STUDIES IN THE RETINA. 47 



then, are due to the driving" out of the staining matter which 

 had accumnlated in the neck of tlie sqneezed-up vesicle. 

 There is no telescoping of the membrane into itself. It is 

 simply another form of the phenomenon shown by asterisks 

 in fig. 0, where the original contents of what has hitherto been 

 thonght to be the tip of the cone become the proximal portion 

 of the contents of tlie Schwalbe's rod, which arises as soon 

 as the vesicle has assumed its cylindrical shape. The vesicle 

 assumes this latter shape apparently in both cases as it 

 becomes more and more turgid with matter received on the 

 one hand from the retina, and on tlie other from absorjitioa 

 of pigment. The stages c and d in fig. 13 require no con- 

 necting links, d being the next stage produced by the filling 

 up of the distal vesicle. Still younger and simpler stages of 

 transformation of vesicles into rods are shown in fig-. 15. 

 They need no comment. 



Many interesting details of observations in relation to this 

 part of the subject might be added, but the task of dealing 

 with the retina of the Amphibia alone threatens to lengthen 

 out so greatly that only points necessary to a clear under- 

 standing of the essential moi-phology of the retinas dealt 

 with can be mentioned. 



How necessary it is to understand the minute details of 

 rod-formation I need hardly insist, that is if we are to make 

 any progress with our researches into the mechanics of vision, 

 for the rods are the specific structures which constitute 

 the retina the specific organ of this sense. Believing, as I 

 do, that all structures are produced both phylogenetically 

 and ontogenetically only in response to physiological needs, 

 I feel confident that in a case like this where the rods are 

 produced in situ, and only when required, their processes 

 of formation must throw light upon the mechanics of their 

 functional activities. Some further details relating to these 

 activities will be found in the next part. 



