendage-Hke oral lobes of the arthropod type, 

 their migration from the neural to the haemal side of the head, and their 

 union to form the transverse unpaired upper and lower jaws of the verte- 

 brate type. From Patten, " The Evolution of the Vertebrates and their Kin." 



Evidence of a similar condition is seen in man, for one of the 

 chief events in the embryonic growth of his face is the concrescence 



