PLATE 8 



EXPLANATION OF FIGURES 



21 Graphic reconstruction of two abdominal spinal nerves from a 28 mm. 

 Squalus acanthias embryo. Note the appearance of the dorsal sensory rami 

 fibers arising from the caudal surface of their respective ganglia at the point of 

 crossing of the dorsal motor rami. They follow the dorsal motor fibers, but for 

 the greater part of their course do not intermingle with them. Likewise the 

 ventral sensory and motor rami run side by side to a little below the level of 

 the aorta, where they intermingle and form mixed rami ventrales. It will be 

 seen that the rami ventrales are much larger and longer than the rami dor- 

 sales, due to a much greater development of the myotomes ventrally. X 25. 



22 Graphic reconstruction of two of the spinal nerves from the lower abdomi- 

 nal region of a 32 mm. Squalus acanthias embryo, which has attained nearly 

 adult conditions in so far as the distribution of the spinal nerves is concerned. 

 Observe, as in figure 21, that the dorsal sensory fibers take exit from the caudal 

 surface of their ganglia at exactly the point of crossing of the dorsal motor rami. 

 It will be seen that the dorsal motor and sensory rami have been carried caudally 

 for a distance of a segment to cross the lateral surface of the following ganglia, 

 where they unite in forming mixed rami dorsales, which traverse the intermus- 

 cular septa one segment behind the one followed by the corresponding rami 

 ventrales. As in the 28 mm. embryo, the motor and sensory rami ventrales run 

 side by side to a little below the level of the aorta, where they join in forming 

 mixed rami ventrales. X 50. 



23 From a dissection of the lower abdominal spinal nerves of a small adult 

 Squalus acanthias from the left side. It is clear that the arrangement of motor 

 and sensory spinal nerve components is practically the same as was described 

 for the 32 mm. embryo. X 4. 



24 Transverse section through the middle of the abdomen of a 7.5 mm. 

 Squalus acanthias embryo, passing through the ventral root, and showing ex- 

 tent and structure of the dorsal and ventral motor nerves. X 83. 



25 From a peripheral section of the more cephalic dorsal motor and sensory 

 rami shown in figure 22. Observe that the nerves are well separated. X 165. 



26 Similar section of the same nerves as figure 25, but taken at exit of the 

 sensory fibers from the spinal ganglion. X 165. 



27 Transverse section through the more cephalic ventral motor and sensory 

 rami shown in figure 22. X 165. 



ABBREVIATIONS 



Ao., aorta R.V.M., ramus ventralis or anterior 



D.Myo., dorsal border of myotomes (motor) 



D.R., dorsal root (radix posterior) R.V.S., ramus ventralis or anterior 



Nc, notochord (sensory) 



Neu., neurilemma or connective tissue Sp.Cd., spinal cord 



cells Sp.G., spinal ganglion 



R.D., ramus dorsalis or posterior V.Myo., ventral border of myotomes 



R.D.M., ramus dorsalis or posterior V.R., ventral root (radix anterior) 



(motor) V.R. (1), cephalic ventral or anterior 

 R.D.S., ramus dorsalis or posterior root 



(sensory) V.R. {2), caudal ventral or anterior 

 R.V., ramus ventralis or anterior root 



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