SPINAL NERVES IN VERTEBRATES 167 



of their course the motor rami ventrales were situated cephaUc 

 and lateral to their corresponding sensory rami; they were 

 readily distinguishable on account of difference in structure, 

 the motor rami consisting of fibers bordered by a few embryonic 

 neurilemma cells, and the sensory rami were composed almost 

 entirely of neural crest cells. A short motor ramus dorsalis 

 had branched off from each motor ramus ventralis to pass caudo- 

 dorsad. between its ganglion and the myotomes, (c) Each 

 spinal nerve possessed a dorsal or sensory root, which took 

 origin from the dorsal pole of its respective spinal ganglion. 

 The earliest embryos disclosed but one ventral or motor root 

 for each nerve; while the later stages had two, some of which 

 arose from more than one rootlet. In position the motor root 

 or roots were always more cephalad than the corresponding 

 sensory root, (d) Later stages were concerned mainly with the 

 prolongation of the above mentioned elements of a spinal nerve, 

 together with a further differentiation of a vertebral sympathetic 

 ganglion on the course of each sensory ramus ventralis, a full 

 discussion of which was given on p. 163. (e) The sensory rami 

 dorsales were found to be very late in appearing, and it was of 

 especial interest to record that they did not leave the ventral 

 poles of their ganglia as in Cyclostomes, but took origin from 

 the caudal surfaces of their ganglia at the intersection with the 

 corresponding motor rami, (f) It will be seen that the order of 

 appearance of the various branches of the spinal nerves is the 

 same as in Polistotrema, namely, motor ramus ventralis, motor 

 ramus dorsalis, sensory :famus ventralis, and sensory ramus 

 dorsalis. (g) Up to embryos of 32 mm., where the various 

 components of a spinal nerve are practically the same as in the 

 adult, the motor and sensory elements have remained separated 

 by connective tissue or neurilemma throughout a considerable 

 part of the proximal portion of the rami dorsales and the rami 

 ventrales. (h) Both the rami dorsales and the rami ventrales 

 have been carried caudad for a distance of a segment as a result, 

 probably, of a more rapid growth of the myotomes than of the 

 skeletal and neural axes. 



