SPINAL NERVES IN VERTEBRATES 161 



preparations would have revealed central processes extending 

 into the spinal cord. 



A reconstruction of two of the more cephalic spinal nerves 

 (fig. 17) shows considerable advancement of all of the spinal 

 nerve components. Thus far only one ventral or motor root 

 (V.R.) has appeared for each nerve. It contains a relatively 

 large number of fibers. The motor rami ventrales (R.V.M.) 

 have grown ventrally along the inner surface of the myotomes 

 nearly to the level of the dorsal aorta. They are composed 

 mainly of fibers lined ^^^th embryonic neurilemma cells (figs. 

 19 and 24) which migrated out of the neural tube with the fibers. 

 Motor rami dorsales fibers (figs. 17 and 24, R.D.M.) have 

 branched off from all of the motor roots or motor rami ventrales 

 and have grown a short distance dorso-caudad between the 

 lower lateral surface of the spinal ganglia and the myotomes. 

 The sensory or receptive portion of the spinal nerves have 

 likewise made considerable progress over the condition shown 

 for the more caudal nerves. The neural crest still persists as 

 a continuous mass of cells extending along the dorso-lateral 

 surface of the spinal cord, although the dorsal border of the neural 

 crest will be seen to be considerably lower and that portion be- 

 tween the ganglia is reduced to a narrow strip of cells. Numer- 

 ous central and peripheral processes have grown out from the 

 spinal ganglion cells. The central processes leave the dorso- 

 median surface of the ganglia to enter the dorso-lateral surface 

 of the spinal cord, constituting the dorsal or sensory roots (figs. 

 17 and 19, D.R.) of the spinal nerves. Some of the peripheral 

 processes pass out of the ventral poles of their ganglia for short 

 distances into a ventral migration of neural crest cells and form 

 the sensory rami ventrales (figs. 17 and 19, R.V.S.). These 

 rami, which at this stage are composed almost entirely of neural 

 crest cells, assume at first a more caudal and median course than 

 the corresponding motor rami, but come in contact with the 

 motor rami about on a level with the central axis of the noto- 

 chord. Their course is then nearly ventrad along the inner sur- 

 face of their respective motor rami, from which they can be 

 readily distinguished (fig. 19, R.V.M. and R.V.S.) by the fact 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 2S, NO. 1 



