Johnston, Forebrain Vesicle in Vertebrates. 



497 



This is the terminal ridge. In front of this is a median pit, the 

 terminal pit. Following the lamina terminalis around the front end 

 of the brain there is found in the dorsal wall a distinct transverse 

 fold, followed by an arched portion and a second more shallow fold. 

 In this fold and for some distance behind it are seen in later embryos 

 the fibers of the posterior commissure and the decussation of the 

 tectum mesencephali. The more anterior and deeper of the two 



Fig. 35. Pig embryo, 6 mm. A., median sagittal section. B and C, para- 

 sagittal sections. Neuromeres numbered in Roman, x 20. 



folds is the velum transversum, as the further description will make 

 clear. In embryos of 6 and 7 mm. the same change of relations in 

 the optic region takes place as has been described above for Squalus 

 and Amblystoma. The terminal ridge becomes prolonged caudo- 

 laterad as the optic ridge and in these later appear the optic chiasma 

 and tracts. The identity of these structures in the classes of animals 

 studied is absolutely clear. 



