MUSCLES OF CERATODUe 



339 



In an embryo of 13-5 mm. (Text-tig. 15) the ventral end of 

 the first branchial muscle-plate has separated and grown 

 forwards to the Ceratohyal, forming the Branchio-hyoideus. 

 The rest of the muscle-plate persists. The lower end of the 

 second branchial muscle-plate has become detached from the 

 pericardial wall and grown inward, forming the Transversus 

 ventralis ii, and is detached from the vertical strip above. 



Text-fig. 14. 



Z brouvch 

 tn.pi. 



'otobr ii 



coracoh^'. 



peruccwcUuMi 



Embryo 12 mm., transverse section. 



The development in the third and fourth arches is similar 

 to that in the second. 



In an embryo of 16 mm. (Text-figs. 18-20) the first four 

 branchial bars have separated into Epi- and Cerato-branchial 

 elements and have chondrified, the process being most complete 

 in the first. The upper end of the first branchial muscle-plate 

 has separated into an inner and outer portion — the inner is 

 the Levator and is inserted into the Epi- and Cerato-branchial i, 

 the outer is the upper end of the Constrictor branchialis, which 

 is continued down through the arch to its lower end. In the 

 second branchial arch separation into these two constituents 

 is not complete, and in the third and fourth branchial arches 

 has barely begun, owing to the progressive retardation in 

 development from before backwards. In a 17-5 mm. embryo 



