sensory fibers are distributed in separate nerves as in 20 mm. embryo, but that 

 the motor nerves have been carried further caudad and are nearer their respective 

 sensory rami. X 50. 



16 Graphic reconstruction of an early developmental stage of several of 

 the lower abdominal spinal nerves from a 7.5 mm. Squalus acanthias embryo. 

 Observe that the motor roots have grown outward and downward for a short 

 distance as ventral motor rami, and- in one instance thesfe is the beginning of a 

 dorsal motor ramus. Ventral prolongations of the neural crest, representing 

 the anlage of the spinal ganglia and the sensory ventral spinal rami, in passing 

 ventrally between the motor components are bent slightly toward the cephalic 

 motor nerves. X 50. 



17 Graphic reconstruction of two more cephalic spinal nerves of the same 

 embryo as figure 6. Note the increased growth of the ventral sensory and motor 

 rami, the two being readily distinguished by the abundance of cells in the sen- 

 sory rami. Observe the vertebral sympathetic ganglia, appearing as swellings 

 at the end of the ventral sensory rami, and the presence of short dorsal motor 

 rami. X 50. 



18 A, B and C transverse sections through some of the largest fibers of the 

 vagus, cephalic ventral spinal, and glossopharyngeal nerves. D and E are longi- 

 tudinal sections through some of the largest fibers of a mixed ramus ventralis 

 and a dorsal sensory ramus of a spinal nerve. X 100. 



19 Transverse section of a 7.5 mm. Squalus acanthias embryo, passing through 

 the motor and sensory components of a spinal nerve. Note the position of the 

 ventral spinal rami and the greater number of cells in the sensory ramus. X 83. 



20 Graphic reconstruction of two spinal nerves in the lower abdominal region 

 of a 19 mm. Squalus acanthias embryo. Observe a notable increase in length 

 of all nerves over the corresponding nerves shown in figure 17. The dorsal motor 

 rami have grown caudally across the lateral surface of their spinal ganglia. The 

 ventral motor and sensory rami have joined opposite the aorta in forming mixed 

 rami ventrales. Sympathetic ganglia have appeared on the ventral sensory 

 rami as the result of a migration of some of the neural crest cells. No dorsal 

 motor rami have appeared thus far. X 50. 



ABBREVIATIONS 



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



Car.V., cardinal vein R.V.M., ramus ventralis or anterior 



C.V., caudal vein (motor) 



D.Myo., dorsal border of myotomes ^■^■^■' ^^^'^^"^ ventralis or anterior 



D.R., dorsal root (radix posterior) 



(sensory) 



Sp.Cd., spinal cord 

 M.T.C., M. transversus caudalis ^^ ^_^ ^^-^^^ ganglion 



Muc.S., mucous or slime sac gyQ_^ sympathetic ganglion 



Nc, notochord y j^ ^ ventral root (radix anterior) 



Neu.C, neural crest V.R. (1), cephalic ventral or anterior 



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



(motor) V.R. (2), caudal ventral or anterior 



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



(sensory) W.D., Wolffian or mesonephric duct 



208 



