Johnston, Forchrain Vesicle in Vertebrates. 517 



primary sensory or motor centers are reduced or wanting owing to 

 reduction or absence of the peripheral organs, the correlating mechan- 

 isms constitute the whole of the zone concerned. The writer has 

 repeatedly (1902, 1905, 1906, 1909) emphasized the fact that the 

 longitudinal zones constitute the most fundamental divisions of the 

 brain and hence the sulcus limitans is the most important landmark 

 in the brain. The two sulci converge at the anterior end of the 

 brain to meet in the lamina terminalis and this meeting-point marks 

 the anterior end of the central axis of the brain. The end of this 

 axis His placed at about the middle of his lamina terminalis, namely 

 in the recessus prseopticus. The facts set forth in this paper show 

 that the chiasma region must be taken from the lamina terminalis 

 and added to the brain floor. Still, in the writer's opinion, the 

 central brain axis has its ending in the recessus prseopticus (Figs. 

 43 and 44). The reasons for this are to be found in the following 

 facts : ( a) the ventral zone of the brain becomes greatly reduced 

 in volume in front of the third nerve by the absence of all motor 

 centers; (b) it is further reduced by the distribution of fiber tracts 

 to various parts of the diencephalon and telencephalon; (c) the 

 sensory centers are represented in the telencephalon by the large 

 olfactory apparatus; (d) the correlating mechanism of the dorsal 

 zone is greatly hypertrophied in connection with the olfactory centers 

 and in higher forms in connection with the somatic cortical centers 

 (neopallium). In other words the ventral zone at the front end 

 of the brain is represented chiefly by the decussating fibers (optic 

 chiasma and commissures of Gudden and Meynert) of the ventral 

 commissural system, while the dorsal zone contains both sensory and 

 correlating mechanisms which are very large. These facts account 

 for the bending down of the sulci limitantes to meet near the ventral 

 border of the lamina terminalis. There is no evidence known to the 

 writer tending to show that the recessus neuroporicus has any sig- 

 nificance in this connection. It is only a convenient practical mark 

 of the dorsal border of the lamina terminalis and the anterior end 

 of the brain roof. 



In the diencephalon the location of the sulcus limitans is still 

 more difiicult. The typical formation of the ventral zone extends 



