322 



THE PINEAL ORGAN 



cells showing mitotic figures are found next the internal limiting mem- 

 brane. There is at first no definite limit demarcating the ependymal 

 zone from the middle or mantle zone. The outer or marginal zone is, 

 however, easily recognized (Fig. 223 B, RZ) by the absence of nuclei 

 and the clear visibility of the glial fibres, which form the bed in which 



PM ELM. RZ 



Fig. 223. — Differentiation of Cells in Wall of Neural Tube. 



A — Section of medullary plate of rabbit embryo before closure of neural tube. 

 B — 7-mm. pig embryo 690, after Hardesty. 



C — 10-mm. human embryo showing ependymal, mantle and reticular zones. 

 D. — Ependymal cells from a part of the wall of the neural-groove of a first-day 

 chick embryo ; Golgi preparation (Cajal). 



C.C. : columnar cell or central canal. 



D.S. : deep surface 



E.C. : primary undifferentiated ependymal 

 cells which will give rise to defini- 

 tive ependymal cells, supporting glial 

 cells (spongioblasts) and nerve cells 

 (neuroblasts) 



E.L.M. : external limiting membrane 



E.Z. : ependymal zone. 



G.C. : germinal cell 



I.L.M. : internal limiting membrane. 



M. : mitosis. 



M.Z. : middle or mantle zone contain- 

 ing neuroblasts. 



N. : nuclei of ependymal cells. 



P.M. : pia mater. 



R.Z. : reticular or marginal zone. 



S.S. : superficial surface later becom- 

 ing the internal or ventricular 

 surface. 



the white medullated fibres are afterwards developed. In a 10-mm. 

 human embryo (Fig. 223, D) the three zones are distinct and consist of : 

 (1) an inner primary ependymal layer, formed of undifferentiated cell- 

 elements containing small, darkly stained nuclei, which are arranged in a 

 radial manner round the central canal and are closely packed ; the nuclei 



