38 H. V. NEAL 



cording to von Apathy, the conducting or neurofibrillar substance 

 just as muscle cells produce muscle fibers. From these 'nerve 

 cells ' the conducting substance grows on the one hand toward 

 the center in the 'ganglion cell' and on the other hand toward 

 the periphery into the sensory cell or muscle cell and so forth. 

 The nuclei of the nerve cells lie in the course of the nerve fibers 

 themselves and form in vertebrates the nuclei of the neurilemma 

 sheath. The protoplasmic body of the nerve cell is, in general, 

 spindle-shaped so that it may be called the nerve spindle. Its 

 membranous boundary is the neurilemma. 



For every motor nerve fiber of a vertebrate, on the other hand, 

 according to Held, there may be distinguished a central and 

 peripheral nerve stretch. The external limiting membrane 

 of the neural tube divides the two. The neurofibrillae of the 

 central stretch are formed before the peripheral. They arise 

 through the unilateral growth of the neurofibrillar substance of 

 a unipolar or bipolar neuroblast which proceeds basalwards 

 in the direction of the Rabl's 'chief axis' and soon transcends 

 the outer limits of the neural tube in its growth toward the 

 terminal organ. The central stretch of the nerve varies with the 

 position of the neuroblast cell. In its extent through the mar- 

 ginal zone the nerve fiber becomes secondarily surrounded and 

 enclosed by glia cells. 



Held distinguishes three chief stages in the development of 

 the peripheral stretch of the nerves: 



1. That of the outgrowing nerve itself, which has not yet 

 reached its terminal organ with its specific substance, but 

 nevertheless is connected with its terminal organ by means of 

 a simple undifferentiated plasmodesmatous strand. Theoreti- 

 cally this stage is the most important. The His doctrine, accord- 

 ing to which the amoeboid neuroblast processes extend through 

 open spaces in the tissues, cannot be correct, since in the Anamnia 

 the growing point of the motor nerve is connected with portions of 

 the epithelium, that is by plasmodesmatous connections, and in 

 the Amniota on the other hand with parts of the mesenchyma. 



2. In the second stage the nerve has reached its muscle an- 

 lage, which it not only touches superficially, but cells of which 



