42 



and, as the prosencephalon grows the median furrow becomes deeper 

 and the point of attachment of the nerve to the brain-wall becomes 

 included within the furrow. 



Figs. 6 and 7 show the pathway (on one side only) of the nerve 

 roots within the brain substance as seen in an embryo 86 mm long. 

 They enter somewhat nearer the dorsal than the ventral surface and 

 pass backward, curving slightly downward, and divide into minute ter- 

 minals which end near the epithelial lining of the brain ventricle. 



Fig. 6. Sagittal section, near the median plane, of the brain of a Squalus embryo 

 86 mm long. From the MiNOT collection, No. 426, sec. 292. X ^c 2, obj. % in. This 

 show the two roots of the nerve, both within and without the brain substance (a thickened 

 portion of the lamina terminalis). Rec.np recessus praeopticus. 



The upper root is composed of fewer strands, and is therefore more 

 slender than the lower root. The fibres terminate a little below, and 

 also, laterally and posteriorly, to a small forward extension of the 

 ventricle, which corresponds, I think, to the place where the neuro- 

 pore closes, and is the recessus neuroporicus of von Kupffer (Fig. 6 

 Bee. np). Fig. 7 shows some finer branches of the lower root. These 

 relations are similar to those in the adult except than in later stages 

 the position of the roots is relatively lower in the brain substance. 



