MOEPHOLOGY OP CRANIAL MUSCLES IN SOME VERTEBRATES. 231 



beliind the premandibnlar Aiilage and internal to the man- 

 dibular myotome (Text-fig. 79). In 2i mm. embryos it 

 apparently is represented by a group of cells lying above 

 the first gill-cleft and continuous behind with cells lying in 

 the hyoid segment — cells which in 3 mm. embryos have diffe- 

 rentiated into myotome and sui'rounding mesoblast. It is 

 probable, therefore, that the abdncensAnlage is formed from 

 the mesoblast of the hyoid segment before the myotome is 



Text-fig. 57. 



IIUJ. 



57. 



Rana. embryo 6 mm., longitudinal vertical section. 



formed. The abducens Anlage begins to separate into rectus 

 externus and retractor oculi in 13 mm. embryos (Text-fig. 96). 

 These observations suggest that perhaps in pig embryos of 

 a younger age than those investigated by Renter a similar 

 series of events may occur. In the later stages of develop- 

 ment there is also a difference between the pig* and the 

 rabbit ; in the former, according' to Renter, the retractor oculi 

 is formed from all four recti, in the latter it is formed from 

 the abducens Anlage only, as is usual in Vertebrates. 



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