MUSEUM OF COMPARATIVE ZOOLOGY. 191 



section is at that point below the preretinal membrane where the rhab- 

 domeres diminish into simple cell boundaries, and the five cells which 

 make a single group are here easily distinguishable. To the left the 

 rhabdomes are much larger, and have assumed their usual outlines. The 

 rhabdomes have increased in size at the expense of the cells. It will be 

 noticed that each of the cells present belongs to some group of rhab- 

 domeres, and consequently all are retinal cells. 



The section (Fig. 3, a) which w^as the next external to the one just de- 

 scribed shows practically the same condition, except that, being slightly 

 nearer the front of the retina, the rhabdomes are not quite so distinct, 

 especially in the extreme right, where in one or two groups scarcely any 

 trace of the rhabdomeres can be seen. Nevertheless, all the cells of the 

 former section can be identified, and moreover between the groups in the 

 upper right hand corner an additional cell is noticeable. This cell, which 

 by a comparison of the two sections is seen to be a supernumerary element, 

 is not a retinal (nerve-end) cell ; but since in maceration preparations the 

 outer expanded ends of the pigment cells were always found near the 

 preretinal membrane, there is every reason for considering this such a 

 cell. Moreover, when sections nearer and nearer the preretinal membrane 

 are examined, these additional cells become more numerous, until finally 

 they are with difficulty distinguished from the retinal cells. The anterior 

 sacs of the pigment cells, then, can be demonstrated on sections as well 

 aa by maceration. 



The rhabdomes never reach the anterior face of the retina, but fall 

 short of it by the thickness of several sections. This space between the 

 rhabdomes and preretinal membrane corresponds to the anterior zone of 

 deep pigmentation seen in longitudinal sections. The pigment in this 

 region is so dense that the outlines of the cells can be traced only with 

 difficulty. 



The phaospheres, as Lankester and Bourne ('83, pp. 185, 186) have 

 called the curious bodies mistaken by Graber for nuclei, are abundant in 

 the nuclear zone of the retina (PI. II. fig. 4, pha sp.). They are as small 

 as the oval nuclei around them, and often smaller, but differ from these 

 in containing usually one, and sometimes two, three, or even four highly 

 refractive dots. Lankester and Bourne state that they are usually behind 

 the nucleus of the retinal cell. In isolated cells I have never succeeded 

 in satisfactorily identifying them, therefore in Centrurus I cannot feel sure 

 of their position. In one section only has a phaosphere occurred in a 

 prenuclear position ; in all others they have been strictly behind the 

 neighboring nuclei. 



