HEAD-MUSCLES IN GALLUS AND OTHER SAUKOPSIDA. 523 



develop from the walls of each lateral vesicle before its 

 cavity is obliterated. 



The muscles developed from the premaiidibular segment 

 are exceedingly uniform throughout the Sauropsida — a rectus 

 superior, rectus inferior, rectus interuus, and obliquus inferior. 

 In Birds and Crocodilia a levator palpebrte superioris is also 

 developed from the rectus superior. 



An orbicularis palpebrarum is developed in Birds, Spheno- 



sup 



Text-fig. 16. — Sagittal section through an embryo of Gallus at end 

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



don, Lacertilia vera, and Rhiptoglossa — probably from the 

 mesoblast of the mandibular segment. 



The myotomes of the head in Gallus are developed from 

 the epithelium lining the dorsal cephalic coelom on either side 

 of the forepart of the alimentary canal (text-figs. 1 and 2, 

 pp. 512, 513). The layers disappear at the sites of the gill- 

 pouches, but thicken in the intervening portions, and form the 

 somites of the head — the corresponding head cavities (dorsal 

 cephalic ccelom) disappearing (text-fig. 3, p. 513). 



