ii PINEAL OKGAN 97 



frequently shown a well-marked tendency to the evolution of eye-like 

 structure. In Molluscs for example we find eyes developing .u the 

 edge of tin- mantle /v/-//:/i), round the tips of tin- siphons (Oardium 

 sp.), on tin 1 <!<>rsul surface of the body (independently in Ch-ttnn and 

 Oncidium) and similar instances might be quoted from other groups. 



Hearing such facts in mind one is compelled to acknowledge the 

 possibility, if imi probability, of such a projecting piece of nervous 

 tissue as t.-hi- piii'-al diverticulum, lying close under the surface of the 

 head on its dorsal side, in the position where light stimulus would be 

 most pronounced, developing secondarily in some cases into an organ 

 of the nature of an eye. 



Discovered by Leydig (1872), its structure investigated by Spencer 

 (1886) and other workers, the development- of the pineal eye has 

 formed the subject of a number of excellent researches. It will be 

 convenient to take as an example that of the common lizards of the 

 genus Lacerta (Novikol'f, 1910). 



The first indication of the organ appears in embryos of about 

 .') mm. in length in the form of a thickening of the thalamencephalic 

 roof, in the region of the mesial plane, and divided by a transverse 

 furrow on its outer surface into a smaller anterior and a larger 

 posterior portion. This thickened part of the brain roof comes to 

 bulge outwards and forms a prominent projection (Fig. 55, A) the 

 groove dividing it externally into anterior and posterior portions 

 being still visible though less distinct. 



The projecting pocket now grows forwards parallel to, and in close 

 contact with, the brain roof (Fig. 55, B), its forwardly projecting 

 portion becoming constricted off from the rest. The constriction in 

 question deepens and the anterior portion (parapineal body) becomes 

 nipped off to form a completely closed vesicle (Fig. 55, C) the rudi- 

 ment of the eye. As the external ectoderm recedes from the brain 

 roof, with the increase in the amount of mesenchyme between the 

 two, the parapineal vesicle remains close to the ectoderm and con- 

 sequently retreats from the brain surface (Fig. 55, D). 



The eye is now seen to be connected with the brain wall by a 

 distinct optic nerve which, in full accordance with the view taken in 

 this book with regard to nerve-trunks in general, is merely a primary 

 bridge which already existed (Novikoff) at a time when the eye vesicle 

 ;ind the brain roof were still in immediate contact and which simply 

 be. -a. me extended in length as the gap between eye and brain became 

 greater and greater (Fig. 56, p.n). Nerve-fibres develop in this optic 

 nerve which pass at their cerebral end into the habenular commissure. 

 Transverse sections through a 9-inm. embryo show that the fibres on 

 entering tin- commissure bend away to the right, passing eventually 

 to the right hahenular ganglion. In this connexion with the right 

 hahenular ganglion Lacerta resembles the other lizard Iguaii" 

 /'(to (lvlineko\\ strom, 1894) but curiously differs from Sphenodon 

 wbere according to I )endy (1899) the connexion is with the left 

 hahenular ganglion. 



VOL. II H 



