584 William Patten 



vacuole, only conspicuous by the constaiicy of its presence and posi- 

 tion, terminate at the same level, and form a gently imdulating line of 

 division (the pseudo-membrane, or sieve-membrane of Car- 

 rière) between the eellular layer of the retina, and the underlyìng rods. 

 The sharpness of this line is enhanced by the fact that the flanged walls 

 of the inner ends of the cells unite with those of the neighboring ones. 

 An exceedingly small space, or canal, surrounding the inner ends of the 

 cells, is thus formed. 



A delicate, structureless wall separates the retìnophorae from their 

 rods, thus tending stili more to produce the Impression that a mem- 

 brane is present. Hensen and Carrière called attention to this effect, 

 in order to warn against it. Hensen attributed it to the fusion of the 

 walls of the neighboring cells, but failed, as did also Carrière, to see 

 the dividing wall. By studying a series of cross sections at the niveau 

 of the pseudo-membrane, some cells will be seen containing, instead of 

 the granular protoplasm, a thin, homogeneous partition, — the ter- 

 minal membrane, — the centro of which is perforated by the axial, 

 nerve fibre (PI. 29, fig. 26 x). 



The large, abnormal folds figured by Hensen, Hickson and Car- 

 rière, especially the two former, are produced by the contraction of the 

 elastic septum, which, in the living condition, is always in a state 

 of tension. • 



Most of the retinophorae bave the peculiar shape indicated in the 

 figures, but a comparatively small number of modified forms, which 

 bave escaped previous observers, are mingled with them. They are re- 

 tinophorae containing the axial nerve fibres and the two nuclei, but 

 differing from the others in being reduced almost to a fibre (PI. 29, 

 fig. 36) . They are not easily seen in sections, but may be found quite 

 frequently among the macerated and detached cells of the retina. 



Although I bave found some very long ones, which must bave ter- 

 minated near the centre of the retina, they are most abundant near the 

 perijìhery, where they become gradually transformed into the short, 

 fibrous cells, without rods, which, for lack of a better name, I shall cali 

 the pseudo-retinophorae (PI. 29, fig. ^S ps. rf.). 



The rods of the retinophorae, which are at once the most in- 

 teresting part of the eye, and that most difficult to understand, bave 

 long resisted ali attempts to harden them in anything like a perfect con- 

 dition. The only method which gives satisfactory results is treatment 

 in hot chromic acid as described at the end of this paper. 



My first studies upon the rods of Mollusca were made upon those 



