PLATE 2 



EXPLANATION OF FIGURES 



2 Graphic reconstruction of the last spinal nerve on the opposite or left side 

 of the same Polistotrema series as figure 1. The space between the two dotted 

 lines denotes the interval traversed in passing through the membranous neural 

 arch. This nerve presents a number of interesting variations. It has no ven- 

 tral sensory ramus or corresponding motor rami. Its ganglion cells are suffi- 

 ciently scattered, one of which is found within the neural canal, to suggest the 

 arrangement found in Amphioxus. X 40. 



3 Graphic reconstruction of the spinal nerves in the region of the caudal 

 heart from an 85 mm. Polistotrema embryo, as seen from the left side. It is 

 evident that the general arrangement of the motor and sensory components is 

 about the same as in figure 1. Observe that many of the dorsal sensory rami 

 have clusters of peripheral ganglion cells opposite the median dorsal cartilagi- 

 nous bar. Also the last ventral sensory ramus has an isolated nerve cell opposite 

 the notochord and the second dorsal sensory ramus has an isolated ganglion cell 

 at the point of branching above the myotomes. X 20. 



ABBREVIATIONS 



C.A., caudal artery 



C.H., caudal heart 



C.V., caudal vein 



D.Mijo., dorsal border of myotomes 



D.R., dorsal root (radix posterior) 



D.S., dorsal spine 



M.C.C., M. cordis caudalis 



M.D.B., median dorsal cartilaginous 



bar 

 Muc.S., mucous or slime sac 

 M.T.C., M. transversus caudalis 

 Nc, notochord 

 N.C., nerve cell 



R.D.M., ramus dorsalis or posterior 



(motor) 

 R.D.S., ramus dorsalis or posterior 



(sensory) 

 R.M.C.C, motor rami for M. cordis 



caudalis 

 R.V.M., ramus ventralis or anterior 



(motor) • 

 R.V.S., ramus ventralis or anterior 



(sensory) 

 Sp.Cd., spinal cord 

 Sp.G., spinal ganglion 

 V.R., ventral root (radix anterior) 



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