524 



F. H. EDGEWORTH. 



The cells forming the somites are at first undifferentiated, 

 then those lying centrally become converted into the myo- 

 tomes, whilst those lying peripherally become part of the 

 general mesoblast. The lower ends of the myotomes are 

 continuous with the epithelium lining the ventral cephalic 

 ccelom (text-fig. 7, p. 515). In the case of the mandibular 

 and hyoid segments the loAver ends of the myotomes become 

 continuous with the mylohyoids, which are formed from the 

 epithelium lining the obliterated portions of the ventral 

 cephalic coelom in those segments (text-fig. 1 , p. 515). The 

 lower ends of the first and second branchial segments separate 

 from the epithelium lining the ventral cephalic coelom. 



The formation of the myotomes of the head of Gallus is 



thyrhy. hrycart. 



Text-fig. 17. — Sagittal section through an embryo of Gallus at end 

 of eighth day. (For explanation of lettering see p. 555.) 



thus similar to that of Scyllium. The difference is that the 

 head cavities in Gallus are not nearly so marked as in 

 Scyllium. In Gallus, too, some of the general mesoblast is 

 proliferated from the epithelium (from both splanchnic and 

 somatic layers) before the formation of the head somites 

 (text-fig. 1, p. 512). 



In Gallus four myotomes are formed — those in the mandi- 

 bular, hyoid, first and second branchial segments (text-fig. 4, 

 p. 514) ; and there are similarly four in Lacerta.^ In reference 

 to this point, it should be remarked that Corning, following 

 V. Wijhe's views, described them as '' Seitenplatten," i. e. as 

 visceral structures. As, however, I have shown that Balfour's 



' Corn in JIT. 



