EXTENT OF THE FLOOR-PLATE OF HIS 131 



SUMMARY 



1. A differentiated floor-plate extends no farther forward than 

 the fovea isthmi. Increased morphological significance is believed 

 to attach to this depression. 



2. The plan of arrangement of the longitudinal zones of His as 

 set forth by him and generally accepted is interpreted as giving 

 a wrong morphological basis for the brain. 



8. Theoretic considerations based partly on the 'blastopore 

 theory' indicate: a) that the floor-plate corresponds to a sutura 

 neurochordalis (His); b) that the floor-plate and notochordal 

 plate are primarily coextensive; c) that the neurochordal suture 

 does not extend to the anterior end of the neural plate; d) that 

 the primary motor and sensory zones are continuous from side 

 to side cephalad of the anterior end of the floor-plate. 



4. The following interpretations are accepted: a) that the 

 optic chiasma marks the anterior end of the neural plate ; b) that 

 the motor zone terminates with the midbrain; c) that the floor- 

 plate terminates at the fovea isthmi; d) that the boundary be- 

 tween motor and sensory zones terminates near the mammillary 

 recess. 



5. A rational basis for the division of the brain into epichordal 

 and prechordal portions is afforded by the interpretation pre- 

 sented. 



6. A number of facts of cephalic and encephalic morphology 

 may be better interpreted on the basis suggested in this paper. 



