184 WILLIAM F. ALLEN 



SUMMARY AND CONCLUSIONS 



The more important points of this paper may be summarized 

 as follows : 



1. Concerning the manner of distribution of the sensory and 

 motor components of the spinal nerves, Polistotrema furnishes 

 an interesting intergradation between the simple arrangement 

 found in Amphioxus and Petromyzon, where the sensory and 

 motor components of the spinal nerves were distributed as sepa- 

 rate rami, and the more specialized condition found in the higher 

 vertebrates, were these components joined in forming mixed 

 nerves. In Polistotrema all of the dorsal and all of the most 

 caudal ventral motor and sensory spinal nerve fibers were dis- 

 tributed in separate motor and sensory rami, while the ventral 

 motor and sensory components of all but the extreme caudal 

 spinal nerves united in forming mixed rami ventrales. A few 

 of the last spinal nerves presented some irregularities in the 

 way of branching. 



2. For a considerable period in embryonic life the motor and 

 sensory fibers of all the spinal nerves of Polistotrema and Squalus 

 passed to the periphery in separate motor and sensory rami. 

 The writer agrees with Onodi that the vertebral sympathetic 

 ganglion cells of Squalus are derived entirely from neural crest 

 cells, which migrated along the ventral sensory rami at a time 

 when they were separated from their corresponding motor 

 rami. The few cells which passed out of the neural tube with 

 the ventral root fibers are apparently concerned only with the 

 formation of neurilemma. 



3. In rather late embryos of Polistotrema and Squalus there 

 are always two ventral foots for each spinal nerve, which re- 

 ceive fibers from several rootlets. In Polistotrema these roots 

 emerge from the membranous neural arch thi'ough separate 

 foramina. 



4. There is considerable variation in different vertebrates as 

 to the place of exit of the central and peripheral processes of 

 the spinal ganglion cells. In most vertebrates the dorsal root 

 fibers leave from the dorsal pole of their respective ganglia. 



