312 H. M. EEKNArvD. 



cannot see them more distinctly. I have been astonished at 

 the difficulty experienced ; section after section was searched 

 in vain ; not a trace of them could be seen^ either because 

 they had been ruptured or else becanse they are so fine that 

 they are easily obscured or otherwise elude our search. 

 Then suddenly a slide showed them perfectly clearly. The 

 chief cause of their visibility will be explained in the sequel. 

 I can no longer wonder at the fact that the few records 

 which have been made from time to time of threads radiating 

 outwards from nuclei in other tissues have, with few ex- 

 ceptions, fallen into discredit, nor that the internuclear 

 connecting filaments of the retina have never been seen 

 before, or, if seen, have been regarded as accidental phe- 

 nomena.^ The fact is, on the contrary, it is their preservation, 

 and, even if preserved, their visibility which is accidental. 

 But, in the course of these researches, so many retinas were 

 studied that the accidental apparitions of the internuclear 

 connecting filaments became in time numerous enough, first 

 to suggest their being of importance, and ultimately to con- 

 vince mo of their universal existence as primary factors in 

 protoplasmic tissues. 



Let us pass the indirect evidence in review, taking each 

 layer in turn, beginning with the largest. 



Evidence for the Existence of the Internuclear 

 Connectinof Filaments between the Nuclei of 

 the Middle Nuclear Layer. 



1. The nuclei of this layer are frequently found somewhat 

 crowded. If the material is well fixed, i. e. shows the intra- 



' I sliould like also to add that a great deal of recent work has been done 

 on the retina, in which more attention has been paid to the staining than to 

 the fixation of the material. The nuclei are seldom shown in the figures 

 with a rcticulaled internal tlructure; tliey are too often mere sacs of dis- 

 organised granules. The metal impregnation methods reduce them to mere 

 smudges. The result of my own work convinces nic that, the missing keys 

 to many unsolved problems will be ultimately found in the Guer structure of 

 the nucleus. 



