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THE AMERICAN NATURALIST [Vol. XL 



between them. Meanwhile the cytoplasm about the nuclei be- 

 comes differentiated to such an extent that by sixteen and one 

 third days the layer is seen to be composed of clearly defined cells 

 each completely filled with pigment granules (Fig. 11). Later 

 all finer structure is entirely obliterated by the pigment, but no 

 granules are ever found normally outside their enveloping cyto- 

 plasm. The significance of these facts in the development of 

 the retina is seen in connection with certain theories which Ber- 

 nard has elaborated. He believes that pigment is a nutritive 

 substance which is constantly being ingested and absorbed by 

 the growing retina. This theory had its oriuin in Miss lluie's 

 article on Drosera, in which she has su(cee(h'(l in estal.hsliing 

 the fact of an intracellular digestion. Bernard "-oes so far as 

 to describe streams of absorbed pigment stretching through the 

 inner nuclear layer and forming the fibers of Miiller. Cameron 

 has accepted this theory and applied it in his study on the amphi- 

 bian retina. The nuclei, he believes, put forth an unformed 

 ferment or enzyme under the action of which the pigment granules 

 are rendered available for the metabolism of the cell. In this 

 way the rod and cone vesicles grow by successive digestions. 



Before the theory is accepted, liowever, it must be shown 

 that pigment is actually a food substance and not as generally 

 supposed a waste product. The theory nnist be harmonize(!, 

 too, with the fact that pigment granules are extremely resistant 

 to the action of all known ferments or digesting fluids as Cameron 

 himself states. A further objection to the theor>' lies in the fact 

 that in the chic'k these granules first form on the side of the pig- 

 before there is any ])igmeiu in their vicinitv If tlicv are able to 

 begin to grow without it, is it not at iea^t |)o..il,!e that thev might 

 continue to develop without it? Fm-tluM-. pi-mient i- never found 

 outside the protoplasm of the j)iunietit cell- noue ever a})[)eafs in 



development. Besides, mav not the mediation of a third >ul)- 



for cannot the rods and cones ol.tain nutritive nn'teri'd for their 

 growth direct from the numerous blood vessels just outside the 

 pigment layer ? 



