FÜNXTIOXAL FACTORS IX THE MORPHOLOGY 

 OF THE FOREBRAIX OF FLSHES 



C . J U D S í) X ti E R R I C K 



HflX I.ABORATORy OF ANATOMY THE USIVBRSITT OF CHICAGO 



In a recently publishecl «Sketch of the Origin of the Cerebral Hemi- 

 pheres» (192 1) I have summarized a few of the salient features of the 

 probable evolutionary history of the forebrain from its simplest begin- 

 nings to the attainment, in the Amphibia, of a form which may be regard- 

 ed as exhibiting the prototype of the cerebral hemispheres of all higher 

 forms. In this paper it is proposed to examine in more detail some of 

 the functional factors which have operated in shaping the forms of the 

 forebrain in lower vertebrates. 



It has been pointed out that two highly developed sense organs, the 

 eye and the nose, have profoundly modified the anterior end of the 

 neural tube in all vertebrates. The retina develops as an evagination of 

 the lateral wall of the neural tube in the región of the diencephalon, and 

 the photoreceptive organ is, therefore, differentiated within true brain 

 substance. Enlarged correlation centers dominated by the visual system 

 are developed in the midbrain and (especially in higher vertebrates) in 

 the thalamus. 



Farther forward the olfactory organ develops outside the brain 

 substance and the receptive and correlation centers related with this organ 

 domínate the rostral end of the neural tube in all lower vertebrates. The 

 modifications of the primitive neural tube [as this is seen, for instance, in 

 Amphioxus and early vertébrate embryos) which are called forth in 

 response to the development of the olfactory organ are of four sorts, all 

 of which are adapted to give added space for the mechanisms ¡nvolved in 

 the correlation of olfactory with other functional systems of neurons. 

 These four types of differentiation are as foUows: il) local thickenings of 



