The Reactions of the Vertebrate Embryo to Stimulation' etc. 573 



fibrils run aucl the absence of cross connections are prominent 

 features. 



In eonclusion it is worthy of note tliat when neurofibrils give 

 off collateral brancbes the calibre of these is apparently the sanie 

 as that of the main trunk. 



e) Summary and Conclusions. 



It is exceediugly difficult either to summarize or correlate the 

 results of the observations recorded in this paper as a number of 

 them are so detached that it is impossible to even conjecture what 

 their relative value or signification may be. The possibility also 

 exists that the i)hraseology employed to describe the physiological 

 events or histological findings may uniutentioually -accentuate the 

 supposed importance of individuai facts. 



In general, hovvever, it may be said, that the functional activities 

 of the body rei)resented by the beat of the heart and the primitive 

 movements of ab- and adduction of the body begin at a time when 

 these phenomena may as yet neither be designated as myogenic nor neu- 

 rogenic in origin. In the case of the heart it may be urged that the 

 movements of this organ, in view of the absence of fuUy differentiated 

 nerve tracts, are to be regarded as the results of muscular activity 

 quite iudependeut of any form of nervous Stimulation. Recent ob- 

 servations, particularly those of Marceau, w^ould seem to substantiate 

 this view. One objection however suggests itself which must be 

 answered before "the myogenic theory" (Engelmann) may be finally 

 accepted. At the time when the Impulses begin to be transmitted, 

 numerous connections between the cells in the heart bave already 

 beeu formed, and it is not at ali improbable that Impulses may be con- 

 ducted and even originate in the undifferentiated tracts of proto- 

 plasm which exist. An aualogous couditiou is found in the spinai 

 cord, the best example of which may be noted in connection with 

 the development of the ventral roots. Attention has already been 

 directed to the fact that definite movements of ab- and adduction 

 take place when the only possible paths for the couduction of motor 

 impulses are along the unditìerentiated Strands of protoplasm con- 

 necting the myotome and periphery on the one side, and the myotome 

 and cord on the other. 



The neurofibrils which ultimately form the ventral root buudles 

 appear first, either in the myotome, in the outer marginai veil of 



