462 Journal of Comparative Neurology and Psychology. 



The determination of the anterior end of the brain will fix the extent 

 of the floor jDlate and roof plate of His and will show the point at 

 which the prolongation of the sulcus limitans must end. In the 

 midbrain, hindbrain and spinal cord the sulcus limitans divides the 

 lateral walls into dorsal or alar plate and A'entral or basal plate. In 

 the di- and telencephalon this sulcus can scarcely be distinguished 

 with certainty, but the fact that zones of widely different functions 

 are separated by this sulcus Mdien present leads one to seek the equiva- 

 lent of these zones and the position of the sulcus in the di- and telen- 

 cephalon. 



Other problems concerning the internal morphology and genetic 

 history of the telencephalon — the origin of the pallium in relation 

 to the primary centers, the evolution of functional localization, etc., — 

 will be discussed in the present paper only so far as necessary in 

 connection with the subject of nomenclature. 



I. Dp:scriptive Part. 

 In this part will be brought together under each class of vertebrates 

 the data which bear upon these problems. As will be seen, the new 

 facts to be brought forward are chiefly embryological (selachians, 

 amphibians, mammals). The summaries of facts which have l>een 

 previously published will be made as brief as possible and the reader 

 is referred to the original papers for more complete accounts. 



1. Cyclostomes. 

 In cyclostomes is found a completely bilobed telencephalon, almost 

 as strongly marked as in any class of vertebrates. The reason for 

 taking especial account of this fact in the telencephalon is that the 

 division into symmetrical lateral halves which is present in all parts 

 of the central nervous system is more pronounced in the telencephalon 

 and in mammals and man becomes one of the most striking features 

 of the whole brain. The cause for the decided division of the telen- 

 cephalon is to be found without doubt in the presence of paired olfac- 

 tory organs and the importance of the sense of smell. In amphioxus 

 the front end of the brain is not bilobed because there is no localized 

 olfactory epithelium. In cyclostomes the olfactory organ is paired 



