172 WILLIAM F, ALLEN 



traded cephalad toward their corresponding motor rami, (c) 

 A later stage of Squalus (figs. 21 and 22) discloses that the dorsal 

 sensory rami (R.D.S.), which are formed considerably later 

 than the other branches of a spinal nerve, take their exit from 

 the caudal surface of their ganglia at exactly the point of cross- 

 ing of their corresponding motor rami (R.D.M.) instead of the 

 dorsal pole as in cyclostomes. That the dorsal sensory fibers* 

 should be given off from this point of the ganglion rather than 

 elsewhere is at least suggestive of an attraction for the sensory 

 fibers, provided that other conditions are equal. 



I am unable to explain why these previously described forces 

 should not be operative in Amphioxus and Petromyzon and 

 cause the various motor and sensory rami in these species and 

 the motor and sensory components of the dorsal and the caudal 

 ventral rami of Polistotrema to unite and form common mixed 

 trunks as they do in the higher vertebrates; unless in Amphioxus 

 and Petromyzon the relatively earlier appearance of the myo- 

 tomes would do away mth a considerable caudal shifting of these 

 nerves. Also it is evident where a few of the caudal ventral 

 motor rami migrated cephalad across their corresponding sen- 

 sory rami to end in the myotomes in front, that there must be 

 much less attraction (chemotropism) between the sensory and 

 motor rami than there is between the myotomes and the motor 

 rami. 



DISTRIBUTION AND POSSIBLE SIGNIFICANCE OF CERTAIN GAN- 

 GLION CELLS ON THE COURSE OF THE SPINAL AND VAGUS 

 NERVES OF POLISTOTREMA 



Numerous investigators have shown that there are no well- 

 defined spinal ganglia in Amphioxus, but that scattered spinal 

 ganglion cells (figs. 28 and 29, N.C.) occur in the dorsal roots 

 and sensory rami from the spinal cord to the skin. From meth- 

 ylen blue preparations Retzius has described certain so-called 

 sympathetic cells in the periphery of the spinal nerves of Myxine 

 and these cells are figured as enclosed in a network of sympa- 

 thetic fibers. Julin ('86) and ('87) found a well-defined vertebral 

 sympathetic system about the aorta in 15 and 18 mm. Petro- 



