THE INTEEBEAIN OH THALAMBNCEPHALON. 



127 



The lamina terminalis of the brain, before turning backward to form 

 the roof of the interbrain, passes first a short distance dorsally, — Lamina 

 supraneuroporica, — and then falls in the usually sail-like, pendulous Tela 

 cJiorioidea, from which, through anteriorly-directed evaginations, the Plexus 

 chorioidei of the yentricle is formed. In several amphibians (Fig. 55, B) 

 and in the Dipnoi, whose brain can scarcely be differentiated from the typ- 

 ical amphibian brain, the Tela grows exuberantly into the central cavity 

 of the interbrain by'the addition of numerous tap-like processes. Close to 

 the epithelium there always lies a rich, vascular plexus. It is probable that 

 the structure is an organ of secretion. Posterior to the Tela chorioidea are 

 one, or frequently several, dorsally directed projections called Paraphysis and 

 EpipJiysis, according to their relative location (see Figs. 76 and 86). 



Kg. 76. — Median sagittal section through the brain of the lizard : Taranus griseus. 



Just anterior to the epiphyseal evagination, the entrance to which is 

 not shown in the figure, lies regularly the Commissura habenularis. It be- 

 longs to the system of fibers which pass into the Ganglia habenulce from the 

 posterior olfactory region, and will be considered later. 



The epiphyseal tube is very constant. In several selachians and in 

 many reptiles this structure passes through a hole in the skull to a sense- 

 organ under the skin which has a striking resemblance to an eye. One can 

 recognize in this impaired parietal organ a cornea, a lens, a retina, and, 

 below this, a pigmental layer. "We are indebted to Graf and Spencer for its 

 discovery. In other vertebrates one finds in the adult stage no connection 

 between the epiphyseal tube and the sense-organ. The epiphysis has with- 

 drawn within the skull, and the parietal eye is so completely lost in the 

 amphibian and reptilian transitional forms that in birds and mammals no- 



