MORPHOLOGY OF CRANIAL MUSCLES IX SOME VERTEBRATES. 169 



a segmented part above, the somite, which subsequently 

 separates from the dorsal edg-e of the coelom and develops 

 into myotome and sclerotome. 



In the bead the conditions in the mandibular and hyoid 

 segments are different from those in the branchial segments; 

 in the latter the lateral plates of mesoderm are at first con- 

 tinuous ventrally with the wall of the pericardium or cephalic 

 coelom^ (Text-fig. 2) ; in the mandibular and hyoid segments 



Text-fig. 1.^ 



Text-fig. 2. 



•M 



I ex OJiX- O-V 



^ ae^k cxxL 



U'^f'. cad 



Text-figs. 1 and 2. — Scyllium, embryo 7 mm., transverse sections. 

 Text-fig. 1 is the more anterior. 



the lateral plates of mesoderm are continuous ventrally with 

 the walls of the mandibular and hyoid sections of the cephalic 

 coelom. These differ from the branchial sections in that no 

 communication from side to side takes place (Text-fig. 1). 

 In the rabbit, however (Text-fig. 76), the cephalic coelom is 

 continuous from side to side in the mandibular and hyoid 

 segments, just as it is in the branchial segments. 



1 The latter name is perhajos in-eferable, as i^robalily the heart origin- 

 ally lay behind the branchial region. 



- For reference letters on Text-figures see p. 314. 



