THE PINEAL GLAND 303 



The cavity of the optic vesicle contains a network of 

 delicate fibres which are attached, on the one hand, to the 

 projecting ends of the rods, and on the other to the inner 

 face of the " lens." 1 



It appears, therefore, that in certain details of structure, as 

 well as in its general form and relations, the pineal eye of 

 Sphenodon closely agrees with that of the lampreys. Especially 

 noteworthy are the projecting apices of the rods and their 

 connection with the back of the " lens " by a network of fibres. 

 There are also, of course, important differences, especially in 

 the character of the retinal pigment, which is dark brown in 

 Sphenodon and white in the lampreys, and in the structure of 

 the upper wall of the vesicle, which in the lampreys is compara- 

 tively simple histologically and by no means lens-like in form. 

 In the lampreys, indeed, as Studnicka has pointed out, the 

 pineal eye is not a " cameral eye " at all ; whether or not it is so 

 in Sphenodon and the Lacertilia may also be questioned. The 

 structure of the " lens " in Sphenodon seems to indicate some 

 other function than that of merely focussing light-rays ; while 

 the presence of pigment in the " lens " of Varanus, as demon- 

 strated by Baldwin Spencer, would seem to render it useless as 

 a lens. In the latter case, however, we may be dealing with 

 modifications in structure consequent upon degeneration. 



If we accept the principle that the degree of morphological 

 and histological differentiation of an organ is an index of its 

 functional activity, we cannot escape the conclusion that in 

 Sphenodon, and also in Petromyzon and Geotria, the "pineal eye" 

 is something a good deal more than a mere vestigial structure. 

 Organs which have lost their function undergo reduction and 

 simplification, as witness the ordinary eyes of many animals 

 which habitually live in the dark (e.g. Proteus). No doubt this is 

 the case with the pineal sense-organs of the vast majority of 

 vertebrates, but in the case of the forms just mentioned we 

 must, at any rate for the present, suspend our judgment. 



It has frequently been pointed out that there is a great 

 morphological difference between the " pineal eyes " and the 

 ordinary paired eyes of vertebrates, inasmuch as the former 



1 The minute structure of the pineal eye in Sphenodon has as yet, so far 

 as I am aware, been investigated in only a single properly fixed specimen, and 

 further observations are needed in order to show how far the results obtained are 

 of general occurrence. 



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