44 n. M. JJERNAED. 



mately to be brought into the typical cyliudrical form. It is 

 easy to see that both these differences are due solely to the 

 fact that where the rods are long cylinders the protrusion 

 has to force its way between them, and only swells out iuto a 

 conspicuous vesicle after getting through. 



It will be seen from the study of these details how important 

 it is to keep the compactness of the layer of rods very clearly 

 before the miud. The rod layer, in fact, arises as the result 

 of the tlirusting out of great numbers of vesicles from the 

 retina, the vesicles only gradually assuming the long, cylin- 

 drical rod shape. The varying forms which the early stages 

 of new rods assume when first protruded, and until tliey are 

 finally developed, depend not only upon the fornis, but 

 also upon the lengths of those among which they 

 have to force their way. We have now seen two of 

 these different series of form-changes, and it will be best in 

 this connection to record the observations made on still 

 earlier stages of growth, when the new vesicles are protruded, 

 not among rods, but among other vesicles which have not had 

 time to become rods. We shall see that whei'eas, when the 

 rods are formed, and their shapes fixed, new vesicles have 

 to adapt themselves entirely to them ; while the rods are 

 still unformed and vesicular the protrusion of new vesicles 

 is able to modify their shapes. In the changes described in 

 Part I we saw that the protrusion of fresh cones altered the 

 shapes only of other cones, helping to change long cones 

 into Schwalbe's I'ods, but that they had no apparent effect 

 upon finished rods. 



The first appearance of rod-vesicles begins very early, as 

 soon as ever the eye begins to function. They can be seen in 

 various sizes in figs. 3 to 7, as round clear spaces against the 

 pigment. At first they are scattered and confused, because 

 all the nuclei do not secrete vesicles simultaneously (see 

 figs. 16, 17, and 18). A little later a stage is reached when 

 they are arranged side by side as large sacs mutually com- 

 pressing one another (figs. 12 and 15). It is at this 

 stage that our sections usually fail us. So long as the vesicles 



