4i6 SCIENCE PROGRESS 



epiblast, and an island tube of epiblast is converted by thickening 

 of its walls into the brain in front and spinal cord behind. 

 Its walls contain numerous masses of protoplasm, or primitive 

 nerve cells, which are called neuroblasts. At first these cells 

 have no processes, but branching projections grow out from 

 each ; these interlace with the similar processes of other cells, 

 and so bring neighbouring cells, and even distant cells, into 

 communication with each other, to ensure harmonious or co- 

 ordinate action between them. But these branching processes 

 or dendrons are not the only outgrowths of the nerve cell, nor 

 are they the first to make their appearance. The first outgrowth 

 to appear is stouter than those that follow ; it grows to the 

 greater distance ; it leaves the central nervous system altogether, 

 and ultimately penetrates to peripheral structures. It is known 

 as the axon, and, with few exceptions, every cell body has only 

 one axon. It ultimately becomes the axial conducting core 

 of a nerve fibre, and a collection of these fibres constitutes the 

 bundle known as a nerve. At first the axon is naked throughout 

 its entire length, but later in development it becomes sheathed. 

 The sheaths of the axon are certainly important for its 

 nutrition, and possibly insulate the axon, much as a telegraph 

 wire is insulated from its fellows by a silk or gutta-percha 

 covering. In the fully formed nerve each fibre then consists 

 of an axon or axis cylinder ; outside of this is a thick white 

 sheath, which contains complex phosphorised fats — this is 

 known as the medullary sheath or white substance of Schwann; 

 outside of this is a delicate nucleated membrane called the 

 primitive sheath of Schwann, or more usually the neurilemma. 

 Certain new fibres, however, never receive a fatty sheath, and 

 are consequently called non-medullated ; these are the fibres 

 which supply the involuntary muscular fibres of the internal 

 organs of the body. 



This view, that each nerve fibre develops as an independent 

 outgrowth from a nerve cell, and grows distalw^ards, finally 

 becoming united to other tissues {e.g. muscle fibres) in the 

 periphery of the body, is associated especially with the name 

 of His, and has been accepted by the majority of embryologists. 



There have been other views held, but it will be sufficient 

 here to mention only one of these; for it is the one which, 

 next to that of His, has been favoured by investigators. Briefly 

 it is as follows : The nerve fibre is not a secondarily formed 



