514 Journal of Cojnparative Neurology and Psychology. 



the brain in the adult as in the embryo. It is no more true to say 

 that the telencephalon is ultra-terminal than to say that it is post- 

 optic or post-velar. The occipital lobe extends as far behind the 

 velum transversum as the frontal lobe extends in front of the lamina 

 terminalis. The whole hemisphere is a great expansion of a part of 

 the lateral wall of the brain between the lamina terminalis in front 

 and the optic vesicles and primitive optic grooves behind. The point 

 in dispute is whether that portion of the preoptic brain segment 

 which is not carried out in the hemispheres belongs in the dien- 

 cephalon or the telencephalon. 



The first step in answering this question is to see clearly that in 

 the early embryo the lateral hemispheres and the median portion 

 exist together undifferentiated as a simple ring or segment in front 

 of the optic vesicles. This segment is bounded from the earliest 

 stages, even before the neural tube is closed, by the sharply marked 

 primitive optic groove and the optic vesicles. It is only some time 

 after the formation of the optic vesicles that the dorsal part of this 

 simple scgTuent bulges out at either side to form the lateral hemi- 

 spheres. If the segment is simple at the start, is there any ground 

 for separating the ventral part and adding it to the diencephalon 

 which lies behind the primitive optic groove ? The only thing to 

 give support to this view is the connection of the hollow optic stalk 

 with the preoptic recess. Since the optic vesicles have always been 

 referred to the diencephalon, their close relation to the lamina termi- 

 nalis through the preoptic recess suggests the inclusion of the lamina 

 terminalis in the diencephalon. l^ow, however, it is shown that the 

 optic vesicles are primarily connected with the post-optic recess and 

 are only secondarily related to the preoptic recess. 



In view of this there remains no ground for separating the median 

 and lateral structures which develop from this primitive first seg- 

 ment. The embryological facts leave only one course open, namely, 

 to consider the lateral hemispheres as the dorsal portion, the region 

 of the optic chiasma as the ventral portion of one segment. 



Finally, it is impossible to harmonize the relations of the velum 

 transversum in lower vertebrates and in all embryos with the view 

 that the lamina terminalis bounds the diencephalon. That the velum 



