SPINAL NERVES IN VEETEBRATES 157 



more cephalic sensory rami, while some of the extreme caudal 

 ventral motor rami will ultimately cross over their more cephalic 

 sensory rami \\ithout exchanging any fibers. 



The following additional points may be recorded for the spinal 

 nerves of the 20 and 27 mm. embryos: (a) As in the adult, most 

 of the spinal nerves have two motor and one sensory roots. 

 They immediately penetrate a mass of mesenchyme which will 

 later form the neural arches. Up to this time the dorsal and 

 ventral rami have been shifted little, if any, caudad, through a 

 more rapid gi'owth of the myotomes and skeletal elements over 

 the central nervous system, (b) It is evident from figure 12 

 that there is a gradual increase in the development of the spinal 

 nerves in passing cephalad. The last nerve is represented simply 

 by a mass of neural crest cells. The next to the last nerve has 

 an irregular-shaped ganglion from which a few central processes 

 have entered the dorso-lateral surface of the spinal cord as the 

 dorsal or sensory root. The second from the last nerve consists 

 of a spinal ganglion, a sensory root, and a single motor root 

 which has separated into a short dorsal ramus and a longer 

 ventral ramus. The third from the last spinal nerve shows 

 some progress over the second. Its dorsal and ventral motor 

 rami have increased in length and fibers are leaving the ventral 

 pole of the spinal ganglion to form the ventral sensory ramus; 

 while in the fourth from the last spinal nerve we have the 

 first appearance of a dorsal sensory ramus. From this point 

 cephalad, for a distance of several segments, there is a gradual 

 increase in length of all the various rami, taking place after the 

 following order — ventral motor ramus, dorsal motor ramus, 

 ventral sensory ramus and dorsal sensory ramus. It will be 

 seen that this order also represents the order of their embryonic 

 appearance, (c) Of the various ventral motor rami which will 

 cross the M. cordis caudalis, the second, third, and possibly the 

 fourth are somewhat longer than several of the preceding motor 

 rami. Opposite the caudal heart they bend inward to enter a 

 proliferation of cells {M.C.C.), which probably represents myo- 

 blasts for the future M. cordis caudalis. (d) A somewhat later 

 stage (fig. 15) shows a conspicuous dorsal and ventral growth of 



