{C'otitiii lied from jkkjc 4^) 



11 A transverse section taken 840 microns behind figure 10. It would pass 

 ( hrou-;!! al)()ut the center of thefirst model (figs. 1 and 2) . Note that the roof ex- 

 pansion, while containing an enormous cavity, is confined almost entirely to the 

 right side. It is decidedly suggestive of a chorioid plexus forming cerebro- 

 spinal fluid. A blood vessel lies in a fold in its wall and about this fold there is 

 a great mass of coagulum. Throughout the first roof plate expansion, the cav- 

 ity, and in many places the central canal itself, is larger than either the third 

 or the fourth ventricle. X 70. 



12 From a section 645 microns behind figure 11, and not far from the caudal 

 end of the first model (figs. 1 and 2). At this point the first roof plate expansion 

 and its enclosed cavity attain their greatest width, which cavity is broadly con- 

 nected with the central canal. X 70. 



13 Transverse section 80 microns behind figure 12 and at the very beginning of 

 model 2 (figs. 4 and 5). The dorsal wall of the first roof expansion is very wide and 

 extremely vascular, and the convexity of its walls and the arch of the cavity bear 

 evidence of moderate internal pressure from cerebro-spinal fluid. The wall, 

 while still quite thick, contains a lesser number of ependymal cells, but more 

 connective tissue. Note especially the absence of any direct connection between 

 the cavity of the roof expansion and the central canal. A chain of ependymal 

 cells still connects the two, and may be indicative of a former embryonic con- 

 nection that has been lost. From this region, caudad, there is no communica- 

 tion between the cavity of the roof expansion and the central canal. That a 

 former connection occurred may be indicated by the fact that at various inter- 

 vals ependymal cells are scattered between the two. A sensory root can be seen 

 sending its fibers inward toward what is believed to be the substantia gelatinosa. 

 X 70. 



13 A, represents a small portion of the first roof plate expansion and its in- 

 closed cavity from a section taken 410 microns behind figure 13. Observe es- 

 pecially the rich blood supply for the dorsal wall and the ease by which diffusion 

 could take place between a blood vessel and the cavity. A large roof plate cell 

 is drawn separately, highly magnified, directly to the left of 13 A. Note the 

 fine granules in cytoplasm, which gives evidence of being secretory. X 125. 



ABBREVIATION.^ 



B.V., blood vessel M.R., motor or ventral spinal nerve 



C.C, central canal root 



C.C.Ex., central canal extension into .V.A., membranous neural arch 



roof plate expansion N.C., notochord 



Ep.N., layer of ependymal nuclei R.P.Ex., roof plate expansion 



M.C., motor or effective cells S.C.F., cerebro-spinal fluid 



M.D.C., median dorsal cartilaginous S.G., substantia gelatinosa 



bar S.R., sensory or dorsal spinal nerve 



M.F., MixUcrian or giant fiber root 



50 



