190 BULLETIN OF THE 



there are at least three kinds of cells. Two can be readily isolated ; the 

 third has been studied only in sections. 



The retina extending from the line of deepest flattened nuclei to its 

 outer margin breaks up into two very distinct forms of cells, — retinal or 

 nerve-end cells, as Lankester and Bourne ('83, p. 182) have called them, 

 and pigment cells. The retinal cells (PI. II. fig. 5) are elongated and 

 rounded at their outer ends ; they terminate below in nerve fibres. From 

 the rounded external end the calibre is uniform till the region of the 

 rhabdomeres is reached. Here the cells increase in diameter, and then 

 continue for some distance uniform in size. Finally, each cell, enlarging 

 slightly at its deep end, rapidly tapers into a nerve fibre. Throughout 

 its whole extent the retinal cell contains pigment, which is principally 

 concentrated, however, at its rounded outer end. 



The pigment cells (PI. II. fig. 6) at their anterior ends, like the pig- 

 mented tops of the retinal cells, abut against the preretinal membrane. 

 From this they pass backward, and in the region of the rhabdome, where 

 the retinal cells enlarge, they contract to thin fibres, which, after the 

 rhabdome has been passed, again expand into irregular pigment sacs at 

 the deep part of the retina. When isolated, they present the appearance 

 (PI. II. figs. 6, 7) of two sacs of pigment connected by a slender rigid 

 fibre. 



The large round or slightly oval nuclei have been identified as be- 

 longing to the retinal cells (PI. II. fig. 7), and the smaller oval nuclei 

 occupy the deep swollen ends of the pigment cells. It is possible 

 that some of the pigment cells may not be prolonged in front of the 

 rhabdomes, and therefore not possess anterior sacs ; but I have never 

 been able to discover such. The filamentous middle portion connecting 

 the two extremities of the long pigment cells is so constant and char- 

 acteristic in maceration preparations, that pigment cells which do not 

 extend to the front of the retina must form the exception, if in fact they 

 exist at all. 



Another method employed in studying the cells of the retina, and one 

 especially instructive for the region of the anterior zone, was by the aid 

 of sections perpendicular to the retinal cells. The retina has the form of 

 a shallow bowl ; consequently in sections perpendicular to its axis the 

 deeper portions of the retina will lie at the periphery of the section, and 

 its centre will be the region nearest the preretinal membrane. 



Figure 3 represents a portion of a retinal section whose centre, and con- 

 sequently highest portion, is toward the right, and whose periphery or 

 deeper portion is toward the left. The relatively higher portion of the 



