SOMATIC MOTOR COLUMN OF AMBLYSTOMA 135 



It is further obvious that the differentiation of the neurones 

 of the motor column and their orientation in the latero- ventral 

 tract are correlated with the more general differentiation of the 

 mesoderm into myotomes. It seems plausible, therefore, that 

 the general and more diffuse process of differentiation within the 

 mesoderm stimulates the differentiation of the motor neurones 

 and their orientation and outgrowth into the tract in a longi- 

 tudinal direction, while the more localized differentiation within 

 the myotome, related directly to muscular activity, stimulates 

 the origin of the collateral, its growth laterad through the limiting 

 membrane of the cord and its advance to the muscle cell. This 

 hypothesis is founded only on the general growth processes. It 

 is susceptible to experimentation by growth of the tissues in 

 vitro and it is hoped that this method may yet be applied to the 

 details of this problem of correlative development. 



2. Questions of cytomorphic and functional development 



It has been noted above that ventral root fibers occur in their 

 full relation between the spinal cord and the muscle some time 

 before the muscles can be stimulated through the sensory field. In 

 one embryo of this stage in development as many as twelve pairs 

 of roots exist. It is clear, therefore, that the physiological prop- 

 erties of these root neurones can in no exact sense be determined 

 through stimulation of the sensory fieid, for they may be actu- 

 ally functional for some time before they come under the influ- 

 ence of the sensory nerves. It is only after all the elements of 

 a reflex arc are established that a reaction can be elicited through 

 it. My observations upon these embryos show that all the ele- 

 ments in the primary and most elementary reflex arc do not 

 become established simultaneously. Both the motor and sen- 

 sory elements in this arc are extensively differentiated and in 

 their usual relation to their end organs for some time before the 

 development of associative neurones puts them into such relation 

 to each other as to make reaction to stimulation possible. It 

 would be illegitimate, therefore, to infer that any particular fea- 

 ture in cytomorphic development within the motor column was 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 23, NO. 2 



