Johnston, The Brain of Petromyzon. 79 



25. The tracts which enter and leave the tectum corres- 

 pond closely to those in Acipenser, except that there is no tract 

 to the cerebellum. All the tracts have a somewhat more simple 

 arrangement than in Acipenser. 



26. A large part of the cells of the central grey of the 

 mid- and 'tween-brain constitute the nucleus of the posterior 

 commissure, homologous with the nucleus described by KoL- 

 LiKEK in mammals. The tibers of the commissure are destined 

 to the medulla. 



27. The ganglia habenulae and bundles of Meynert cor- 

 respond in all important features to those of Acipenser, except 

 that the right bundles contain ascending fibers which end in the 

 right ganglion. 



28. The pineal apparatus probably functions as a light 

 percipient organ, it is in relation with the left ganglion haben- 

 ulae only. 



29. The hypothalamus is less highly differentiated than 

 in Acipenser. Its tracts cori-espond in general with those of 

 the hypothalamus of Acipenser. 



30. There is a centrifugal tract to the saccus vasculosus 

 which has the same relations as in Acipenser. 



31. The tractus lobo-epistriaticus crosses in the p<^st optic 

 decussation instead of the anterior commissure. 



32. The form of the fore-brain has become greatly mod- 

 ified by pressure from the buccal apparatus. The striatum oc- 

 cupies the base, the epistriatum the dorsal part of the lateral 

 wall. The olfactory lobes and areas have been telescoped back- 

 ward and to the sides of the striatum and epistriatum. This 

 has led to modifications of the cavity which give the appearance 

 of lateral ventricles. 



33. The epistriatum is an olfactory nucleus and sends 

 short neurites into the striatum ; it also forms part of a coordi- 

 nating apparatus for impulses coming from the tectum by way 

 of the hypothalamus. 



34. The olfactory area, which constitutes the great lateral 

 expansion, receives fibers from the olfactory lobe and sends its 



