FORE-BRAIN 



532 THE EYE IN EVOLUTION 



to form an ectodermal tube enclosing an axial canal, ^ the anterior end of 

 which dilates markedly to form the brain while the remainder forms the 

 segmented dorsal spinal cord ; the latter acts as a reflex centre, while in 

 addition to this, the former assumes controlling and integrating functions 

 of ever-increasing importance so that as evolution proceeds the entire 

 mechanism shows a progressive degree of cephalization. 



At an early period the embryonic cerebral vesicle shows two constric- 

 tions dividing it into three primary bulb-like vesicles — the fore-brain 

 (prosencephalon), the mid -brain (mesencephalon) and the hind -brain 

 (rhombencephalon) (Figs. 699 to 709). During the course of vertebrate 

 evolution these three primary vesicles differentiate as follows : 



'TELENCEPHALON — olfactoiy lobes, cerebral cortex (pallivim), basal 



nuclei of the corpus striatvun. 

 DiENCEPHALON — thalamus, epithalamus, hypothalamus, ejDiphysis, 

 hypophysis. 



Tectum (and optic lobes or, in Mammals, corpora 

 MID -BRAIN c^uadrigemina), tegmentvim (and, in Mammals, 



cerebral peduncles). 



{METENCEPHALON — cerebclKun and part of medulla oblongata (in 

 Mammals, the pons). 

 MYELENCEPHALON — remainder of the medulla oblongata. 



In some fishes this division into five main segments is maintained, but in 

 most Vertebrates the telencephalon grows out into two paired lobes (the 

 cerebral hemispheres), each containing the cavity of a lateral ventricle 



(Fig. 702). 



It is interesting that in Cyclostomes the histological structure of the central 

 nervous system is extremely primitive and its organization allows for the most 

 part only total movements of the whole body (mass reflexes) rather than complex 

 adjustments involving precise coordination. It is thus unspecialized, j^lastic and 

 capable of differentiation in any direction — a very suitable primordial ancestor for 

 the Vertebrates (Herrick, 1921). The central nervous systems of selachian and 

 teleostean Fishes, on the other hand, show systems of nuclei and fibre-tracts as well 

 defined as those of Mammals ; they are precisely adapted on a reflex plane to a 

 particular environment, and although they are thus able completely to dominate this 

 habitat, their central nervous systems are less capable of free adjustment to other 

 conditions. They therefore form terminal branches of the phylogenetic tree.^ The 

 ancestors of the Amphibians were the more primitive Crossopterygii, nearly related to 

 which are the lung-fishes (Dipnoi). Their more plastic central nervous system and 

 the more complete evagination of the fore-brain into cerebral vesicles, particularly 

 at the caudal rather than the olfactory end, allowed them to become adapted to the 

 lessened oxygen supply in stagnant swamps and ultimately to emerge on land. On 

 this relatively primitive fish-brain further evolution was therefore based. 



We shall now outline the main evolutionary changes in the development 

 of the h'lin of Vertebrates with particular reference to their visual systems 

 (Figs. 7 = ^ to 715). 



1 p. 239. 2 p. 234. 



