CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
951 
itself by a reference to the longitudinal section (fig. 1), and obviously this is a 
difference of degree and not of kind; next we have on each side a wing which goes 
out into a much more complicated arrangement than in L. maculatus. 
We are now in a position to explain the morphology of the “peculiar organ” in 
Mormyrus. It is easy enough to imagine that the ancestors of the Mormyridce had a 
brain resembling that of the Ballan Wrasse, possessing a valvula cerebelli of no larger 
size; but there is nothing to prevent this valvula being endowed with a capacity for 
indefinite growth. It may be seen that the external extremity of the molecular layer 
on the right side in the figure (fig. 15) has begun to turn down; it might go on 
growing, and might form another fold like the one already present, and after that 
another, and so on until it breaks through the tecta lobi optici, and this going on to an 
indefinite extent in every direction would eventually produce such a brain as is 
possessed by the Mormyridce of the present day. 
To solve the problem of the tuberculum impar we must ton to an entirely different 
genus of Fishes ; the well-known Gold Carp, such favorites with some people as 
ornaments for ponds and fountains, present the key to this difficulty. 
These Fishes have a tubercle placed in the fourth ventricle behind the cerebellum; 
in the species that I investigated there were two tubercles in this position, one placed in 
front of the other. On examining the figure (fig. 16), which is a section across the 
medulla oblongata of a Gold Fish ( Cyprinus sp. ?), it is found that there is a tubercle 
in the centre, and on each side a sharp ridge ; the composition of these beginning on 
the outside of the ridges, which are the vagal tuberosities, is as follows : — 
First, there is a layer of small cells. 
Secondly, a layer of finely granular material. 
Thirdly, a layer of nerve fibres. 
Fourthly, a granular layer with large cells dispersed in it. 
All these are in the vagal tuberosities; then comes the tuberculum, which occupies 
the space between them ; it consists of— 
Fifthly, a layer of small cells; and 
Sixthly, a layer of granular material, which occupies the greater part of the 
tubercle in the centre. 
At the junction of this tubercle with the vagal tuberosities there are sections of a 
few bundles of nerve fibres. 
We have now to consider how the tuberculum impar of the Mormyrus may be 
explained by a consideration of these facts. 
The organ in question is, as we have seen, placed immediately behind the cerebellum, 
and at first sight this seems to be the only circumstance that it has in common with 
that of the Carp, since there are in the Mormyrus no sharply projecting ridges, and 
the succession of tissues does not apparently correspond ; but a little consideration will 
MDOCCLXXXII. G F 
