150 WILLIAM F. ALLEN 



Except for the extreme caudal region, the distribution of the 

 sensory rami in figure 1 is essentially the same as in a more 

 cephalic region. The primitive condition of the last seven 

 nerves, where the sensory and motor rami remain entirely 

 separated, has been noted. A second possibly primitive condi- 

 tion remains to be recorded for the caudal spinal nerves, namely, 

 a tendency, as in Amphioxus, for the spinal ganglion cells (N.C.) 

 to migrate peripherally along the course of the nerves. Th^y 

 are found along the course of many of the dorsal and on some of 

 the ventral rami, and are most numerous on the extreme caudal 

 nerves. The dorsal and ventral sensory rami entering the last 

 four spinal ganglia exhibit considerable variation as to number 

 and arrangement. It frequently happens that more than one 

 sensory ramus mil enter the dorsal or ventral surface of one of 

 these ganglia or two or more nerves may unite before a ganglion 

 is reached. In a number of instances there is evidence of a 

 partial fusion of two sensory rami, and several other irregulari- 

 ties are shown in figure 1 for the last dorsal sensory rami. At 

 the base of the fin the sensory rami unite and receive a sort of 

 plexus from the fin. The right side (fig. 1) has one less sensory 

 nerve than the left side. Its ganglion receives a sensory ramus 

 from the rear in addition to others from above and below. The 

 last sensory nerve on the left side (fig. 2) is of a very different 

 type than any of the others. It has no ventral ramus and its 

 root fibers enter the spinal cord at least a segment behind the 

 last sensory root on the opposite side. To trace peripherally 

 from the spinal cord, this nerves passes laterally to the neural 

 arch, where it has one large nerve cell (N.C). Its course is 

 then dorso-caudad for some distance within the membranous 

 neural canal and the position of its foramen in the neural arch 

 is indicated by two dotted lines. Outside the neural arch it 

 has six nerve cells, which may represent its spinal ganglion. 

 On the contrary, its extreme dorsal position, the scattered ap- 

 pearance of its few cells, together with the relatively larger 

 number of fibers than cells, are striking differences from ordi- 

 nary spinal ganglia. To compromise for the absence of a ventral 

 ramus, the so-called spinal ganglion of this nerve receives three 



