The brain of Acipenser. 155 
The crossed fibres are as diffuse as possible and the fibres of the 
two sides mingle indistinguishably with one another and with the 
ascending fibres from the hypothalamus, so that it is impossible to 
follow them after crossing. They may be distributed to all the 
olfactory nuclei, but I think that a very large part of them go to 
the epistriatum. The centrifugal fibres to the olfactory lobe, if 
present, are mingled with the centripetal fibres and can not be 
distinguished. 
2. Interpretation. Review of Literature. 
A. Hind Brain. 
a) Medulla. 
1. Base of the Medulla. 
This calls for little comment in the present state of investigation. 
The chief problems at present in connection with the hind brain 
have to do with the cranial nerves and their sensory centers, and the 
morphology of the cerebellum. In the base of medulla there is to 
be noticed, I think, a tendency for commissural and tract cells to 
become modified to form special cell groups. This is perhaps to be 
seen in certain groups of cells mentioned in connection with the 
secondary vagus tract (page 89). The cells of the nucleus of Mry- 
NERT’s bundle are perhaps to be regarded in this way, although in 
a more advanced state of transition. These cases may be considered 
as early or intermediate stages in the formation of nuclei such as 
exist in the higher parts of the brain, while in those parts (e.g. in 
the central grey of the ‘tween brain) it is probable that there 
are cells which retain the primitive character of commissural and 
tract cells. 
The ventral root of the VII nerve and its nucleus of origin 
present a somewhat more primitive condition than in other fishes. 
VAN GEHUCHTEN (794) describes these in the trout as having almost 
the same arrangement as in man. In Acipenser the ascending root 
and knee of the facial do not contain all the fibres of the motor 
root. The nucleus is elongated and some of the fibres pass directly 
from their cells of origin out in the root (page 74). A similar con- 
dition is found in the ventral V in Acipenser. The elongated form 
of the nucleus of the V and its lateral situation have not before 
been described, I believe. This partial isolation of the nuclei of the 
ventral V and VII is perhaps to be regarded as a precursor of 
the nucleus ambiguus in higher Vertebrates. 
