NERVE-TISSUE. 303: 



is nothing to support the assumption that nervous tissue origi- 

 nates from " cells." Unquestionably, there are large masses of re- 

 ticular bioplasson in which by a growth of living matter, chiefly 

 in one direction, axis-fibrillae originate, while the medullary 

 investment is a much later formation. At first there is no trace 

 of ganglionic elements. We know that these elements make their 

 appearance only after the third month of intrauterine life. The 

 motor elements of the spinal cord particularly are first observed 

 during the third and fourth months, corresponding to the time 

 when the embryo begins to manifest signs of life. Further, the 

 ganglionic elements cannot arise from cells, as they are no cells, 

 but from portions of living matter arranged in clusters, in which 

 all the plastids remain interconnected. This mode of development 

 is indicated by the inflammatory changes of the ganglionic ele- 

 ments, when they return to their embryonal condition. 



In the human embryo two months old, the intervertebral 

 ganglion is composed of medullary tissue, with relatively large 

 fields of myxomatous basis- substance. The nerves are non- 

 medullated, which proves that the myeline investment must be 

 formed at a later period than the axis-cylinder. (See Fig. 130.) 



For a successful investigation of the development of nerv- 

 ous tissue the recognition of two points is, in my opinion, of 

 fundamental importance: viz. : first, that the nervous sys- 

 tem is a formation originating in the middle embryonal 

 layer j and second, that no isolated " cells" take part in the pro- 

 duction of nervous tissue. With the knowledge of these facts, 

 we can understand that in an elongated group of intercon- 

 nected plastids the central portion may remain unchanged 

 living matter, the axis-cylinder j while a more peripheral por- 

 tion may become reticular (horny ?) and infiltrated with mye- 

 line as a kind of basis-substance, and that the most peripheral 

 portion, by a solidification of the bioplasson liquid, may be trans- 

 formed into the myeline sheath with a nucleus, in about the same 

 manner as a fat-globule arises from myxomatous connective 

 tissue. 



METHODS FOR THE PREPARATION OF NERVE-TISSUE. 



Successful examination of the nerve-tissue depends on a suit- 

 able mode of preservation. Teasing, tearing, pulling, and making 

 specimens " half dry " are methods unworthy of being named. 



Small pieces of the brain or the spinal cord should be placed 



