HEAD-MUSOLES IN GALLUS AND OTHER SAUEOPSIDA. 541 



coelom gradually retreat from the head. The mandibular and 

 hyoid mylohyoid muscles are formed from the obliterated 

 sections of the ventral cephalic coelom in those segments. 



In early stages of Gall us the cephalic section of the ccelom 

 similarly exists in the mandibular and succeeding segments 

 (text-fig. d, p. 513), but its relations to the vascular system are 

 a little different. The heart is situated more anteriorly, lying 

 in part in the cephalic and in part in the body portion of the 

 coelom; the truncus arteriosus is in the first branchial seg- 

 ment, and there is a very short ventral aorta between the 

 origins of the hyoid and mandibular aortic arches, the three 



Text-fig. 36. — Sagittal section through an embryo of Tropido- 

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



branchial aortic arches being subsequently given off from the 

 posterior end of the aorta. A little later the ventral aorta, 

 mandibular, and hyoid, aortic arches disappear, the ventral end 

 of the hyoid aortic arch being left as the ventral carotid 

 artery. The mylohyoids are formed, as in Scyllium, from 

 the walls of the obliterated portions of the ventral cephalic 

 coelom in the mandibular and hyoid segments. The subsequent 

 retreat of the heart and coelom from the head are also exactly 

 parallel events to those occurring in Scyllium. The condition 

 of the heart, truncus arteriosus, and ventral aorta in the early 

 embryo of Grallus is evidently a secondary one. 



