STRUCTURE OF PARIETAL ORGAN OF SPHENODON 253 



stage of development becomes incorporated with the end vesicle of the 

 pineal outgrowth, and thus fails to form an independent parietal organ " 

 (Klinckowstroem) ; or whether, as Studnicka believed to be more pro- 

 bable, the upper subdivision from the first represents the end vesicle 

 of the pineal outgrowth, the parietal organ being absent. 



Quite recently an interesting paper by G. W. H. Schepers has been 

 published in the J. of Anatomy (Vol. 72, 1938), in which he describes two 

 epiphyseal vesicles in the brain of a South African land tortoise, Testudo 

 geometrica. In this animal the epiphyses were situated in the usual 

 position between the cerebral hemispheres and midbrain ; the smaller 

 vesicle was antero-inferior in position, whereas the large vesicle was postero- 

 superior. They were enveloped in a large dorsal sac or parencephalon 

 (Fig. 180), and were situated between the ependyma and pia mater. No 

 stalk or nerve-fibres were found connecting the vesicles with the central 

 nervous system. The larger, superficial vesicle lay in a depression in the 

 roof of the dorsal sac, in close relation with the vessels of the choroid 

 plexus ; while the smaller, anterior vesicle lay in relation with the anterior 

 wall of the sac, below and between two anterior horns of the sac. The 

 walls of both cysts, but more especially the anterior, showed signs of 

 cellular degeneration. Schepers concludes that the vesicles are neural 

 derivatives and that the larger structure is comparable with the pineal 

 body ; the smaller with the " parapineal body." 



Structure of the Parietal Organ of Sphenodon 

 A. The Retina 



Beneath the thin inner fibrous capsule (Fig. 181), there is a well-defined 

 external limiting membrane ; while next to the lumen is an internal limiting 

 membrane pierced by the projecting inner ends of the sensory cells. 

 Between the two limiting membranes is a supporting tissue formed by 

 the elongated bodies of the ependymal cells, the outer ends of which are 

 expanded and continuous with the external limiting membrane. The 

 neurosensory elements consist of an inner layer of sensory cells, the 

 cylindrical bodies of which extend from the internal basal membrane 

 towards a middle layer of nerve-fibres. These course parallel to the 

 surface and midway between the two limiting membranes ; as the cell 

 bodies pass towards the middle stratum of nerve-fibres they taper into 

 fine processes which appear to end in this layer. External to the nerve- 

 fibre layer is found a stratum of nerve-cells, some of which are large and 

 contain a clear vesicular nucleus with a well-defined nucleolus. These 

 are the ganglion cells, processes of which join the nerve-fibre layer. 

 Besides the nerve elements just described there are in the internal layer 



