I. INTRODUCTION. 



The vertebrate brain has long been regarded as made up of transverse rings or 

 segments of morphological significance. Burckhardt ('94) first pointed out that 

 the neural tube may be considered as made up of longitudinal plates, or zones for 

 the most part bilaterally arranged. In the primitive brain, as in the cyclostomes, 

 the median dorsal zone is essentially ependymal and non-nervous, while the border- 

 ing lateral zones are highly nervous (PL XXIX, Figs. 8, 10, 15). This median zone 

 retains to a greater degree than any other portion the primitive characteristics of 

 the neural tube, remaining fairly constant both in its ontogeny and phylogeny through- 

 out the vertebrate series. This is particularly true of the pars intercalatus, that 

 portion of the median roof of the diencephalon lying anterior of the posterior com- 

 missure and between it and the base of the epiphysis, and designated by Burckhardt 

 as the " Schaltstiick " (Figs. 15, 20-24, prs. i'cal.). It is made up of elongated ependy- 

 mal cells inclined obliquely cephalad. In the teleosts it is in an especially primitive 

 condition containing no nervous elements except the fibres of the epiphysial decus- 

 sation which pass through it near its dorsal surface (Fig. 24, dec. e'phy.}. 



The median zone of the mesencephalon is in most teleosts, however, probably 

 more complex than in any other group, due to the development of the dorsal decus- 

 sation of the tectum opticum (Figs. 20, 24, 25, dec. d.} and the crowding into the 

 median plane of the structures derived from the lateral zones. 



The teleost brain presents a structural form considerably removed from the 

 more generalized types found in ganoids, selachians, and amphibians. This large 

 and variable group presents many types of brain structure, assuming in the more 

 highly differentiated members of the group a considerable degree of complexity. 



The mesencephalon is complicated by the presence of the torus longitudinals. 

 In the adult this consists typically of a medianly grooved ridge extending downward 

 from the thin median portion of the mesencephalic roof (Figs. 6-19, tor. 1g.). It 

 usually extends from the posterior commissure through the length of the optic lobes, 

 but is best developed at its anterior end (Figs. 20-23). The form and relative size 

 of the torus and consequently its relations to the surrounding structures (tectum 

 opticum, posterior commissure, valvula cerebelli, etc.) vary greatly in the different 



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