GENERAL CONCLUSIONS 469 



indications are : (1) the frequent absence or disappearance during the 

 later stages of development of the nerve or nerves connecting the parietal 

 eye or epiphysis with the central nervous system ; (2) excessive develop- 

 ment of pigment in or around the retinal cells, or development of pigment 

 in the lens or cornea ; (3) degeneration of the retinal epithelium of the 

 parietal organ ; in the epiphysis of anamniota degeneration of the lining 

 epithelium of the pineal stalk ; or in the pineal organ of adult birds, 

 degeneration of the epithelium lining the follicles, accompanied in some 

 cases by obliteration of the lumen of the follicles ; and in the pineal organ 

 of adult mammals frequent degeneration of the parenchyma cells. The 

 degree of degeneration of the parenchyma cells in adult mammals varies 

 both in different individuals and in different parts of the organ in the same 

 individual. In the latter case it is common to find areas in which the 

 parenchyma cells have disappeared altogether and been replaced by 

 neuroglial plaques or bands. These often break down in the centre to 

 form cysts, and deposits of calcareous salts are frequently seen in the 

 walls of the cysts or in the trabecular or capsule. See Figs. 221, A, B, C, 

 2 %5> 317? and 318. 



8. In addition to the evidence in some extinct and living vertebrates 

 of a single pair of pineal organs which are united, either partially in the 

 stem of a Y-shaped organ bearing two terminal vesicles, or completely 

 fusion having taken place throughout the whole length of the stalk which 

 terminates in a single composite vesicle, there are indications, according 

 to certain authors, of the existence of two pairs of parietal organs arranged 

 serially, one pair lying in front of the other. Thus in the Palaeozoic fish 

 Bothriolepis (Fig. 319), Patten describes, in addition to the median eye 

 tubercle situated on the pineal plate between the two orbital cavities, a 

 pair of bilateral impressions which are visible only on the internal aspect, 

 and are present on the deep surface of the post-orbital plate (Fig. 320). 

 These he believed lodged a pair of posterior median or parietal eyes. 

 The three impressions or pits form a triangular group disposed in a 

 similar manner to the median eyes of many invertebrates, and, more 

 particularly, the triplacodal entomostracan eye which is found in certain 

 Branchiopods, e.g. Apus and Branchipus (Figs. 248, 250), and in the 

 " carp louse " Argulus foliaceus, which is typical of many other crus- 

 taceans. Another interpretation of the meaning of these two impressions 

 is given on p. 472 by Stensio, who suggests that they are produced by 

 the attachment of paired muscles of the lateral eyes. The existence and 

 exact position of two pairs of retinal placodes which will give rise to the 

 median eyes of vertebrates and which lie one in front of the other on each 

 side of the open medullary plate, has not, we believe, been definitely 

 established, nor is there agreement with respect to their exact position 



