REMOVAL OF NASAL PITS AMBLYSTOMA EMBRYOS 49 



of the support given by the nasal placode to the mesenchymal 

 tissue in which the cartilage is formed. 



The effect of the absence of the nasal sac on the brain is a re- 

 duction in size of the telencephalon. 



The removal of the nasal epithelium deprives the developing 

 forebrain of a stimulus necessary for its complete development. 

 This is evidenced by the fact that the forebrain of the six months 

 old larva from which one placode had been removed, showed 

 considerable differences in the size of the two hemispheres, the 

 operated side being the smaller. This difference however is 

 found only after the nasal sac of the unoperated side has become 

 functional. This fact indicates that there exists in the central 

 nervous system as, indeed, in other organs a "potential for differ- 

 entiation" which carries the development of the parts to a point 

 at which all parts are present although not in their final form 

 or size, <"he further growth being dependent on the functional 

 activity of the end organs of the brain. 



