226 C. JUDSON HERRICK 



loses its connection with that of the third ventricle. The sub- 

 commissural organ of the midbrain roof (see p. 281) is continued 

 forward through most of the length of the pars intercalaris 

 diencephali, almost to the recessus pine alls. 



Kupffer ('06, p. 174), on embryological grounds, divides the 

 dorsal portion of the diencephalon of Necturus into two trans- 

 verse regions. The more anterior of these (parencephalon, 

 Nebenhirn) includes the habenular bodies, dorsal sac, and 

 epiphysis; the more posterior (synencephalon, Schalthirn) in- 

 cludes the region between the epiphysis and the posterior com- 

 missure. The more ventral parts of the diencephalon he calls 

 the hypencephalon. 



Johnston ('09, p. 489), in describing the development of 

 Amblystoma, recognizes two neuromeres in the diencephalon. 

 The first is largely consumed in the formation of the optic 

 vesicles; the second gives rise to most of the other diencephalic 

 structures. There is no agreement, however, among embry- 

 ologists regarding the metameric subdivision of either the dien- 

 cephalon or the mesencephalon; and any scheme of subdivision 



of these regions based on primitive metamerism is as yet very 



> 

 precarious. 



In my opinion the attempt to elaborate a final analysis and 

 subdivision of the diencephalon of vertebrates on the basis of 

 present knowledge is premature. The immediate need is for 

 a precise analysis of the adult functional relations in a series of 

 representative species; this pattern should then be read back- 

 ward in the ontogeny as far as possible. Only after this has 

 been done are we in a position to separate the palingenetic 

 from the cenogenetic factors in the problem. 



In this communication I shall follow the topographic sub- 

 division of the diencephalon which was outlined in my paper 

 on. the morphology of the forebrain ('10, p. 431). The limits 

 of these subdivisions are not clearly marked on the external 

 surface of the brain, but on the ventricular surface they are 

 very evident. The sculpturing of the ventricular surface of 

 the mesencephalon and diencephalon of Necturus is illustrated 

 in figure 5 of McKibben's paper on the nervus terminalis ('11) 



