Johnston, Forebrain Vesicle in Vertebrates. 5-^^ 



usually pass over chains of three neurones. The existence of neurone 

 chains of only two links connecting the peripheral sensory surface 

 with the cortex is somewhat in dispute, but it is certain that such 

 chains are relatively few in number. The general cortex provides 

 in its structure the means for association and correlation between 

 the areas concerned with various modes of sense impressions. This 

 general cortex has its efferent pathway over the cortico-spinal tract 

 and other bundles descending through the cerebral peduncle. 



In lower vertebrates, in which the general cortex is not yet known, 

 the telencephalon seems to consist of olfactory centers and corpus 

 striatum and it is generally believed that the first cortex to appear 

 was olfactory in function. The writer was the first to attempt a 

 definition of the olfactory cortex (1901, p. 239). It was pointed 

 out that the cortical center receives olfactory fibers of the third 

 order, not of the second order. The olfactory pathway consists of: 

 fila olfactoria — bulbus and tractus olfactorius — lobus olfactorius and 

 its efferent fibers — cortex. This definition of the cortex has since 

 been adopted and further developed by Kappers (Kappers and Theu- 

 nissen 1908, Kappers 1908). However, olfactory fibers of the 

 third order run to other centers in addition to the cortex. In the 

 diencephalon the nucleus habenul£e and hypothalamus, and in the 

 telencephalon itself, the epistriatum (nucleus amygdalae in mam- 

 mals), receive olfactory fibers of the third order (Edinger 1896, 

 Johnston 1898, 1901, Kappers 1906, 1908). The epistriatum fulfills 

 this definition in selachians, ganoids and perhaps teleosts when there 

 is no other part of the forebrain that does meet the conditions. How- 

 ever, in higher vertebrates (Kappers 1908) a part of the epistriatum 

 becomes gTadually pushed back until it finally occupies a position 

 in immediate proximity to the pyriform lobe (nucleus amygdalae), 

 while a true cortical formation appears in the roof of the hemi- 

 sphere in dipnoans (Elliot Smith 1908) and all higher classes. 

 ISTow the epistriatum of forms above fishes, whose history has been 

 so beautifully traced by Kappers (1908) does not represent all of 

 the formation to which he gives the name epistriatum in selachians. 

 The writer has shown that the epistriatum in Petromyzon (1902) 

 and Acipenser (1898, 1901) receives an ascending tract from the 



