223 



which penetrates to where the lieuiispheres lie against each other 

 or are connected with each other. This gi-oove grows shallower and 

 fainter in a candal direction and finally disappears altogether (Cf. 

 figs. 3 — 6). 



The coalescence of the hemispheres is by no means a superficial 

 one, since it is accompanied by radical changes in the position of 

 the nuclei and in the course of the fibre tracts. Some of the fibres, 

 indeed, which in other fishes decussate in the commissnra anterior, 

 here decussate above the ventriculus medianus. The small size of 

 the ventral commissura anterior of these fishes as compared with 

 that of other Teleosts, is hereby explained. 



I wish to point out that it is a common feature that a part of 

 a commissure may cross more dorsally if a suitable commissure-bed 

 is present (cf. the development of the psalterium in reptiles and of 

 the corpus callosum in matnmals). 



The nuclei and tracts w the fore-hrain. 



The nuclei and tracts of the fore-brain have been frequently 

 described, and Sheldon in particular has given a most minute and 

 accurate account of it. It is therefore not my intention to describe 

 them all again here, the more so as the position of the nuclei has 

 already been spoken of in discussing the morphology. I will only 

 say a few words concerning some fibre-tracts which differ from the 

 normal type in their course, and concerning the corpus striatum 

 which has been almost quite pushed away from the surface by the 

 other portions of the fore-brain (vide supra). 



At the frontal part of the telencephalon, the corpus striatum is 

 seen for a short distance on the medio-dorsal side of the liemispheres 

 between the septum and the epistriafum (fig. 1). F'urther caudally (he 

 growth of the septum pushes it quite away from the surface, lis shape 

 then is oval, in consequence of which in a cross section through this 

 region the septum appears narrowest in the middle (fig. 3). Further 

 caudally the striatum becomes broader; it spreads further in a 

 median direction, dislodging the two septa. This spreading of the 

 striata goes so far that at the level of the posterior boundary of 

 the commissura anterior, they grow together over the median line, 

 whereby the septum becomes divided into a dorsal and a ventral 

 part (fig. 4). We can here distinguish a median and two lateral 

 portions in the striatum. 



Further caudally the median connecting portion of (he striata is 

 separated from the lateral parts more or less. This separation is 



