318 THE PINEAL ORGAN 



The processes of the parenchyma cells are especially well seen in the mar- 

 ginal plexus, at the periphery of the pineal lobules (Fig. 220). According 

 to the prevailing view held by the more recent authors, e.g. del Rio- 



PE.Z 





V CT 



Fig. 220. 



-Diagram representing Stages in the Development and Differ- 

 entiation of the Pineal Organ. 



A — Early stage, showing part of the epithelial wall of a lobule of the primary 

 pineal diverticulum, with a vascular strand of connective tissue separating 

 it from an adjacent lobule on the right. 



B — Later stage, the external limiting membrane has disappeared, and the vessels 

 with their connective tissue sheaths have penetrated the epithelial wall. A 

 differentiation of the primary ependymal cells has now taken place, a middle 

 zone of pale cells with vesicular nuclei now being present between the small 

 darkly staining cells and the reticular zone. 



C and D — The differentiated neuro-epithelial cells of the adult organ — ependy- 

 mal, C ; glial and parenchymatous, D. 



Ar. : arteriole. 



C. : capillary vessel. 



C.T. : connective tissue. 



E.L.M. : external limiting membrane. 



E.Z. : ependymal zone. 



G.l. : glial cell. 



I.L.M. : internal limiting membrane. 



M.Z. : mantle zone (large pale cells). 



P.C. : parenchyma cells. 



P.E.Z. : primary ependymal zone. 



R.Z. : reticular zone. 



V.C.T. : vascular connective tissue. 



Hortega, the majority of the processes end in club-shaped swellings 

 which are attached to the walls of the blood-vessels running in the trabe- 

 cular of connective tissue, while others join with each other in the forma- 



