462 H. M. BERNARD. 



divided into two compartments by a cross-membrane ; ^ and 

 as they assume their definitive shapes they become gradually 

 filled with a staining reticulum, which, omitting the ellipsoid, 

 develops especially strongly in the outer and, in the adult 

 Amphibian rod, more impoi'tant of the compartments. 



This account seems to justify the description of the rods as 

 prolongations of the " visual cells." It is obvious that each 

 may be regarded as a prolongation of the cytoplasm belong- 

 ing to each rod nucleus, a prolongation at first filled with 

 fluid, but sooner or later containing also the usual reticulum 

 which ramifies through the cytoplasm of ordinary cells. My 

 only objection to this description is to the term " visual 

 cells." My researches long ago compelled me to abandon 

 the usual conception of the retina as composed of cells, and I 

 now regard it as a syncytium, in which the nuclei 

 are arranged in layers, not as fixed morphological 

 units, but solely as centres of physiological acti- 

 vities which may at times require them to migrate 

 outwards, ultimately, if life lasts long enough, to 

 become rod nuclei. The evidence for this is, to my mind, 

 so convincing that I have no hesitation in making the state- 

 ment, even though it stands in such startling contrast to the 

 conclusions of nearly all the most recent workers on the 

 retina, such as Ramon y Cajal, Dogiel, and others, and though 

 a criticism of the method and results of these authors is here 

 out of the question. In the first part of this paper, p. 43, I 

 referred to the migration of nuclei from the middle nuclear 

 layer to the outer nuclear layer, and showed that, even if we 

 could not see evidence of it in our sections, it would be 

 necessary to assume it ; and I here add figures of nuclei 

 passing through the outer reticular layer in different Am- 

 phibia (figs. 21 — 23, 25, 26) ; while, again, in fig. 24 one or 

 pei-haps two nuclei have moved outwards together, leaving a 

 space vacant in the middle nuclear layer, and apparently 



• I have not yet been able to ascertain for certain the time of appearance 

 of this membrane. As we shall see below, it probably appears before the 

 ellipioid 



