140 WILLIAM F. ALLEN 



the dorso-lateral surface of the spinal cord. The important 

 pecuharities in the arrangement and course of the sensory fibers 

 in Amphioxus are: (1) there is no centralization of ganglion 

 cells in the dorsal root to form a spinal ganglion, although scat- 

 tered ganglion cells are found (figs. 28 and 29, N.C.) in the dorsal 

 region of the spinal cord, in the dorsal root, throughout the 

 ventral ramus to the skin, and in the dorsal ramus. (2) The 

 dorsal and ventral sensory rami do not follow a circuitous course 

 along the inner surface of the intermuscular septa and around 

 the myotomes to reach the skin, but penetrate immediately the 

 intermuscular septa. 



From the work of Ransom and Thompson, Schneider ('79), 

 Julin ('86) and ('87), Johnston ('05), and others it is evident 

 that the arrangement of the motor and sensory fibers of the 

 spinal nerves of Petromyzon agrees with Amphioxus, except for 

 the passage of the dorsal and ventral sensory rami around the 

 intermuscular septa and myotomes to reach the skin and the 

 concentration of ganglion cells in the dorsal root directly out- 

 side of the neural arch to form a spinal ganglion. Julin ('86) 

 .and ('87) found in a 15 to 18 mm. Ammocoetes that the ventral 

 sensory raini from the region of the heart to the anus received 

 visceral fibers from a ramus communicans, and that sympathetic 

 cells were present near the aorta as superficial and profundus 

 sympathetic ganglia. 



2. Conditions in various regions of adult Polistotrema 



J. Mliller found that the spinal nerves of Myxine arose from 

 two roots which united in crossing the notochord. He was un- 

 able to determine whether these roots were motor or sensory, 

 nor was he certain of the presence of ganglia, but thought that 

 they were present on the most cephalic nerves. Ransom and 

 Thompson state that in certain regions of Bdellostoma there 

 are two anterior (ventral) roots to one posterior (dorsal) root. 

 The posterior root is said to possess a distinct spinal ganglion, 

 which divides into a dorsal and a ventral ramus. Each of the 

 two anterior roots was found to give off a dorsal branch, which 



