34^^ Journal o¥ Comparative Neurology. 



In assuming an entirely distinct function for the two 

 radices and the structures supplied by them, we meet an 

 apparent difficulty in the fact that in those fishes whose 

 olfactory bulb is exserted the radices are both produced from 

 the cerebrum in the elongate crura. It must be remembered, 

 however, that we have already shown that in these cases the 

 ventricle continues out with the radices, so that, morpho- 

 logically at least, the bulb contains even in this case a part of 

 the cerebrum, i.e., a distinct pes. 



PROSENCEPHALON. 



The study of the cerebrum of fishes is likely to furnish the 

 best and most convenient basis for determining their position 

 among the classes of vertebrates. The skeleton is less trust- 

 worthy than usual, by reason of its specialized nature. The 

 development of a vast number of membrane bones, with the 

 marvelous changes in the whole bodily configuration and 

 structure dependent thereon, can, at most, only be regarded 

 as superficial characters as compared with those entirely 

 independent changes in the brain within a spacious cavity 

 shielded from all direct external influences. 



Accepting the fundamental homologies laid down by em- 

 bryology, our morphological study proceeds from a condition 

 when the prosencephalon consists of a single dilation of the 

 neural tube, the primitive prosencephalic vesicle. Any 

 attempt to construe vertebrate brains which fails to recog- 

 nize this primary midividcd and closed character of the 

 cephalic cavity must fail. It is possible for the requisite 

 increase in cerebral substance to be produced in at least three 

 different wSys. First, there maybe developed from the sides 

 of the tube thick excrescences in which cells may proliferate. 

 Second, the vesicle may increase in size as a whole, retaining 

 its undivided character. Third, the vesicle may enlarge in 

 certain portions, while elsewhere it may retain its simple 

 original form. In Teleosts the first and the second methods 

 chiefly prevail in contrast to the higher vertebrates, where 



