THE PINEAL SYSTEM OF IGUANIDiE 277 



slope slightly outward as they ascend to the margin of the lens. The 

 retina is composed of a single layer of cells containing pigment in their 

 rod-like inner extremities, while the outer ends terminate in conical 

 expansions the bases of which are continuous with the external limiting 

 membrane ; oval nuclei were present in the bodies of the cells near the 

 centre. The lens was biconvex and consisted of elongated cells, the oval 

 nuclei of which lay for the most part in a single plane. A thick layer of 

 corium lay between the parietal organ and the epidermis. This and 

 the epidermis with the overlying corneal scale were destitute of pigment. 

 The corium was remarkable with regard to the exceptionally large size 

 of the papillae which entered the epidermis. The corneal scale was 

 slightly arched upwards, like the glass of an old type of watch. No 

 nerve was described, and it is worth while noting that the absence of 

 a nerve in association with such a simple structural type of retina is 

 what might be expected, the differentiation of the cells from the primary 

 single layer of ependymal elements having advanced very little beyond the 

 stage of deposit of pigment in the inner ends of these cells. 



A nearly allied species, Leiolcemus tenuis, also described by Spencer, 

 showed very similar conditions to those described above. 



Plica umbra (Spencer). 



The pineal system of this animal is remarkable for the extraordinary 

 length of the epiphysis, which, arising from the roof of the diencephalon 

 in the usual situation, is seen to consist of a proximal nearly vertical 

 part and a horizontal segment which comprises the greatly elongated 

 stalk, at the distal end of which is a small pigmented vesicle of doubtful 

 nature, which has been regarded by different authors as the end vesicle 

 of the pineal organ ; as a part of, or offshoot of the parietal organ ; or 

 an accessory organ. Both the upper and lower walls are pigmented. The 

 true parietal organ is a small flattened vesicle lying far forward and in 

 an exceptionally large parietal foramen. The superficial wall, corre- 

 sponding to the lens in other specimens, is unpigmented ; the lower 

 or retinal segment is deeply pigmented. There was a specially large 

 corneal scale in the middle of which was a small, slightly arched parietal 

 speck. 



Iguana tuber culata. 



The pineal system of Iguana has been specially studied by Spencer 

 (1886), Leydig (1896), and Klinckowstroem (1895). The latter author 

 also followed out in detail the developmental history of Iguana. His 

 conclusions from the general standpoint are so important that we shall 

 give them in full, in place of detailed descriptions of individual specimens. 



