HEAD-MUSCLES IN GALLUS AND OTHER SAUROPSIDA. 535 



(test-fig. 15, p. 522). The outer part divides into tlie 

 quadrato-maxillaris which keeps its primitive origin from the 

 pterygoid process, and the temporal which grows up to gain 

 an origin from the skull. The inner part divides into the 

 external pterygoid Avhich keeps its pterygoid-quadrate origin 

 (text-fig. 16, p. 523), and the internal pterygoid which arises 

 from the hind end of the palato-pterygoid bone (text-fig. 16, 

 p. 523). 



The lower part of the mandibular myotome in Reptiles 

 similarly divides into inner and outer portions — pterygoid 

 and temporal (text-fig. 27, p. 530). The development of the 

 outer portion is much the same as in Birds. It is at first a 

 muscle passing from the pterygoid process to the lower jaw 

 (text-fig. 24, p. 528) ; subsequently, whilst retaining an 

 attachment to the quadrate, it also extends up to the side of 

 the skull. The part, however, which retains its original 

 upper attachment is not differentiated as a separate muscle 

 (quadrato-maxillaris) lying internal to the temporal as in 

 Birds. In Chama3leon the development is a little excep- 

 tional : the outer portion separates into a temporal (which 

 becomes digastric in condition) and a quadrato-mandibularis 

 which arises from the front of the quadrate (text-fig. 35, 

 p. 539) ; the position of the latter muscle, below and not inter- 

 nal to the temporal, suggests that it is not homologous with 

 the quadrato-maxillaris of Birds, but is merely the lower part 

 of the temporal — that part which does not become digastric. 



The development of the internal, pterygoid portion is very 

 diverse in the various groups of the Reptilia. The simplest 

 condition is present in Chelone, where there is during 

 development a single undivided pterygoid muscle with two 

 heads, one (inner) attached to the hind end of the palato- 

 pterygoid bar, and the other (outer) attached to the pterygoid 

 process of the quadrate. These two heads extend upwards, 

 the former gaining an additional origin from the descending 

 plate of the parietal, the latter an additional one from the 

 prootic. The muscle does not become divided into two bellies. 



In Sphenodon and Agama the pterygoid Anlage becomes 



