405 



follow the history of the metameres through the later stages. In 

 fig. 8, the auditory vesicle (au) has been formed and the first visceral 

 slit has broken through. The anterior metameres, lying in front of 

 the one marked 6, are no longer distinguishable from surface study. 



Fig. 8. Head of embryo after formation of the auditory vesicle, the first five neural 

 segments no loDger discemable. na nasal epithelium, mb beginning of the mid-brain in 

 the position previously occupied by the second accessory optic vesicle. 



Fig. 9. Head of embryo^ somewhat older than the one in Fig. 8. nV beginning of 

 the 5th nerve. Other reference marks as before. 



