96 EMBEYOLOGY OF THE LOWER .VERTEBRATES CH. 



Graham Kerr, 1907) they bulge inwards. lu 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 <>f ;is the corpus striatum 

 in these fish- 



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

 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 tin- pineal eye lie- 

 embedded. 



The evidence of embryology indicates that the most archaic con- 

 dition !' the pineal organ was a simple diverticnlnni of the brain 

 projecting towards the skin on the dorsal surface nf the head. 

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

 culum. But we do know from the study of invertebrates that \\ln-re 

 tissue rich in ner\' ( 'lcni.-nt,s comes to he exposed to light, ihere is 



1 An adiiiiral.li- arrount of tin- .structure ami ilc\ rlujmn'iit of this r.-^ioii of tin- 

 braili l.y Slu.lnicka will b<- fouii'l in Oppd l'.05). 



