100 EMBEYOLOGY OF THE LOWEK VEETEBEATES ch. 



thalamencephalon. In each of these the lower wall shows histological 

 characteristics of retinal tissue and each is in continuity with the 

 brain — in the case of the parapineal organ directly and in the case of 

 the pineal by an elongated stalk containing nerve-fibres. 



The parapineal organ lies in some cases (Geotria — Dendy, 1907) 

 slightly to the left of the pineal and its nerve-fibres have been traced 

 into the left habenular ganglion while those of the pineal organ have 

 been traced to the right habenular ganglion. In neither case does 

 the outer wall of the vesicle show any signs of thickening to form a 

 lens — so that neither organ can form an image — but the overlying 

 tissue is comparatively transparent so that diffuse light stimulus can 

 reach it. 



According to Studnicka the two organs develop as evaginations 

 of the brain roof one (parapineal) in front of the other. The para- 

 pineal evagination soon loses its lumen and becomes solid and it is 

 noteworthy that at first it is continuous on each side with the habenu- 

 lar ganglion of that side. Later on it becomes by differential growth 

 shifted far forwards, away from the region of the habenular ganglia, 

 and it loses its connexion with the right ganglion while it remains 

 connected by nerve-fibres with the left. 



The two questions of special interest which present themselves 

 in regard to the pineal and parapineal organs are (1) were they 

 originally ocular in structure and function and (2) were they paired 

 or unpaired ? 



(1) It is obvious that the presence of an eye-like pineal or para- 

 pineal organ in certain Eeptiles and in Lampreys, and of a large 

 parietal foramen in the skull of various extinct Vertebrates suggests 

 the possibility of these organs having had the form of visual sense 

 organs in the ancestral Vertebrate. Against this however must be 

 set the fact that in all other Vertebrates than those mentioned, 

 including such relatively archaic forms as Elasmobranchs, Cross- 

 opterygians, Dipnoans and Urodeles, there is no trace whatever of 

 eye structure. 



It seems highly improbable that a well-developed visual organ 

 once present on the dorsal side of the head in the ancestors of 

 Vertebrates should have disappeared without leaving a trace in all 

 the varied groups, with their very different modes of life, outside 

 the limits of the Lampreys and Eeptiles. To the present writer 

 it does not appear that the evidence, so far as it exists at present, 

 is anything like convincing that the pineal eye is an ancestral 

 feature of Vertebrates in general rather than a mere secondary 

 development. 



(2) Various recent investigators of the pineal organs are inclined 

 to look upon them as being originally paired structures, the pineal 

 organ in the strict sense being the right-hand member of the pair 

 and the parapineal organ the left. This is perhaps most clearly 

 suggested by the Lampreys in which the parapineal organ is con- 

 nected by nerve-fibres with the left habenular ganglion and the 



