516 "Journal of Comparative Neurology and Psychology. 



is widest here. This seems to the writer to be due to the with- 

 drawal of nervous material from the dorsal part of the lateral brain 

 walls to form the optic vesicles. This withdrawal of retina-sub- 

 stance leaves a gap which is filled by membraneous tela only. The 

 tela in the median region of the telencephalon is perfectly evident 

 in any brain, embryonic or adult, so far as the writer is acquainted. 

 From the posterior part of this tela next to the velum transversum 

 arises the paraphysis or the rudimentary paraphysal arch, and 

 forward from that the tela stretches across the median ventricle be- 

 tween the interventricular foramina. It is equally evident that at 

 all stages of development these foramina are roofed by lateral pro- 

 longations of the tela. This is true of all forms with the apparent 

 exception of cyclostomes, teleosts and ganoids. In cyclostomes this 

 is probably due to the compression of the forebrain by the oral 

 funnel and olfactory organ. In ganoids and teleosts the interven- 

 tricular foramina have been widened beyond recognition by the 

 eversion of the lateral walls. In amphibians and higher forms the 

 prolongation of the tela over the interventricular foramina to become 

 the roof of the lateral ventricles has great importance for the forma- 

 tion of the lateral plexuses. The beginning of these has been de- 

 scribed and the only further comment which the writer wishes to 

 make is that the complexity and mystery which the text-books throw 

 around the relations of the velum interpositum and the lateral 

 plexuses should be brushed aside for the sake of the student, who 

 finds the subject difiicult enough without artificial stumbling blocks 

 being put in his way. The student should be told simply that the 

 median tela extends laterally as the roof of the lateral ventricle and 

 this becomes infolded to form the lateral plexus. This process con- 

 tinues around the side wall just in front of the junction of the 

 hemisphere and thalamus. 



6. Dorsal and Ventral Zones in Diencephalon and Telencephalon. 

 — The sulcus limitans of His marks the boundary between alar plate 

 (dorsal zone) and basal plate (ventral zone). The dorsal zone 

 throughout the central nervous system is sensory, the ventral zone 

 motor. Both zones include gray matter and fiber tracts belonging 

 to the correlating mechanisms, and in those segments in which the 



