136 JoURNAL OF COMPARATIVE NEUROLOGY. 
it is also entirely possible that the phenomena in question are 
without such deep significance, having been caused by the 
greater number of obstacles in the path of the fibre as it grew 
through the outer levels. 
4. Phylogeny of Midbrain Structures. 
The midbrain has ever been a stable part of the neural 
tube. Marked out early in ontogeny from the other brain-seg- 
ments, the midbrain of Mustelus retains many features of organ- 
ization which are really primitive in character. 
The central gray matter is the most archaic of the mid- 
brain structures, and the newer additions of the outer levels are 
derivable from it. The central gray matter is to be compared, 
both as to general functions and morphological topography, to 
the gray matter of the spinal cord before the latter has devel- 
oped its specialized cornua. Ina broad way, the ventral region 
of each is motor, and the dorsal part a series of sensory centres. 
The homology is most readily traceable in the ventral region. 
The neurones of the III and IV nerves are true somatic motor 
neurones, corresponding entirely to those of the ventral cornua 
of the cord. 
The dorsal region of the midbrain has become more and 
more specialized as an optic termination. At a phylogenetically 
early period, the optic fibres grew backward from their original 
relations to establish terminations here. Probably the most 
primitive connection is the one with the giant neurones consti- 
tuting the apparatus of the roof-nucleus and REISSNER’s fibre 
Through this means, the optic neurones were chained directly 
to the somatic motor neurones innervating the body muscula- 
ture. Later, the midbrain roof became thickened by the wan-, 
dering outward of neurones from the central gray matter, and — 
by the development of new optic terminations associated with 
them. Thus has arisen the tectum mesencephali, an end-station 
which has remained important in the vertebrate series asa 
visual centre until secondary connections were established with 
the pallium. Hence the tectum is of great magnitude in the 
lower vertebrates, where the pallium is weak, but becomes 
= 2 a 23 a 
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