STUDIES IN THE EETINA. 43 



material filling the rod ; the short innermost darker portion 

 clearly corresponds with the old staining tip of the cone 

 (of. the sections indicated by asterisks). The small numerals 

 1—7 show a continuous developmental series illustrating the 

 transformation of long-neched cones into Scliwalbe's rods. 



It was stated in Part I that the groAvth-forms (q, Co, Cg) of 

 the new rods shown in PI. 3; fig. 4, of that paper were due 

 to the fact that each new element on being protruded 

 had to force its way between tightly packed cylindrical 

 rods ; obviously the new vesicle could only swell into a 

 sac after getting through, that is. against the pigment. 

 If this explanation be correct we should expect to find 

 other form-phases where the rods Avere not so long. A 

 much more direct transformation of cones into rods waSj 

 indeed, figured in the series of elements supplied by the 

 axolotl, Part 1, PI. 3, fig. 8.^ In this animal the rods are 

 very thick, and, compared with their thickness, very short. 

 Now it is interesting to note that we have at the sides of 

 tadpole retinas, where the rods get progressively shorter, a 

 very similar process of direct transformation of cones into 

 rods to that which we found in the axolotl. The distal 

 portion of the protruded, cone seems to be neatly rounded off 

 (fig. 11), as if there had never been any swollen vesicle at 

 its tip. It is further quite distinctly striated longitudinally. - 

 Here, then, we have the cones changing directly into rods by 

 the absorption of the refractive globule and the lengthening 

 of the outer limb at the expense of the inner. If we compare 

 this process closely with that occurring among the long rods 

 in the central regions of the retina (figs. 9 and 10), we find 

 that it differs in two points : (1) there is no long thin neck, 

 and consequently (2) there would appear to be no con- 

 spicuously swollen vesicle at the tip which would have ulti- 



' To make that figure true to life the tips of the cones in a and d sliould 

 have been drawn with delicate vesicles, but all traces of such vesicles had 

 been destroyed in the actual sections. 



- Compare tlie dots been on the distal vesicles shown in figs. 9 and 10, also 

 the remarks on the striation of the cones in Part II, p. 455. 



