164 WILLIAM F. ALLEN 



have been carried some distance caudad, presumably through a 

 greater growth of the myotomes in those regions, (e) Nothing 

 in the way of a sensory ramus dorsalis has appeared in this 

 stage. 



A more advanced stage in the development of a spinal nerve 

 is shown in a graphic reconstruction (fig. 21) of two upper 

 abdominal spinal nerves from a 28 mm. Squalus. It is evident 

 from this reconstruction that a number of important changes 

 have taken place since the pre\dous stage, and that the ar- 

 rangements of the spinal nerves in this stage have much in com- 

 mon with the adult previously described. All of the various 

 rami that were described in the previous reconstruction will 

 be found to have increased considerably in length; this is espe- 

 cially true for the rami ventrales, which have nearly doubled in 

 length. The motor and sensory rami ventrales {R.V.M. and 

 R.V.S.) have remained separated as far as the lower surface of 

 the aorta or a little below, where they unite in forming mixed 

 rami ventrales {R.V.), which can be traced along the inner sur- 

 face of the myotomes or their intermuscular septa nearly to the 

 floor of the abdomen. Figure 35 will disclose how sharply a 

 layer of connective tissue or neurilemma separates the motor 

 from the sensory rami ventrales fibers in the region of the dorsal 

 aorta. As in the previous stage there are two ventral or motor 

 roots (V.R.). The mode of origin and distribution of the motor 

 rami dorsales (R.D.M.) is about the same as in the previous 

 stage, with the exception that they have extended a little farther 

 dorsad, nearly to the level of the upper surface of the spinal 

 cord, and more important, after crossing the spinal ganglia 

 they are accompanied by the sensory rami dorsales fibers, 

 which took origin from the caudal surface of their respective 

 spinal ganglia. Most of the sections show the motor and sen- 

 sory portions of a ramus dorsalis to be separated by connective 

 tissue cells, but in places these bundles come in contact so that 

 it is impossible to distinguish them. It is of especial significance 

 to note that the sensory rami dorsales left the caudal end of 

 their respective ganglia at exactly the point where a motor 

 ramus dorsalis crossed. Also at this stage the motor and sen- 



