STUDIES IN THE RETINA. 39 



those required travel along the middle nuclear layer from the 

 undifferentiated rim of the retina where nuclear division is 

 active during growth. Thus a stream of nuclei travels 

 inwards from this undiiferentiated i-im towards the axis alono* 

 the middle nuclear layer; (5) this stream of nuclei lays 

 the foundation for the two reticular layers of the retiua ; the 

 cytoplasmic trailings of the nuclei being, as it were, swept to 

 the sides of the stream, accumulate, but while the inner 

 accumulation persists the outer is mostly used up, probably 

 in the formation of the rod-vesicles. 



Before leaving the subject for the present, I should like to 

 call attention to the conviction which I expi'essed in Part II, 

 p. 452, that the retina is a syncytium, in the reticulum of 

 which nuclei are suspended, and that it is almost impossible 

 to speak of "cells" in connection with its component 

 elements. The streaming of the nuclei and the trailing- 

 behind them of cytoplasmic tangles, which trailings 

 accumulate as the eye grows, may, I think, be regarded as 

 complete justification for this conviction. I had not for- 

 gotten and do not forget the large '''ganglionic cells," 

 which appear to supply an easy refutation. On the contrary^ 

 it was a prolonged study of these same '^ cells " which first 

 led me to this conclusion, as I shall relate in detail in a 

 future paper. 



Lastly, I should like to venture the suggestion that the 

 principle here established for the retina may be of wide 

 ap[)lication, although I cannot hear of any other exemplifica- 

 tion of it as yet known. The principle is this : an organ 

 has to continue to grow after it has begun to func- 

 tion. Assuming that nuclei or cells are incapable of mitotic 

 division when once specialised for some highly complex 

 function, we should be compelled to postulate an undifferen- 

 tiated region which would persist as long as growth lasts. 

 From this region, which would be the centre of active nuclear 

 or cell division, the new elements required by the functioning 

 and growing area would have to migrate, through longer 

 or shorter distances according to the exigencies of the 



