STUDIES IN THE RETINA. 45 



outwards their sac-like processes, which are destined to 

 become new rods. These, therefore, would represent a still 

 earlier stage in the development of new rods even than that 

 shown in fig. 4, c^. They might almost be called intra-retinal 

 stages. 



Returning now to the positions of the cone nuclei in the 

 Amphibia, if we compare them with those of the " cone " 

 nuclei in Mammalia, we see at once that the cones in the 

 Amphibia cannot, at least off-hand, be regarded as the mor- 

 phological equivalents of the cones in the mammals, or even 

 of the giant cones in the eyes of the cod. In both these the 

 cone nucleus has, if anything, a more advanced position near 

 (or even protruding through) the memb. limitans ext. than 

 the nuclei of the rods, which is the very reverse of what we 

 find in the Amphibia. Whether, again, the cones of the 

 human eye are the morphological equivalents of the giant 

 cones of the eyes of the cod is a further problem, which I 

 shall endeavour to decide in another paper. I shall not be 

 surprised if these, and, indeed, not a few other retinal 

 elements which have hitherto been regarded as special struc- 

 tures of definite morphological value, turn out to be passing 

 form-phases due to functional activities. Indeed, much of 

 the diversity which occurs in the records of past observations, 

 a diversity which is usually attributed by each observer to the 

 defective preservation of the material used by his less fortu- 

 nate or skilful colleagues, will prove ultimately to be due to 

 the fact that different physiological conditions are accompanied 

 by striking variations in the appearances of the tissues con- 

 cerned. 



The one point which we have here endeavoured to establish 

 is that in the Amphibia the "cones," which have always 

 been regarded as highly specialised structures with definite 

 sensory functions, superior even to those of the rods, are not 

 definitive structures at all, but simply developmental stages 

 in the production of new rods. 



