MEDULLA OBLONGATA OF AMBLYSTOMA 385 



The gray substance of the oblongata shows progressively more 

 clearly defined areas of functionally distinct neurones as we pass 

 from the spinal end forward. And the anatomical arrangement 

 suggests that the capacity for diversified reactions is greater in 

 those parts of the body innervated from the anterior end of the 

 oblongata. Physiological observation, of course, shows that 

 this is the case. 



In Ambly stoma of different ages various forms of reflex connec- 

 tions are now known and these can be arranged in a graded series, 

 starting from the primitive reflex apparatus of the' very early 

 swimming larva, which may be conceived of as suggesting some 

 of the steps in the phylogenetic evolution of the mechanism of 

 correlation as found in the brains of higher vertebrates, 



C'oghill has shown ('14) that in the simple swimming reflex of 

 very early developmental stages of Amblystoma there is very little 

 evidence of specificity of function even in the peripheral sensory 

 neurones. Each of these neurones (the transitory giant cells of 

 Rohon and Beard in the spinal cord) may connect peripherally 

 with both the skin and the myotom, the cutaneous and the muscle 

 sense innervation cooperating to maintain the swimming reflex. 

 These transitory cells are in later developmental stages replaced 

 by those of the spinal ganglia, in which the neurones concerned 

 with cutaneous and deep sensibility are probably distinct as in 

 higher vertebrates. 



In the spinal cord of the half grown larva, however, the neu- 

 rones of the gray substance show very imperfect functional locali- 

 zation, each of these neurones apparently being physiologically 

 related to all types of peripheral sensory nerves. Here also any 

 stimulus whatever on the trunk will evoke a simple swimming 

 movement. 



In the medulla oblongata of these half grown larvae the periph- 

 eral neurones show a high degree of functional specificity, and 

 the central neurones of the second order tend to be grouped 

 around these special sensory roots. But the functional localiza- 

 tion of these secondary neurones is not complete, each neurone 

 having a dominant relation to some particular peripheral root but 

 subsidiary connections also with other functionally distinct roots. 



