i8o PROTOPLASM 



they become. On this, therefore, depends also the fact that 

 the pronounced fibrous structure of the ganglion cells, and the 

 striations of the epithelial cells, axis-cylinder, etc., were dis- 

 covered at such a relatively early period, while the reticular 

 structures were not discovered till much later. If a system 

 of long parallel lines be drawn at equal distances apart, 

 and joined together by vertical, irregularly arranged con- 

 necting lines of equal thickness, as shown in Fig. 6, 

 Plate XII., the following fact can be observed from a con- 

 sideration of the drawing. When the figure is looked at 

 from a moderate distance, so that it can be plainly discerned, 

 and then the interval between it and the eye gradually 

 increased, a distance is finally reached in which only the 

 long parallel lines can still be plainly distinguished, while 

 the cross connections, on the contrary, have disappeared from 

 the vision of the observer. 1 



Just the same state of things is presented by the micro- 

 scopic image of an axis-cylinder, which gives in my opinion 

 a sufficient explanation of the fact, that the longitudinal 

 fibrillce were discovered here at such a relatively early 

 period, while their cross connections were not seen till 

 so late, and even now require to be studied carefully and 

 to have their structure rendered very distinct, if they 

 are to be clearly observed. The same fact is true, however, 

 of the protoplasmic reticulum generally. If a drawing of such 

 a structure be made, at a certain distance the true filaments 

 of the reticulum will become more indistinct, and finally only 

 the thicker and darker nodal points will remain, provided 

 they are sufficiently darker than the filaments of the net- 

 work, so as to produce, of course, the appearance of granula- 

 tion. If, however, certain tracts of the network are ranked 

 one behind the other in a line for some distance, they will 

 remain distinct for a correspondingly longer time, and one 



1 It is also possible to observe at the same time the vanishing of the longi- 

 tudinally directed shorter lines, from which, as has been said, it follows that, 

 other conditions being equal, the distinctness of the lines varies with their 

 length. The observations described are just as successful, in fact even 

 better if anything, if the figure be approached gradually from a greater 

 distance. 



