1910.] and Adjacent Parts of the Brain in the Tuatara. 633 



niedullated 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 investigated 

 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 hence- 

 forth 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 belonging 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 find only 

 three kinds of histological elements : (1) radial supporting fibres, (2) ganglion- 

 cells and nerve-fibres, (3) sense-cells. 



The radial supporting fibres are comparable to the Miiller's fibres in the 

 lateral eyes, and probably extend right through from surface to surface of 

 the retina, their inner ends forming the well-developed internal limiting 

 membrane, and their outer ends abutting agahist the inner capsule of the 

 eye. Their nuclei appear to .be all lodged in their outer portions, which 

 have the misleading appearance of a layer of short conical cells. 



The ganglion-cells are numerous, and are readily distinguished by their 

 large spherical nuclei, finely granular cytoplasm (with usually one large 

 projection), and the shrinkage cavity which surrounds them. 



The sense-cells are slender, elongatedly spindle-shaped, with large oval 

 nuclei. Their outer ends run into the layer of nerve-fibres. Their inner 

 ends project slightly into the cavity of the eye, but are covered with little 

 conical caps, formed apparently by extension of the internal limiting 

 membrane. 



Ill most respects the structure of the retina agrees closely with that of 

 Anguis and Lacerta as recently described by Nowikoff. That author, 

 however, gives a somewhat different account of the projecting ends of the 

 sense-cells and of the distribution of the pigment. 



Baldwin Spencer considered that in Sphenodon the pigment was especially 

 associated with the sense-cells ; Nowikoff, on the other hand, maintains that 



