Johnston, Forchrain Vesicle in Vertebrates. 



499 



inferior lobe becomes dra^vni down in funnel shape and the lateral 

 portions become thickened as the tuber cinereum. It is one of the 

 peculiarities of the mammalian and human brain that the infun- 

 dibulum is drawn doM-n close behind the chiasma and forms a deeper 

 and narrower funnel than in lower vertebrates. There is no funda- 

 mental difference of relations. 



The evidence that the fold in the roof which has been called the 

 v^elum transversum is correctly identified may be seen in Figs. 34 



Fig. 37. Pig embrjo, 12 iiuii. Mediau .sagittal section, reconstructed from 

 several sections, x 15. 



to 39. The posterior commissure is a point about which there is no 

 dispute. In the arch (neuromere) in front of it appears later the 

 epiphysis (Fig. 38). In front of the epiphysis and in the same arch 

 appears the commissura hahenularis. This arch then is the roof of 

 the diencephalon. It is bounded in front in all other classes of 

 vertebrates by the velum transversum. The fold to which this name 

 has been given lies in the proper segmental position. This is further 

 supported by its relations to the structures in front of it. Immedi- 

 ately in front of the velum the roof is raised in a distinct arch. 



