464 



O. RYLANDS DE BEER 



2, and 1 (which will be drawn off into the service of the eyeball), 

 acquire a more anterior position. 



Similarly the more posterior somites, 6, 7, &c., move relatively 

 backwards. By measuring somites in the region of the fifth 

 and sixth, it can be seen that they are stretched and occupy 

 more space along the long axis of the embryo than the remainder. 



Text-figs, 7, 8. 



F ^p Gi 



Text-fig. 7. — Embryo 8 mm. long. Ventral roots are present from 



the seventh somite backwards. 

 Text-fig. 8. — Embryo 10 mm. long. The tenth segment is the 



most anterior to develop a fully- formed mixed nerve. The vagus 



is represented as truncated to reveal the ventral ]Jortion of the 



sixth somite, which lies median to it. 



Text-fig. 6 shows a number of embryos in successive stages of 

 development, all drawn to the same scale. The corresponding 

 somites of each embryo are interconnected by dotted lines. It will 

 be seen that the somite which laps round the vagus and forms the 

 first permanent myotome in the 10 mm. and all later stages 



I 



