184 H. L. WIEMAN 



in the results of the experiment, namely, the development of 

 ascending tracts from the cord to the brain awaits and therefore 

 depends upon the formation of motor tracts. Once the stumps 

 have been bridged by descending fibers, all semblance of repulsion 

 between the posterior stump and the transplant disappears, and 

 the way is prepared for ascending tracts to grow forward. 



Hooker ('15), after severing the cord of the larval frog in the 

 cervical region, found that when the wound surfaces are not 

 apposed the first steps leading to a reunion and return to nearly 

 normal form and structure are brought about by, 1) the develop- 

 ment of nerve fibers from the motor cells of each segment of the 

 cord and, 2) the growth of sensory axones from the cut surface 

 of the posterior stump. In other words, the motor connections 

 precede the sensory. Coghill ('13), in his study of the primary 

 ventral roots and the somatic motor column of Amblystoma, has 

 shown that the ventral root fibers occur in their full relation 

 between the spinal cord and muscle some time before the muscle 

 can be stimulated through the sensory field. The physiological 

 properties can in no sense be determined through stimulation 

 in the sensory field, for they may be actually functional for some 

 time before they come under the influence of the sensory nerves. 

 Thus Coghill's work indicates that in the formation of the early 

 reflex arcs in the cord the motor connections are established 

 before the sensory. My results similarly point to the formation 

 of the long motor tracts from the brain to the cord before the 

 sensory tracts. 



It is also to be noted that the restoration of nervous connec- 

 tions through the transplant takes place in the direction of 

 the normal metabolic gradient of the embryo. Such a gradient 

 is also exhibited in the transplanted tube, the original anterior 

 end invariably exhibiting evidence of more vigorous growth and 

 developmental energy than the posterior end. This is true even 

 after a connection has formed between the anterior stump and 

 the transplant. The fact that this connection is always made 

 at a point nearer to the anterior end of the transplant might 

 explain the difference in behavior of the two ends of the trans- 

 plant, were it not for the fact that a similar difference in the two 



