CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
955 . 
of Fishes not only with one of the pairs of the corpora quadrigemina but especially 
with the anterior pair, as Stieda long ago pointed out. The ventricle of the optic lobe 
would be simply the remains of the ventricle which is always found in the mesencephalon 
of the embryo in Yertebrata. It does not seem to me that Fritsch’s homologies have 
any force as against these arguments. 
The torus semicircularis could not homologise with the thalami optici, but would 
seem more likely to correspond to the corpus geniculatum externum,* a ganglion in 
the course of the optic tract, which according to ForelI becomes larger in the lower 
Mammalia. 
The region round about the posterior commissure in Fishes homologises better with 
the thalami optici; the part wliich extends behind that commissure in Mammalia 
becomes much less developed, as ForelJ has also pointed out, in descending the scale. 
The torus longitudinalis, if this line of argument carries any force, could not homolo¬ 
gise with the fornix, as the former is a nervous centre, closely resembling the granular 
layer of the cerebellum in structure, and the latter is simply a set of longitudinal 
commissural fibres. With these principal points, the minor interpretations of Fritsch 
of course fall to the ground. Finally it seems to me that as Fishes retain so many 
marks of being in an embryological position, they would possess a brain in accordance 
with these conditions, and to find all the parts of a Mammalian cerebrum even in a 
rudimentary state, much more in the comparatively developed form insisted upon by 
Fritsch, would be, so to speak, an anachronism. The brain would not be in accordance 
with the rest of the organisation. 
Explanation of Plates. 
The following letters have the same signification throughout:— 
a . Arteries. 
aq.Sy. Aqueduct of Sylvius. 
a.v.c. Ala of the valvula cerebelli. 
a. id. Anterior wings of the valvula cerebelli. 
c.a. Commissura ansulata. 
c.c. Crura cerebri. 
c.ca. Central canal of the spinal cord. 
* I Lave only just received the interesting article by P. Maysek on the brain of Teleostei (Zeitsch. f. 
Wiss. Zool., Bd., xxxvi., 1881.) I am glad to find that in most cases he is in accord with me. Curiously 
enough, he compares the tori semicirculares with the corpus geniculatum internum, 
f Loc. cit., pp. 29 and 30. 
X Loc. cit., p. 30. 
