68 THE PINEAL ORGAN 



towards the light is clear, as in the eyes of Patella, Trochus or Murex 

 (Figs. 33-35). This distribution of the pigment in the cells of the retina 

 is similar to that found in the retina of the parietal eye in some reptiles, 

 e.g. Anguis and Pseadopus Pallasii. It must be borne in mind, however, 

 that although the resemblance in the distribution of pigment and the 

 general disposition of the cells of the retina in the examples given is very 

 close, these eyes differ in other respects and cannot be regarded as homo- 

 logous. But the close resemblance in minute structural details, with 

 regard to the deposit of pigment in two types of cell and the similarity of 

 its disposition in the eyes of widely different classes of animals, does point 

 to a general homology with regard to histological structure among these 

 classes — a point which is fully corroborated by resemblances in histological 

 structure which occur in other tissues. 



Pigmentation of the Eyes of Vertebrates 



This subject may be considered under two headings : 



1. Pigmentation occurring in the pineal organ. 



2. Pigmentation of the lateral- or paired-eyes. 



Pigmentation of the pineal apparatus may further be considered in two 

 sections, namely : 



A. Pigmentation of the parietal sense-organ, or " pineal eye," and the 

 accessory structures connected with it. 



B. Pigmentation of the " epiphysis," conarium, or " pineal gland." 

 Commencing with the description of the pigment found in the pineal 



eyes of cyclostomes, we have to distinguish between the pigment found in 

 the retina of the parietal sense-organ and that which is found in the tissues 

 surrounding this. The pigment cells of the retina are distinct from the 

 sensory cells and have been accurately described by Dendy in the late 

 velasia stage of the New Zealand lamprey — Geotria (Fig. 45). They are 

 long, tapering cells, having a wide inner end directed towards the cavity 

 of the vesicle. The free end of this inner segment is rounded in form and 

 marked off from the body of the cell by a clear line which corresponds in 

 position to a limiting membrane ; this membrane can be clearly seen in 

 depigmented specimens. The nucleus is oval and situated near the outer 

 end of the cell ; beyond this, where the cell divides into thin tapering 

 processes, pigment granules are absent. The granules are almost uniform 

 in size and appear dark-brown or black by transmitted light. Pigment 

 cells are not found in the left parietal sense-organ — " parapineal organ " 

 of Studnicka — and they are absent in the single parietal sense-organ of the 

 hag fish, Myxine glutinosa. 



The appearance of the pineal region of an adult lamprey, P. planeri> 

 as seen from above is indicated in Fig. 47, after Studnicka. This author 



