96 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



Graham Kerr, 1907) they bulge inwards. In the typical Teleosts 

 what apparently corresponds to the hemisphere forms simply a solid 

 mass projecting into the cavity of the fore-brain, the structure which 

 is usually and probably erroneously spoken of as the corpus striatum 

 in these fishes. 



A part of the brain which is of very special morphological interest 

 is the thalamencephalon — which persists with singularly little change 

 throughout the series of Vertebrates. Even in Amphioxus sagittal 

 sections through the front end of the neural tube present appearances 

 which vividly suggest the thalamencephalon of the more typical 

 Vertebrates (Kupffer) and raise the question whether — as is probable 

 enough on other grounds — the head region in Amphioxus is degener- 

 ate and once possessed a brain. 



Amongst the structures connected with the thalamencephalon 

 special interest attaches to the pineal body. 1 So far this has been 

 alluded to merely as a comparatively simple diverticulum of the 

 thalamencephalic roof. In the majority of Vertebrates it remains 

 comparatively uncomplicated and its main function appears to be 

 that of forming a peculiar internal secretion. 



In two sets of Vertebrates — the Lampreys on the one hand, and 

 Sphenodon and many Lizards on the other — there becomes developed 

 in relation to it an organ possessing a close resemblance to an eye, of 

 the " camera " type, possessing a retina and in some cases a lens. 

 The organ appears to be functional as the tissues overlying it are 

 commonly free from pigment and its retinal cells on exposure to light 

 show a change of position in their pigment granules similar to what 

 is commonly found in visual organs. Though functional it does not 

 follow that the organ serves for the detection of what we call light : 

 it may be that its sensitiveness is rather towards radiant energy of 

 other wave-lengths than that included within the range of the visible 

 spectrum. 



There is a general tendency amongst those who have carried out 

 researches upon the pineal eye to regard the eyelike condition as a 

 relatively archaic condition of the pineal organ — a tendency which is 

 encouraged by the evidence of palaeontology that certain ancient 

 Tetrapods of the palaeozoic and mesozoic periods possessed a highly 

 developed pineal organ — the skulls of these animals possessing a 

 relatively huge parietal foramen, corresponding with the foramen in 

 the roof of the skull of modern lizards in which the pineal eye lies 

 embedded. 



The evidence of embryology indicates that the most archaic con- 

 dition of the pineal organ was a simple diverticulum of the brain 

 roof projecting towards the skin on the dorsal surface of the head. 

 There is no clue whatever as to the original meaning of this diverti- 

 culum. But we do know from the study of invertebrates that where 

 tissue rich in nerve-elements comes to be exposed to light there is 



1 An admirable account of the structure and development of this region of the 

 brain by Studnicka will be found in Oppel (1905). 



