346 H. M. BERNAKD. 



rod as one of its normal functional activities. In Part TI, 

 fig. 17, this discliarge of small globules of fluid into the rods 

 can be seen ; the same also can be seen in the newt;, Part II, 

 fig. 30. In both cases the fluid clearly comes from the 

 nuclei. In all the mammalian eyes I have examined, the 

 existence of globules of clear fluid can be seen in the rods. 

 I am further inclined to regard the large inner limbs of a 

 certain number of the rods in the human retina, Avhich, on 

 that account, have been called cones and considered as of 

 special morphological value, as simply due to the fact that 

 they have just received, or are in the act of receiving, a 

 discharge of fluid. In all essentials they are ordinary rods, 

 and, so far as appearance goes, differ in no other respect 

 than that their inner limbs are swollen up with fluid. The 

 mammalian cone is thus apparently simply a normal physio- 

 logical phase in the life of the rod. All the rods of the 

 human retina ]n'obably pass, every now and then, through a 

 cone phase, most rapidly and frequently where functional 

 activity is greatest, i. e. in the region of the fovea centralis, 

 where the rods are of increased length.-^ 



Further evidence for the origin of this fluid from the 

 nuclei may sometimes be seen at the rim of the retina in 

 young tadpoles where rod-formation is beginning. Fig. 24 

 shows a most instructive series, frequently but not always 

 seen. The large columnar cells inside the iris contain distally 

 a great fluid space, the nucleus, with some cytoplasm, 

 being confined to the proximal end of the cell. The 

 chromatin network invades this fluid space at one end, and 

 pigment granules at the other. This is, in brief, the descrip- 

 tion which can be given of the rods, except that the latter 

 protrude beyond the membrana limitans externa. Where 

 the iris passes into the retina the cells become transformed, 



' As shown in earlier papers, the cones are not homologous or even 

 analo,i,^ous strnetures in difTcrent eyes. The cones of the Amphibia are young 

 rods ; tlie cones in tiic fish have an entirely different sigiiilicance (see Part V), 

 while the cones in the human retina admit of the simpler explanation given 

 above in the text. 



