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tion i. e. parallel to the surface but seem to turn radially with the 
growth of the tectum. It does not seem possible that all the optic 
fibres thus end in a cell. The peripheral process of these cells divides 
dichotomously repeatedly and mingles in the fine nervous reticulum of 
the ectal layer of the tectum which is accordingly the real transfer 
station for optic stimuli. 
The tracts entering the tectum are 1) the optic, which contain 
all the fibres of the optic nerves. None of these fibres pass to 
the geniculatum or to other parts of the thalamus but, after dividing, 
the two portions pass to the cephalomesad and caudo-lateral regions 
of the tectum respectively and spread out in its second layer. 2) The 
lemniscus tract which arises, as stated, in the basal processes of the 
inner ganglion layer and passes to the lateral region of the medulla. 
3) Closely associated with this tract are the fibres which, after decus- 
sation in the ponsregion, enter the middle crura of the cerebellum. 
Part of these fibres also seem to form the communication with the 
III, IV, and possibly the VI nerves. 4) The anterior and posterior 
brachia connected indirectly with the transverse and ventral commis- 
sures as described more in detail in the forthcoming paper. 5) Com- 
missure fibres of the tectum (Sylvian commissure) rising in the ectal 
reticular zone and crossing via the torus. 6) Gelatinous fibres which 
spring from the epithelium of the torus and terminate in the stroma 
of the ectal part of the tectum. 
Regarding the histogenesis of the frame our material is still in- 
complete but is sufficient to show that the nerve cells arise every- 
where from the epithelium of the ventricles. The process of cell- 
multiplication goes on for a long time after birth but is limited to 
special stations or factories (germinative 
areas). In the brain of Trutta fario 
nine days old!) karyokinetic figures are 
found in all segments of the brain but are 
exceedingly variable in frequency. In the 
medulla the complete compliment of cells 
seems to have been formed while the cere- 
bellum is still rapidly growing. In the latter 
Fig. 10. A portion of the volvula of Trutta fario, 9 days old, showing the 
karyokinetic figures of the neuroblasts (a) among the epithelium cells (6). 
1) My thanks are due to Professor J. FRENZEL for his generosity in 
supplying this material. 
