Johnston, Forchraui Vesicle in Vertebrates. 461 



the roof. In cyclostonies this is only slightly marked and has been 

 clearly described recently by Sterzi. In mammals the velum has 

 not been recognized hitherto and therefore the bonndary line in ques- 

 tion is wholly uncertain in mannnals. 



3. The problem of the membraneous roof and its relations to the 

 massive walls and the nen^ous pallium. This question naturally 

 presented itself to the earlier neurologists because of the obvious 

 great differences between the mammalian or human brain in which 

 the ventricles were apparently wholly covered by massive nervous 

 structures and that of lower forms, especially teleostean fishes, in 

 which a relatively great expanse of membraneous roof covers a broad 

 ventricle in the telencephalon. A discussion of this problem natur- 

 ally involves consideration of the next. 



4. The problem of the median and lateral ventricles in the telen- 

 cephalon and the interventricular foramina. Does the telencephalon 

 possess a median ventricle ? One interpretation of the mammalian 

 brain would assign the whole median ventricle in front of the pos- 

 terior commissure to the diencephalon (third ventricle). The walls 

 bounding this ventricle then belong to the diencephalon and the 

 boundary between di- and telencephalon is assumed to be the inter- 

 ventricular foramina. " If, however, the pars optica hypothalami and 

 lamina terminalis belong to the telencephalon, as was held by His, 

 they must carry with them into the telencephalon the adjacent front 

 part of the Illd ventricle. In the brain of a fish or even of an amphi- 

 bian there appears at first sight to be a large median ventricle belong- 

 ing to the telencephalon and it is more or less difficult to fix the 

 location of the foramina leading into the lateral ventricles. In the 

 teleostean and ganoid brain only the olfactory tracts and bulbs appear 

 to possess distinct cavities which could be called lateral ventricles 

 and the whole of the olfactory centers and corpora striata are found 

 in the lateral walls of the median ventricle. There is, therefore, 

 need to determine whether a part of the median ventricle belongs 

 to the telencephalon and to fix for the different classes of vertebrates 

 the homologue of the inteiwentricular foramina. 



5. Subdivision of the di- and telencephalon with reference to the 

 longitudinal zones laid down by His in other segments of the brain. 



