28 H. V. NEAL 



bridges, a conclusion reached by Neal ('98), Dohrn ('07) and 

 Gast ('09), there appears no good reason why he should not 

 accept the conclusion that spinal somatic motor nerves become 

 differentiated and connected with their myotomes in the same 

 manner. If a nerve like the trochlear, which traverses a great 

 distance in reaching its peripheral organ, can do so without the 

 aid of a plasmodesmatous bridge, there would seem very little 

 ground for assuming a different mode of histogenesis for spinal 

 somatic motor nerves. That, as a matter of fact, their genesis 

 is essentially alike is supported by the evidence presented in 

 this paper. 



Held ('06, '09) Hke Paton ('07) emphasizes the importance of 

 the plasmodesmatous connections of neural tube and myotome, 

 and concludes, in agreement with 0. and R. Hertwig ('78), that 

 these plasmodesmatous bridges are secondarily formed as cell 

 outgrowths. Not only cells of the myotome and spinal cord 

 but also those of the chorda are supposed by Held to partici- 

 pate in their formation. Held ('09, pp. 91-92) refers to the 

 network between chorda^ tube and myotome as 'Scilly's Faser- 

 netz' and characterizes it as usually a cell- or nuclear-free tissue, 

 since it consists exclusively of the basal cell processes of the 

 germ layers united together. Secondarily, mesenchymatous cells 

 wander into this network. 



He further states (p. 281) that at the time of the appearance 

 of the first motor nerves, which have grown from the neural 

 tube, there is everywhere present an abundance of fine con- 

 nections. The process of transformation of this — a pre-nervous 

 plasma path — into nerves depends upon the sj^ecific power of 

 His' neuroblasts. 



With regard to their primary origin, he says (p. 277) that 

 Hensen's hypothesis that all nerves have arisen by the incom- 

 plete separation of the origin- and end-cells cannot be correct, 

 since it cannot be thoroughly applied to the details of the devel- 

 opment of definite organs in their reciprocal relations. The 

 motor spinal nerve of a frog-larva, for example, can be devel- 

 oped from no mitotically-formed primitive nerve. On the con- 

 trary, Held, in agreement with the Hertwigs, regards the plas- 



