STUDIES IN THK RETINA. 51 



of the persistence of the colour right into the retina will be 

 given below. 



Before continuing to consider whether and how the matter 

 can escape from these inner limbs, one or two points may be 

 noted in passing. (1) The "giant cones" of the eyes of fish 

 are not the morphological equivalents of the cones in the eyes 

 of the frog. The latter are almost the earliest stages of rod 

 formation; the former are not only fully developed rods, but, at 

 least so far as growth proportions go, the most highly deve- 

 loped elements known in vertebrate eyes; their great size 

 will have some bearing upon the question as to the length of 

 the life of individual retinal elements when that question 

 comes to be put. 



(2) A very large proportion of these " giant Cones " are 

 double. Although the dividing line in the swollen inner 

 limb may become very faint, and sometimes only traceable 

 in tangential sections, the presence of two nuclei and of 

 two outer limbs will always show whether any large "cone" 

 is double. I have already shown that the peculiarities of the 

 double cones in the frog are due to the fact that two 

 protrusions of the retina start side by side almost simul- 

 taneously, and their forms are due to mutual pressure, both 

 being subject at the same time to the general pressure 

 which we have to assume to account for the ordinary cone 

 phases of rod formation. In these great double "giant 

 cones " we merely have two rods very close together, and about 

 the same age. The fusion of their inner limbs will take place 

 sooner or later, as these inner limbs swell with matter. The 

 only part of the phenomenon which requires investigating is 

 why these rods of similar age should be so frequently in 

 pairs, or, tracing it a stage further back, why it is that as 

 new nuclei arrive to send out their protrusions to form new 

 elements for the growing retina, they leave so many pairs of 

 nuclei between which they do not or cannot force their 

 way. 



(3) The remarkable change which takes place in the 

 forms of the elements of the growing fish retina, from an 



