300 The Ohio Journal of Science [Vol. XX, No. 8, 



One of the earliest advances that we need to note was the 

 definite determination that all ganglia are derived from ecto- 

 derm rather than from mesoderm. This is significant, because 

 it determined once for all the course of progress in this line. 



Second, and of more significance to us today, is the fact 

 that nearly all of the early work on ganglia was done on the 

 spinal ganglia, and from this fact arose a very definite attitude 

 toward the whole problem. Anyone at all familiar with this 

 problem knows how easy it is to secure a series of stages in the 

 development of the spinal ganglia. Since these structures are 

 arranged segmentally, one embr3^o properly chosen furnishes 

 a whole series of ganglia in the various stages of development, 

 from very early to fairly late stages, so that the question of 

 securing material for investigation is relatively simple. Most 

 of the problems concerning the spinal ganglia were settled 

 satisfactorilly, and there has been no marked modification of 

 these early views in recent years. Authors agreed on most 

 points, as to the mode of origin, and also as to the final fate of 

 the ganglia. The definiteness of these results, however, gave 

 a very decided bias to the conclusions drawn in regard to the 

 cerebral ganglia. The ease with which the origin of spinal 

 ganglia was determined and the uniformity with which they 

 arise from the neural crest, and the simplicity of the neural 

 crest in this region, influenced, unquestionably, the conception 

 of the position and the behavior of the neural crest in the head. 

 It is undoubtedly true that too much emphasis was given at 

 first to the role played by the neural crest in the head. As a 

 result, other ganglion forming structures were either overlooked 

 or underestimated at first. For some years the tendency was 

 very marked to attribute to the neural crest in the head too 

 much importance in the formation of cerebral ganglia. It was 

 not fully realized that in addition to the neural crest there were 

 other ganglion forming structures, and, further, that the neural 

 crest itself might behave differently in different types in the 

 head region. It is just this fact that has obscured many of the 

 descriptions of the formation of the cerebral ganglia. The 

 neural crest is not at all uniform in its behavior in different 

 types. This bias was not corrected until a different point of 

 view had become established as a result of the nerve component 

 work. 



