Johnston, Forebratn Vesicle in Vertebrates. 513 



adult brain arise from the recognition of the boundary suggested 

 above which is clearly marked in the development. 



5. The Ventricles and the Tela. — An essential part of the question 

 of the boundary between diencephalon and telencephalon is the prob- 

 lem of the median ventricle ; specifically, does any part of the median 

 ventricle belong to the telencephalon ? The view held by His was 

 that the anterior part of the median ventricle belonged, with the 

 pars optica hypothalami, to the telencephalon. The view which 

 makes the lamina terminalis the anterior wall of the diencephalon 

 assigns the whole of the median ventricle in front of the aqueduct 

 to the diencephalon. It is obvious that the writer must agree with 

 His in recognizing a median ventricle in the telencephalon, although 

 a shorter part of the median ventricle is included than was included 

 by His. The above diagrams (Figs. 43 and 44) show the boundary 

 line of His and that adopted by the writer and it is clear that the short 

 part of the median ventricle between this line and the lamina 

 terminalis belongs to the telencephalon and makes communication 

 with the lateral ventricle through the interventricular foramina. 



The view which regards the lamina terminalis as the anterior wall 

 of the diencephalon and of its ventricle denies the existence of any 

 median portion in the telencephalon. This means one of two things : 

 either the diencephalon is the terminal segment of the brain and the 

 telencephalon lies lateral to it as two hemispheres, or the dien- 

 cephalon is terminal and the telencephalon consists of ultra-terminal 

 hemispheres, l^either of these is true. Aside from the fact that the 

 latter view involves a contradiction in terms, it cannot be considered, 

 because all the evidence shows that the hemispheres are lateral struc- 

 tures, (a) In the ontogeny of all vertebrates the hemispheres arise 

 as evaginations or expansions of the lateral brain wall behind the 

 lamina terminalis; (&) the lateral ventricles thus formed remain 

 always as lateral prolongations of the ventricle and the median ven- 

 tricle always extends forward beyond the interventricular foramina ; 

 (c) when the hemispheres by their great growth push forward beyond 

 the lamina terminalis, as they do in most vertebrates, they are still 

 connected with the lateral wall of the brain stem and in the middle 

 line the lamina terminalis is alwavs the most anterior structure of 



