BRAIN. 147 



habenular ganglion and the posterior commissure, at the boundary 

 between 'twixt- and mid-brains, sometimes as two distinct structures, 

 sometimes as the result of division of a single outgrowth of the roof. 

 The anterior of these is the parietal organ or eye; the other the 

 pinealis or epiphysis proper. The two vary in development in dif- 

 ferent vertebrates, the parietal eye being well-marked only in cyclos- 

 tomes, Amia, teleosts and most lizards (fig. 151), while the pinealis 

 is almost invariably present. 



In its fullest development in lizards and Sphenodon the parietal 

 organ extends as a slender stalk, hollow at first, through the parietal 

 foramen of the skull, expanding beneath the skin to a vesicle, above 

 which the integument is usually thin and transparent, forming a physi- 

 ological cornea. The distal wall of the vesicle is thickened in the 

 middle, forming a lens, while the cells of the proximal side elongate, 

 each becoming differentiated into a distal, rod-like end and a proximal 

 portion which contains the nucleus and is connected with a nerve fibre. 

 Pigment is deposited between these cells so that the whole forms a 

 retina. An important point, to be better appreciated after the con- 

 sideration of the paired eyes, is the fact that these parietal eyes are 

 like those of most invertebrates in having no inversion of the retina. 

 How far these eyes are actually functional is not settled. Even in 

 Sphenodon, where it is best developed, experiments have resulted in no 

 decided reactions. 



In other vertebrates the parietal organ does not pass outside the 

 skull, and even may not appear transitorily in development. The 

 pinealis to some extent may take its place and often shows, as in certain 

 lizards, traces of a visual structure. In the anura its tip approaches 

 the skin and later is cut off from the brain by the development of the 

 skull, forming the so-called frontal organ, visible from the exterior. 

 Pineal and parietal organs differ in their nerve supply, the parietal being 

 connected with the superior commissure of the 'twixt-brain, the 

 pinealis and its derivatives with the posterior commissure. In the 

 higher vertebrates the epiphysial structures are completely covered by 

 the backward growth of the cerebrum. The large parietal foramina 

 in many extinct reptiles would seem to indicate that they had well 

 developed parietal or pineal organs. The roof of the brain in this 

 region, behind the lamina terminalis, also gives rise to a chorioid plexus 

 like that of the fourth ventricle, a part of which invades the third 

 ventricle and another portion, the inferior plexus, sends branches 



