194 



THE PINEAL ORGAN 



nerve-fibres, while at the distal end near the atrium (Fig. 137, b) the 

 greater part of the section was occupied by epithelial cells ; the nerve- 

 fibres here being confined to the lower and lateral parts in the periphery 

 of the section. In the middle of the nerve (Fig. 137, c) the nerves were 

 in the lower part of the section, the cells above ; the nerve-fibres were 

 evenly distributed, not yet being collected together in cords. In the 

 adult Petromyzon in the last part of the stalk, the nerve-fibres are arranged 

 in definite layers ; moreover, outside the nerve-fibres in the fully developed 

 pineal nerve, separate cells are still present which Studnicka regarded as 

 being vestiges of the original tissue of the stalk. These cells are found 

 either in the centre (Fig. 137, e) or at the periphery. The special pia- 



cpr 



po. air 



pp.o. / ,N pin 



M.Br 



Fig. 137. 

 A — a. : Longitudinal section of the pineal nerve of a 35-mm. Ammoccetes ; 

 b, c, d : Transverse sections through different parts of the pineal nerve from 

 a similar specimen ; b : near the atrium ; c : about the middle of the 

 nerve ; d : through the proximal part ; e : transverse section through the 

 pineal nerve of an older Ammoccetes. (After Studnicka.) 

 Sagittal section through the parietal region of the brain of an older Ammoccetes. 

 The atrium of the parietal organ is exceptionally large, and is situated at 

 the same level as the lumen of the parietal organ. 

 atr. : atrium. po. : parietal organ. 



cp. : commissura posterior. pp. : paraphysis. 



c.p.a. : commissura pallii anterior, pp. o. : parapineal organ. 



B 



ch. hob. : chiasma habenularis. 



M. Br. : midbrain. 



N. pin : nervus pinealis. 



o. 

 r. hab. : right habenular ganglion. 

 tr. hab. : tractus habenularis. 



