424 
in young animals) the cells are all obviously discrete while in others 
it requires great care to make out the cellular components of the 
spots. The most con- 
spicuous elements in the 
nidulus are large multi- 
polar switeh-cells which 
are especially abundant 
about the margin and 
serve to unite fibres from 
two distinct tracts (fig. 2). 
The remainder of the cel- 
lular elements are smal- 
ler and either variable 
or constant in size. Some 
are free while others tend 
to associate themselves 
in groups. All transitions 
can be observed between 
the latter and the opaque 
clumps above mentioned. 
The number of blood ves- 
sels is greater here than 
in any other part of 
the brain. These vessels 
come into intimate con- 
nection with the cells. 
Everything indicates a 
tremendous vital activity 
and a complex inter- 
change of nervous sti- 
muli. The „nucleus ro- 
tundus“ is perforated by 
the tract of the horizonta: 
commissure which seems 
however, to have little or 
Fig. 1. A portion of the nidulus” ruber (nucleus rotundus, FRITSCH) from adult 
specimen of Lucioperca. c Large switch-cell connecting with the dorsal peduncular 
tract and other regions. 5 a clustre of small cells, @ a similar clustre within the 
gelatinous spots characteristic of the area. 
Fig. 2. Cells from the margin of the nidulus. 
