254 THE PINEAL ORGAN 



cells containing pigment granules, which are mostly arranged in a radial 

 manner parallel to and between the sensory cells ; pigment granules are 

 also found in and between the sensory cells and in the outer layer, where 

 they are often deposited in large round cells called " pigment balls " ; 

 small neuroglial cells with branched processes are also present. More- 

 over, a considerable amount of pigment is contained in the connective 

 tissue around the parietal organ, namely, between the inner and outer layers 

 of the fibrous capsule and in the loose areolar tissue beyond the capsule. 



B. The Lens 



The lens varies markedly in form both in different species and in 

 different examples of the same species. It is usually biconvex, but one 

 side may be more convex than the other or it may be approximately plano- 

 convex. In the latter case the flatter surface may be directed super- 

 ficially, or it may be directed towards the cavity of the parietal organ. 

 Other lenses are irregular in form ; in some cases sections of the lens 

 may show what appears to be a central notch, lying beneath a clump of 

 rounded pigment cells as in Fig. 176, showing the parietal eye of a specimen 

 of Varanus giganteus, described by Spencer. In a similar case in which 

 pigment was present in the centre of the lens in a specimen of Anguis 

 fragilis, described by Beraneck (Fig. 182), there was a peculiar seam on 

 the superficial surface of the lens, which suggests that the lens was formed 

 by the growth in contact with each other of the rudiments of two lenses. 

 In some cases the lens may be flattened and very wide, more especially in 

 those cases in which the whole parietal organ is wide and appears to be 

 dorso-ventrally compressed. It is especially in this flattened, wide type 

 of lens that central pigment cells have most frequently been observed. 

 An exception to the above statement, however, occurs in an example of 

 a very narrow, almost cylindrical parietal organ in Anolis, an American 

 type of Iguana described by Spencer. In this specimen the vertical or 

 axial diameter of the lens is little less than the transverse diameter, pigment 

 cells were nevertheless present in the middle of the inner half of the lens. 

 The growth of the whole organ appears to have been arrested at an early 

 stage of development, and the retina to have been composed simply of 

 elongated ependymal cells separated by wide intercellular spaces. It 

 may be noted also that the parietal foramen in this specimen was especially 

 narrow, being little wider than the parietal organ. With reference to 

 these conditions it may be mentioned that in a nearly related type of 

 Anolis Spencer found a dorso-ventrally flattened form of parietal eye, and 

 that the occurrence of such opposite extremes in the shape of the parietal 

 organ and its lens in the same genus merely exemplifies the tendency to 

 variation which exists in degenerating organs, and does not contradict 



