114 B. F. KINGSBURY 



This last statement applies particularly to the floor-plate. The 

 roof-plate, due to the general interpretation that the choroid 

 telas and plexuses express in each instance expansions of it, 

 has received more consideration. Although little attention has 

 been devoted to the floor-plate, the writer believes that the struc- 

 ture, extent, and significance of this portion of the developing 

 brain and spinal cord are matters of marked importance in un- 

 derstanding the fundamental morphological plan of the brain, 

 and it is the purpose of the present paper to emphasize particu- 

 larly the theoretical significance of this structurally insignificant 

 portion of the neural tube. 



Descriptions of its structure are scanty, although its structural 

 appearance is well known to every one working with the ver- 

 tebrate embryo. The portion of the neural tube that it repre- 

 sents is apparently devoid of neuroblasts, consisting of neuroglial 

 elements alone, which as an ependymal plate are often conspicu- 

 ously evident. Processes of these cells and subsequentlj^ neuro- 

 glial fibers extend* to the surface of the neural tube. It possesses 

 thus purely negative characters for one interested primarily in 

 the neuronal composition of the nervous system. Streeter ('11)^ 

 describes it concisely, as also does Strong ('16) .^ It is indeed this 

 differentiation of the lateral wall referred to by Streeter and 



^ (P. 6.) "In the region of the anterior median fissure of the cord and the 

 median raph6 of the hindbrain, corresponding to the Boden-platte of His, the 

 neuroglia maintains its primitive ependymal type of simple radial fibers extend- 

 ing from the lumen to the surface of the tube. It is this region that is traversed 

 by the fibers of the anterior white commissure of the cord and the transverse 

 arcuate fibers of the hindbrain. The persistence of this simple type of neuroglia 

 may be explained by the absence of any mantle or nuclear layer with its consequent 

 complications at this place." 



(P. 7.) "Thus in the adult we find that the ependymal neuroglia is persistent 

 only as septa in the anterior and posterior median planes of the nervous system 

 and as a lining membrane for its central canal and ventricles." 



2 (P. 453.) "Two points are to be noted: Firpt, that the neural plate is a 

 bilateral structure and the future development of the tube will naturally take 

 place principally in th i side walls or lateral plates of the formed tube; second, that 

 the primary connection between the two side walls is the ventral median plate, 

 the dorsal median plate having been produced by a secondary fusion. This 

 being the case, the ventral connection between the two lateral plates will natur- 

 ally be more extensive and possibly more primitive than the dorsal." 



