Johnston, Forehrain Vesicle in Vertebrates. 463 



in the embryo and the mode of development of the mouth, hypophysis 

 and olfactory pit shows that the paired definite olfactory organs are 

 older than the circular mouth. These organs have, therefore, exerted 

 an influence on the brain for a long time previous to the period when 

 cyclostome characters became permanently fixed, and the result is 

 the bilobed telencephalon. This is seen in the presence of massive 

 lateral walls which are connected above, below and in front by rela- 

 tively thin membranes which are thickened only l\y transverse fibers 

 and nowhere contain gray matter. It is seen in the forward projec- 

 tion of the olfactory bulbs into which the paired olfactory nerves 

 enter. The olfactory bulbs are sessile ; there is no extended olfactory 

 tract or peduncle. It is seen further in the lateral extensions of the 

 ventricle to which attention has been called by Studnicka and the 

 ^Ai'iter (1906). These are rather wdde cavities extending into the 

 olfactoiy bulbs and connected with the median ventricle by narrower 

 openings, the intcrventricuJar foramina. Owing to the pushing back- 

 w^ard of the telencephalon in cyclostomes the lateral ventricle presents 

 a posterior prolongation similar to but not homologous wdth the 

 posterior horn of the ventricle in mammals. The tela chorioidea of 

 the Illd ventricle forms in front of the nuclei habenulie a broad low 

 dorsal sac whose roof in some species is depressed by the epiphysial 

 bodies lying over it. In front of this is a slight transverse fold 

 (better seen in embryos than in the adult) first described by Sterzi 

 (1907) which this author homologizes with the velum transversuin of 

 other vertebrates. This velum is continued laterally around the 

 brain by a constriction (groove) without and a fold (ridge) within 

 which marks the boundary between the diencephalon and telenceph- 

 alon. In front of the velum transversum the roof continues for some 

 distance as a thin membrane, the lamina supraneuroporica of Sterzi, 

 and then is thickened by the fibers of the superior olfactory decus- 

 sation. According to Sterzi, the point corresponding to the neuro- 

 poric recess of other vertebrates is at the front or lower border of this 

 decussation. 



The conditions in cyclostome embryos (solid nerve cord, solid con- 

 nection of brain with ectoderm instead of open neuropore, compact- 

 ness of structure) are not favorable for the study of the anterior 



