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



The simplest way to approach this problem is to return to 

 the neural crest as we find it in the trunk, and then call attention 

 to the variations in its mode of behavior in the head. The 

 classical description of the mode of origin of the neural crest 

 in the trunk is as follows: A mass of cells derived from the 

 ectoderm and lying in the re-entering angle between ectoderm 

 and neural tube becomes detached, migrates ventrally, and 

 assumes the position of the adult spinal ganglia. This con- 

 ception was established early and has never been materially 

 modified for spinal ganglia. The spinal ganglia send fibers 

 to two general regions, first to the skin forming the somatic 

 group of fibers and second to visceral structures directly from 

 the spinal ganglion or from its derivative the afferent sym- 

 pathetic ganglia forming the visceral afferent system. 



In the head region, however, two other types of behavior 

 of the neural crest are present. It should be emphasized in 

 passing that the neural crest in the head gives rise to exactly 

 the same elements, viz., the general somatic or tactile, and 

 the general visceral ganglia, as does that in the spinal cord 

 region. This is a generalization of the first importance in our 

 effort to establish general principles. We need not enter into 

 the old discussion of the derivation of the head from the body 

 or the reverse. The homology of ganglia derived from the 

 neural crest in the head and in the body is well established, 

 regardless of one's conception of this disputed question. Nor, 

 need we raise the question of the segmental value or position 

 of the cerebral ganglia. That is another problem that is 

 frequently dragged into the discussion of the origin of the 

 cerebral ganglia, to the confusion of the student. Leaving 

 aside the disputed question of cephalization, including the 

 segmentation of the head, we can examine the behavior of the 

 neural crest in the head region as a distinct problem. 



The neural crest behaves then in three different ways in the 

 head region. It may behave just as it does in the spinal cord 

 region, that is, become located in the re-entering angle between 

 the ectoderm and the brain tube and, later becoming detached, 

 move ventrally to a position lateral to the brain tube. This 

 whole process is identical in type with the behavior of the spinal 

 cord neural crest, and, as mentioned above, gives rise to the 

 same components. If we accept the current descriptions, this is 

 probably the most common type of behavior among vertebrates. 



