* SPINAL NERVES IN VEETEBRATES 171 



ordinarily situated cephalad if their corresponding sensory 

 portion "wdth which they later join in forming a mixed ramus. 

 Hence the more cephalic motor ramus will be exposed for a 

 longer period of time to this force which would tend to carry 

 both rami caudad, which factor might be sufficient to cause the 

 motor elements of the spinal nerves to approximate their corre- 

 sponding sensory elements. Furthermore, in the majority of 

 embryos, especially of the simpler vertebrates, the sensory 

 branches lie median to their corresponding motor branches. 

 Consequently the rapid growth of the myotomes on the outside 

 together with the increase of the connective tissue within 

 would tend to bring the motor and sensory portions of a nerve 

 nearer. 



Another mechanical factor which might contribute to the 

 approximation of the motor and sensory branches of a spinal 

 nerve is the more rapid and in some cases a more continuous 

 growth of the skeletal than the neural axis, which would neces- 

 sarily carry both the motor and sensory roots caudad. Since 

 the motor roots are formed first and are more cephalic in posi- 

 tion, they would be subjected longer to this force which would 

 tend to carry both roots caudad, and as a result the motor roots 

 should approach their corresponding sensory roots. 



2. Chemical attraction (chemotropism) 



That there may be such a mutual attraction between the 

 sensory and motor components of a spinal nerve is suggested 

 by the following observations on the development of the spinal 

 nerves in various vertebrates: (a) Certain of the caudal ventral 

 motor rami (figs. 1, 3, 12 and 13, R.V.M.) in Polistotrema have 

 migrated cephalad, counter to the force of the growing myo- 

 tomes which would tend to carry them caudad, and have joined 

 their corresponding sensory rami (R.V.S.) in forming mixed 

 rami ventrales. (b) In the 7.5 mm. Squalus embryo it was shown 

 that the ventral prolongations of the neural crest, representing 

 the beginning of the spinal ganglia and the ventral sensory rami 

 (fig. 16, Sp.G. and R.V.S.) have the appearance of being at- 



