530 



F. H. EDGEWORTH. 



tion of a strip to form the depressor palpebrao inferioris in 

 Sphenodon/ Agama, Chelone, and Alligator) either forms a 

 muscle tjiug the pterygoid process to the side of the skull, or 

 is inserted into the (membranous) palato-pterygoid bar or 

 atrophies. An insertion into a membrane bone must be a 

 secondary feature. Reptiles must therefore be descended 

 from forms which, like Selachians and Birds, had a movable 

 ptery go-quadrate. Birds preserved this feature, and with it 

 the primitive insertion of the upper part of the mandibular 

 myotome, whereas in Reptiles the pterygo-quadrate became 



retractoc. 



pty an. 



Text-fig. 27. — Transverse section tlirongh an emliryo of Sphe- 

 nodon, in "stage P" of Howes and Swinnerton. The upper part 

 of tlie section is more anterior than the lower. (For explanation 

 of the lettering see p. 555.) 



fixed, and the upper part of the myotome can be seen, during 

 development, undergoing secondary modifications. 



In Alligator (text-figs. 21, 22, p. 527) and Chelone the upper 

 part of the myotome atrophies without ever having had an 

 insertion into the palato-pterygoid bar ; the pterygoid process, 

 either in continuity with the quadrate (Alligator) or after 



^ The depressor palpebrse inferioris of Sphenodon is inserted into the 

 reflected angle of the conjunctival sac (text-fig. 29, p. 532), that of Chelone 

 and Allisrator into both lids. 



