110 Development of Occipital Nerves in Human Embryos 



a typical Froriep ganglion with a ventral root that would liave doubtless 

 joined with the hypoglossus fibres. In Fig. 9 a slight indication of a 

 ganglion is present, though it is not labelled in the diagram. In such 

 cases one cannot say whetlier it belongs to the spinal group or to the 

 accessorius root ganglia of the cranial group. These two seem to develop 

 from the same crest, and it could be expected that the oral tendency of 

 the former and the caudal tendency of the latter might cause in some 

 cases a fusing of the two; such an instance is seen in Fig. 12. Where 

 the retrogression of the spinal elements is advanced, the Froriep ganglion 

 is absent, and the first cervical also then shows abortive tendencies. If 

 Fig. 5 is compared with Fig. 7, it will be seen that there the spinal 

 reduction extends an entire segment further caudad ; instead of a rootless 

 Froriep ganglion, as in Fig. 7, there is in Fig. 5 a rootless first cervical 

 ganglion. 



It is evident that there is a great irregularity in the degree of reduction 

 of the occipito-spinal dorsal roots and ganglia in different individuals. 

 By comparing individuals of different ages we cannot therefore estimate 

 the retrogression undergone in the development of a single individual; 

 one cannot say, for instance, that because a Froriep ganglion is present 

 in an embryo of 7.0 mm. and is not present in another embryo of 14.0 mm. 

 that it has in the latter case disappeared. It was found in case of the 

 accessory root ganglia that ganglion masses once present persist through- 

 out life, though they may early reach a point beyond which they do not 

 further develop. The same is doubtless true as regards the Froriep 

 ganglion. 



Comparative Morphology. 

 In considering the phylogenetic significance of the nerves of the occipi- 

 tal region it becomes apparent that we are here dealing with structures of 

 two different sources; on the one hand, the cranial nerve elements rep- 

 resented by the glosso-pharyngeus, the vagus, and the accessorius — a 

 portion of the vagus, and on the other hand the elements of spinal origin, 

 the upper cervical nerves and the hypoglossus. The literature concern- 

 ing the comparative anatomy of these structures is voluminous, and par- 

 ticularly their involvement in the various theories proposing a segmental 

 origin of the vertebrate skull. A complete review of this literature and 

 discussion of the morphological bearing of the cranial nerves is given by 

 His. 87, and again later by Bahl. 92. Since then has appeared the im- 

 portant work of Fiirhringrr, 97, supplemented by tlie omhryological 

 investigations of Braus, 99, and Froriep. 02. ]\Iention should also be 

 made of the work done on the accessory ncnwe by Luhosch, 99. The 

 general facts as known may l)o stated as follows: 



