THE PINEAL ORGAN OF MAMMALS 



321 



of epithelial cords into the surrounding vascular connective tissue and 

 an intimate blending of the epithelial and mesodermal elements. The 

 resemblance lies chiefly in the mode of differentiation of the definitive 

 cellular elements from the primary ependymal cells of the developing 

 neural tube. It will be remembered that in the development of the 

 central nervous system the medullary plate at first consists of a single 

 layer of columnar cells, between which on the primarily superficial surface 

 (Fig. 223, A), there are scattered here and there cells which are undergoing 

 mitosis. The medullary or neural plate is soon converted into the neural 



Pr.Sp. 



Fig. 222. — Section through a Perivascular Space in an Adult Human Pineal 

 Organ showing Minute Droplets of a Finely Granular Material 

 probably of a colloid nature and indicating an early stage in the 

 Formation of a Corpus Arenaceum. (R. J. G.) 



Cap. : capillary containing red blood corpuscles. 

 Pv. Sp. : perivascular space. 



groove, which afterwards by fusion of its margins along the mid-dorsal 

 line becomes the neural tube. The primarily superficial surface of the 

 plate is now the internal or ventricular surface ; and the primarily deep 

 or under surface is external and in relation with the vascular mesenchyme 

 which will become the pia mater. The wall of the neural tube rapidly 

 increases in thickness and the cell-elements lengthen out into protoplasmic 

 strands containing small, oval, nuclei (Fig. 223, B and C) ; an external 

 and an internal limiting membrane is developed and the mesenchyme 

 lying outside the external limiting membrane condenses to form the pia 

 mater. The radiating protoplasmic strands become joined by the union 

 of lateral processes and thus give rise to a continuous network of fibres 

 enclosing spaces. Imbedded in the radiating strands are deeply stained 

 oval nuclei, which occupy the inner and middle zones ; whereas the large 

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