STUDIES IN THE RETINA. 27 



the cone, measures 10^, aud is, as is well known, of the same 

 length as the inner limb of the rod. The second section also 

 measui'es 10 fx, with slight variations it is true, yet with a 

 constancy suflBcient to demand explanation. Section 3 at 

 times also appears to be well marked, but this may be purely 

 accidental. The same lengths are found for these three basal 

 sections in the cones c^ and c^, there being no doubt in these 

 cases of the existence of section 3. On the other hand, sec- 

 tions 4 and 5 are more than doubtful, and the last 20 fx of the 

 tips of the cones runs without natural divisions to the pigment 

 in which the most distal portions are always embedded. 



Keturning now to the forms of these different types of 

 cone, it is obvious that, so far as the three inner sections are 

 concerned, the cone C.^ is deducible from Cg by the simple 

 addition of a section; for cone C3 differs from Co in having 

 the thick basal section, with its large vacuole. Both form 

 and measurement thus seem to indicate that, so far as the 

 three basal sections are concerned, cone Cg is but a further 

 development of cone Cg, i. e. a new portion has grown out 

 beyond the memb. limitans ext., causing section 1 of cone c^ to 

 become section 2 of Cg, and section 2 of Co to become section 

 3 of Cg. This is, as we shall presently see, what has actually 

 taken place. 



With regard to the more distal sections the changes are 

 more complicated. While it is very difficult to get absolute 

 demonstration of the terminal vesicle in C2, I am quite con- 

 vinced that such a vesicle exists. Fig. 2, a, shows such a 

 vesicle with its proximal half clear of pigment ; but this 

 belongs to a cone which I take to be at a still earlier stage 

 than Co ; while fig. 2, d, f, g, shows what appear to be terminal 

 vesicles covered over, almost in a basket-like fashion, with 

 pigment granules. The special sticking of these granules to 

 cones is always very marked, and will be noted again. Max 

 Schultze long ago called attention to the clinging of the pig- 

 ment granules to the ' double cones ' in Fish.^ More easily 

 demonstrable, because longer, and therefore more likely to 

 1 • Archiv mikro. Anat.,' iii, 1867. 



