THE HUMAN PINEAL ORGAN — DEVELOPMENT 385 



lying in rows between parallel nerve-fibres, have long been recognized 

 in specimens stained with the ordinary nuclear dyes such as iron haema- 

 toxylin. The appearance of these small cells in rows between the nerve- 

 fibres is strongly suggestive of multiplication by amitotic division, more 

 especially as mitotic figures are not often seen at this stage of development 

 and the nuclei frequently lie in pairs. It also seems possible that they 

 represent a modified or immature form of astrocyte rather than a distinct 

 type, the modification from the usual astrocyte form into the interfascicular 

 type of cell found in the white matter being due to the presence of the 

 medullated white fibres, which limit the expansion and formation of pro- 

 cesses in certain directions. The existence of transitional forms between 

 the oligodendrocytes and the astrocytes among the cells in the afore- 

 mentioned rows is an additional point in favour of this interpretation. 

 Oligodendrocytes are also found in relation with large nerve-cells, such 

 as those in the anterior cornua of the spinal cord and the pyramidal cells 

 of the cortex cerebri (Fig. 263). These are often spoken of as satellite 

 cells or perineuronal cells, whereas the oligodendrocytes found between 

 the medullated nerve-fibres of the white matter in the brain and spinal 

 cord are known as interfascicular cells. The small round granules lying 

 in or on the processes of both types of oligodendrocyte which are termed 

 gliosomes are believed to be concerned in the formation of the myelin 

 sheath of nerve-fibres, and the relation of the perineuronal and inter- 

 fascicular oligodendrocytes to the nerve cells and nerve-fibres suggests 

 an homology of the oligodendrocytes of the central nervous system with 

 the ganglionic capsular cells and the cells of the sheath of Schwann in the 

 peripheral nervous system. The small branched cells with acidophil 

 granules which are found in relation with the parenchyma cells of the 

 pineal body possibly represent oligodendrocytes in this organ. 



The mode of development of neuroglial fibres and the question of 

 the existence of an intercellular substance we shall discuss later in the 

 description of the structure of the normal pineal organ and the changes 

 which it undergoes in disease or as a result of involution, but, as we have 

 thought it advantageous to allude to some of the characteristic features 

 of neuroglial tissue in the central nervous system before entering on the 

 description of the pineal organ itself, so also we think that it will be 

 profitable to discuss now the structure and some of the principal modifica- 

 tions of the normal ependyma of the brain and spinal cord, and the 

 changes which it undergoes in disease or as a result of degeneration. 



In the fully developed pineal organ the definitive ependyma is limited 

 to the cells lining the pineal recess, but under certain conditions remnants 

 of the original ependymal lining of the primary cavity of the pineal 

 diverticulum or of its secondary outgrowths may persist as the lining 



25 X^^l 



