SPINAL NERVES IN VERTEBRATES 185 



while in Polistotrema each dorsal root takes exit from the median 

 side of its ganglion not far from the center. From the text it 

 is clear that all of the ventral sensory rami fibers in all verte- 

 brates leave their ganglia by way of the ventral pole, that the 

 dorsal rami fibers in Polistotrema leave from the dorsal pole, 

 that the dorsal rami fibers of Squalus leave from the caudal 

 surface of their ganglia, and in the higher vertebrates the dorsal 

 rami leave with the ventral rami fibers through the ventral pole 

 of their ganglia. It appears, then, that there may have been a 

 gradual shifting of the point of exit of the dorsal rami fibers 

 from the dorsal surface of the ganglia in simple vertebrates 

 to the ventral surface of the ganglia in the higher vertebrates, 

 and that this change in the place of exit of the fibers took 

 place around the caudal surface of the ganglia rather than around 

 the cephalic surface. 



5. The order of appearance of the various spinal nerve rami 

 of Polistotrema and Squalus is as follows: motor ramus ventralis, 

 motor ramus dorsalis, sensory ramus ventralis, and sensory 

 ramus dorsalis. The last mentioned appears considerably later 

 than the others in Squalus, not until after the motor ramus 

 dorsalis had grown caujdad across the lateral surface of its corre- 

 sponding ganglion. 



6. Two interesting observations were recorded in connection 

 wdth the development of the spinal nerves in Squalus. First, 

 a graphic reconstruction of several spinal nerves of an early 

 Squalus embryo demonstrated that the ventral expansions of 

 the spinal ganglia, representing the anlage of the ventral sen- 

 sory rami and the vertebral sympathetic system, in passing be- 

 tween the motor roots and the ventral motor rami, approached 

 the more cephalic motor components as if attracted by them. 

 Second, a sensoiy ramus dorsalis always arises from the caudal 

 surface of its ganglion at exactly the point of crossing of the 

 corresponding motor ramus, with, which it later forms a mixed 

 ramus . 



7. While there appears to be no exchange of fibers in my early 

 turtle and pigeon embryos, the motor and sensory components of 

 the spinal nerves were in no instance completely separated by 



