162 WILLIAM F. ALLEN 



that the motor rami are composed of fibers, bordered by a few 

 embryonic neurilemma cells; while the sensory rami, especially 

 their distal extremities, are made up almost entirely of cells. 

 It mil be seen from an examination of figure 17 that the cells 

 of the sensory ventral rami have extended some little distance 

 ventrad of the fibers of the ventral motor rami and that they 

 have accumulated in a mass opposite the dorsal aorta, which 

 undoubtedly represents the anlage of a vertebral sympathetic 

 ganglion. This reconstruction of the spinal nerves of a 7.5 

 mm. Squalus certainly supports Onodi's hypothesis that sympa- 

 thetic ganglia are formed of cells migrating from the spinal gan- 

 glia. It is highly improbable that the few scattered cells on 

 the outer surface of the ventral motor rami take any part in 

 the formation of sympathetic ganglia. At this stage the anlage 

 of the sympathetic ganglia will be seen to be not only median, 

 but considerably below the level of the motor rami. The cells 

 about the motor rami are probably concerned only in the forma- 

 tion of neurilemma and connective tissue coverings. 



Figure 20, which is a graphic reconstruction of two spinal 

 nerves from the lower abdominal region of a 19 mm. Squalus, 

 presents the following points of developmental progress: (a) 

 From a comparison with figure 17 it is evident that this advance- 

 ment has been concerned largely with the increase in length of 

 the nerve elements already formed. The motor rami dorsalis 

 [R.D.M.) have crossed obliquely the outer surface of their 

 ganglia and have grown along the inner surface of the myotomes 

 in a general dorso-caudal direction to the region of the dorsal 

 pole of the ganglion of each succeeding spinal nerve. The motor 

 and sensory rami ventrales {R.V.M. and R.V.S.) have extended 

 caudo-ventrally along the inner surface of the myotomes to the 

 level of the caudal vein and the Wolffian duct. They have 

 remained entirely separate as far as the lower margin of the 

 dorsal aorta, with the exception that for a short distance in the 

 region of the upper surface of the aorta they have come in contact 

 with each other, but are prevented from intermingling by a layer 

 of connective tissue cells or neurilemma. At the level of the lower 

 border of the dorsal aorta the motor and sensory rami ventrales 



