STUDIES IN THE EETINA. 327 



The diagram leaves one interesting point undecided. In 

 some retinas, e. g. of man, it appears as if the fibrils from the 

 rods run back as distinct rod-fibres into the middle layer (see 

 the asterisk in the diagram), Avhereas, in other eyes (see 

 fig. 20 e), no such isolated bundles appear to run through the 

 layer of rod-nuclei, but the nuclei are all connected together 

 into a network. Evidentl}^ both conditions occur. It is also 

 fairly clear that all rod-fibres are not nervous. Two very 

 important constituents of such fibres have already been 

 described in Part V, viz. the strands of the syncytial frame- 

 work and the pigmentary matter. Evidence that the nerve 

 or protomitomic fibrils also take part in their formation, 

 as was suggested by Max Schultze, will be given lower down. 



From the vantage ground of this discovery of the protomi- 

 tomic system, which we have now shown, not only morpho- 

 logically but physiologically, to be of fundamental importance 

 in the retina as a protoplasmic structure, several vistas of 

 inquiry are opened out. It is perplexing to have to decide 

 which to follow. The prominent position into which the 

 doctrine of nerve discontinuity has sprung naturally tempts 

 to criticism from the new point of view. This course, 

 however, began to lead me into a treatise on the morphology 

 of the nervous system, and threatened to extend this paper 

 into a book. I had to decide to deal with it separately in an 

 article to be published elsewhere. 



Then, again, the " protomitomic system " may be expected 

 to throw fresh light upon several burning questions in 

 cytology. But here also a special treatise would be required. 



The simplest and most useful plan will be to confine myself 

 in this paper to the retina, and to give an account of the 

 observations which I have been able to make as to the 

 relations between the protomitomic system and some other of 

 the retinal structural elements. It is by such observations 

 alone that we can supplement what we know of the mor- 

 phology of the protomitomic system by some insight into its 

 physiology. 



