( 902 ) 
the frontal growth of the valvuia is clear. It does not oceur in 
animals without valvula (e.g. sharks), and in Lophius, where the 
datt tr B immenwamd v. tectum em 
omlan- ploot 
| 
eld, mar Buiten wand vu A. omslag ploot 
ml sj Buiten wand oly valvuta Pe, 
Va loop daa IL wortel in de 
Valvuloa Ceacbhk 
Fig. 5. Gadus. 
valvula cerebelli is extremely small, there is neither any question 
of a frontal shifting of a part of the trochlear fibres. Nevertheless 
the conditions in this animal furnish us with the explanation. 
The diagrams given here show how the said root dispersion is to 
be deduced from the simple position. In type a (shark) the decussation 
of the fibres occupies but a small space. As the velum is very thin 
there is only one decussation. In 4 the velum is considerably 
thickened, principally by the growth of the molecular and Purkinje 
layer over it. (Type Lophius). 
Although the decussation remains on one vertical level, a distinction 
‘an nevertheless be made between the fibres which decussate close 
to the aquaeduct, and those which decussate more or entirely on 
the surface of the molecular layer. 
In ¢ the molecular and Purkinje-layer has grown still considerably 
further under the tectum opticum and exhibits more folds; a conse- 
quence of this is the enlargement of the distance between the fibres 
with a more peripheral and with a more central decussation, which 
attains its maximum in d, practically agreeing with the conditions 
as shown in Gadus. 
