3 o2 SCIENCE PROGRESS 



these results we may conclude, with a considerable degree 

 of probability, that the anterior and posterior pineal organs 

 of the Lacertilia, as well as of the Rhynchocephala, represent 

 the vestiges of an original pair of sense-organs. 1 



The structure of the "pineal eye" in the adult Sphenodon was 

 first investigated by Baldwin Spencer in the memoir so often 

 referred to already. Spencer showed that it exhibits an 

 unusually high degree of histological differentiation ; but the 

 use of more recent methods of preparation has brought to light 

 still further details of structure, and necessitated some modifi- 

 cation of Spencer's views. 



The organ has essentially the same structure and relations 

 as in those Lacertilia in which it is still well developed, con- 

 sisting of a hollow vesicle lying in the parietal foramen beneath 

 a " parietal plug " of specially modified connective tissue. The 

 upper wall of this vesicle is greatly thickened to form the so- 

 called " lens," which is almost disconnected from the " retina." 

 The retina consists of (i) slender rod-like cells, which lie next 

 to the cavity of the vesicle with their inner extremities projecting 

 freely into the latter for a short distance, while their outer ends 

 are connected with slender nerve-fibres ; these rods have con- 

 spicuous nuclei and are separated from one another by numerous 

 minute pigment granules of a dark brown colour ; the cells in 

 which the pigment granules are developed have not yet been 

 made out ; (2) behind the rods lies a layer of nerve-cells and 

 fibres, and (3) at the outer limit of the retina is a layer of cells 

 which are probably supporting elements. 



The nerve of the pineal eye is very well developed, even in 

 the adult, and can easily be traced into continuity with the 

 layer of nerve-cells and fibres behind the rods. 



The "lens" is biconvex inform and also of complex histological 

 structure. Its outer half is made up of nucleated columnar cells 

 lying at right angles to the surface ; the inner half is composed 

 of somewhat similar cells which, however, appear to contain 

 numerous delicate fibrillae and bodies resembling centrosomes. 

 The most remarkable feature, however, is the presence of a large 

 central cell of irregular form, resembling a ganglion cell. 



1 It is possible that, as de Klinckowstrom suggests, the proximal portion of 

 the " stalk " may be common to the two organs ; in this case the apex of the stalk 

 only would represent the right pineal sense-organ. The structure of this apex 

 in Sphenodon, with its pigmented lower wall, tends to support this view. 



