280 C. W. PRENTISS 



The present lack of segmental arrangement displayed by these 

 ganglia does not preclude their metameric origin. Their develop- 

 ment begins considerably later than that of the spinal nerves, 

 and the rapid growth of the region they occupy, before they make 

 their appearance, may cause them to shift their positions with 

 relation to their myotomes. They certainly appear in regular 

 series and their early development is similar to that of the spinal 

 ganglia. 



As to the number of dorsal ganglia represented in the hypoglos- 

 sal series, no absolute statement can be made. The evidence of 

 comparative anatomy goes to show that four or five spinal nerves 

 have been added to the cranial series as a result of the union with 

 the cranium of a corresponding number of vertebrae. Meeks 

 ('09) finds in an Acanthias embryo three rudimentary spinal gang- 

 lia located between the vagus and the first spinal nerve, the gang- 

 lion of which would correspond to Froriep's ganglion in mammals. 

 According to this evidence, four dorsal ganglia have become rudi- 

 mentary structures in mammals and the corresponding ventral 

 roots have united to form the hypoglossal trunk. Regarding 

 each pair of the eight ganglionic nodules found in the 13 mm. embryo 

 as homologous to a single spinal ganglion, then we would have the 

 same number of ganglia, four, represented between the first cervi- 

 cal and the jugular. The more anterior roots of the hypoglossus, 

 which are found in the early embryos but disappear in the later 

 stages, represent ventral roots of the vagus and glossopharyngeal 

 according to the observations of Bremer ('08). 



CONCLUSIONS 



1. The jugular and superior ganglia of the vagus and glosso- 

 pharyngeal nerves, the hypoglossal ganglia and ganglia of the 

 spinal nerves arise in the pig embryo from a continuous neural 

 crest, as observed by Streeter in human embryos. 



2. The hypoglossal ganglia are retarded in their development, 

 but appear in embryos of 13 mm. as a series of eight connected 

 cell masses of nearly equal size. 



