MEDULLA OBLONGATA OF AMBLYSTOMA 367 



gray substance. Some of the root fibers leave their respective 

 fascicles to enter the bundles of arcuate fibers, probably to termi- 

 nate in the tegmental areas of the same and the opposite sides. 

 Golgi sections show many free arborizations of arcuate fibers 

 ending in the motor tegmentum and especially in the motor nuclei 

 of the cranial nerves. The number of such fibers is small in the 

 young larvae and most of the root fibers evidently end in relation 

 to the neurones of the stratum griseum of the adjacent parts of 

 the sensory region on the same side of the oblongata. 



In the adult the general pattern of the root fibers is similar to 

 that of the larva, the same arrangement of longitudinal tracts 

 being present; but in this case the related areas of neuropil are 

 larger, the number of medially directed root fibers in the arcuate 

 tracts is greater, and there are some other important changes, 

 though all features of the adult appear to have been derived 

 directly from the larval relations here described, which I regard 

 as primitive for Amphibia. 



THE TERMINAL NUCLEI AND SENSORY TRACTS RELATED TO THE 

 SENSORY ROOTS OF THE CRANIAL NERVES 



In all of the more highly differentiated brains the various sen- 

 sory roots of the cranial nerves terminate in special gray centers, 

 each of which serves as the end-nucleus of a special functional 

 system. As we have seen above, the arrangement of the sensory 

 roots of larval Amblystoma as they enter the oblongata confomis 

 to the typical vertebrate pattern and each root maintains its 

 individuality to its ultimate terminus. But the neurones of the 

 second order w^hich form the terminal nuclei of these root fibers 

 are practically all found in the primitive relation as a central gray 

 layer, within which in ordinary histological preparations there is 

 very little evidence of any specialization. But if this central 

 gray substance were functionally equipotential, as it appears to 

 be, the question would arise, how could the central analysis of 

 sensory stimuli be effected? 



To answer this question I have examined a large number of 

 Golgi preparations of larval and adult Ambtystoma brains with 

 results which are somewhat surprising. Previous to the exact 



