52 



ZOOLOGY 



SECT. 



chord. It is roughly triangular in transverse section : anteriorly 

 it ends abruptly, some distance behind the anterior end of the 

 notochord, while posteriorly it tapers to a point over the hinder 

 end of the latter. It is traversed by an axial cavity, the neurocoele 

 (Fig. 752, cent, c.), connected with the mid-dorsal region by a 

 longitudinal cleft the dorsal fissure. At the fore-end of the nerve- 

 tube the neuroccele becomes dilated, forming a considerable cavity, 

 the encephaloccele or cerebral ventricle (Fig. 752, en. cce., Fig. 756, c.v.), 

 and a little behind this the dorsal fissure widens out above to form 

 a trough-like dorsal dilatation (dil.) covered only by the delicate 

 connective-tissue sheath which invests the whole nerve-tube. The 

 anterior end of the neuron, containing these two cavities, is to be 



FIG 756. Amphioxus lanceolatus. A, brain and cerebral nerves of a young speci- 



' men ; B, transverse section through neuropore ; C, behind cerebral ventricle ; D, through 



dorsal dilatation, ch. notochord; c.v. cerebral ventricle ; dil. dorsal dilatation ; c. eye-spot ; 



np. neuropore ; olf. olfactory pit ; /, //, cerebral nerves. (From Willey, after Hatschek.) 



looked upon as the brain, although not distinguishable externally 

 from the remaining portion or spinal cord. 



The anterior and dorsal region of the brain is produced into 

 a small, hollow, pointed pouch which comes into relation with the 

 olfactory organ and is called the median olfactory lobe. In its 

 posterior and ventral region a depression has been described 

 which appears to correspond with the infundibulum of the Craniata 

 (vide, p. 99). In the young animal the cerebral ventricle opens 

 above by an aperture, the neuropore (Fig. 756, B, np.), which sub- 

 sequently closes up. 



The neuron- is mainly composed of longitudinal nerve-fibres with 

 abundant nerve-cells mostly grouped around the neurocoele. At 



