THE PINEAL ORGAN OF MAMMALS 323 



are about five or six deep and are almost uniform in size ; (2) the middle 

 or mantle zone — this is the forerunner of the grey matter and contains 

 large, rounded cells with pale vesicular nuclei. These are the neuroblasts 

 and are comparable with the large pale cells with vesicular nuclei of 

 the pineal organ in late foetal life and early infancy, both as regards their 

 origin from the primary ependymal cells of the inner zone and with 

 respect to their further differentiation, namely, the neuroblasts of the 

 central nervous system into nerve cells ; the large pale cells with vesicular 

 nuclei of the pineal organ into parenchyma cells with processes resembling 

 the processes of nerve-cells, but lacking an axis cylinder or myelinated 

 nerve-fibre. A differentiation also takes place of the primary ependymal 

 cells into the definitive ependyma, consisting of cubical or columnar 

 cells lining the ventricular cavities and central canal of the spinal cord, 

 and the spongioblasts which develop into the supporting or neuroglial 

 cells. The primary ependymal cells of the central nervous system thus 

 give rise to three types of cells : (1) the definitive ependyma, (2) the 

 neuroglial cells, and (3) neuroblasts which differentiate into nerve cells. 

 In the pineal organ a similar differentiation takes place of three types of 

 cell from the primary ependymal zone, namely : (1) the definitive 

 ependyma, lining the pineal recess ; (2) spongioblasts giving rise to glial 

 cells ; and (3) large pale cells with vesicular nuclei which give origin to 

 the branched parenchymatous cells of the adult organ. 



As a rule the lumen of the epiphysis in mammals and the cylindrical 

 ependymal epithelium which lines it disappear entirely during the later 

 periods of foetal life ; remnants of the lumen may, however, persist in 

 the adult animal, in the form of minute, often microscopic, cysts, of which 

 an example is shown in the section of an epiphysis of an ox (Fig. 304 

 (Dimitrowa), Chap. 32, p. 452). 



We do not propose to do more than allude here to some of the general 

 relations of the pineal organ to neighbouring structures and to note some 

 points of interest with regard to the relative size and form of the 

 pineal organ in some types of the Mammalia, since the variations in micro- 

 scropical structure can be most conveniently dealt with in the chapter on 

 the structure of the human pineal organ. 



Relations of the Pineal Organ in Mammals 



The pineal body has been found to be present in nearly all species 

 of mammalia, from the Prototheria or Monotremes, including the duck- 

 bill or Ornithorhynchus (Fig. 224) and the spiny ant-eater or Echidna, 

 up to man. Its attachment to the roof of the thalamencephalon between 

 the anterior and the posterior commissures is always the same, but there 

 are differences with regard to its general form. As a rule it is conical, 



