212 THE PINEAL ORGAN 



cells often protrude into the lumen of the tube between the inner ends of 

 the ependymal cells ; here they form cylindrical or club-shaped swellings, 

 with rounded free extremities and sometimes show a constriction at the 

 level of the free ends of the ependymal cells. They frequently contain 

 fine granules and threads, which are probably mitochondrial in nature. 

 A few large ganglion cells are found, chiefly in the vicinity of the external 

 limiting membrane ; they are feebly stained, the cell-body being clear or 

 finely granular. The nucleus is also palely stained, but shows a well- 

 defined nucleolus. The neuroglial cells are small, mostly spindle-shaped, 

 with tapering processes ; the amount of cytoplasm relative to the size of 

 the nucleus is very small, and the nucleus is deeply stained. Nerve- 

 fibres are seen singly or in bundles, mostly lying parallel to the surface, and 

 fine neuroglial fibres are also present. 



The proximal part of the pineal organ of the sturgeon, as well as 

 the stalk and end-organ, thus appears to have a structure, which is com- 

 parable with that of the parietal sense-organ of the Cyclostomes and 

 reptiles rather than with that of a secreting gland. 



With regard to the destination of the nerve-fibres ; these were studied 

 by J. B. Johnston in Acipenser. He described a nerve-fibre layer which 

 passes from the dorsal surface of the pineal sac to the roof of the brain, 

 where the fibres passed across from one side to the other, dorsal to the 

 habenular commissure, in what he called the " epiphyseal commissure." 

 He was able to follow the fibres on into the region of the nucleus anterior, 

 and he considered that these fibres at least in part had the value of neurites of 

 the cells of the epiphyseal sac. Other fibres of the " decussatio epiphysis " 

 were seen to end freely between and in contact with the cells forming the 

 wall of the stalk. In the decussatio epiphysis a third type of cell was 

 demonstrated, namely, cells, the fibres of which simply decussated and 

 could be followed into the region of the nucleus anterior in front of the 

 ganglion habenulas. 



Other fibres were traced by Johnston from the proximal end of the 

 stalk which turned laterally over the upper side of the epiphyseal sac to the 

 upper border of the decussatio epiphysis ; some of these fibres then 

 crossed forward and passed into the ganglia habenulae, about an equal 

 number being distributed to each side. 



This bundle of commissural fibres is probably the same as that 

 described by Eycleshymer and Davis in the epiphyseal region of Amia 

 calva (p. 213, Fig. 149, E). 



The pineal organs of some special types of fish may now be alluded to 

 with the view of drawing attention to certain points which are of value in 

 the study of the pineal system as a whole. Among these is the primitive 

 Ganoid fish Amia calva, which is commonly known as the American 



