HEAD-MUSCLES IN GALLUS AND OTHER SAUROPSIDA. 539 



from the upper part of the myotome; this may be regarded 

 as that portion of the hyoid myotome which retains its primi- 

 tive insertion, whereas the main mass has gained a secondary 

 one. The upper end of the depressor mandibulge is attached 

 to the postero-lateral aspect of the skull; in Chamfeleon 

 and Tropidonotus (text-fig. 35, p. 539) it has additionally an 

 origin from the quadi'ate. 



Myotome of the First Branchial Segment. — The 

 upper part of the first branchial myotome of Gallus dis- 

 appears, whilst the lower part develops into the branchio- 

 maxillaris muscle which extends forwards from the first 

 branchial bar to the lower jaw (text-fig. 16, p. 523). 



j.imx an. ' quad mand 



f> ext orb palp 



Text-fig. 35. — Sagittal section through an older embryo of 

 Chamteleon. (For explanation of lettering see p. 555.) 



The muscle is present in all the groups of the Sauropsida. 

 Its primary condition is that of a muscle — the branchio-hyoid 

 — which connects the first branchial bar with the ceratohyal 

 (text-figs. 25 and 31, pp. 529, 534). This condition is the per- 

 manent one in Sphenodon, and is found as a temporary one in 

 Gallus, Chelone, Agama, Chamaeleon, and Tropidonotus. (The 

 embryo of the Alligator was too far advanced in development 

 for this stage to be seen.) In all these groups the muscle 

 subsequently extends forwards to the lower jaw. This exten- 

 sion forwards is not dependent on loss of the continuity of the 



