MORPHOLOGY OF CEANIAL MUSCLES IN SOME VERTEBRATES. 189 



The myotome of the mandibular segment of Rana separates 

 from the lateral half of the iiitermantlibularis in 5 mm. 

 embryos; it extends backwards in 7 mm. embryos, dividing 

 into internal and external portions (Text-fig. 58). The myo- 

 tome thus comes to lie in a nearly horizontal position internal 

 to the muscles developed in the hyoid segment. The internal 

 portion develops into the pterygoid muscle, the external into 

 the temporal, sub-temporal, extra-temporal, and masseter 



Text-fig. 17. 



— .tlt.mArv. 



sJUc. "-^-^.A-. ^^^'^'-^ 





17. 



Scyllium, embryo 30 mm., longitudinal vertical section. 



(Text-figs. 59, 60). The masseter and extra-temporal ai-ise 

 from the internal surface of the processus muscularis of the 

 palatoquadrate bar. The anterior end of the pterygoid shifts 

 outwards beneath the anterior ends of the other muscles and is 

 inserted into the outer end of Meckel's cai-tilage. The tem- 

 poral is inserted into the inner end of Meckel's cartilage ; the 

 masseter is inserted into Meckel's cartilage a little distance 

 from its outer end ; the subtemporal is inserted, by two 

 tendons, into Meckel's cartilage and the superior labial car- 

 tilage ; the extra-temporal divides into two j)ortions, one of 



