406 



We have now reached the stage, approximately, at which the observ- 

 ations of previous writers begiD — that is the stage just after the 

 appearance of the auditory vesicle. 



In fig. 9, some characteristic changes are to be noted, the 

 auditory vesicle has shifted backwards until it occupies a position 

 opposite the 11th metamere. The 8th metamere, still serves as a 

 landmark, there are now two clearly marked metameres in front of it, 

 and three behind it. The only metameres discernible from surface 

 views, are those belonging to the hind-brain. The mid-brain is greatly 

 increased in expanse, and the first accessory optic vesicle has been 

 crowded forward into the region of the thalamencephalon. The accessory 

 optic vesicle is also reduced in size. In the middle of the primary 

 optic vesicle is seen the infolding to form the lens, and the choroid 

 fissure has also been formed. The 5th nerve, is already well begun, 

 and nerve fibres are also given off from the segments numbered 9 

 and 10. The question of what nerves these represent lies outside tbe 

 domain of this paper. 



In fig. 10, the neural segments are undergoing some changes in 

 outline, that are likely to lead to confusion in identifying them, in 

 later stages. If we look along the lower margin of the segmented 



'."(V - 



Fig. 10. Head of embryo showing the beginnings of elevations on the upper margin 

 of the segments. A. op 1 the first accessory optic vesicle now in the region of the thalamen- 

 cephalon — with its fellow it affords the material from which the epiphysis is developed. 



