NERVOUS FIBRES. 1 13 



explain the relation which they bear to the cells. Remak 1 

 describes the early condition of the nerves in the following 

 manner : " The substance of the cerebro-spinal nerves of the 

 rabbit, in the third week of embryonal existence, consists of 

 corpuscles, some of which are irregularly spherical, others 

 slightly elongated, having a very delicate filament adhering 

 to them ; they arc mostly transparent, and arranged in rows 

 without, however, presenting any distinctly perceptible fibrous 

 structure." And 1. c. page 153, he says, " A structureless and 

 general globular mass is the original form, from which the 

 primitive fibres of the cerebro-spinal nerves are developed. 

 These primitive fibres are at first varicose, and contain no 

 medulla; most of them pass into the cylindrical form, through 

 the intermediate stage of transitional fibres." 



I have investigated the development of nerve in the fcetal 

 pig. The nerves of the foetus have not the shining white 

 colour, presented by those of the adult animal, but are gray 

 and transparent, and the younger the embryo the more strik- 

 ing are these appearances. We are, therefore, quite prepared 

 to find that microscopic investigation show r s the white sub- 

 stance of the fibres to be less perfectly or not at all developed. 

 If a nerve, taken from a foetal pig of about six inches in length, 

 be spread out, in the usual mode of preparation by tearing it 

 under water, some fibres are seen which very much resemble 

 those of the adult animal, and which are furnished witli 

 outlines almost as dark. The greater part of the substance, 

 however, does not form connected fibres, but consists of separate 

 round globules, or more or less long, irregular little cylinders, 

 arranged with their long axes in the direction of the course of 

 the nerves, having outlines, however, quite as dark as those of 

 the nervous fibres. These appear to be what Remak refers 

 to in the description previously quoted. In addition to them, 

 however, a substance of quite another appearance is seen, 

 which has not the dark outline, docs not appear pellucid but 

 granulated, and in which the celh nuclei arc distinctly recog- 

 nisable. 



When the other constituent parts predominate, the nuclei 



1 Miiller's Arcliiv, 183G, p. 148. Respecting the microscopic structure of the 

 brain and spinal cord of the foetus, see Valentin, Entwickelungsgeschichte, p. 183. 



