Johnston, Forcbrain Vesicle in Vertebrates. 509 



ougiilj understood as to need no further comment (Gaupp 1898, 

 Minot 1901, Johnston 1905, 190G). 



The velum transversum has been described in cyclostome embryos 

 by Sterzi (1908) and in mammals in the foregoing pages, so that 

 the boundary line sought for is now clear in the brain roof in all 

 classes of vertebrates. From the velum transversum a groove or 

 constriction continues around the sides of the brain. Owing to the 

 early evagination of the optic vesicles this constriction in the dorsal 

 half of the brain occupies the space left vacant, so to speak, by the 

 withdrawal of the retinal tissue. Ventrally the groove is to be 

 thought of as lying in front of the neuromere to which the optic 

 vesicle belongs. The diencephalon consists in its dorsal half of but 

 one neuromere after the withdrawal of the optic vesicle ; in its ventral 

 half it includes two neuromeres, the more posterior of which is 

 narrow while the more anterior one forms the depression of the brain 

 floor which I have called the primitive inferior lobe. The boundary 

 between the diencephalon and the telencephalon in the brain floor has 

 been in dispute because of the obscurity which has existed over the 

 optic recesses and the anterior end of the brain. 



His placed the boundary behind the infundibulum and assigned 

 the pars optica hypothalami to the telencephalon. He was led to this 

 by his conviction that the telencephalon consisted of a complete brain 

 ring or segment and by his belief that the end of the brain axis was 

 in the Basilarleiste or infundibular recess. As shown above, the 

 optic chiasma is formed in the terminal ridge and therefore occupies 

 the extreme anterior border of the floor plate of the neural tube. If 

 the telencephalon is a complete transverse segment of the brain, as 

 His always insisted, there is no alternative but to include the optic 

 chiasma within it. The primitive optic groove which bounds the 

 optic chiasma behind belongs to the same neuromere with the optic 

 vesicles and therefore is included in the diencephalon. The telen- 

 cephalon can include no more than the optic chiasma and the associ- 

 ated decussations in the brain floor which lie in the terminal ridge. 

 The boundary Ix^tween the diencephalon and telencephalon is marked 

 by the velum transversum above and by the primitive optic groove or 

 postoptic recess below. In adult mammals, in which both these land- 



