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G. RYLANDS DE BEER 



the somites lying anterior to them, and it will be shown that 

 this is the only view which avoids weighty assumptions and 

 discrepancies. 



As development proceeds the interpretation becomes more 

 difficult, and for two reasons : 



1. The fourth and fifth somites lose their distinctness and 

 the fourth breaks up unrecognizably into mesenchyme. This 

 is possibly due to the pressure of the auditory sac, which appears 



Text-fig. 5, 



The somite beneath the auditory capsule. 



between the facial and glossopharyngeal nerves, overlying the 

 fourth somite. As the sac extends bac"kwards the fifth somite 

 also begins to break up, though some remnants of its muscle- 

 fibres persist. 



2. The somites appear in later stages to be situated more 

 posteriorly with regard to the gill-slits. This is due partly to 

 the development of the latter, which push them backwards, 

 and partly to the fact that owing to the slight curvature which 

 the head undergoes, the line of mesodermic somites finds itself 

 situated on the outer side of the circumference of this curvature. 

 Since the centre region of the head mesoderm (somites 4 and 5) 

 is broken down into mesenchyme, the more anterior somites 3 



