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over the roof of the dorsal sac and over the upper part of the para- 

 physis. Id the adult it is a relatively large organ and takes an im- 

 portant part in the formation of the pineal complex. It remains 

 tubular, but its walls become greatly folded and much thickened. They 

 are supplied with blood by the anterior and posterior pineal arteries 

 and drain into the sinus longitudinalis. There is little or no evidence 

 that the pineal sac is a glandular body, but, on the contrary, its 

 histological structure points to a sensory function. Its thick wall is 

 made up of nucleated radial supporting fibres, numerous ganglion-cells 

 and nerve-fibres, and numerous sense-cells whose inner ends project 

 slightly into the lumen of the organ. These constituents are identical 

 with those which occur in the retina of the pineal eye, and their 

 arrangement is essentially the same. In one case, in which the tip 

 of the pineal sac projected unusually far forwards, so as to come under 

 the influence of the light passing through the transparent parietal 

 plug, a pigmented evagination of the wall of the pineal sac was formed, 

 and the resemblance to the retina of the pineal eye became still more 

 obvious. These observations, confirming and extending earlier ob- 

 servations by Hoffmann, Gisi, and myself, greatly strengthen the 

 view that the pineal sac and pineal eye are bilaterally homologous 

 structures. 



The pineal sac is provided with a well-developed nerve, composed 

 of non-medullated fibres, which runs in the "pineal tract" and joins 

 the brain-roof in the middle line between the superior and posterior 

 commissures, which remain perfectly distinct throughout life. 



The histological structure of the pineal eye itself has been in- 

 vestigated with especial care, and various methods of fixation and 

 staining have been employed for the purpose. The sharp distinction 

 between lens and retina appears at a very early date, and though 

 they remain in contact with one another throughout life, the actual 

 connection between the two is henceforth very slight, and the transition 

 from the one to the other is perfectly abrupt. 



At a very early stage the development of the nerve-fibres divides 

 the retina into a thick inner and a thin outer layer, with the nerve- 

 fibre layer between them. The inner layer contains many nuclei be- 

 longing to sense-cells, and also nuclei which belong to ganglion-cells. 

 The outer layer contains only a single layer of nuclei, belonging to 

 the radial supporting fibres. Later on the ganglion cells come to lie 

 more to the outside of the nerve-fibre layer, next to the nuclei of the 

 radial fibres. 



In the adult retina, omitting for the moment the pigment, we 



