EXTENT OF THE FLOOR-PLATE OF HIS 115 



which is lacking in the floor- and roof-plates that sets off both 

 these dorsal and ventral medial zones. 



The roof-plate and the floor-plate are comparable structures 

 and this should be borne in mind in the interpretation of the 

 latter. The roof-plate arises from the fusion of the edges of the 

 neural plate when it forms the neural tube. It is thus primarily a 

 bilateral structure, the two halves from the method of its forma- 

 tion united in an ideal dorsal line of concrescence. Awhile in the 

 spinal cord, medulla oblongata, and roof of the third ventricle, 

 it differentiates as non-nervous material — dorsal septum and 

 epithelial tela — in the cerebellum and roof of the midbrain it is 

 so speedily obliterated that it is difficult to say that it exists at 

 any time as a differentiated structure — a septum of non-nervous 

 (neuroglial) elements uniting two primarily nervous plates. 

 This would in no way affect the originally bilateral value of the 

 cerebellum and tectum mesencephali. The same considerations 

 apply to the floor-plate. Theoretical considerations to be dis- 

 cussed subsequently indicate that the floor-plate, like the roof- 

 plate, has its two halves united by an ideal plane. In early 

 stages of the neural tube, it is impossible to determine how much 

 of the floor of the neural tube is non-nervous (ependymal). It 

 is only through the growth and differentiation of the neural tube 

 that the floor-plate becomes clearly demarcated and attains the 

 characteristic structural features above referred to. It is quite 

 possible that it is primarily quite slight. Throughout the extent 

 of the spinal cord and medulla oblongata it remains relativel}^ or 

 completely free from nerve cells, although in certain regions (e.g., 

 as in the pons) what appear as medial migrations of neuroblasts 

 may quite completely transform it. 



The histologic structure of the floor-plate and its significance 

 in separating the right and left halves of the spinal cord (as the 

 neuroglial basis of the anterior commissure) and (as the septum 

 medullae) the caudal portion of the brain stem require no addi- 

 tional description or discussion It is only when the question is 

 raised as to how far forward into the cranial portion of the neural 

 tube it may be traced that we reach questions touching intimately 

 the morphology of the vertebrate central nervous system. 



