SPINAL NERVES IN VERTEBRATES 169 



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rami. In the higher vertebrates it was found that the motor 

 and sensory components of a spinal nerve unite about as soon 

 as they are formed; while in selachians they remained separate 

 for some little time in embryonic life. Between these two ex- 

 tremes, where the motor and sensory nerves never unite and 

 those where they unite in embryonic life, Polistotrema forms a 

 most interesting link. In this representative of cyclostomes it 

 was noted that the motor and sensory components of the most 

 caudal nerves and the motor and sensory elements of the dorsal 

 rami always remain separate, while the motor and sensory ele- 

 ments of each cephalic ventral ramus join in forming a mixed 

 ramus ventralis. 



From the fact that the union of the motor and sensory com- 

 ponents of the rami ventrales occurs before it does in the rami 

 dorsales in selachians, taking place before the sensory portion 

 of the dorsal rami are formed, and that the motor and sensory 

 portions of the dorsal rami of Polistotrema never fuse, it is fair 

 to assume that the union of the motor and sensory elements of 

 the rami ventrales is phylogenetically older than the union of 

 the motor and sensory components of the rami dorsales. 



The first intermingling of the motor and sensory fibers of the 

 rami ventrales was found to be below the level of the dorsal 

 aorta in the most primitive (caudal) region of adult Polistotrema 

 and in Squalus embryos. Also in the higher vertebrates, where 

 the motor and sensory rami come in contact about as soon as they 

 are formed, there is a considerable period of time in which there 

 is no exchange of fibers in the rami ventrales down to a level 

 with the aorta: The same is true for the motor and sensory 

 components of the rami dorsales of Squalus; they may come in 

 contact in several places, but the first intermingling of fibers is 

 in the periphery, not far from the dorsal border of the myo- 

 tomes. For the majority of vertebrates the generalization will 

 hold that the union of the motor and sensory portions of a spinal 

 nerve occurs at first in the periphery and then takes place 

 gradually in a central direction. 



As a rule in the embryo and in the simpler vertebrates, where 

 the motor and sensory components of a spinal nerve were sepa- 



