ORIGIN OF THE CEREBRAL HEMISPHERES 447 



ation without the collateral source of oxygen furnished by the 

 cerebrospinal fluid. 



The thin walls of the forebrains of Polypterus and the lung- 

 fishes, accordingly, have two possible sources of oxygen supply, 

 in contrast with the thickened masses of the brain of' teleosts 

 which have but one, and if the supply is deficient this may be 

 sufficient to maintain the hfe of the tissue during critical periods 

 of drought. Indeed, in the total suppression of gill breathing 

 skin respiration may be adequate to supply the minimal amount 

 of oxygen necessary to keep such a brain alive while the animal 

 remains inactive. This minimum of oxygen might well be quite 

 inadequate to prevent asphyxiation of a brain of teleostean type. 



Now the form of the forebrain is not an essential factor in 

 this situation in the case of lowly organized fishes of sluggish 

 habit, provided only that the walls are thin and highly vascular 

 and there are extensive choroid plexuses. The everted primitive 

 endbrain of Polypterus, the dilated form of Ceratodus, and the 

 evaginated cerebral hemispheres of Protopterus and Lepidosiren 

 appear to be equally competent to meet the harsh conditions 

 imposed. But only in the evaginated forms does there reside 

 the potentiality of indefinite further differentiation under more 

 fortunate conditions of life. 



On the basis of these considerations it may be assumed that 

 a primitive ganoid type in late Silurian or early Devonian times, 

 whose brain form was not far from that indicated in figure 3 

 or that of the modern petromyzonts and whose histological 

 structure was primitive and generalized, was subjected to periodic 

 drouth. This climatic change is believed to have been very 

 pronounced and widespread at this time (Lull, '18, p. 121). In 

 adaptation to this environmental change the forebrain of the 

 generalized type in question differentiated in one or several of 

 the directions presented by modern ganoids and lungfishes. 

 One such form, viz., the thin-walled evaginated hemisphere, 

 proved capable of further progressive differentiation in air- 

 breathing Amphibia and in their later descendants. 



By reason of the hmitationS imposed by their modes of life 

 these animals in the early stages of this differentiation undoubt- 



