156 WILLIAM F. ALLEN 



dorsal or ventral i-amus. (i) The last nerve to enter or leave the 

 spinal cord is a sensory nerve on the left side (fig. 2) . It lacked 

 a ventral ramus and a typical spinal ganglion, but a few scat- 

 tered ganglion cells were found outside the neural arch and one 

 within it. (j) As was noted in' a previous paper, the extreme 

 caudal end of the spinal cord is non-nervous, consisting of sup- 

 porting cells and undeveloped embryonic cells. It culminates 

 in several processes, which terminate in a mass of connective 

 tissue at the end of the membranous neural canal. 



3. Embryonic arrangement in Polistotrema 



Serial sections of 20, 25, 27, 57, 60, and 70 mm. embryos were 

 available for this study. The spinal nerves of the 20 to 27 mm. 

 embryos were found to be in an embryonic state; while those of 

 57 to 70 mm. had reached pi'actically adult conditions. Un- 

 fortunately the interesting gap between these extremes could 

 not be filled. Of the material representing a purely embryonic 

 arrangement of the spinal nerves, reconstructions were made of 

 the spinal nerves of the tail of the 20 mm. embryo and of two 

 nerves from segments a little behind the anus from both the 20 

 and the 27 mm. series, the latter representing a region in the 

 adult where the ventral motor and sensory rami always unite 

 in forming mixed rami ventrales. 



An examination of the 20 mm. reconstruction (fig. 12), which 

 includes the distribution of the spinal nerves of the tail region 

 and two nerves from the neighborhood of the anus, disclosed an 

 arrangement of the motor and sensory components which is 

 directly comparable in many respects to the primitive disposi- 

 tion found in Petromyzon and Amphioxus. It is clear from 

 this reconstruction, that for the two most cephalic spinal nerves 

 (shown here) to attain adult conditions, the ventral motor rami 

 {R.V.M.), which traverse the inner surfaces of their respective 

 myotomes about equidistant between two sensory rami {R.V.S.), 

 must sometime in their future join their following sensory rami. 

 In like manner the first two or three ventral motor rami in the 

 caudal portion of the reconstruction will have to unite with their 



