I'LA'i'i'; .-) 



KXl'I.ANATIONT OK KICUIUCS 



II I'loiii a Iraiisvcrsc section lakcn I!) microns hcliiTid limine 

 seiits a condition i'oiuul tluouKliout the entire posterior end ol 

 expansion, where a series of cavities are connected with one ano 

 root is seen issuing from the cord. X 70. 



15 to 17 Three transverse sections, anterior, middle, and po 

 what has been designated as the second outcroi)i)ing of the riK 

 in model 2 (figs. 4 and 5) appears immediately behind the first 

 already figured. In figure 15, which is through the anterior end of this roof 

 plate expansion, the outcropping is one of cells only. In figure 16 a distinct cav- 

 ity containing coagulum is visible, connected below with the central canal. Ob- 

 serve the vascularity of its walls, of the ependyma surrounding the central canai 

 and the diverticula of the same, which doubtless serve as a modified chorioid 

 plexus for producing cerebro-spinal fluid. In figure 17, which is through the 

 posterior end of ths roof expansion, a cavity is still present. Its walls, which 

 are mainly connective tissue, are rich in blood vessels. A few scattered ependy- 

 mal cells and fibers represent a possible connection with the central canal. A 

 third outcropping of the roof plate has been described, but not figured in trans- 

 verse section. It is located behind the second expansion, and the fact that no 

 ependymal cells are encountered between the first and the second, and the second 

 and third roof expansions, except immediately surrounding the central canal, 

 favors the view that these separate outcroppings of the roof plate in the adult 

 were never connected in an embryonic state. X 70. 



18 and 19 Two sections some distance caudad of figure 17 and some little 

 distance apart. They show the spinal cord to l)e jjcrfectl}^ normal, l)ut to be 

 gradually tapering down in size. Figure 18 is from that part of the cord repre- 

 sented by model 3. The ependymal area in the center has maintained its origi- 

 nal size, the reduction that has occurred in size is to be found in the nervous 

 portion. Not only does the ependymal area broadly divide the spinal cord into 

 two halves, but it has here as elsewhere in a number of places, entirely obliter- 

 ated the central canal for a space of a few microns. Figure 19 is some distance 

 caudad of figure 18, passing through the sensory nerve roots one segment anterior 

 to the abnormal sinus terminalis (model 4, fig. 9, S.T.). The spinal cord will 

 be seen to be fast losing its nervous structure, for no motor cells will be seen in 

 this section or in any further caudad. X 70. 



ABBKEVIATIO.N'S 



B.V., blood vessel M.V.C, median ventral cartilaginous 



C.C., central canal bar 



C.C.Ex., central canal extension into A^^l., membranous neural arcli 



roof plate expansion or cast of the Nc, notochord 



same R.P.Ex., roof plate expansion 



Ep.C, ependymal cells R.P.Ex. (2), second roof plate expun- 



Ep.N., layer of ependymal nuclei sion 



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



bar S.R., sensory or dorsal spinal nerve 



M.R., motor or ventral spinal nerve root 



root 



52 



