PLATE 1 



EXPLANATION OF FIGURES 



1 Parasagittal section of sub-commissural organ. 49 days. X 325. 



2 Parasagittal section of sub-commissural organ. 60 days. X 400. 



3 Parasagittal section of sub-commissural organ. 100 days. X 325. 



4 and 5 Single ependymal cells obtained by dissociation. The diameter of 

 the fibrillar process apparently depends on its length. X 1608. 



6 A somewhat diagrammatic view of a sagittal section of the spinal cord 

 (neural tube) near the middle of the bodJ^ A connection between Reissner's 

 fiber and the fibrils of ependymal cells is simulated. The cell in the lumen of 

 the tube is helping to form, or to repair (probably the latter) Reissner's fiber. 

 60 days. X 750. 



7 Sagittal section of spinal cord near the middle of the body, showing 

 Reissner's fiber apparently free from the walls of the neural tube. The dorsal 

 wall is not shown in its whole thickness. 100 days. X 750. 



8 Sagittal section in the region of the sinus terminalis. X 325. 



9 Posterior end of the section shown in figure 8, at a slightly different focus 

 and more highly magnified. Several cells are seen to be in process of meta- 

 morphosing into the posterior end of Reissner's fiber; others, apparently, are 

 just entering the lumen of the neural tube. X 750. 



10. Sagittal section through the sinus terminalis, showing the terminal 

 plug. There is only one cell in process of metamorphosing into the Reissner 

 fiber, and this is, apparently, in continuity with the connective tissue lying 

 beyond the postei-ior end of the neuraj tube. X 325. 



ABBREVIATIONS 



a, anterior fhrl." , cross-sections of fhrl.' 



cl. e'end., ependym.al cell i., internal limiting membrane 



cl. R., cells in process of forming Reiss- ivlr., sheath oi fbrl.' 



ner's fiber nl. e'end., ependymal nucleus 



coag., coagulum nl., nucleus 



co'ms. p., posterior brain commissure nl. n., nucleus of nerve cell 



d., dorsal obt. trm., terminal plug of neural tube 



ex., external limiting membrane p., posterior 



fbr. p., fibers of posterior commissure par. n., wall of neural tube 



fbr. R., Reissner's fiber sb-co'ms., sub-commissural organ 



fbrl., fibrils from deep (proximal) ends lis. co'nt., connective tissue 



of ependymal cells v., ventral 

 fbrl.' fibrils from superficial (distal) 



ends of ependymal cells 



226 



