46 H. V. NEAL 



lie definitely free, and in the spaces between them there is not 

 the slightest amount of protoplasm to be seen. Held denies 

 the accuracy of this description as based upon unsatisfactory 

 methods of preservation and staining. Cajal ('06, '07) in Held's 

 opinion has been led into error for the same reason. On the 

 other hand, Ko Hiker ('05) finds the fibers of the embryonic 

 trochlearis nerve in a calf surrounded by sheaths of an inter- 

 mediate substance. Gurwitsch ('00) mentions a lamellar net- 

 work which grows into the bundle of 'naked' fibers from the 

 surrounding mesenchyma sheath and divides up the nerve bun- 

 dle, while according to Kappers ('04) the nerve bundle is coarsely 

 split or divided by fine processes of the neurilemma cells. 



In his search for evidence to support the hypothesis of the 

 plasmodesmatous origin of nerve fibers, it seems not to have 

 occurred to Held that the protoplasmic envelope of an embryonic 

 nerve fiber may be produced by the same neuroblast that formed 

 the fiber. The simple fact of a protoplasmic sheath around the 

 neurofibrillae no more proves the existence of nerve 'paths' than 

 it does the hypothesis of neuroblastic outflow. The evidence is 

 in harmony with either supposition. In Squalus, embryonic 

 nerve fibers are 'naked' in the sense that primarily they have 

 no cellular sheaths. But it is also a fact that they have granu- 

 lar, protoplasmic sheaths, visible, not only in sections, but in 

 cover glass preparations of the living nerve fiber. The substance 

 of the sheath stains lightly under the same treatment which 

 stains the fiber intensely. For this reason the protoplasmic 

 envelope is easily — and has been generally — overlooked. The 

 thickness appears to vary in proportion to the length of the 

 neuraxon process. In nerve anlagen, like the trochlearis, for 

 example, in which the fibers are especially long and slender, the 

 protoplasmic sheaths of the fibers are proportionally^ thin. In 

 spinal somatic motor nerves the sheaths are relatively thick, 

 becoming thinner as the nerve fibers increase in length. The 

 ingrowth of processes of neurilemma cells described by Gurwitsch 

 ('00) and Kappers ('04) occurs in later stages and adds to the 

 interfibrillar protoplasmic substance. 



